x \ I W M ~ \( 1 ‘ AK — k____, ,A Wl A RAND METHOD FOR DETERMENING THE MAGNESiUM f CALOUM RATiOS OF WELL SAMPLES AND iTS USE AS AN A80 ZN PREDICTEHG STRUCTURE AND SECONDARY MOSITY 5N CALCAREOUS FORMATKONS Thesis éor tho Doom 6? M. S. MICH‘EGAN STATE COLLEGE Richard Louis Jacky 1954 y m mm In HII mm mm 1|th MM“ 3 1293 1 This is to certify that the thesis entitled A rapid method for determining the magnesium/ calcium ratios of well samples and its use as an aid in predicting structure and secondary porosity in calcareous formations presented by Richard Louis Jodry has been accepted towards fulfillment of the requirements for Master of Science degree iDWd Geography 787‘0 owl; Major professor Date May 27L 1954 0-169 7? . I '- -~ m L - i .. L n 9 ‘1. is: t i ["5 F. ""‘P v“ . .. x; L . f ‘..E F .‘ x v"! 1 mm P Sills-84981930 1 Lemmas A RAPID METHOD FOR DETERMINING THE MAGNESIUM/CALCIUM RATIOS OF WELL SAMPLES AND ITS USE AS AN AID IN PREDICTING STRUCTURE AND SECONDARY POROSITY IN CALCAREOUS FORMATIONS BY RICHARD LOUIS JODRY A. THESIS \ t l\\'.\\‘*“ Submitted to the School of Graduate Studies of Michigan— ' State College of Agriculture and Applied Science in partial fulfillment of the requirements for the degree of MAS TER OF SCIENCE Department of Geology and Geography 1954 IHEsui ACKNOWLEDGMENTS The writer wishes to express his sincere thanks to Dr. B. T. Sandefur, of the Department of Geology and Geography, Michigan State College, for his many suggestions for the improvement of this report. He also wishes to give grateful acknowledgment to Dr. S. G. Bergquist and other members of the staff of the Department of Geology and Geography, Michigan State College, for their help in checking the manuscript. The Geological Survey Division, Michigan Department of’ Con- servation; the Sun Oil Company, Mt. Pleasant, Michigan; and the Cities Service Oil Company, Mt. Pleasant, Michigan, furnished many of the well samples used. Their kindness in furnishing samples is g re atly app re ciated . ii 338858 ABSTRACT Fields producing oil from the Rogers City dolomite in the Michigan Basin are becoming increasingly more difficult to discover. To aid in new discoveries, a method of sample analysis based on the lVersenate method, which quickly gives the magnesium/calcium ratio of well samples, has been devised. A study of these ratios shows that dolomitization of the Rogers City limestone occurs at two places: (1) on the apices of folded structures, as a result of solution and replacement by ascending waters along tension fractures; and (Z) in long narrow bands, the result of high-magnesium waters ascending faults and master fissures. These dolomitized areas have distinct flank-zones of decreasing magnesium/calcium ratios, which aid ma- terially in predicting the location of dolomites, and their distance from control points. Secondary dolomites in the Rogers City lime- stone are usually porous and serve as good reservoirs for the pos— sible accumulation of oil and gas. This method of magnesium/calcium ratio analysis should aid materially in predicting the presence of local secondary dolomitiza- tion in any calcareous formation and in any area, and thus help to iii locate more easily possible oil production in reservoirs created in secondary dolomite 3. iv TABLE OF CONTENTS Preparation and Digestion of the Samples ........... Determination of Calcium ...................... Determination of Magnesium .................... Calculations ................................ ORIGIN OF DOLOMITE IN THE MECOSTA COUNTY AREA ...................................... RELATIONSHIP BETWEEN DOLOMITIZATION AND POROSITY ................................... POSSIBILITIES FOR OIL PRODUCTION IN THE MECOSTA COUNTY AREA ....................... CONCLUSIONS ................................ BIBLIOGRAPHY ............................... TABLE II. LIST OF TABLES Formation tops and well data ....... Individual well sample analyses results vi Figure LIST OF FIGURES Page Columnar section of the Devonian System ........ 6 Photograph of the laboratory setup ............. 19 Ratio map, average magnesium/calcium ratios .................... ' ........ rear pocket Structure contour map of the Michigan ”Stray" sand ....................... rear pocket Structure contour map of the Rogers City formation ...................... rear pocket Relationship of thickness of the Bell shale to dolomitization of the Rogers City formation .............................. 37 Relationship between dolomitization and porosity ............................ 44 vii INTRODUCTION In fields producing oil from the Rogers City formation, in the central Michigan Basin, production is generally limited to dolomite zones. This formation is normally a fossiliferous, marine limestone, . . 1 with no primary dolomite in the central Michigan area. In its normal 1 G. V. Cohee and L. B. Underwood, U. S. Geol. Survey Oil and Gas Investigations, Preliminary Map 38, "Lithology and Thickness of the Dundee Formation and the Rogers City Limestone in the Mich- igan Basin” (1945). form, the Rogers City formation apparently has no effective porosity capable of containing oil or water. Yet, fortunately, when the forma— tion is penetrated by wells on structure, it is often found to be an extremely porous and permeable dolomite. This dolomite is common in the western half of the central portion of the ”Basin," particularly in Clare, Osceola, Mecosta, Isabella, and Montcalm Counties. The area has been well defined by Cohee and Underwood. Ibid. ** Often the oil fields producing from the dolomite zones in the Rogers City formation are very small in area, and are difficult, if not impossible, to locate other than by closely spacing subsurface con- tour points. In some localities the dolomite zones are not directly related to folded structure, but extend in narrow, straight bands often less than one-quarter of a mile in width and sometimes several miles in 3 length. Such zones are impossible to locate by conventional methods, 3 K. K. Landes, "Porosity Through Dolomitization," Bull. Amer. Assoc. Pet. Geol., Vol. 30, No. 3 (Mar., 1946), pp. 306-307. and are found mainly by chance. Dolomite on top of folded structures, and in bands unrelated to structures, is not limited to the Rogers City formation, nor to the 4 central portion of the Michigan Basin. Landes describes a narrow- 4 Ibid.. pp. 305-309. banded type of dolomitization, unrelated to structure, in the Adams and Deep River pools in Arenac County, Michigan, and also mentions the dolomitization in Trenton structures of Ohio and Indiana. Sim- ilar types ofvlocal dolomitization apparently occur in many limestone areas throughout the United States. Because of the thick