—..'__~— ROBINSON RANCH FIELD, CRQOK COUNTY. WYOMlNG Thai: for the Degree of M. S. MICHiGAN STATE UNEVERSITY Rabat? F. Ohihaber 1960 L [B R A R Y Michigan St:- . Univenity ROBINSON RANCH FIELD, CROOK COUNTY} WYOMING BY Robert F. Ohlhaber A Thesis Submitted to the Department of Geology and the Graduate School of Michigan State University in Partial Fulfillment of’Reqniramente for the Degree of Master of Science Michigan State university East Lansing, Michigan May, 1960 PREFACE Exploration for commerical petroletm reserves in the Powder River Basin of Wyoming has continued in various stages of activity since oil seeps were first noted on the flanks of the Black Hills in the mid 1800's. Activity reached a peak during the early 1900's, which included the discovery and development of three of the largest fields in the basin; Salt Creek (1908), Big Muddy (1916) and Lance Creek (1918). Changing market conditions forced a standstill of exploration and production from the mid 1920's until World War II. Exploration and development since this time has continued at a fairly constant rate to the present day, with emphasis on particular pay-horizons prevalent during certain periods. This paper is concerned with one such horizon, the Perm-Pennsylvanian Minnelusa formation, which is currently undergoing active investigation in the northeast portion of the basin, and one field in the area from which it produces, the Robinson Ranch Field. By studying the charac- teristics of this known producing field, the writer hopes to bring to light certain criteria which might aid in future exploration for similar oil accumulations in the Powder River Basin of Wyoming. ACKNOWLEDGEMENTS The writer wishes to express his gratitude to Dr. J. H. Fisher, of the Department of Geology, Michigan State university, fbr his guidance and efforts in seeing that this paper was completed, and that the facts herein presented are essentially correct. Sincere thanks also to Dr. C. E. Prouty, Head, Department of'Geology, who made it possible to complete the study in absentia from Michigan State Uni- versity, to Dr. B. '1‘. Sandeiur, a very encouraging member of my com- mittee, and to Tennessee Gas and Oil Company, fer the time, facilities and help, particularly from Jack Bogrett. Finally, my sincere appreci- ation to my wife, Joanne, for her constant encouragement and invaluable assistance in editing this paper, and to Cecelia Hutchcraft, for her secretarial efforts in getting the manuscript in final form. LIST OF FIGURES AND TABLES LIST'OP ENCLOSURES . . . . IMWTI ON C I O O O O O WATION O O O O O O O O 0 TABLE OF CON'DBN'I‘S GEOLOGIC SEWING AND REIONAL HISTCBY. . 8mm WY. 0 O O O O O O O O O O O smnm O O O O O O O O O O 0 O O O Cretaceous.............. Jurassicand'rriassic. . . . . . . . Lower Triassic andPermian. . . . . . Lower Permian - Pennsylvanian . . . . Upper Minnelusa . . Middle Minnelusa. . Lomr Minnelusa . . STRUCTLRE........ Surface...... sub-smac° e e e e Pierre "A" Horizon Fall River Sandstone Mi nnekahta Limestone and Minnelusa Formation DWATIONALHISMY............... Mdence for Early Permian Deformation. . . . . iv Page Vi vii 22 23 25 ' 25 Lyidence Against Early Permian Deformation Mdence of Ieramide Deformation PRODUCTION............ History............ Data............. Oil Characteristics. . . . . . Trap and Reservoir Control . . Estimated Recoverable Reserves fixture Developnent and Explorat ion . . . SIGNIFICANCE or THE ROBINSON RANCH m. . Minnelusa Oil Accumulation . . . . . . . Key to Future Oil Exploration. . . . . . . BImIW O I O O I 0 O C 0 O O O O O O 0 P880 ‘8 33 39 39 kl ‘61 1*3 mm #0.» h) H FIGURES Page IndexMap-Regional....................3 Indestp-NortheastWycming ..... h .Indexhp-RobinsonRanchArea.............. 5 . Generalized Stratigraphic Section - Powder River Basin . . . 9 GraphicColmmarSection ................lO Comparison of the GooseEggSection inTwoWells . . . . . . 28 TABLES . Spudded and Completion Dates of Wells in Robinson Ranch Area-vithApproxi-ste Initial Potentials . . . . . . . . . .3k—35 Plate I Plate II Plate III Plate IV Plate V Plate VI Plate VII Plate VIII Plate IX' vii ENCLOSURES Structure Map - on tap of the Pierre Shale "A" Horizon Structure Map - on top of the Fall River Sandstone Structure Map - on tap of the Minnekahta Limestone Structure Map - on top of'the Minnelusa.rormation IsOpach Map - Spearfish Formation ~ . IsOpach Map - Combined.Minnekahta and Opeche Members of the Goose Egg Formation leapach.Map - Goose Egg Formation exclusive of the Minnekahta and Qpeche Members Electric Log Cross Section - North-South Electric Log Cross Section - East-Vest IMQDUCTIW The Robinson Ranch Area, as covered in this paper, comprises some twenty square miles in the northeastern portion of the PowderuRiver Ba- sin, in the vicinity of, and including the town of, Mborcroft. The field is a recent discovery, and at the present time is still in the early stages of development. Robinson Ranch field can be considered one of the more significant oil field discoveries in Hyoming in recent years. The importance of the field is not necessarily related to its' size, but rather to the zone from which production is obtained and the tools and methods employed in its discovery. Production is from the Permian-Pennsylvanian.Minnelusa formation. Until OctOber, 1958, the discovery date of the Robinson Ranch Field, the only other significant Hunnelusa.production in this portion of the Pow- der River Basin was from the Donkey Creek Area, same five to six.mdles to the west. The Adon Field, which lies some thirty miles northeast of Robinson.Ranch, did have some minor Minnelusa.produotion when first developed in 19h8, but is now abandoned. Pan American PetroleumCorporation is responsible for the discovery and Operation of the field. At the present time, there are ten Minnelusa producing wells and six dry 1...... in the area. Two of the dry holes were drilled by P. M. Oil Enterprizes, two by Davis Oil Company, and the other two by Pan American. In addition, there are four dry Dakota formation tests, and one dry shallow test which did not penetrate below the Pierre shale. These five dry holes were drilled before any serious thought was given to the possibility of Minnelusa oil in the Robinson Ranch Area. This paper will attempt to explain the occurrence of Minnelusa oil at Robinson Ranch, and in so doing, point out various geologic fea- tures anomalous to the region which may be used in future petroleum exploration in the area. Subsurface methods are employed almost exclu- sively, since surface work is of little value in the area. Data used in the subsurface interpretations was derived from available electric logs, sample logs, samples, and various reports of the wells drilled in the area. The sources of all other information contained in this paper came from direct contact with the oil companies concerned, petroleum reporting services, and numerous references cited at the end of this paper . LOCATION Robinson Ranch Area is located in Crook County, Wyming, in the northeastern corner of the state (Figures 1 8- 2), some fifty miles from the Montana and South Dakota borders. The area includes Sections 19-22, 27-3h in Township 50 North, Range 67 West, and Sections 3-10 in Town- ship h9 North, Range 67 West. The field proper lies northeast of the town of Moorcroft, which is included in the Robinson Ranch Area as covered in this paper. Accessibility to this area is good, with U. S. Highway 16 running through Moorcroft and county roads leading out of town directly through the field. NORTH DAKOTA MONTANA ' -‘O— —a—. SOUTH DAKOTA . fl . a - WYOMING I NEBRASKA __.__,__,|-___ COLORADO I ROBINSON RANCH AREA FIGURE I REGIONAL INDEX MAP NSON FIGURE 2 INDEX MAP NORTH EASTERN WYOMING R67W 0 u 02 I9 22 o! On ‘Im I? T 50 § 0'3 .IO 3 N ? ,, y x 9 no A .IO 0l5 o20 AIS 3| 34 MOORCROF T l : 97 6 3 ¢‘ ¢, T ¢, 49 N 02! 7 IO ¢9 INDEX MAP OF ROBINSON RANCH AREA, WYOMING LIST OF WELLS WELL a LOCATION M). no I PARRENT - BUCKMILLER NO.I NE SE SW SEC. 22 TSON RSTW II GARIEPY - BRIGGS no.2 C NW NE SEC. 29 TSON RSTW 2 PBM - BUCKMILLER no.2 SW NW SEC. 2| TSO'N RS7W I2 PAN AM -ROBINSON NO. S-S SW NE SEC. 29 TSON RCTW 3 P SM - BUCKMILLER NO.I C NW SE SEC. 2B TSON NS7W I3 PAN AM ' STATE NO. C-I C NW SE SEC. ZS TSON RSTW 4 TEXACO -ROSINSON NO.I NW NW NE SEC. 9 TQSN RSTW I4 PAN AM - ROBINSON NO.I-I SW SE SEC. 29 TSON RSTW S TEXACO - ROBINSON NO. 2 NW NW SE SEC. B TASN RSTW IS PAN AM - ROBINSON NO.I C NW NE SEC. 32 TSON R67W 6 FERLEY -DUTTS NO.I NI/2 NW SW SECS T4SN RSTW IS PAN AM -ROB|NSON no.2 C SW NE SEC. 32 TSON RSTW 7 W S M - ROBINSON NO.I NW NW NE SEC. 5 T4SN RSTW I7 PAN AM -ROBINSON NO. B‘Q SE NE SEC.” TSON RSTW B PAN AM- ROBINSON NO. C-I NW SW SEC.‘ TQSN RSTW IB PAN AM -ROOINSON NO. B-2 NE SE SEC. ZS TSON RS7W 9 PAN AM - STATE NO. C-2 SE SW SEC. 29 TSON RSTW IS PAN AM-ROBINSON NO. B-S C SE SE SEC. 29 TSON R67W IO PAN AM -' HOFFINE OOV'T. NO.I NE NW SEC. 32 TSON RSTW 20 PAN AM -ROB|NSON NO. A-3 NE NE SEC. 32 TSON RSTW 2| DAVIS-CORDELL NO.I SE NW SEC.I9 T50N RSTW 22 DAVIS -ROBINSON NO.I SE NE $60.9 T49N RS7W SCALE . r'- «00' N. F. ONLNABEN Fig. 3 NOVEMBER 20. USS. GEOLIIIIC SETTING AND REGIONAL HISTGY Crook County, Wyoming, includes two geologic features within its boundaries - the Black Hills and the Powder River Basin. The Robinson Ranch Area lies on the northeastern margin of the basin, a short dis- tance west of the fairly gentle (a few to 20 degrees) dips of the flanks of the Black Hills. A detailed account of the regional history of the area is given in Darton (1901, 1909), Darton a. Page (1925), and the Wyoming Geological Associations' 13th Annual Field Conference "Powder River Basin Guidebook (1958)". The Black Hills are a'broad domal uplift, some 120 miles long and 60 miles wide at the widest portion. A Pre-Cambrian core trends nearly north-south, flanked by upturned and truncated Paleozoic and.Mesozoic sediments. The anticline extends beyond the crystalline core, trending northwest approximately 120 miles. The uplift is slightly asymmetrical; the east flank has dips of MS degrees or greater, the top nearly flat, and the west flank fairly gentle with dips of 20 degrees or less. Adjacent to the west flank of the Black Hills, and extending west- ward to the Big Horn Range, is the broad Powder River Basin. Cretaceous, Paleocene, and Eocene sediments make up the surface of the basin, with early Tertiary beds reaching a thickness up to 6,000 feet at the deepest portion, as evidenced by seismic work. With the exception of a few well established anticlinal structures found along the western and southern margin. of the basin (Salt Creek, Billy Creek, Big Muddy and Lance Creek), almost no reversals of dip are to be found in the Powder River Basin. Using Darton's (1925) estimate of 9,000 feet of uplift for the Black Hills, the structural relief between the bottan of the Powder River Basin and the top of the Black Hills, is approximately 20,000 feet. Because the Paleocene and Eocene deposits of Lax-snide age are separated from the uplift by upturned and truncated Cretaceous out- crops, no overlap exists, and the age of deformation can only be ap- proximated. The source of the Upper Cretaceous Lance and Fox Hills formations in the Powder River Basin may hm been from the inalift, thus dating the dming as late Cretaceous. The distribution of the Eocene Wasatch formation seems to indicate that the uplift had occur- red prior to that time, and was furnishing some of the sediments. On a geologic basis, it is doubtful that the major uplift of the Black Hills occurred prior to the uplift of the Bighorn Mountains, which can be dated as post Ft. Onion (Paleocene) Pre-Hasatch. Scattered outcrops of White River sediments in the Pumpkin Buttes Area, Township Mt North, Range 76 West, and abundant rocks of White River age in the Hartville, Lance Creek Area (southeastern.margin), in- dicates the basin was receiving sediments as late as Oligocene time. SURFACE GEOLOGY The Upper Cretaceous Lance formation is the surface formation in the Rebinson Ranch Area. The re: 3111- (Upper Cretaceous) sandstone crops out approximately one and one-half miles to the east of the field prOper, and Fort Union Tertiary sediments lie some twenty miles to the west . The Lance fomation consists of a gay to yellowish gray sand- stone and gray shale, with a few thin beds of carbonaceous shales. T0pography is generally rough, with a series of ridges and val- lays striking approximately na'thwest by southeast, parallel to the trend of the Black Hills. Relief within the area averages 150 feet, with maxim relief approximately 300 feet. The field itself lies in a fairly flat valley bottom fomd by an intermittent stream drain- ing north. Drainage of the area is by intermittent stremns flowing north by northwest into the Belle Fourche River, making it part of the Missouri River watershed. MIGRAPHY The stratigraphy of the Robinson Ranch Area is typical of the eastern margin of the Powder River Basin in Wyoming. Figure h represents surface and subsurface rock units present in this region. The thicknesses given from the surface Lance formation through the Skull Creek formation represent averages for that portion of the Pow- der River Basin; those given from the Fall River sandstone through the Minnelusa formation indicate the actual range of thicknesses found with- in the Robinson Ranch Field. . Only those members of particular interest to the petroleum indus- try are differentiated within formational units. Formations such as the Pierre and lower Sundance contain msppable members not usually con- sidered in subsurface exploration, and for that reason they are not TL'HICECJ 1.33 3Y1? 5,3’1‘3/ SERIES GROUP FORELATIOI“; fill—1821’? FEET \L'LJQ LI'ITHOLOGY Shale, gray, and sandstone gray to yellowish Lance GOO - 750 :r y. Fox Hills 150 - 200 Sandy shale and siltstone; light gray sandstone. 0 1.8.4.8 Pierre 3.250 - 21-70 Shale, dark: {gray to ;_-;ray. thzmmlomimflmnmmssfiflenmenms Viobrara 100 be tonite beds. Shale, black to dark gray; abundant calcareous Upper _:age Ereafis 200 so cretioxs. Cretaceous Colorado Carlile anr;er Sapd 200 500 3a QBtODBLithi; bedded. Shale, dark gray, numerous limestone and Unnamed Shale lOO iro stone co cretions. Shale, light gray, calcareous, interbedded Cree horn 300 — 350 :arl. Shale, black and gray, fissle, abundant Belle Fourc e #50 ~ 600 heutonite and ferruginous concretions. shale, black-brown black, siliceous, be tonite lowry 150 ~ 130 sandstone stringers, fish scales Sandstone, silty, medium grain and shale, Hewcastle O — 30 sandy, some lignite and bentonite. Shale, dark bluish gray, scattered lenticular [ Lower Skull Creek 2 - clay - iron concretions. , Sandstone, brown, fine to coarse :rained, ‘ Cretaceous Tnyar Fall River 55 — 65 massive cross bedded. Kara Shale, varigated, red, maroon, gray and buff- Fuson 3O - 35 sandstone, gray to yellow. Sandstone, gray, coarse grained, massive cross Lanota M5 - 55 bedded, conglomerate. Upper fiorrison YO ~ 1H0 Shale, green, gray-green, maroon, waxy, sandy. Shale, gre n-gray, silty, glauconite and Jurassic Hyper Sundance lhO - 150 interbedded glauconitic sandstone. snnIe, gray to green-gray, sandstone, gray Lower Sundance loo - 210 to tan, fine grained, thin limestone, sandy. ' Shale, siltetone and sandstone, soft, red, Triassic pipearfish #15 - 590 argillaceous. “oose Egg 3l5 - 355 Shale, siltstone and sandstone, red to ocher, linnekahta hgw Dolomite and limestone tan some gypsum and “ ‘ 1 haylite. Permian 0986116 37 hale, sandy, red and brown. , Upper #00 1 Minnelusa 1150 Series of sandstones, carbonates, anhydrites, A and red shales. 1 Pennsylvanian Liddle 200 lower 350 NOTE - Chose thicknesses from the Fall River formation through the Minnelusa.arelrepmeoentative of the Robinson Ranch Area. All other represent average thickness in the northeastern Powder River Basin. PAN AMERICAN ROBINSON NO. I C NW NE SEC. 32 T50N RGTW KB 4254 svsrcu . umomcv I ROCK mm [ LATEROLOG J I ! 3:12-32: { i g F“: linu— I \ ‘5 1:3 ’ no 7 " to ‘ r o: m / sums mun KARA 5P. 4 Ju @722: 5 ~' — F—a‘ 1—. : w >§< - A J. .1 . . V E: 3;. ': - l:_"— K— C" =13 =1 _’f ‘f: f n gefii-{Jis uoamsou Fu ('1 *&31§E~f§ a Ey...‘:__*z;;v r. . iii—$24135 4.": — ’-"-- "w are ”5—. 2::- . .45 :—-.-.—:—..—_7—_:_.- u. sunonuce Fu. 0 ‘5 f ::'£gf_.:_ ; :“5'::—:~1—‘L— t5 ‘ (n I '.:..; 9'4; ' ‘ Fir—cat??? . L g m ‘5 ' 'v—u '- 5 0 - Z" 7'1.-. " vb. --v “ ~§+ fit-5.7". .- "F- " ns :..-3:} '_."‘ :g‘fffi' -. L. suuonucc m. f. ‘- w I .-._, .- 5:22; 15;}? ~§- raises: _ ‘2 :1:- - 8 ‘ .7. 532 :'_-:-'5‘“ spunnsu m. - Rik 2 . E e E w i .- ‘2- '§l s. q," :5" -¢ GOOSE EGG FM. ~"‘._.__.—-.=.- rg" c117..- - F.-£ -g. E ____________ ._ _ .4 2. - ., Z .. ' g MINNEKAHTA man. 4-4" a . _-_-_,_____ ___ __--._. _ a: u ‘L‘.#I~u - . one": man. é 31' \ “W \“).".\\" ‘l‘xx ‘— 4 _:_\;1._~._.9.-.o;,.3._v ( 3 E ‘ H" \ \‘ \ "an :3 MINNELUSA m. -.- .a ,4» awakens—rec- .__ , ., ..-.- -*-.-'.--: a L H ‘42:) ' a? a " GRAPHIC COLUMNAR SECTION CORRELATED WITH LATEROLOG LOWER CRETACEOUS TO LOWER PERMIAN I}. OHLHAIEI IOVEIOER “J,” E Ioo' OOLOMITIC SANOSTON SALT aunvonmc uuosrouc 50' [:3 eemomre oomumc sun: 0. ouucomre Aunvonmc SHALE mm“ "M :1 “E" noouus - DOLOIIITIC “moan: COAL [2,: OIL snow - POOR “”370“ E on. snow - coco Fig. 5 ll mentioned here. Since this paper is not a stratigraphic study, only the productive Minnelusa. formation will be discussed in detail. The remainder of the stratigraphic colmnn is covered in general tens, with sane emphasis on a few zones of interest. CRETACEGJB Marine gray to black shales dominate the stratig-aphic section throughout Upper Cretaceous time, These shales are interhedded or interfinger with discontinuous sandstone further vest in the basin. A few or the sands are continuous through this portion or the basin and produce oil. The Turner sand member of the Carlile formation has pro- duced stall quantities of oil, and can be correlated to the first Hell Creek sandstone found westward in the basin. The lower Cretaceous section represents a greater variety or depo- eitional environments. In the Black Hills proper, marine sediments from the Mowry Shale through the Fall River sandstone are separated from the varied continental facies of the Mon-Lakota sequence by a disconrornity representing the advancing Cretaceous Sea. Within the Puller River 3.