glacial drift covering the surface of central Michigan, no geophysical techniques have been developed which will satisfactorily indicate dolomitized structures in this area. To'obtain core-test data, wells must be drilled to a depth of approx— imately 1,200 feet in order to reach reasonably diagnostic marker- beds. Anticipated oil production from the small reservoirs which might be expected to remain undiscovered cannot justify the eXpense of extensive core-test programs. It appears, then, that to find further production economically from the Rogers City formation a new simplified technique must be developed which will reveal the presence of potential reservoirs. This paper describes the development of such a technique: the use of magnesium/calcium ratios, to indicate the proximity of porous zones resulting from secondary dolomitization of calcareous forma— tions . LOCALIZATION OF THE PROBLEM In order to demonstrate this new technique clearly, a relatively small and closely drilled area seemed desirable, so that an intensive study of geologic data might be made. The area chosen was the east-central portion of Mecosta County, Michigan, including Wheat- land (T.14N.,R.7W.), Morton (T.I4N.,R.8W.), Sheridan (T.15N.,R.7W.), and Martiny (T.15N.,R.8W.) Townships. This area lies directly south- west of the Coldwater oil field, Michigan's most prolific structural- 5 type Rogers City field. It lies west of the Sheridan field, and four 5 C. R. Criss and R. L. McCormick, "History and Performance of the Coldwater Oil Field, Michigan," Paper Z78-G, Amer. Inst. of Mining and Metal. Eng. miles south of the Fork field, another prolific producer from the Rogers City formation. Favorable structure and reservoir develop- ment are indicated by the forty-four wells penetrating into the Rogers City formation and by forty-five shallower wells. However, only two single well pools have been discovered in the area. The apparent abundance of control is not ample to contour the area satisfactorily. Five separate structural interpretations, each with convincing logic, have been seen by the writer. STRATIGRAPHY The study in Mecosta County is centered on the Rogers City formation of Middle Devonian. It is overlain by the Bell shale mem- ber of the Traverse Group and lies conformably on the Dundee for- mation (Figure 1). Because of similar lithology of the Rogers City and Dundee formations in some areas, the term "Rogers City" is seldom used in oil field parlance, but the two formations are spoken of collectively as the "Dundee." In some areas the formations are colloquially designated as ”Dundee” for the upper, or Rogers City, formation, and "Dundee Restricted" for the lower, or true Dundee formation. In this report the formations will be referred to by their correct names. In the area under consideration the Dundee grades from a tan, primary, nonfossiliferous marine dolomite, which in places is porous and permeable, to a limestone with the same general characteristics. 6 as. The Rogers City, as described above, is a marine limestone from 25 to 40 feet thick. FIGURE I GENERALIZED COLUMNAR SECTION OF THE DEVONIAN SYSTEM IN THE MICHIGAN BASIN AFTER HELEN H. MARTIN MICHIGAN GEOLOGICAL SURVEY SYSTEM GROUP FORMATION SOUAW BAY THUNDER BAY POTTER FARM NORWAY POINT TRAVERSE FOUR MILE DAN ALPENA NEWTON CREEK GENSHAW FERRON POINT ROCKFORT QUARRY BELL SHALE ROGERS CITY CAZENOVIA . DEVONIAN LUCAS AMHERSTBERG DETROIT FLAT ROCK SYLVANIA R|VER BOIS BLANC GARDEN ISLAND ANALY TICAL TE CHNIQUES In attempting to establish a new technique for locating addi- tional areas of oil accumulation, a careful chemical analysis of well samples from the Rogers City formation was made to determine accurately the relative percentages of magnesium oxide and calcium oxide present. The resultant percentages are expressed as a mag- nesium/calcium ratio. These ratios were plotted areally on a map, 7 W. C. Krumbein and L. L. 51055, Stratigraphy and Sedimentation, Freeman and Co., San Francisco (1951), Chapter 13, "Stratigraphic Maps.” to demonstrate the lateral extent of dolomitization, and thus may be used to predict dolomitized reservoirs within a definable distance of existing wells. Standard methods of determining magnesium and calcium content of limestone rocks are normally tedious and time-consuming, . 8,9 involving precipitation and separation of the rock constituents. 8 Assoc. Official Agr. Chemists, "Official and Tentative Methods of Analysis,” 6th ed. (1945), pp. 42-50. 9 L. M. Powell, ”Calcium Carbonate—Magnesium Carbonate Ratios Of the Pinconning Field," Thesis, University of Michigan. The method employed in this study was described recently by Cheng, 10 Kurtz, and Bray. It is a rapid, simple, and accurate procedure to 10 K. L. Cheng, T. Kurtz, and R. H. Bray, ”Determination of Cal— cium, Magnesium, and Iron in Limestone,” Anal. Chem., Vol. 24 (Oct., 1952.), p. 1640. directly determine calcium and magnesium, without precipitation. Except for the preliminary work of Cheng, Kurtz, and Bray, this study of magnesium/calcium ratios would have been more difficult. In using this method, a set of well samples may be analyzed chem- ically for calcium and magnesium in little more time than is needed to examine them microscopically. Because this method has been outlined in chemical publications, not readily available to geologists, it is outlined here in detail. Following the outline of the basic method of analysis is a description of its adaptation Specifically for magnesiur’n/ calcium ratio determination. Preparation and Digestion of t_h_e__S_a_rnpl_e_§ 1. Samples should be washed and dried. The ”junk" iron may be quickly separated from the sample with a imagnet. Separate manually any obvious pieces of "cavings." If a ratio is the result desired, as in this paper, it is not necessary to remove small flakes 9 of shale (unless extremely limy) and other relatively inert substances. If exact percentages of calcium and magnesium oxide are desired, all foreign substances must be removed. 2. Place a LOGO—gram (approximately, if a ratio is desired) sample into a ZSO-milliliter beaker and add cautiously 10 milliliters of perchloric acid. 3. Heat the solution gently until it becomes colorless, then evaporate to dryness. After complete evaporation the sample should be cooled. 4. Dissolve the cooled residue by adding 3 milliliters of l- to—l hydrochloric acid and 10 milliliters of water. Filter the solution and dilute to 250 milliliters with water. It is essential that distilled water be used for all aqueous solutions, and that all equipment be 11 washed thoroughly with distilled water. 