- sin, however, the Mon-Mote sequence is at least in part marine, indi- cating e. transgressive sea within the basin at this tin. The Mcwry and Skull Creek formations are marine shah facies, sepa- rated by the marginal marine and deltaic deposits of Newcastle sandstone. The Newcastle sandstone is an excellent oil reservoir and produces at various fields near the Robinson Ranch Area. The Fall River sandstone, which, along with the Newcastle sandstone, has accounted for virtually all the oil production in the northeastern 12 portion of the basin prior to the discovery of Minneluse oil, overlies the Fusen-Lakote sequence nentiomd above. It is common practice, es- pecially within the oil industry, to call the Fall River sandstone the Dakota sandstone. However, since Dakota is such a general term and ap- plies to various parts of the entire Inyan Kara Group at different re- gional localities, this term seems undesirable. JURABSIC MD MASSIC The Jurassic rocks within the area include both continental and marine deposits. The Upper and Lower Sundance fornations are urine sediments, overlain by the continental Harrison fomation. The contact between theilorrison and Upper smashes is a .11th one to pick both at the surface and in the subsurface, due to the re-worked nature of the Upper-Sundance sediments. The first occtn'rence of a glenconitic sand is generally conceded to indicate the Upper Bundance, and it is on this basis that the contact was determined for this paper. A great regional unconfomity exists between the Jurassic sequence and the Triassic 8pesrfish fomation. The thickness of'the Bpearf‘ish varies greatly because of the unconfomity, even in the limited area covered in this paper. The Triassic rocks are easily identifiable in the region as a series of soft, red, sandy ehales, siltstones, and ar- gillaceous sandstones. mum TRIASSIC AND mm The 150 - 200 foot sequence of anhydritic red siltstones, anhydrites, red siltstones, and thin linestones above the Minnekahta limstone and 13 Qpeche shale formerly were included in the Spearfish formation. Sub- sequent work by c. A. Burk and H. D. Thomas (1956) indicated that this interval is widespread, easily recognized, and can be correlated.with the upper Phosphoria and Dinwoody formations of western Wyoming. This unit and the underlying Minnekahta limestone and Qpeche shale is now known as the Goose Egg formation. Thomas (l93h) described the boundary between the Permian and.Triaesic systems as heing:msrked.by the top of the Ervay limestone member, from foesil evidence at the type locality in the northern Rattlesnake Hills of central wyomdngt Although the Ervay limestone is not present in the Rebinson Ranch Area, there is a distinct lithologic break within the Upper Goose Egg fermstion, from a red shale to a'bed of anhydrite. Correlating the anhydrite with the Ervay lime- stone, the Permian-Triassic boundary is placed 60 - 90 feet below the top of the Gome Egg formation. The lower portion of the formation is Middle Permian as determined by fossil studies (Burk, c. A., 3. Thomas, H. D., 1956) and unconfbrmahly overlies the Pennsylvanian and early Permian Minnelusa formations. The Minnekahta limstone and Opeche shale numbers have been carried in geologic literature through the years as fbrmstional units. It is this writer's intention, however, to fbllow the‘wycmdng Geological Association's most recent terminology as stated in their 1956 publication "Wyoming Stratigraphy", and assign these units as members within the Goose Egg formation (Figure t). These members, particularly the Minnekahta, which is dominantly a dolomite rather than a limestone, are easily recognized in the subsurface from.electric and sample logs, and are used.extensively as mapping horizons in subsurface exploration. 1h WEB PMIAN ~ PENNSYLVANIAN Various names have been applied to the sequence of sandstones, carbonates, evaporites, and red shales that outcrop between the Permian red beds and the Mississippian Madison formation on the various uplifts surrounding the Powder River Basin. The Casper, Bartville, and Minnelusa formations may all be considered as rock equivalents, representing facies and thickness changes from one part of the basin to another. This paper will use only the name Minnelusa, and apply it to the above mentioned sequence in all regions of the basin. The Tensleep and Amsden formations of the Big Horn Mountains are not, however, rock equivalents. They are equivalent in time only to the lower and middle members of the Minnelusa formation, and must be treated as such. ' The Minnelusa formation can be divided into three separate time rock units on the basis of regional unconformities; the Upper, Middle, and Lower members. The entire formation is marine, with the basal sand- stone of the Lower member being a possible exception. By outlining the general regions of production within the Powder River Basin for each mem- ber, it becomes readily apparent the south-eastern portion of the basin accounted for almost all Minnelusa production prior to the discoveries of Donkey Creek and Robinson Ranch fields to the north. UPPER mm The Upper Minnelusa member consists of sandstones, carbonates, (pre- dominantly dolomite) and anhydrites, totaling approximately'hOO feet in thickness. The percentage of sandstone to carbonates and evaporites averages 50 - 80 per cent sandstone as against 20 - 50 per cent carbonates and evaporites . 15 The Black Hills Area appears to have been a slight positive fea- ture at the time of Upper Minnelusa deposition as evidenced by the high percentage of sandstone within the region. The source of the sandstone, however, was apparently from the west rather than the east. The nearly identical lithologic character of the Upper Minnelusa sandstones (wolrcampicn) to the Middle and Upper Pennsylvanian Tensleep sandstone in the Big Horn Basin, indicates the older sandstone (Tensleep) was deposited in western and central Hyuning, uplifted and eroded, then re-deposited as Upper Minnelusa to the east. Paleontological evidence, as derived from various surface sections throughout the state, determines, the age of the Upper Minnelusa as woii‘cemp (Agatston, R. 3., 195%), or Lower Permian. It has been a some- what false conception, particularly within petroleum circles, to consider the Minnelusa synoncmous with Pannsylvanian, although recent discoveries such as the Robinson Ranch Field, are bringing to light what appears to be a more correct age interpretation. As shown graphically on Figure 5, two sandstone beds, separated by an evaporite and carbonate section, occur near the tap of the Upper Minnelusa. These sands apparently correlate with the Converse sandstone of the Lance Creek Field, located in the soutlnastern portion or the basin, and are named the First and Second Converse sands. The Converse sandstones are excellent reservoirs, and constitute the only productive zones within the Minnelusa in the northeastern region of the Powder Riwr Basin, as evidenced at Donkey Creek and Robinson Ranch Fields. l6 MImLE MINNEIMSA The Middle Minnelusa is bounded, both top and bottom, by regional unconformities. The uppermost unconformity, separating the Upper and Middle Minnelusa meinbers, is marked by a red shale marker bed. The basal unconformity separates Middle Pennsylvanian (Desmoinesian) from Lower Pennyslvanian (Atokan). The lower unconformity can readily be picked in the subsurface by characteristic kicks on both electric and radioactive logs, caused by a definite lithologic change at this point. Sandstones and carbonates, with a few persistant thin beds of black shales, dominate the Middle Minnelusa section. Because they are con- tinuous and have characteristic radioactive log kicks, the black shale beds are a valuable aid in long range correlation of the section. The mateat percentage of sandstones are found in the western portion of the basin, indicating a salt ea in that direction. The oil producing sands of the Middle Minnelusa in the southeastern portion of the basin are called the "Leo" sandstones, and represent eastern extensions or tongues of the main sand bodies to the west. The age of the Middle Minnelusa, as determined by fusulinid'dsta from surface sections (Agatston, R. 8. - 19510, is Virginian, Missourian, and Desmoinesian (Upper and Middle Pennsylvanian). Not all the formations are present in various parts of the basin due to truncation. Except for two pronounced variations, the section generally increases in thickness from northwest to southeast, with maximn deposition occurring adjacent to the Hartville Uplift. A marked thinning is apparent within this thick section at Lance Creek, indicating contemporaneous uplift with deposition. A second pronounced sons of thinning is found in the north- eastern portion of the basin caused by Tertiary intrusives. 