11 To determine the effect of common tap water or unclean appara- tus on the results, an anlysis was made using 10 m1. of tap water in place of the digested rock sample. The tap water analysis showed that a positive error of 4.6% CaO and 1.0% MgO would result if tap water were used instead of distilled water. This error approaches a maximum of 20%. 10 Reagents Egr§ena£e solutign. Dissolve 4 grams of the disodium salt of 12 (ethylenedinitrilo) tetraacetic acid in 1 liter of water. Standardize 12. Versenate, Murexide, and F241 indicator may be obtained from the Hach Chemical Co., Ames, Iowa. this solution against a standard calcium solution by titrating 10 mil- liliters of the standard calcium solution with Versenate in the manner described below. Standard calcium solution. Dissolve 2.500 grams of reagent- grade calcium carbonate in approximately 5 milliliters of l-to-l hydrochloric acid (warm gently if necessary) and dilute to exactly 1 liter with water. This solution contains 1 milligram of calcium pe r millilite r. Potassium hydroxide. Use a 20-percent aqueous solution. Calcium indicator powder. Mix thoroughly 40 grams of pow- dered potassium sulfate and 0.2 gram of Mirexide. 11 Titration 1. Pipette a IO-milliliter aliquot of the solution to be analyzed into a ZOO—milliliter porcelain dish, then add approximately 20 milli- liters of water, 1 milliliter of potassium hydroxide, and a tiny scoop (20 to 30 mg.) of calcium indicator powder. 2. Stir the solution and titrate with the standardized Versenate. The end point is reached when the color of the solution changes from pink to violet. De te rmination of Magne sium Reagents Versenate solution. Prepare as above. Buffer solution. Dissolve 60 grams of ammonium chloride in approximately 200 milliliters of water; add 570 milliliters of con- centrated ammonium hydroxide, and dilute to 1 liter with water. Potassium cyanide. Prepare a 10-percent aqueous solution. F 241 indicator. Dissolve 0.15 gram of Eriochrome Black T- (F 241) and 0.5.gram of sodium borate in 25 milliliters of methanol. 12 Titration l. Pipette a 10—milliliter aliquot of the solution to be analyzed into a ZOO—milliliter porcelain dish, then add 25 milliliters of water, 2 to 3 milliliters of buffer solution, a few drops of potassium cyanide solution, and 8 drops of F 241 indicator. 2. Stir, and titrate with the Versenate solution. The end point is reached when the color of the solution changes from wine— red to clear blue. Standardization of the Versenate solution for magnesium is calculated below. Calculations If 1 gram of sample is “made up'I-to a 250-milliliter solu- tion, and a 10-milliliter aliquot is taken for titration, calcium and magnesium can be calculated as follows: 25 x 1.4 x B 1,000 I> x x 100 = percent calcium oxide Cx25xl.66xLD-B) 1,000 x 100 = percent magnesium oxide A = milligrams of calcium per milliliter of Versenate solution (see Example 1 for standardization). B = milliliters of Ve rsenate solution used in titration, with Mure xide a s indicato r . 13 C = milligrams of magnesium per milliliter of Versenate so- lution (see Example 2 for standardization). D = milliliters of Versenate solution used in titration, with F 241 as indicator. Example 1. Standardization with standard calcium solgt_i_o__r_1_. The standard calcium solution prepared contains 1 milligram of calcium per milliliter of solution. Twenty—three and seven— tenths milliliters of Versenate solution were necessary to ti- trate, to the end point, 10 milliliters of the standard calcium solution. 10 mg. calcium (solution) ——-{> 23.7 ml. Versenate. ‘Versenate = 10 mg. Ca/23.7 m1. = 0.422 mg. Ca/m1.("A” in e quation) . Example 2. Computation .Jf milligrams of magnesium per milliliter of Versenate (Standardization). Versenate = 0.422 mg. Ca/ml. Versenate (above). Atomic weight of Mg _ 24432 Atomic weight of Ca 40.08 Versenate = 0.422 x 0.607 = 0.256 mg. Mg/ml. Versenate (”C" in equation). 14 Essmls .3_-_ __C_0_rsps.t_et_ies_2_f_£e 52992139392291 _a_n_d__ 13.6.13: 6 n t C alsiu m; —-—————_— ——-—-————— If these procedures were followed, and in the analysis 24.2 milliliters of Versenate were used in titration with Murexide and 37.8 milliliters of Versenate were used in titration with _F 241, and using standardized solutions, then: A. = 0.422; B = 24.2; C = 0.256; D = 37.8. Substituting into the formulae above: 0.422 x 25 x 1.4 x 24.2 1,000 x 100 = 35.7% CaO 9.256 x 25 x 1.66 x (37.8 — 24.2) 1,000 x 100 = 14.4% MgO Magnesium/calcium ratio = 14.4/35.7 = 0.403. If weights and measures are made with reasonable care in the preparation of the standard calcium solution and in each prepara- tion of Versenate solution, the quantities ”A.” and ”C” in the formulae will tend to become constants. The results were identical in each of twenty standardization tests made by the writer, using the same stand- ard calcium solution and successive solutions of Versenate: A = 10 mg. calcium (solution) -—-{>" 23.7 ml. Versenate = 0.422 mg. Ca/ml. C = 0.422 x 0.607 = 0.256 mg. Mg/ml. 15 When standardization results in these values they may be substituted in the formulae as constants, greatly simplifying compu— tations . Thus: (l._4_22x25x1.4x_B_ 100 _ tCO 1,000 x — percen a , or 1.477 x B = percent CaO. 0.256 x_2_5 x 1.66 x (D - 1,000 B l x 100 = percent MgO, or 1.0624 x (D - B) = percent MgO. Reasonable care in this analysis should give values for CaO and MgO within 0.5 percent accuracy. The magnesium/calcium ratio is desirable to express the results. of analyses, since the figure may be readily used as a point of contour or graph control. The use of the magnesium/calcium relationship (as opposed to calcium/magnesium) produces a ratio which variesbetween 0.000 and 1.000 for limestones and dolomites, thus keeping within well-defined limits. The magnesium/ calcium ratio accentuates small changes toward the dolomitic rocks, whereas changes in rocks of a predominant calcite nature are mini- mized. Variance in a calcium/magnesium ratio is from 1.000 to infinity for dolomites and limestones, thus making contouring or graphing impractical. Emphasis is placed on minute changes in rocks containing principally calcium. MASS ANALYSIS TECHNIQUES It may be noted that in the computations each individual sam- ple was analyzed for percentage of magnesium oxide and calcium oxide, and these results were expressed as a ratio. If the desired 'result of analysis is a ratio, much of the foregoing process may be eliminated. The desired magnesium/calcium ratio may be expressed as follows: A. . . EX%-$E-%§% x 25 x 1.66 x (D - B) “lease. = 1.000 x 10° ”/0 CaO A x 251 303.4 x B x100 [At.Wt.M [AT—WT_C§l x 1.66 x (D - B) = 1.4 x B This may be expressed as (D - B)/(c x B) where ' 1.4 C ‘ leafless] x 1 66 [At.Wt. Ca] ' [arms-Mg] . In Example 2, [At.Wt.Ca] is 0.607. Thus, c = 1.389 In its simplest form, the formula for determining the magne- sium/calcium ratio for a given sample may be expressed as follows: 16 l7 eszsssLMsQ _ D - B —_————-- percent CaO — 1.39 B This basic formula permits the ratio computation without the use of a specific weight of sample (1.000 gram in the original formula) and without standardizing the Versenate solution for either magnesium or calcium. It is desirable to use approximately 1 gram of clean sample and a Versenate solution near the strength described only because these proportions give an optimum reaction without waste of reagents. Where mass analysis of samples from specific formations or strata covering a large area is desired, there is no apprent reason why all samples through the section cannot be analyzed together. Care must be taken to use a representative sample. This can be 13 done by mechanical means, or by mixing thoroughly all samples 13 W. C. Krumbein and F. J. Pettijohn, Manual of Sedimentary Petrograghy, Appleton-Century, New York (1938), Chapter 3, ”Preparation of Samples for Analysis." ' after digestion and using a proportionate aliquot of the solution di- luted to 250 milliliters for analysis. To analyze a number of samples it is best to first prepare and weigh all samples. Samples may be digested in groups as large as evaporation facilities will permit. Perchloric acid fumes are 18 EXTREMELY DANGEROUS, and evaporation MUST be done with proper ventilation. After evaporation, samples can be kept in their beakers for an indefinite period in a dry place. Many samples, usually eight to twelve, may be conveniently handled and redissolved at one time. It is desirable to use two or more 250-milliliter calibrated flasks for preparing the sample. The dissolved sample may be filtered directly into the calibrated flasks, and by using two or mwre, filtering may continue while the preceding sample is “made up" and titrated. A. lO—milliliter graduated pipette is used to take the aliquot for analysis (Figure 2). To determine calcium, a quantity of the potassium hydroxide solution is mixed with 20 times its volume of water. Twenty milli- liters of this solution and indicator powder are added to the aliquot before titration. A graduated 50-milliliter burette is best for titra— tion. To determine magnesium, the buffer solution, water, and potassium cyanide may be combined in the ratio of 26 parts water, 3 parts buffer, and 1 part potassium cyanide. Thirty milliliters of this solution and 8 drops of F 241 indicator are added to the aliquot before titration. A. number of samples may be analyzed in a very short time by using this method. FIGURE 2 LABORATORY EQUIPMENT I ‘ I _.' ‘ . . i o- -' 1' O l \ \N‘ _ \ ‘ At left is shown the 50—milliliter burette and re- agents necessary for titration. The 250-milliliter cali- brated flask, with funnel inserted for filtering, and the 10---milliliter pipette for measuring the aliquot of sample for analysis are in the center of the photograph. Evap- orated samples, in their beakers, are at the right. The large liter graduate holds distilled water. 19 ANALYSIS OF THE MECOSTA COUNTY AREA Well samples, Schlumberger electric logs, and any other per- tinent data available were examined to pick formation tops in the area. These data are presented in Table I. Where wells did not penetrate to the top of a formation an ”X" is placed in the table. A dash is used where tops could not be determined accurately. Column 11 of Table I indicates the number of feet below the top of the Rogers City formation at which water (or oil) was logged. Samples of the Rogers City rocks, preserved from thirty—five of the wells drilled in the area, were analyzed as described in the text from the top to the bottom of the formation; or into the Dundee formation in the deeper wells. Samples from wells drilled with both rotary and cable-tool rigs were used. No cores from wells in the area were available. Individual analyses of each sample from each well series were made. The results of the analyses of these samples are shown in Table II. The average magnesium/calcium ratio for the first 25 feet of the Rogers City formation was computed and is presented in Column 12, Table I. This average is limited to the upper 25 feet of formation in order to preclude the possibility of including the primary dolomites of the Dundee. 20 21 TABLE I FORMATION TOPS AND WELL DATA Well Name Town- Sec- Stray ship tion Sand Darke - Bergelin ................. Wheat- 3 -415 Merrill and Darke — Guy ........... land 4 -406 Fisher—McCall - Ockert ............ ” 6 -390 Michigan Consolidated ~ Rettinger ..... “ 6 -388 Sohio - Rettinger ................. " 6 -382 Dye Creek — Lehnert .............. " 7 --406 Moco - Ortwein .................. " 7 -402 Rand and Neyer - Mayer ........... " 7 -389 Rex and Skelly - Mayer ............ ” 7 -405 Smith—Ortwein ................... U 7 - 397 Lueder - Doan ................... ” 8 -384 Lueder - Walker-Skalitzky .......... ” 8 -375 Rex - Skalitzky .................. " 8 -375 Rex - Skalitzky No. 2 .............. ” 8 -384 Rex - Snider .................... " 8 -402 Carter — Flachs .................. " 9 -400 Carter - Smith Petroleum Co. ........ “ 9 -380 Carter - Snyder .................. ” 9 —385 Lueder - Flachs ................. " 9 -401 McGuire - Oliver ................. " 11 -476 Michigan Consolidated - Keller ....... ” 16 -395 G011, Graves & Mechling - Ruetz ...... " 17 -421 Moco - Minkel ................... “ 18 -382 Mehrtens - Beaumann .............. " 21 -443 Southwest Development — Hewlett ...... " 26 -584 ‘——.——_ ..-—_. 22 TABLE I (Continue (1) Base Trav- Trav- Rogers Feet Mg/Ca erse Shale City _ of For— erse Bell Thick- For- to Ratio Mar- Lime— Shale Wa— (avg . lst shall Ta‘ stone “8 SS "3”" te r 25 feet) tion tion -716 -2136 -2173 -2708 37 -2745 1 0.654 -700 -2125 -2156 -2662 41 -2703 4 0.599 -704 -2120 -2154 —2647 43 -2690 3 0.656 X X X X X X X X -709 —2121 -2156 -2643 41 -2684 1 0.662 -713 -2119 -2154 X X —2687 14 X -716 X -2148 -2636 43 -2679 l X -718 -2110 -2155 -2635 47 -2682 2 0.652 -705 -2115 ‘-2151 -2639 49 -2688 2 X -709 —2113 -2147 —2639 42 -2681 1 X -698 X -2154 -2652 46 -2698 6 X X X X X X X X X -690 -2106 -2141 -2629 42 -2671 1 X -695 -2103 -2134 -2632 42 -2674 3 X -707 -2122 -2157 -2663 37 -2700 1 0.706 -701 -2117 -2162 -2658 41 -2699 4 0.505 -707 -2108 -2147 -2643 43 -2686 3 X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X -731 -2121 -2161 —2649 45 -2694 none 0.458 -698 —2100 -2142 ~2626 44 -2670 l X X X X X X -2723 none X X X X X X X X X 23 TABLE 1 (Continued) Well Name Town- Sec— Stray ship tion Sand Collin - Bowmann ................ Wheat- 29 -382 Pure - G. T. 41 ................. land 36 -421E Gulf — Gale ..................... Morton 3 -366 Colmur - Bell ................... ” 6 -297 Colmur - Sapp ................... " 6 —325E Chapman — Minkel ................ " 7 —290 Michigan Consolidated — 131 Austin . . . " 7 -299 Michigan Consolidated - 133 Austin . . . . " 7 -32;' Colmur - Mecosta Lakes ........... " 8 -342 Michigan Consolidated - Stickler ...... " 15 -308 Michigan Consolidated - Patrick ...... ‘ " 16 -321 Rex - Mecosta Lakes .............. " 17 ~311 Michigan Consolidated - Austin 9 ...... " 18 -306 West Michigan Consolidated - Minkel . . . " 18 -312 Collin and Berlin - Ockert .......... " 19 -331 Ohio - Thurkow .................. " 19 —328 Mercer - Norman ................ " 21 -318 McGuire - Smith ................. " 22 -310 Michigan Consolidated - Berry ....... " 22 -328 Michigan Consolidated - Smith ........ " 22 -315 Leonard - Minkel ................. " 24 —349 Benedum-Trees — Truman ........... " 26 —354_ Smith - Rean .................... " 29 -349 Taggart - McKeough ............... Austin 2 -290 Taggart - Barton ................. " 11 -296 TABLE 1 (Continue (1) 24 Base Trav— Trav- Rogers Feet Mg/Ca erse Shale City _ of For- erse Bell Thick- For- to Ratio Mar- Lime- Shale Wa- (avg. lst, shall ma- stone ness ma- ter 25 feet) tion tion -700 -2083 —2122 -2612 45 -2657 none 0.149 X X X X X X X X - I. -2094 -2611 39 -2640 - 0.135 - -2034 -2070 -2562 40 —2602 none 0.178 X X X X X X X X - - -2043 -2522 39 -2561 23 0.355 X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X —584 -2022 -2060 -2529 49 -2578 none 0.104 X X X X X X X X X X X X X X X X -595 —2032 -2072 X X X X X -600 -2010 -2056 -2529 47 -2576 none 0.064 -589 --2029 -2067 -2553 35 —2588 none 0.073 —577 -2007 -2056 -2554 35 -2589 2 0.559 X X X X X X X X X X X X X X X X - -2055 -2092 -2567 56 -2623 5 0.630 - - -2085 -2557 48 -2605 6 0.665 X X X X X X X X -569 -1979 ~2016 -2519 44 —2563 none 0.057 -570 -1994 -2039 —2512 45 -2557 48 0.089 25 TABLE I (Continued) Well Name Chapman, Union - Proctor .......... Teater - Wood ................... Daily - Hutchins ................. American Gas - Brehm ............ Obenauer - Musgrave .............. King - Gingrich .................. Sherrit - Gingrich No. 3 ............ Gould and Cross - Starks ........... Sherrit - Otterbein ................ Sherrit - Scheibe ................. Sherman - Scheibe ................ Benedum-Trees - Grove ............ Gould and Cross - Barrows ......... Gould and Cross — Grove ........... Red Man - Floria ................ Michigan Consolidated - Bliss ........ Gulf - Warner ................... Anderson - Dobben ................ Benedum-Trees - Brand ............ Collins and Berlin - May ........... Merrill and Darke - Fate ........... Gulf — Landis ................... Hebard - Weeks .................. Michigan Consolidated - Carpentier Anderson - Farwell 8: Loyd .......... Town- Sec- Stray ship tion Sand .Austhi 12 -288 Sheridan 2 -450 . " 8 -379 ” 10 -460 " 11 IX " 12 -451 " 12 -452 “ 13 —449E " 13 -448E ” 13 -439E " 13 -457E " 14 ~45? ” 14 -458E " 14 —467 " 14 -452E ” 17 -423 " 21 -452 " 22 -473 ” 24 -449 " 26 -460 " 27 -459 " 30 -441E “ 32 -432‘ “ 35 -452 ” 36 -488 26 TAB LE I (Continue (1) Trav- Rogers Base erse Trav- Shale City Feet Mg/Ca of For- ‘ erse Bell Thick— For— to Ratio Mar- Lime- Shale Wa- (avg. lst shall ma— stone ness ma- ter 25 feet) tion tion -570 -1992 -2027 -2503 51 -2554 42 0.137 -757 -2162 -2203 -2743 49 «2792 none 0.055 -686 - -2135 -2671 47 -2718 none 0.040 X X X X X X X X X X X X X X -774 - -2203 -2729 42 -2771 none 0.051 X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X - - -2200 -2714 57 —2771 none 0.064 X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X -766 -2148 —2190 -2719 43 -2762 none 0.063 X X X X X X X X X X X X X X X X -773 -2157 -2200 —2717 40 -2757 1 0.698 -742 -2176 -2211 —2718 40 -2758 4 0.557 X X X X X X X X -743 42153 -2195 -2688 46 -2734 6 X X X X X X X X X -736 -2139 -2197 -2694 50 —2744 1 0.702 27 TABLE I (Continued) Well Name Town- Sec- Stray ship tion Sand Chapman - Bark ................. Martiny 1 —373 Gordon - Engleman ............... " 9 -438 Pure - Smith .................... " 10 -371 Michigan Consolidated - Alderman ..... " 11 -409 Rex - Bouck .................... " 11 -382 Gordon and Chapman - Soule ......... " 12 -386 Gordon - Soule .................. " 12 -384 Michigan Consolidated - Evans ....... " 12 -378 Michigan Consolidated - Evans No. 2 . . . " 12 -389 Michigan Consolidated - State ........ " 13 -383 Gordon - Bush ................... H ' 14 -385 Ross — Oliver ................... " 14 -350 Gordon - Helmer ................. " 17 -427 Self - Percy .................... " 22 -383E Hall, LaMee - Oliver .............. " 23 —381E Lakeside - Dye .................. " 23 -398 Rand and Neyer -. Baker. ........... " 33 -350 28 TABLE I (Continue d) Trav— Rogers Base erse Trav- Shale City Feet Mg/Ca of For— erse Bell Thick— For- to Ratio Mar— Lime— Shale Wa- (avg. lst shall ma- stone ness ma- ter 25 feet) tion tion - - -2122 -2661 45 —2706 none 0.088 X X X X X X X X -666 —2075 -2119 -2645 44 -2689 none 0.074 X X X X X X X X -688 -2083 -2131 -2668 36 -2704 1 0.389 X X X X X X X X -667 ~2087 -2125 “2657 38 -2695 none 0.054 X X X X X X X X X X X X X X X X X X X X X X X X -671 -2101 -2144 -2667 48 -2715 show I 0.240 X X X X X X X X -723 - - -2675 42 -2717 none 0.040 X X X X X X X X X X X X X X X X X X X X X X .X X -640 -2048 -2088 -2583 47 -2630 none X TABLE II INDIVIDUAL WELL SAMPLE ANALYSES RESULTS 29 Feet 5 _ am Below Pct. Pct. Mg/ Well ple Top of CaO M O Ca No. Rogers 6 Ratio City Darke, 1 Bergelin 6A 1 - 8 24.3 15.9 0.654 Sec. 3, T.14N.,R.7W. Merrill & Darke, 1 Guy 3A. 0 - 4 21.0 11.2 0.533 Sec. 4, T.14N.,R.7W. 3B 4 - 8 27.5 17.0 0.618 3C 8 -11 28.6 18.5 0.647 Fisher-McCall, 1 Ockert 7A 0 - 5 25.6 16.0 0.625 Sec. 6, T.14N.,R.7W. 7B 5 -17 28.4 19.7 0.694 Sohio, 1 Rettinger 27A 0 - 3% 24.5 15.1 0.616 Sec. 6, T.14N.,R.7W. 27B 344 25.8 17.7 0.686 27C 14 -26 26.7 18.3 0.685 Rand 8: Neyer, 1 Mayer 5A 0 - 1%- 25.1 16.8 0.669 Sec. 7, T.14N.,R.7W. 5B 1%- 2 24.3 14.9 0.613 5C 2 - 4 27.6 18.6 0.674 Rex, 1 Snyder 13A. 0 — 1 16.0 11.3 0.706 Sec. 8, T.14N.,R.7W. Carter, 1 Flacks 8A 1 - 6 20.4 10.3 0.505 Sec. 9, T.14N.,R.7W. Goll, Graves, Mechling, 2A 0 - 8 35.4 13.7 0.387 1 Reutz 2B 8 -11 33.0 18.5 0.561 Sec. 17, T.14N.,R.7W. 2C 11 -16 38.8 12.7 0.327 2D 16 -21 34.1 16.3 0.478 2E 21 -25 32.4 17.4 0.537 2F 25 -30 30.8 19.4 0.630 30 TABLE II (Continued) Feet S - B or am elow Pct. Pct MD/ Well ple Top of CaO M O Ca No. Rogers '3 Ratio City Collin, 1 Bowmann 28A 0 ~ 6 35.9 5.0 0.139 Sec. 29, T.14N.,R.7W. 28B 6 ~12 34.3 7.0 0.204 28C 12 ~18 39.9 4.1 0.103 28D 18 ~33 37.2 