17 Oil production from the Middle Minnelusa Leo sandstones is presently confined to the southeastern portion of the Powder River Basin, primarily the Lance Creek Area. Lawns moat The Lower Minnelusa member constitutes the mind» of the formation, from the regional unconformity at the base of the Middle Minnelusa to the erosional top of the Mississippian Madison formation. It correlates with the Ansden type section of the Big Horn Mountains, and is Atokanian and Morrowanian in age. The tsp of the Lower Minnelusa is usually determined in surface sections or samples by the presence of an abundant pink or orange chart, and in subsurface by a marked decrease in magnitude in the curves of electric and radioactivity logs, indicating a thin bedded sequence of shales and carbonates. The Lower Minnelusa is usually divisable into three units ) upper cherty carbonates and red shales; cherty limestone; and basal sandstone. All the units are fairly constant in areal extent and thickness with the exception of the basal sand, or "Bell" sandstone as it is known. The discontinuity and variable thickness of this sandstone is probably due, at least in part, to the underlying, karat-type topography of the Madison formation upon which it was deposited. The Reclamation limestone, a fairly clean carbonate directly above the basal sandstone, accounts for the only oil production to date from the Lover Minnelusa, this being restricted to the southeastern portion of the basin at Lance Creek and West Mule Creek fields. 18 SURFACE There is not direct evidence of a structural feature at the sur- face in the Robinson Creek Area. The discontinuous gray my. and sandstone lenses of the Lance fermation make surface measurements next to impossible, even in a restricted area such as is covered here. The outcrop pattern of the Fox Hills sandstone, however, may indi- cate a northlest plunging nose and synclins as observed in Sections 9 and 10, Township #9 North, Range 67 west. The re-entrant or the rox Hills into the basin could well be the surface expression of the sub- surface structure responsible for production at Robinson Ranch.Field. The general outcrop pattern of the Fox Hills sandstone, taken from U.8.G.8. Map»OMhl9l, is outlined on the four subsurface structure maps, -Plates I, II, III, and Iv, and the probable relation to the structure demonstrated. BUBSURPICE The subsurface structure of the area is shown in Plates I, II, III, and IV. Pan American Petroleum.Corporation first discovered a subsurface structural anomaly in the Rdbinson Ranch Area in August, 1957, using seismic methods. subsequent drilling has verified the high and the boundaries of the anticline have been more definitely defined. The structure is a slightly asymmetric anticline, having a generally north south axis. The anticline apparently resolves into two separate highs, with depth, as seen on the contour maps of the Minnekahta and Minnelusa horisons, Plates III and IV. The amount of closure variesfrom the different horizons, but in general is somewhere around no to 50 feet, possibly' more. Formation taps and correlative points used in the construction of the structure contour maps were derived from.awailsble electric logs run on the wells. These logs are of three types; Schlumberger Electric (E) log, Induction-Electric (I-ES), and Laterolog (LL). The Laterologs were available only on those wells which penetrated to the Minneluss. All well elevations were derived from a common source - The Power Sur- veying Company of Denver, Colorado. Distant well spacing, and in some instances, no well control at all, necessitated a large amount of interpolation. for example, in the southern region of the area (Township h9 North), only one well out of five was drilled to the Minnelusa, that being Pan American's Rebinson 0-1, in Section 8. The fact that no wells have been drilled north of the field, makes it virtually impossible to determine the northern ex- tent and configuration of the structure. P. M. Oil Enterprises Buckmiller No. 2 Uhit, SH SH, Section 21, Township 50 North, Range 67 West, shown as a location, would help to solve this problem if completed. At last report (March, 1960) the well was spudded and surfhce casing set, but some sort of legal entanglement has prevented the drilling of the well to its proposed.Minnelusa depth. various petroleum industry reporting services still carry the well as a location, and until it is reported as an abandoned location, the possibility of it being completed still exists . Although there is no direct evidence of a syncline between the . P. M. Buckmiller No. 1 well in Section 28, Township 50 North, Range 67 Rest, and the field wells to the west, this writer believes that it does exist, and in the approximate location shown on the structure map. The attitude of the various horizons mapped at the above location indi- cate they are more canpatible to the regional dip than to the structure 1 and, a surface expression of a syncline can be demonstrated in the out- crop pattern of the Pox Hills sandstone in Section 10, Township 1&9 North, Range 67 West. Further discussion of this problem follows, as each mapped horizon is reviewed separately. PIERRE BHALE "A" MIZON - PLATE I Plate I represents the structure contour map of the Pierre"A" hori- zon. The ”A" horizon is an arbitrarily chosen correlative point within the Pierre shale, easily recognizable on the logs of all the wells in the area. The purpose of mapping such a point is to determine if the structural feature present at depth is also reflected in the attitude of the younger rocks nearer the surface. The depth from surface to the "A" horizon averages from 2,700 to 3,000 feet within the area. Several factors prevented the choice of a horizon closer to the surface. They are as follows: 1. The varied lithology of the surface Lance formation results is uncertain correlations within that fonation. . 2. The absence of a horizon fran one well to another due to tepogrsphic relief. 3. The various depths to which surface casing was set. Elec- tric logs cannot be made through casing; therefore, a hori- zon seen in the log of one well may be masked by casing in another, even though it may be present at both locations. h. Lack of a definite correlative point in the Upper Pierre shale. ' . The choice of the "A" horizon is further Justified in that it has a characteristic log curve of a bentonitic zone or bed and represents a time-boundary marker. The basic attitude of the structure at this horizon, as well as the other mapped horizons, was obtained by both well control and by following the general configuration of the Fox Hills sandstone outcrOp. A reversal of the regional west dip is indicated between P. M. Buckmiller No. 1 and the Pan American nun wells, Robinson A-3, 13-2, 3-3, and 3.1:. If the P. M. Bucloniller No. 1 well was placed so as to be on the flank of the structure, a marked broadening of the contours would result, causing a terrscing of the east flank of the anticline. This would also involve using one point to detemine a structural low, which should be avoided when possible in subsurface work. The importance of this map 1. that it demonstrates that the structure is continuous through the Upper Cretaceom, even though it is not evident on the surface. This has important implications concerning future petro- leum exploration in this portion of the Powder River Basin, where un- disturbed Tertiary deposits and continental Cretaceous deposits at the surface make surface exploration unreliable, and many of the wells from which data must be used for subsurface exploration have penetrated only the shallower Cretaceous formations . FALL RIVER mam, PLATE II The structural picture of the Fall River sandstone is essentially the same as discussed above for the Pierre "A” horizon. A reversal of dip amounting to more than 90 feet is indicated, with the exact amount of closure unknown. The anticlinal axis has migrated slightly to the west and south with depth. The westward migration is to be expected, since the struc- ture is located on regional west dipping strata of the eastern margin of the basin. The structural high, or crest, appears to be in the imp mediate vicinity of Pan American's Robinson B-l, while the discovery well, Pan American Robinson no. 1 is 1h feet lower at this horizon. The Fall River tests, located in Township h9 North, Range 67“West, were drilled off structure, and offered no clue, before seismic work was initiated, that such a structure existed in the area. This illustrates that unless well control is unusually dense, possible structural features may be missed entirely'byvmapping on the Pall River sandstone or shallower horizons. Although the Fall River sandstone is an important reservoir for oil in the Powder River Basin and produces at nearby Donkey Creek, it is apparently'barren at Robinson Ranch. To this writer's knowledge, only .three of the Minnelusa tests (discounting the four Fall River tests in Township #9 North) had any oil shows within the section and either test- ed or cored this interval. The wells and results are as follows: 1. Briggs-Gariepy No. 2, Center NW NE, lection 29, Township 50 North, Range 67 West. Cored . recovered It} feet of stained sand. No test. 23 2. Pan American Robinson No. 1, Center nu ma, Section 32, Town- ship 50 North, Range 67 went. Drill Btu Test. Recovered 3,850 feet of fresh water. 