9.5 0.255 Gulf, l Gale (Rotary) 31A 1 ~ 8 24.8 3.6 0.145 Sec. 3, T.14N.,R.8W. 31B 8 ~16 25.8 3.8 0.147 31C 16 ~26 30.0 3.4 0.113 Colmur, 1 Bell 30A 1 - 9 34.4 7.3 0.212 Sec. 6, T.14N.,R.8W. 30B 9 ~21 38.5 5.5 0.143 30C 21 ~33 34.1 13.5 0.396 Chapman, 1 Minkel 18A 0 - 5 28.7 8.5 0.296 Sec. 7, T.14N.,R.8W. 18B 5 ~13 35.7 14.4 0.403 18C 13 ~20 36.9 13.5 0.366 Rex, 1 Mecosta Lakes 35A 0 ~14 37.4 3.9 0.104 Sec. 17, T.14N.,R.8W. Ohio, 1 Thurkow 22A 0 - 2 38.4 4.0 0.104 Sec. 19, T.14N.,R.8W. 22B 2 ~ 6 45.0 3.8 0.084 22C 6 ~10 48.2 2.7 0.056 (Coral L.S. in Rogers City) 22D 10 ~18 47.0 1.3 0.027 22E 18 ~25 49.2 2.4 0.049 Mercer, 1 Norman 4A. 0 ~ 9 47.8 3.0 0.063 Sec. 21, T.14N.,R.8W. 4B 9 ~15 38.5 3.0 0.078 4C 15 ~23 44.4 3.4 0.077 TAB LE II (Continue (1) 31 Feet Sam- Below Pct. Pct. Mg/ Well ple Top of CaO MnO Ca No. Rogers ‘3 Ratio City McGuire, 1 Smith 12A 0 - 5 10.5 1.8 (0.171) Sec. 22, T.14N.,R.8W. 12B 5 - 9 9.9 6.2 0.626 (12A mostly cavings) 12C 9 ~22 12.8 6.3 0.492 Leonard, l Minkel 34A. 0 ~ 3 39.3 18.9 0.481 Sec. 24, T.14N.,R.8W. 34B 3 ~ 7 26.1 18.6 0.713 34C 7 ~15 21.0 13.8 0.657 34D 15 ~24 27.3 18.3 0.670 34E 24 ~32 26.1 17.6 0.674 34F 32 ~38 25.0 17.5 0.700 34G 38 ~46 31.0 20.1 0.648 34H 46 ~52 25.4 16.3 0.642 Benedum-Trees, 1 Truman 1A 0 ~25 26.2 17.7 0.665 Sec. 26, T.14N.,R.8W. 1B 25 ~44 29.5 20.1 0.681 1C 44 ~49 27.1 19.3 0.712 1D 49 ~55 29.6 19.6 0.662 Taggart, 20 McKeough 21A 0 ~ 6 37.7 3.7 0.098 Sec. 2, T.14N.,R.9W. 21B 6 ~13 47.4 1.8 0.038 (Coral L.S. in Rogers City) 21C 13 ~28 49.5 1.7 0.034 21D 28 ~34 48.0 3.5 0.073 21E 34 ~41 45.0 3.2 0.071 Taggart, 1 Barton 24A 0 - 9 35.9 3.9 0.109 Sec. 11, T.14N.,R.9W. 24B 9 ~20 43.0 3.2 0.074 24C 20 ~26 46.8 4.0 0.085 Chapman, 1 Proctor 23A 0 ~ 3 33.2 4.6 0.139 Sec. 12, T.14N.,R.9W. 23B 3 ~10 41.4 6.5 0.157 23C 10 ~15 34.7 4.0 0.115 32 TAB LE 11 (Continue (1) Feet S ~ e am B low Pct. Pct. Mg/ Well ple Top of CaO M O Ca (7 No. Rogers 0 Ratio City Teater, 1 Wood 11A 0 ~ 2 39.3 4.1 0.104 Sec. 2, T.15N.,R.7W. 11B 2 ~ 9 49.9 2.2 0.044 11C 9 ~17 48.3 2.1 0.043 11D 17 ~25 52.0 1.4 0.027 11E 25 ~35 44.9 4.1 0.091 Daily, 1 Hutchins 9A 23 ~30 40.1 1.6 0.040 Sec. 8, T.15N.,R.7W. 9B 30 ~37 40.2 2.1 0.052 King, 1 Gingrich 17A 0 ~10 41.5 2.3 0.055 Sec. 12, T.15N.,R.7W. 17B 20 ~25 41.1 1.9 0.046 17C 25 ~31 45.5 2.8 0.062 Benedum-Trees, 1 Grove 15A 0 ~ 8 28.9 2.8 0.097 Sec. 14, T.15N.,R.7W. 15B 8 ~10 31.6 2.9 0.092 15C 10 ~14 48.7 2.0 0.041 15D 14 ~20 47.7 2.7 0.057 15E 20 ~26 48.7 1.6 0.033 Gulf, 1 Warner (Rotary) 10A 3 23.6 .3 0.097 Sec. 21, T.15N.,R.7W. 10B 14 ~19 34.1 .8 0.053 10C 19 ~24 35.0 .4 0.040 Collin & Berlin, 1 May 32A 0 ~ 3 24.5 17.1 0.698 Sec. 26, T.15N.,R.7W. Merril & Darke, 1 Fate 25A 1 ~ 5 25.3 14.1 0.557 Sec. 27, T.15N.,R.7W. TAB LE 11 (Continue (1) 33 Feet Sam- Below Pct. Pct. Mg/ Well ple Top of CaO M00 Ca No. Rogers ° Ratio City Anderson, 1 Farwell 8: Loyd 33A 1 ~ 3 26.9 19.2 0.714 Sec. 36, T.15N.,R.7W. 33B 3 ~11 27.3 19.8 0.725 33C 11 ~21 23.8 15.9 0.668 33D 21 ~30 25.7 17.2 0.669 33E 30 ~37 25.4 17.3 0.681 33F 37 ~49 26.0 18.2 0.700 33G 49 ~54 25.0 16.5 0.660 33H 54 ~64 26.6 18.9 0.711 331 64 ~78 22.7 15.4 0.678 33.] 78 ~86 25.8 15.9 0.616 33K 86 ~91 24.5 18.2 0.742 Chapman, 1 Bark 26A 1 ~14 43.4 4.2 0.097 Sec. 1, T.15N.,R.8W. 26B 14 ~21 47.6 4.2 0.088 26C 21 ~33 45.2 3.6 0.080 26D 33 ~44 38.7 7.6 0.196 Pure, 1 Smith 16A. 0 - 2 38.4 3.9 0.102 Sec. 10, T.15N.,R.8W. 16B 2 ~ 8 48.6 3.7 0.076 16C 8 ~16 49.0 2.1 0.043 16D 27 ~37 48.0 3.5 0.073 Rex, l Bouck 29A. 0 ~ 2 18.5 7.2 (0.389) Sec. 11, T.15N.,R.8W. (Sample contaminated with cavings) Gordon, 1 Soule 20A 0 ~ 5 46.2 2.5 0.054 Sec. 12, T.15N.,R.8W. 34 TABLE II (Continued) Feet S - . am Bdow Pct. Pct. Mg/ Well ple Top of CaO M 0 Ca (7 No. Rogers ° Ratio City Gordon, 1 Bush 14A 0 ~ 4 36.2 9.9 0.273 Sec. 14, T.15N.,R.8W. 14B 4 ~12 31.3 6.2 0.198 14C 12 ~18 37.1 5.5 0.148 14D 18 ~29 29.2 9.9 0.339 14E 29 ~38 26.3 18.5 0.703 Gordon, 1 Helmer 19A. 0 ~19 41.7 1.9 0.046 Sec. 17, T.15N.,R.8W. 19B 19 ~23 43.6 1.5 0.034 w. 35 The ratio averages were plotted on a base map, from which data a ratio contour map of the area was constructed (Figure 3~~ rear pocket). This map shows dolomitization to occur in two long bands extending northeast, joined by a band extending southeast. In this part of the Michigan Basin the dominant structural trend is southeast, with minor trends extending northeast. There is little control in T.15N.,R.8W., but the marked trends of dolomite in the other townships and the magnesium/calcium ratio of 0.240 in Section 14 seems to justify the interpretation given to that township. If these zones of dolomitization are along structural trends, the area may be contoured to show that the shallow horizons (Figure 4~~rear pocket), the Rogers City formation (Figure 5~~rear pocket), and the dolomitization agree closely (Figure 3). If the assumption that dolomitization coincides with the structural trends is correct, a study of dolomitization in an area which is difficult to contour by other methods should aid materially in eliminating many structural mi sinte rp re tation s . ORIGIN OF DOLOMITE IN THE MECOSTA COUNTY AREA There are two common theories for the origin of porosity in the Rogers City formation in the Mecosta County area. One theory is that the structures are reefs and that dolomitization occurs through replacement. This theory, once strongly considered, is seldom held today. If these were reef structures, one would expect that the over- lying Bell shale should show thinning over the dolomite zones, and that a gradual diminishing of structure should be found in overlying formations. Neither is true. There is no correlation between the thickness of the Bell shale and dolomitization within the underlying Rogers City (Figure 6). Even the shallow beds show the same struc- ture as the Rogers City formation (Figures 4 and 5). Samples from three wells listed in Table II (the Colmur, 1 Bell; Ohio, 1 Thurkow; and Taggart, 20 McKeough) indicate coral limestone in the Rogers City, but only the Colmur well samples are dolomitized even slightly. As noted earlier in this report, the Rogers City is an extremely fos~ siliferous limestone, and probably does contain some reefs, but there is no evidence that such reefs are associated with either structure 0 r dolomitization . 36 MAGNESIUM/CALCIUM RATIO OF THE ROGERS CITY FORMATION FIGURE 6 37 RELATIONSHIP OF THICKNESS OF THE BELL SHALE TO DOLOMITIZATION OF THE ROGERS CITY FORMATION .300 I .700 L—l-‘r- f + .600 .500 L‘ .400 i .300 .200 .IOO .000 34' 30' 42' 40' so' 54' st 38 The writer believes that dolomitization occurs along tension 14 fractures formed at the apices of folded structures. These _-———_——. .—. -— _———._————————————. 