3. Pan American Robinson C-l, 1W 6W, Section R, Township 1&9 North, Range 67 West. Drill Stem Test. Recovered 15107 feet of water. One apparent anomalous feature of the structure at this horizon is the abnmt northwest swing of the contours indicated by the control af- raided by Briggs-Gariepy No. 2, north of the field. Without additional drilling in this area, it is difficult to determine if the structure actually starts trending northwest at this point, or if this is si¢ly a local feature of the Fall River sandstone, independent of the structural trend. ummmm mam AND MINNELUBA llitliliATIOliJ PLACE III m PLATE IV The tOp of the Minnelusa formation represents an erosional surface which is sometimes difficult to correlate on electric logs. The Minne- kahta limestone, on the other hand, has a characteristic log kick, and can be correlated accurately over larm distances. .The two horizons are discussed together, since they are separated only 60 or 70 feet strati- graphically and the stmctures mapped on both are essentially the sense. A major change in the subsurface structurel pattern is evident when mapped on the Minnekahta and Minnelusa horizons. At this depth the anti- cline apparently resolves into two highs, separated by a saddle in the vicinity of Section 32, Township 50 North, Range 67 West. This inter- pretation is based on indirect evidence, as listed below. 2h 1. There is a very slight difference in elevation of the Minne- kahta limestone and Minnelusa formation from Pan American Robinson No. 2, Center SW NE, Section 30, Township 50 North, Range 67 Vest and Pan American Robinson C-l, Center NH $4, Section 1+, Township 1&9 North, Range 67 West. The horizontal distance between the two wells is over 1,100 feet, yet the depth of the Minnekahta and Hinnelusa varies only 25 feet. If a second high were not present as shown, an abrupt terrac- ing of the south plunging nose of the anticline would exist. 2. It is believed that Pan American Petroleum Corporation drilled . their Robinson C-l well (location given above) to test a second structu- ral high indicated by their seismic records. Since Pan American will not release any seimic data, this is Just an assmnption and cannot be confirmed. There are, however, few other valid geologic reasons for drilling this well, which is more than a mile fran established production. The fact that the well was dry does not discount the presence of such a feature, as the well apparently missed the structure. Another feature of the structure at this horizon shows the anti- clinel axis swinging to the northeast near the crest of the main or larger high. Whereas the structure appeared to trend northwest when mapped on the Fall River sandstone, it now appears to trend slightly east of north. This is probably the basis for locating P. M. mickmilhr No. 2 in Section 21, Township 50 North, Range 67 West. At least 30 feet of closure can be demonstrated, with another 20 to ho feet probable. The crest of the anticline is approximately in the same location as it is at the Fall River sandstone horizon, except that it now extends farther south of include Pan American Robinson No. l, 25 Section 32 - the discovery well. It can be seemed Pan American lo- cated this well according to seismic data, to drill into the crest. DWIWAL HISTGRY ORIGIN , The origin of the structure at Robinson Ranch is questionable, even upon the consideration and integration of the structural, sedi- mentary, and stratigraphic data available within the area. Little doubt exists that the structure was affected by the Laramide orogeny, but it may actually have originated much earlier in mologic time, during the Post Upper Minneluaa, Pre-Gooee Egg period. Discussion both for and against the concept of early Permian de- formation is presented below. As an aid in determining the sequence of events, three isoPach maps are included in this pqer. Plate VII represents the thickness of the combined Minnekahta and Opechc nuabers; Plate VI the thickness of the Goose Egg "shales" (undifferentiated) and exclusive of the Minnekahta and Opechc members) and Plate V, the thickness of the Spearfish formation. EVIDENCE FOR EARLY-mm WIGN 1. Plate VII, indicates that the thinning of the combined Minne- kahta and Opechc members coincides with the crest of the anticline. Al- though the magnitude of thinning is not great (15 feet), it is sufficient to be mapped on 5 foot contours. Since no evidence of truncation of these units is apparent, this zone of thinning can be interpreted as the result of deposition over a structural high, dating the structure as Post Upper Minnelusa - Pro-Opechc (Goose Egg). 2. its isopach map of the Spearfish formation, Plate V, also clear- ly shows a marked thinning over the structure. Disreganiing the nominal eastward regional thinning of the formation, the minimum thickness with- in the area occurs aJnost directly on the anticlinal crest. This fea- ture is in direct accord with the attitude of the Opeche and Minne- kahta members discussed above, and supports the theory of structural deformation within the area prior to Larmids movement. 3. A more positive approach to dating the stnicture as having originated at this time would be an isopach map of the Minnelusa, or at least of the Upper Minnelusa member within the Robinson Ranch Area, but lack of well data prevents this from being done. A regional study of the Upper Minnelusa, however, demonstrates that deformation within the basin possibly did occur during this time on a much larger scale (Foster, 1958). A marked thinning within the Upper Minnelusa may represent a buried, truncated, anticline, trending in a north—west direction along the eastern side of the Powder River Basin. If this interpretation is correct, it is probable that the smaller feature at Robinson Ranch originated at the same time. h. The fairly rapid changes in thickness over relatively short distances of the Goose Egg formation within the basin indicates deposition on an erosion surface of moderate relief (Privrasky, Strecker, Grieshaber, and Byrne, 1958). This change in thickness of the Goose Egg is apparent in the Robinson Ranch Area, Plate VI, but its attitude is such that dis- cussion is included in another section of this paper to demonstrate evidence against early Permian defamation. . S. A widespread breccia zone at the top of the Upper Minnelusa may represent erosion of that unit. A truncated unconformity can be observed on the surface between the Goose Egg (Opechc shale) and the Upper Minnelusa in the Sundance-Beulsh Area of Crook County, Wyming (Brady, 1958) an}, Section 8, Township 52 North, Range 61 West. Here, the horizontal Opechc shales overlie Minnelusa strata which dip ap- proximately 8 degrees southwestward. In the basin, however, evidence of truncation depends entirely on individual interpretation and cor- relation of subsurface data (samples, electric logs, seismic), and thus is Open to argmnent. EVIDENCE AGAINST EARLY-PERMIAN W01! 1. The isopaeh map representing the thickness of the Goose Egg "shales' exclusive of the Minnekahta and Opechc members, Plate VII, can be interpreted as evidence of a depositional trough or low during Upper Permian time, and in the approximate position of the present structural high. An east-west section of the isopach shows the Goose Egg thickening towards the anticlinal crest. A north-south section generally parallel to the structural axis does, however, show some thinning at the crest. Note Figure 6, a canparison of the Goose Egg section between two wells in the Robinson Ranch field. Pan American Robimon B-l represents a location at or near the crest of the anticline - - Robinson 3-2 a well on the eastern flank of the structure. The two wells are typical of the area, and reflect similar occurances in all the other well lo- cations . Pan Amrican Robinson B-l Pan Merican Robinson 3-2 i - __ 7\ ___. ___.“ __ im- Egg Shales _ L- » Increase in Thickness :.- Mostly Confined. to This Zone L P :3 ‘__) - i __ .—-———- ""‘""' '21:": ‘b— "" Minnekahta Member :- ___—___.”- I Figure 6. Conparison of the Goose Egg sections of two Robinson Ranch Field wells showing the sons of thickening. The following facts can be observed fmm Figure 6: (a) An increase in thickness of to feet from Robinson 3-2 to 3-1, or fran the flank to the crest of the anticline. ' (b) The change in thickness is mainly confinsdto one some- an evaporite sons. (c) The daninantly elastic sones are of a fairly constant thickness. Since no samples or sample descriptions are available for the two wells cited above, it is asstmed, by laterolog correlation, that the entire Goose Egg section is lithologically similar to the section at Pan Amrican Robinson No. 1, which was described fran samples and is shown graphically on Figure 5. The lithologic character of the Goose Egg indicates deposition on a broad low land subject to alternate wetting and drying conditions in an arid, restricted, basin type of environment (Krumbein a Glass, 1951). The zone which accounts for most of the thickness change is domi- nantly a massive, dolomitic anhydrite, with a few thin stringers of red shale and siltstone. The question now arises as to what might account for the increased precipitation of anhydrite from one location to another at Robinson Ranch. The precipitation of salts from sea water is a complex process involving many factors, both physical and chemical. Under drying conditions during Goose Egg time, sea water may have con- centrated in lows or depressions, accounting for the increased thickness of evaporites in restricted areas. This explanation is based on Clark's (1921;) interpretation of the origin of methadone thick salt deposits in areas having generally thinner, constant, evaporite section. If this is fact, and there was such a depression in the area during Goose Egg time, the origin of the Robinson Ranch anticline would probably have to be attributed to Laramide movement. 2. Deformation can be attributed to a cone of weakness in the basement complex. Such a weakness must have been in evidence during the Late Pennsylvanian or early Permian period if the Robinson Ranch structure originated at that time, and probably remains a zone of weakness to the present time. It is difficult to imgine why such a zone would not urder- go greater defamation than what is apparent, due to the great forces that must have been active in that area during the Laremide orogeny. EVIDENCE OF LARAMIDE MOVDIENT The Powder River Basin had its origin in Late Cretaceous-Early Tertiary time, contemporaneous with the Laramide orogeny which elevated the mountain masses surrounding it (Eardley, 1951). Deformation was most intense along the western margin, near the steep west flank adjacent to the Big Horn mountains. Local defamatians are also prevalent along the eastern margin, however, as evidenced by the structures associated with production at Donkey Creek and Robinson Ranch. These structures were apparently influenced to a major degree by the Black Hills uplift. (Strickland, 1958). Upturned and truncated Late Cretaceous Lance sediments indicate deformation occurred some time after these continental sediments were deposited. Apparently undisturbed weeetch (Eocene) sediments are dis- tributed around the uplift (Black Hills) in such a manner to indicate the uplift had occurred prior to their deposition and was furnishing some of the sediments for these rocks (Eardley, 1951). It is evident that local defomation within the basin, such as the structures at Robinson Ranch and Donkey Creek, either underwent additional movement or originated during this period of logs scale deformation. These structures, especially along the eastern margin of the basin, ex- hibit similar stratigraphic and structural characteristics, and may be considered to have identical geologic histories. 31 PRODUCTION HISTORY Oil production in the northeastern Powder River Basin dates back to before the turn of the century. The years 1885 to 1900 saw many temporary oil booms around the town of Moorcroft. The presence of oil in the region had been known since the gold rush days in the Black Hills, as oil seeps were noted by prospectors on the western flanks of that large domal uplift. The presence of shallow Cretaceous oil in commercial quantities satisfied most of the oil companies Operating in this portion of the basin, although oil in the Minnelusa was known to exist for some time. In 1923, the United States Geological Survey released a press statement, part of which reads: ‘ "Perhaps the most promising formation is the thick.Mdnnedusa sandstone, and no test well reaching the deeper sands in the region should stop short of the farmation . . . . some oil has been found in it, its great thickness and.porous character fit it admirably for a reservoir, and the great mass of’marine limestone below may well be a source of oil. All oil fOund in the Minnelusa so far is black and heavy." Until very recently, however, the testing of the Minnelusa formation was confined to the flanks of the Black Hills, while wells in the Basin preper tested only the shallower Cretaceous pay sands. The Texas Company made the first important.Minnelusa discovery in the northeastern Powder River Basin with its Adan NO. 1 Uhit in Section 2, Township 52 North, Range 72 West, in January, 19h8. The well had an initial potential of 2th barrels per day at 31.5 A.P.I. gravity oil. Three subsequent wells 32 were dry, and the one producer was abandoned in 1956 after a total cumulative production of 32, 513 banels of oil. The method of dis- covery was seismic, disclosing an anticlinal trap with approximately 100 feet of closure. The discovery of Minnelusa oil at Donkey Creek was the next step toward proving the economical feasibility of producing oil from the deep- er horizons within the basin. Donkey Creek was discovered in 1953 as a Dakota (Fall River) Lower Cretaceous producer. The presence of a possible oil pool was noted as early as 1950, when the Mule Creek Oil Company drilled a dry hole to evaluate geophysical anomalies, both gravity and seismic. Drill stem tests of the Dakota formation recovered substantial amounts of water and slight shows of oil. The area remained inactive until True and Bram Producers (now True Oil Canpany) drilled the Dakota discovery well in April, 1953. Development of the Dakota horison has continued to the present time. It was not until March, 1957, that True Oil Company drilled their Minnelusa discovery well. This well, the Burrows B-6, C NW NW, section 8, Township 19 North, Range 68 West, flowed at the rate of 28 barrels of 31° A.P'.I. gravity black 011 per hour firm a 20 foot section in the top of the Minnelusa‘. Development of Minnelusa production within the Donkey Creek area has proceeded continuously since that time, with 15 wells now pro- ducing. Pan American Petroleum Corporation acquired their first of two productive Robinson Ranch leases in May, 1957, Just two months after the Minnelusa discovery at Donkey Creek. Subsequent seismic work, started the following August, revealed a structural anomaly, which was drilled 33 in October, 1958. The second productive lease, Robinson "B", was acquired in January, 1958. The delay in drilling is attributed to clearing up the minor mineral interests on the leases. The discovery well, Robinson No. 1, c nu ma, aootion 32, Township 50 North, Bang. 67 West, had an initial production of 29h barrels of 26.1; A.P.I. gravity oil per day. Altogether, Pan American acquired four leases, Robinson "A", W Katrine, Robinson "B", end State or Hyuning "c". All leases were acquired before the discovery well was drilled, and all have production. Robinson Ranch differs from Donkey Cnek in that the latter was pri- narily developed for Lower Cretaceous Dakota (nu River) fonation pro- duction, end only after dewelomcnt or this sons had continued for a period or four years was the possibility of Minnelusa-oil production ex- ploited. Robinson Ranch, W, was a Minnslusa play from the start, and is the only field in the northeastern Powder River Basin producing exclusively from the Persian-Pennsylvanian horison. Eamon DATA Pan American Petrolem Corporation has released only those pro- duction figures for the last two months of 1958 and the first eleven months of 1959. (The first full year of development of the Robinson . Ranch field). Boom» the data is in tom of production by leases by months, it is necessary to integrate from Table I the completion dstes of each well in various leases, and show which wells are included in the monthly production totals. Both oil and water production figures are shown in terms of barrels per month. Table I follows on the next two pages. ‘EEEESEI Pan American Pan American Pan American Pan American Pan American Pan American Pan American Pan American Pan American Pan American Pan American Pan American TABLE I Spudded well No. ngpleted Robinson B-l l-2h-59 2-19~59 Robinson 3-2 3-1-59 3-31-59 Robinson 3-3 3.20-59 1542-59 Robinson B-h hole-59 5-10-59 Robinson B-S 5-13-59 6‘27-59 Robinson 1 10-12-58 11-28-58 Robinson 2 1.9-59 2-8-59 State 0-1 12-h-58 1-12-59 State 0-2 2-10-59 3.2u.59 Katrine l 3-7-59 h-l9-59 Robinson A-3 2-2-59 -2-59 Rdbinson Col 5-29-59 6-18-59 Buckmiller 1 5-5-59 8-8-59 Buckmiller 2 Abandoned Location Bottomed In Minnelusa Minnelusa Minnelusa Minnelusa Minnelusa Minnelusa Minnelusa Minnelusa Minnelusa Hinnelusc Minnelusa Minnelusa Minnelusa 3h .12 Sum 376 30?!) 