14 M. P. Billings, Structural Geology, Prentice-Hall, New York (1942), Chapters 6 and 7. 1 fractures probably formed during a period of gentle folding. 5 The 15 C. R. Criss and R. L. McCormick, 22.__c£. movement of brine from the underlying Dundee along the fractures recrystallized the minerals, dissolved the fossils, and enlarged the 16 fractures. .Landes describes in great detail the development of 16 K. K. Landes, _p.__c_i_t. porosity through dolomitization, giving special emphasis to dolomiti- zation along faults, fractures, and joints. He concludes: The writer believes that local diastrophism has produced master fissures in the limestone-containing section; that an artesian circulation has been developed which has carried waters through deeper dolomites and up into the limestone; and that these waters have replaced some of the limestone by (blomite that is locally porous where there was an excess of solution over pre- cipitation during the replacement process. The Rogers City limestone has been altered to a very dark brown, massive dolomite, and the numerous vugs, fractures, and channels formed in the process are lined with white crystals of 39 dolomite. Since these tension fractures apparently do not extend off structure, lateral dolomitization is limited, and is seldom off struc- ture. Dolomitization along master fault or fracture planes is the notable exception. Few faults of consequence have been found in Central Michigan, but those known to penetrate limestone beds fre- quently provide channels for local dolomitization, developing reser- voirs which produce oil prolifically. The development of porosity through dolomitization along faults and tension fractures is apparently the same. Since the throw of these faults in Michigan is apt to be very slight they are generally difficult to locate and follow by normal subsurface methods. The only major fault with appreciable throw'now known in the Michigan Basin is along the west flank of the Howell anticline. Current drilling on this anticline (spring, 1954) indicates that the Trenton limestone is locally altered to dolomite and contains oil and gas. This dolomite possibly occurs along fault planes associated with the master Howell fault. It is the conclusion of the writer that dolomitization similar to the type found at Howell is found also in the Adams and Deep River pools. These fault planes ‘ 17 have been called "master fissures" by Landes. 7 Ibid., p. 318. 40 In the western half of the central portion of the Michigan Basin where the tops of structures have been dolomitized, the underlying Dun- dee in places is a primary dolomite, generally not more than 100 feet thick. In the eastern half of the central basin, the Dundee is not nor- mally a dolomite. It is much thicker, and varies from 100 to 400 feet. Here Rogers City dolomitization is rare on structure, but may be found along fault zones. This suggests that expansion fractures on structure may have a limited vertical extent, and that high-magnesium waters percolating from below, working along small fracture systems, have not had sufficient time to dolomitize appreciably more than a 100-foot vertical section of limestone. This hypothesis is supported by the fact that in areas where Dundee limestone is thick the lower 100 feet of the Dundee formation is often dolomitized on structure. The source of high-magnesium waters for this process .is apparently from the pri— mary dolomites of the Detroit River formation, which lie below the Dundee limestone. In this area of thick Dundee limestone, the Rogers City is dolomitized where faults or master fissures penetrate both the Dundee and Rogers City formations, thus allowing direct communication with zones of high-magne sium brines below. It would be interesting to compare the horizontal extent of dolomitization away from a fault plane to the vertical extent of the process along a system of expansion fractures to determine if they are equal. 41 In Mecosta County a distinct pattern of lateral dolomitization exists (Figure 3). At the top of structural trends the magnesium/ calcium ratio approaches or slightly exceeds 0.700. This area is dolomitized quite uniformly (Table II).. On the flanks of the struc- tural trends dolomitization was uneven, with the higher-ratio zones probably representing the more infrequent expansion fractures along which the process began. Nowhere in the area does dolomitization extend laterally more than 1% miles from the top of a structure, at which distance the ratio drops below a value of 0.100. It is inter- esting to note that the magnesium/calcium ratios coincide closely 18 _with percentage figures quoted by Landes as necessary for effective 18 Ibid., p. 310. porosity in other areas and formations. Using the conventional dilute hydrochloric acid test for dolo- mite, rocks with a ratio distinctly above 0.400 give evidence of dolomitization, while those below produce a typical limestone reac- tion. Thus, in the conventional examination of well cuttings, the Rogers City formation appears to change abruptly from dolomite to limestone, with no indication of decreasing ratios along the flank~ zone. It is this flank-zone which may be important in predicting 42 the proximity to dolomitized structures, and the proximity to and strike of dolomitized fault planes. The primary limestones in the area studied show a ratio varying from 0.027 to 0.100. Where ratios exceed 0.100 they con- tinue upward to above 0.500 nearby. No isolated zones of low dolo~ mitization appear to exist which cannot be connected reasonably with structural trends where the formation is dolomitized. It would seem logical to assume that if a well were drilled into the Rogers City formation in any place one might expect similar dolomitization. Thus, in an area where little or no structural control exists a wildcat well which shows a ratio of 0.100 or more would seem to indicate a dolo- mitized structure within 1% miles. Similarly, as the ratio increases, one might logically expect that a structure exists at proportionately closer distances. Preliminary investigation indicates that similar magnesium/ calcium relationships exist in other areas of secondary dolomitization, and that ratio—distance correlations may be established in them. RELATIONSHIP BETWEEN DOLOMITIZA TION AND POROSITY A comparison of water zones to the magnesium/calcium ratio (Figure 7) shows that 0.458 is the highest average ratio at which no effective porosity occurs, and that generally the ratio must be above 0.500 before effective porosity results. In some wells slight effective porosity was found where the magnesium/calcium ratio average was below 0.500, but it appears that such porosity was developed along a single fracture from a nearby structure. The Rex, 1 Bouck well, Section 11, T.15N.,R.8W., has extremely effective porosity and shows a ratio of only 0.389. This low ratio, however, is due to the great number of fragments of crinoidal limestone derived from the base of the Bell shale which seriously contaminate the sample. In zones where dolomitization is negligible, the analyses show that the first flank-indication of dolomitization is an increase of magnesium in the top few feet (Table II). Small increases, however, may not be sig- nificant, and apparently never develop porosity. 