39080?!) 580 BOPD 63 no #73an 29h BOED 300 BOP!) 228 BOPD 360 no 102 BHPD 10% no 75 Burn DeA D & A D & A Table I (continued) Parrent Briggs-Garpiely Lester Perley Texaco Texaco ‘W & M Davis Davis Buckmiller 1 Robinson 1 Butts 1 Robinson 1 Robinson 2 Robinson 1 Cordell 1 Robinson 1 10-6-53 11-17-53 11-9-56 5-27-57 5-18-59 6-1-59 items 5-31-13 6-11MB 7-19-1.8 8-20-55 9-13-55 2-3-60 2-18-60 2-2ho60 3-15-60 Pierre Fall River Fall River Fall River Fall River Fall River Minnelusa Minnelusa D 35 & A & A & A & A & A & A & A See Index Map, Pig. IP BOED BHPD D & A Initial Barrels Barrels 3, for locations Production 011 Per Day Water Per Day Dry and Abandoned L. H. ROBIRSON "A" LEAN Year and Month 21.]; Water 1958 November 1, 11-85 0 December 7.955 15 January 1&197h .30 February 18, 263 1:073 March 20,h10 8,ulk April 17.219 10:03“ ‘ W 18,723 12,772 June 18. 591 17,010 July 19.?“ 11303 August 23,989 18:279 September 20: 51" 221078 October 16:“‘3 2h,876 November 16,339 22:903 Total ' m 2M2: Wells Pan Am «- Robinson No. 1 Pan Am - Robinson No. i We, 19% - "We, 1959 Feb., 1959 - NOV-g 1959 L. H. ROBINSON "B" LIASI _Ygar and Month 9}}, M 1959 1" chum 5: 531 50 April 36: 227 11 81-6 MW 58, 316 29: 186 June 118, 1081 22, 329 July #9. 198 30. 963 August 1‘7: 375 a) 093 Septeiber 1‘2:305 291310 com": u, am all “£9 am: 33,923 38.3% Total M m 3.1)., 1959 - Me, 1959 April, 1959 - Ram, 1959 April, 1959 - Mu 1959 May, 1959 - Rom, 1959 July, 1959 "' no": 1959 Wells Pan Am - Robinson B-l Pan Am - Robinson 3-2 Pan Am - Robinson B-3 'Pan Am - Robinson B-h Pan Am - Robinson B-S 37 STATE OF HYUHING "C" LEASE Year and Month 31; Water 1959 January 7.665 93 February 13,391+ 192 Mb . 181513 39983 April 211978 2,910 m 21: 589 219% June 17. 333 15:59? July 18,772 18,035 August 20,868 21,313 September 16,522 11,390 October 15.253 16.9% November 16,089 215516 w 421218 m wells Pan Am - State 0-1 Jan., 1959 - Nan, 1959 P” All " gut. 0-2 “Itch, 1959 - Me, 1959 E.M.,WLIASI Year and Month Q_i_l_ m 1959 April 1,230 3,870 May 1) 53“ 5: 301 June 916 9:889 July 1:055 29805 August 82 3,007 30pm 832 2:91-0 October 553 19275 November 651 l, 70 Total 6,823 any: Wells Pan Am - Hofflne NO. 1 April, 1959 - Nah, 1959 'Ibtal cmlatiwe production for the Robinson Ranch field during this period mmtod to 1,310,939 barnels or fluid -- 780. 591 barrels of oil and 530,3108 barrels of water. In ter- of percentages, oil accounted for 59.5 per cent of the total fluid, and water 10.5 per cent. The 38 average production of oil per well is 6,500 barrels per month or 215 barrels per day. This figure may be misleading, as many averages are. For example, the one producing well within the E. M. Hoffine Lease -. Hoffine No. l, averages a little more than 20 barrels of oil per day, compared with 500 barrels of oil per day for Robinson No. l in the L. H. Robinson "A" lease. It is interesting to note the relationship between the emolmt of water recovered and the location of a well relative to the structure on the Minnelusa horizon (Plate IV). The discovery well, Robinson No. l, is located on or near the crest of the anticline, within the L. H. Robinson "A" Lease. It produced nearly 2h,500 barrels of oil and only #6 barrels of water for the first three months of production (see production figures).. Robinson No. 2, on the some lease, was completed 2 months later in February, 1959 , approximately 50 feet lower strati- graphically than Robinson No. 1. Observe, from the production data, how the amolmt of water recovered increased to over 1,000 barrels per month for this lease after Robinson No. 2 was completed. Similar occurrences can be shown for each of the other leases. Hoffine No. 1, stratigraphically the lowest well in the field, has pro- duced approximately four times as much water as oil. 0n the L. '11. Robinson "B" lease, water recovery was held to 50 barrels a month when Robinson 3-1 was completed. This well is stratigraphically the highest in the field, located on the anticlinal crest (Minnelusa horizon). Upon the completion of the four other wells on the "B" lease -- Robinson 3-2, 13-3, 13-h, and 3.5, on the flanks of the structure, the total mount of water recovered per month is now above oil production. 39 OIL CHARACTERISTICS The character of the oil produced at Robinson Ranch from the Minnelusa formation generally conforms in composition to oil indigenous to Pemaian rocks throughout the Powder River Basin. The general characteristics of Robinson Ranch oils are listed below: API Gravity --.---------.------. 27 (Robinson Ranch . 26A) Sulphur Content t -----------... 2.0 Base ---..-..----------.-------.- Armatic - Raphthene Gasoline Content $ .-----..----.. 2.5 Carbon Residue $ --------------- 5.8 (Btrickland, J. 14., 1958) The sulphur content (2.050 and carbon residue (5.8%) is high, canpared to the low (2.5” gasoline content. Persian oils also have a high asphaltene content. This oil is, on the average, of a better quality than oil of the same age found in the Big Horn Basin. TRAP AID RESERVOIR CONTROL Basically, Robinson Ranch Field produces from an anticlinal trap, approximately one mile long, having between 50 and 100 feet of closure. Pressures are supplied by a water drive, obviously effective as evidenced by the large amounts of water being produced with the oil. Indirect evidence of a tilted water table may be demonstrated by the varying amounts of water produced at the different locations. Robinson A-3, Section 32, Township 50 North, Range 67 west, is a dry hole located only one quarter of a mile east of Robinson No. l, the discovery well. 110 Hoffine No. 1, a producer, is located a quarter of a.mile west of the discovery well. Structurally, Rebinson Ae3 is approximately 30 feet higher than Roffine No. l and yet produced only water when the pay zone areas perforated and tested. Consequently, the pay none at Robinson A-3 must be downdip from the oil-water contact. The ratio of oil-water - 12h, produced at Hoffine No. 1 indicates it too is close to the oil-water contact. Thus, there is a difference in elevation of 30 feet in the oil-water contact from the east flank of the anticline to the west flank. That is, the contact is 30 that higher to the east. Similar situations can.be shown throughout the field. 0n the basis of the above data, i.e., relative amount of water and oil produced from each well, an oil-water contact is shown on Plate IV, the structure mapped on top of the Minnelusa.f0rmation. This is only an approximate outline of the contact, but it clearly shows a definite tilt of the water-oil interface to the west. To map this con- tact more accurately, additional well data, such as micrologs or macro- laterologs are required. Pan American Petroleum Corporation, however, will not release this data at the present time. The apparent tilted water table at Robinson Ranch is not an anomalous feature of’oil fields in the basins of'Hyoming. Similar occurrences are numerous, particularly in the Big Horn Basin. One noteble‘example is . the Frannie Field in Park County, which exhibits a tilt of over 600 feet in the water-oil interfece from the east flank to the west flank of the anticlinal trap responsible for production. The theory of tilted water tables, as described‘by'Mt K. Hubbart (1953) results frah water entering a formation at the outcrop and moving kl down the regional dips basinward. This water flow may displace oil trapped in anticlinal structures, and "push" it basinward until it is off center on the structural high. This is probably what happened at Robinson Ranch. The Minnelusa formation outcrops some 35 miles east of the Robinson Ranch area in the Black Hills. Water entering the fomtion at this point flows down the regional west dip into the Powder River Basin. This water flow displaced the oil trapped in the anticline, and mod it off-center to the west, thus accounting for the tilted contact . EBTIMA'ED RECOVERABLI am Until additional data is released by Pan American Patrolman Corpor- ation, an accurate figure of the recoverable reserves within the Robinson Ranch Field cannot be given. The January 25, 1960, issue of "The Oil and Gas Journal" estimated the primary recoverable reserves at 9, 225,000 barrels of oil. As of January 1, 1960, 818,000 barrels of oil had been produced from the field. housing "The Oil and Gas Journal" estimate to be correct, more than 8 million barrels of oil remain to be recovered. NI'URB 13W AND WITHIN Future develOpment within the area will probably be confined to exploration of unproven acreage outside the field itself. Drilling on loo-acre spacing, which is the required spacing of wells in the area, would necessitate locating tests fairly well off structure and downdip from the oil-water interface, as shown on Plate IV. ha The best play appears to be the indicated second high south of the field in Sections h and 5, Township #9 North, Range 67 West (See Plates III and IV). Pan American attempted to drill this structure with their Robinson C-l Minnelusa test in Section h, but apparently missed, since the well was reported dry with no shows. Based on this ‘writer's interpretation of the structure, a.Minnelusa test should.be drilled sanewhere in the vicinity of C NE NE Section 5, Township #9 North, Rang. 67 west. The man wells drilled in the area were two Minneluss. tests by Davis 011 oompcny, both dry holes. Davis-Cordell No. 1, c as aw Section 19, Township 50 Earth, Range 67'Hest, did not encounter any oil or gas shows and was not tested. This well was drilled some distance off structure, probably to evaluate the possibility of a stratigraphic trap within the Minnelusa.formation. The second well, Davistdbinson No. 1, c as m Section 9, Talnship 1.9 North, Runs- 67 west, was muse to test a possible nosing of the Minnelusa, indicated on the surface'by the outcrop pattern of the Fox Hills sandstone. SIGNIPTCANCE 0? TBS ROBINSON RANCH FIELD Prior to 1958, exploration for oil along the eastern margin of the Powder River Basin was mostly confined to the relatively shallow, Cretaceous sandstones, namely the frontier (Hall Creek), Newcastle, and Fall River. Discovery and.production of.Minne1usa.oil at Adon and.Donkey Creek fields heightened interest in the search for petroleum from this ' deeper horizon, which was climaxed.by Pan American's discovery in late 1958 of the Robinson Ranch Field. In 1959, intensified deep drilling into the #3 Paulo-Pennsylvanian Minnelusa formation resulted in the apparent dis- covery of at least one new Minnelusa field. Champlin Oil and Refining completed a Minnelusa well in Section 36, Township 51 North, Range 68 West, known as Prong Creek, with initial production of 500 barrels of oil per day. Lack of control in the area prevents this writer from determining if acclmmlation is controlled by structure or stratigrqhy, although subsurface regional control suggests structure . MINNELUSA OIL ACCUMUIATION Oil accuzmlation within the Minnelusa formation at Robinson Ranch is related directly to an anticlinal trap. Similar anticlinal structures along the eastern portion of the Powder River Basin produce, or have produced, Minnelusa oil, including Aden (abandoned), Donkey Creek, and Lance Creek Fields. Thus far, no Minnelusa production ha been obtained by a purely stratigraphic type of trap. This is understandable, as the Converse pay sand (upper member of the Minnelusa formtion) is a continuous, blanket type of sand and apparently does not pinch' out within the basin. The anticlinal structure responsible for the entramnt of Minnelusa oil at Robinson Ranch originated either during Post Minnelusa - Pre Goose Egg time or during the Laramie orogeny - with movement during Laramide time well established. The Hinnelusa oil at Robinson Ranch is probably indigenous to the carbonate-evaporite facies of the upper member from which it is produced. Movcsent of the oil into the anticlinal trap occurred contemporanemisly with deformation - either during early Permian or late Cretaceous time. Curtis, Strickland, and Busby (1958) mention an apparent 11-h relationship between oil distribution and marked "thins" represent PreoGooee Egg vertical movements along which oil migrated and was trapped during Goose Egg deposition. According to Figure 2 of Foster's Minnelusa paper (1958) , Robinson Ranch is located within a Minnelusa "thick", and is a direct contradiction of this theory. ROBINSONRANCH-ABAKEYTOWOILWIW Future caloration in the eastern portion of the Powder River Basin will probably disclose many anticlinal structures similar to the one found at Robinson Ranch. The varied geologic phenomena which occurred in the Robinson Ranch Area down through geologic time were active throughout the region, as evidenced by regional studies of the Powder River Basin and the Black Hills. hirther study (of some of the geologic conditions found at Robinson Ranch and discussed in this paper may warrant their use in the continuing search for Minnelusa oil in the Powder River Basin. The discovery of the Robinson Ranch Field must be attributed to seismic work, with no assistance from photogeology or surface mapping. A photogeologic map of the area does not show any strong evidence of structures, nor is the drainage pattern anomalous to the region. The importance of surface or photogeology should not be minimized, however, as it was probably the slight hint of structure offered by the outcrop pattern. of the Fox Rills sandstone that initiated the seismic program. Areas which show rapid variations in the thickness of the Spearfish formation should be studied carefully. The thinning of this unit over the structure at Robinson Ranch may be indicative of similar structures throughout the region. 165 Delineation of structure by the usual subsurface methods (i.e. electric logs and sample logs) is very improbable because of the sparse well control in the region. Anticlinal structure was not evident at Robinson Ranch prior to seismic work, although four Fall River formation tests had been drilled in the immediate vicinity years before. Exploration should not be confined to areas where regional. studies disclose "thinning" of the Minnelusa forntion. The concept that the "thins" represent Post-Kinnelusa, Pre-Ooose Egg vertical movemnts, and that oil accmlated in the him may be valid, but the very fact that Robinson Ranch is located in a regional Minnelusa "thick" indicates oil accmmnlation is not confined to these areas. Finally, allowances for the migration of the structural axis with depth must be considered on anticlinal structures discovered at the shallower horisons. Careful study of the possible effects that hydro- dynamic conditions may have on the acctmulation of oil within the structure should be made, as evidenced by the tilted water table at the Robinson Ranch Field. amass AGATSTON, R. 8., 1951;, "Pennsylvanian and Lower Permian of northern and. Beaten] “Ming, H Bulls Amre ASSOC. Patrols Mel V01. E 132 508-583- BRADY, F. H., 1958, "Eyeporite deposits in the Minnelusa formation in the Wee-Beulah Area, Crook County, Wyoming", Rye. Gaol. Assoc. Guidebook , 13th Annual Field Conference. ' BURK, C. A., and moms, B. D., 1956, "The Goose Egg formation (Perme- Triaseic) of eastern Wyoming, " Oeol. Sunny of WW Investigations No.. 6. CLARK, r. w., 1921;, "The Data of Geochemistry," u. s. Oeol. Survey Bull. 770. owns, a. F., smxcum, J. w., and mm, a. 0., 1958, "Patterns of oil occurence in the Powder River Basin, Ryming" in Habit of Oil, AMI. MOO. htmle Gale EB $8-222s BARTON, N. 3., 1901, "Preliminary description of the geology and water resources of the southern half of the Black Hills and adJoining regions in South Dakota and "young," U. 8. Cool. Survey Ann. Rept. 21. at in pp l$97-60“!- 1909, "Geology and water resources of the northern Wmoimm regions in South rum. and Uyuning,” U. S. Gaol. Survey Prof. Pm. am, N. 11., and PAIGE, 5., 1925, "Description of the Central Black Hills," U. 3. Cool. Survey Atlas, Folio 219. RARDLBY, A. J. , 1951, Structural Geolog of North American, pp 21-33;. roam, D. I., 1958, "Sunni-y of the stratigraphy of the Hinnelusa formation, Powder River Basin, Wyoming," Eye. Geol. Assoc. Guide- bookI 11th Annual Field Conference. HUBBERT, M. K., 1953, "Entrapment of petroleum under hydrotynamic con- ditions," Bull. Amer. Assoc. Petrol. Oeol., Vol. 1Q Ro.:8, pp 12514- 2026. mm, w. c., and ms, L. :.., 1951, strstim and ssdinsntstion, pg 13, fig, 3_6_2. PRIVRAm, II. C., mom, J. R. W, C. R. and arm, F., 1958, "Preliminary report on the Goose Egg and Chugwater fbnaations in the Powder River Basin, liming," yo. Oeol. Assoc. Guidebka 3:33 , Annual Field Conference. ' STRICKMRD, J. W., 1958, "Habitat of oil in the Powder River Basin," [10. Cool. 511°0- Ouidebook, lath Annual Field Conference. >.( }/!.f SUPPLEMENTARY HATER! A! ,IHIWHHllllHlW’ljllllllllllllllll'llillll'ilhllllr 92092