43 DISTANCE FROM THE TOP OF THE ROGERS CITY WATER TO TOP OF ROGERS CITY 15' 30' N0 WATER RELATIONSHIP BET AND FIGURE WEEN 7 DOLOMITIZATION POROSITY 44 0 "’9'? .0 LIMESTONE AVERAGE MAGNESIUM/CALCIUM ROGERS CITY FORMATION I.O DOLOMITE RATIO OF THE POSSIBILITIES FOR OIL PRODUCTION IN THE MECOSTA COUNTY AREA Several places in the Mecosta area may have possibilities for oil production; this possibility based on the magnesium/calcium ratio interpretation. Perhaps the most interesting prospect is in Section 8, T.14N.,R.7W. The Rex, 1 Skalitsky, well has every indication of be- ing on the southwest flank of a small, closed structure, which extends into the north half of Section 9. Both structural and dolomitic condi- tions seem favorable for oil accumulation, despite the seven dry holes which surround the present producing well. A similar area, but somewhat less pronounced, is in the southeast corner of T.15N.,R.7W., at the common corner of Sections 25, 26, and 35. A_ strong structure trends across T.14N.,R.8W., with three possible areas of oil accumulation. The most likely is centered at the corner of Sections 25, 26, 35, and 36. Here dolomitization is widespread. The trend diagonally across T.15N.,R.8W., especially around Sections 15 and 32, has possibilities for oil accumulation. The well in Section 17, exhibiting strong limestone characteristics, casts doubt over the conditions at the center of the township. However, the ratio 45 46 0.240 in Section 14, and the ratio 0.178 in Section 6, T.14N.,R.8W., strongly suggest dolomitization in the immediate vicinity. Apparently there is a small, closed dolomitized structure centered in the south- west part of Section 1, but this area is inaccessible for drilling be— cause of the lakes, swamps, and extreme ruggedness of terrain. The dolomitization in the southwest of T.15N.,R.7W., suggests structure, but there is no evidence of closure between this area and the small producing area in Section 7, T.14N.,R.7W. CONCLUSIONS Many geologists who have considered problems of dolomitiza- tion have concluded that a need exists for: (l) a method. of quick analysis of limestone-dolomite rocks; (2) a ready method of classifi- cation for the degree of dolomitization; and (3) a simple method to present this information. The Versenate method of analysis of lime~ stones and dolomites, and the expression of the results of analysis as a magnesium/calcium ratio may fulfill these needs. The uses of such a method are many. As in the Mecosta area considered here, magnesium/calcium ratios should greatly fa- cilitate the prediction of dolomitization on structure, and should aid in locating dolomitized structures. Ratio determination should be particularly useful in locating and following dolomitization, either along faults or in areas not re- lated to structure. The Pinconning Field, Fraser and Pinconning Townships, Bay County, Michigan, exhibits the real need for such a tool. In 1944, the Shell Oil Company drilled a well which intersected and produced oil prolifically from a dolomitized fault zone in the Dundee formation. In succeeding years over twenty offset wells were 47 48 drilled, all dry, before the strike of the fault was found and a second producing well obtained. The field now extends more than three miles along the strike of the fault and is still being extended. It is the writer's belief that a study of the magnesium/calcium ratios in this area could have indicated the dolomitized fault zone with only a frac- tion of the number of offset wells which we re drilled. Such a study should be valuable in determining extent and direction of dolomitiza- tion in other areas of Michigan having similar problems. Preliminary work by the writer indicates that this geologic technique is useful in determining formation boundaries and correla- tions, and may be very helpful in distinguishing between lagoonal and deep sea facies in reef areas. It is hoped that this technique will provide the basis for a better understanding of limestone-dolomite deposition and of second- ary dolomitization, wherever it occurs. BIBLIOGRAPHY Association of Official Agricultural Chemists, "Official and Tentative Methods of Analysis," 6th Edition (1945), pp. 42-50. Billings, M. P., Structural Geology, Prentice-Hall, New York (1942). Cheng, K. L., T. Kurtz, and R. H. Bray, ”Determination of Calcium, Magnesium, and Iron in Limestone," AnalLtical Chemistry, Vol. 24, No. 10 (Oct., 1952), p. 1640. Cohee, G. V., and L. B. Underwood, U. S. Geological Survey Oil and Gas Investigations, Preliminary Map 38, "Lithology and Thick- ness of the Dundee Formation and the Rogers City Limestone in the Michigan Basin” (1945). Criss, C. R., and R. L. McCormick, ”History and Performance of the Coldwater Oil Field, Michigan,” Paper 278-G, American Institute of Mining and Metallurgical Engineers (Oct., 1953). Krumbein, W.. C., and F. J. Pettijohn, Manual of Sedimentary Petro— gagghx. Appleton-Century, New York (1938). -.\" “ \. "\ Krumbein, W. C., and L. L. 81055, Syratigraphy and Sedimgntatbna \ Freeman and Co., San Francisco (1951). f \. \ Landes, K. K., "Porosity. Through Dolomitization," Bulletin pf\--.the American Association of Petroleum Geolggistg, Vol. 30‘, No. 3 (March, 1946), pp. 305-318. ‘ - glx. - \ ‘ Powell, L. M., "Calcium Carbonate ~ Magnesium Carbonate Ratios . of the Pinconning Field,” Thesis, University of Michigan. 49 FIGURE 4 20' ONE MILE AM CONTOUR INTERVAL STRUCTURE CONTOUR MAP MICHIGAN STRAY SAND h — ‘31“ ( '71-2123 /o-unk( COLDIATER nuo ' L// / so" ~ 31“ ' 273! 12730 (31., S‘OOI \§ ¢ .(VL'ZMI -gyo¢ [#2008 0. '0 O o 0; '00'00 (00 9 o o) 40.12 -01 0-04 239 {I 292 ‘ 5-290 .QL 2%6 ‘ 200 3303 2” —/ 293 'ZNWzoso {x 020- 279 ~zIs p, n-mz? a 79‘ fi '92 (12" a STRUCTURE CONTOUR MAP ROGERS CITY FORMATION CONTOUR INTERVAL: 20' l A ONE MILE s-aoso TL'USQ 0‘2687 S ‘08 TL'ZISG 0‘2703 :( 8‘80! tL-zmo '2702 a I\ s 110-4100 o-zno OIYL'ZIII ; o-zru i 9 / FIGURE 5 S‘CZIE .. . a .c O - f Y A ‘ut '[x' _/\ —._-__. ‘. “(Ly/{.4}. MICHIGAN STATE UNIVI Moi 2‘9 I .. l 93 I l 304 IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII 02774233