THESIS

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This is to certify that the

thesis entitled

Niagaran Reefs
Northwestern Michigan

presented by

G. Daniel Orr

has been accepted towards fulfillment
of the requirements for

Master' 5 degree in 990109)!

  

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NIAGARAN REEFS
NORTHWESTERN MICHIGAN

by

G. Daniel Orr

A THESIS

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

MASTER OF SCIENCE

Department of Geology

1984

ABSTRACT
PINNACLE REEFS:NORTHWESTERN MICHIGAN
BY

G. DANIEL ORR

Stratigraphic, structural, and lithological analyses of
lower Salina - Niagaran units in off-reef wells were done to
determine if changes evidenced in the analyses were related
to production within the northwestern Michigan pinnacle reef
belt.

Analyses support the model that A-l Carbonate
sedimentation represents a restricted marine tidal flat
deposit and pinnacle reefs that underlie these tidal flats
have been fully or partially dolomitized. Initial production
in reefs below these areas gauged the highest flow rates.
The tidal flat environments of the A-l Carbonate are best
depicted by lithologic changes occurring at the base of the
A-2 Evaporite. Clean halite becomes a basal anhydrite
indicating a shallowing in water depth over the tidal flats.

Lithological changes noted can be used as a tool for

limiting exploration efforts to better productive areas.

ACKNOWLEDGEMENTS

Special thanks to my advisor, Dr. James H. Fisher, for
direction and guidance on this project; to the geological
staff at the DNR for their assistance in the data collection
process; to Trendwell Oil, Hunt Energy Corporation, and
Sullivan and Co. for financial assistance; to the geology
department at Western Michigan University for their
assistance in photo reduction of plates; to Chuck and Cari
Ailes for the typing and editing of the manuscript; to my
wife, Catherine, for her love and support; and most of all
to the Lord God who, upon my request, gave me a generous

dose of wisdom without finding fault (James 1:5).

ii

TABLE OF CONTENTS

LIST OF FIGURES ----------------------------------- iv
LIST OF PLATES --------------------------------------- v
INTRODUCTION ---------------------------------------- 1
Area and Purpose of Study ---------------------- 2
Method of Investigation ------------------------ 3
Previous Work ---------------------------------- 4
GENERAL STRATIGRAPHY AND SEDIMENTATION -------------- 8
STRUCTURAL HISTORY --------------------------------- 15
FACTORS CONTROLLING PETROLEUM OCCURENCE ------------ 19

DESCRIPTION AND DISCUSSION OF MAPS
Niagaran Structure Map ------------------------ 21
Brown Niagaran Isopach and Lithofacies Map ---- 22
A-l Evaporite Isopach and Lithofacies Map ---- 23
A—l Carbonate Isopach Map --------------------- 24

A-2 Evaporite Isopach and Lithofacies Map ---- 27

A-2 Carbonate Structure Map ------------------- 29
A-Z Carbonate Isopach Map --------------------- 30
B-Unit Isopach Map ---------------------------- 31
PETROLEUM PRODUCTION ------------------------------- 31
CONCLUSIONS ---------------------------------------- 32

BIBLIOGRAPHY --------------------------------------- 37

Figure 1:

Figure 2:

Figure 3:

LIST OF FIGURES

Stratigraphic Succession in Michigan

Niagaran

Depositional

Michigan Basin

Michigan

Elements

Basin

iv

and

Environments in the

Surrounding

Structural

Plate 1

Plate 2

Plate 3

Plate 4

Plate 5

Plate 6

Plate 7

Plate 8

Plate 9:

Plate 10:
Plate 11:
Plate 12:

Plate 13:

LIST OF PLATES

Brown Niagaran Structure Map
Brown Niagaran Isopach Map
Brown Niagaran Lithofacies Map
A-l Evaporite Isopach Map

A-l Evaporite Lithofacies Map
A-l Carbonate Isopach Map

A-2 Evaporite Isopach Map

A-2 Evaporite Lithofacies Map
A-2 Carbonate Structure Map
A-2 Carbonate Isopach Map
B-Unit Isopach Map

Oil and Gas Production Map

Cross-Sections

INTRODUCTION

 

The search for Niagaran pinnacle reefs has been active
in the Michigan Basin since the 1950's. Exploration was
concentrated in southeastern Michigan on the St. Clair
platform and remained active there into the 1960's.
Gravimetry was highly successful in locating pinnacle
structures and was the primary exploration tool. More
notable discoveries on the platform were the Berlin, Peters,
Boyd, Ray, and Belle River Mills fields. Two important
discoveries made in the early 1950's were the Hamlin field
in Mason County and the Chester field in Otsego County. The
fact that these two discoveries were part of a reef trend in
the Northern lower peninsula was not recognized then

(Hartsell, 1982).

Interest shifted from southeastern Michigan to the
north in 1968 when Pan American and Northern Michigan
Exploration Company jointly drilled the Pan Am Drasey #1 in
Presque Isle County. This well, though non-commercial,
produced oil from the drill-stem test, and spurred a flurry
of drilling activity for pinnacle reefs across northern
Michigan (Fisher, 1973). Gravimetry was used in the early

1

2

stages of exploration, but gave way to reflection
seismolong enabling explorationists to delineate reef

positions more accurately at greater depths.

The Northern pinnacle trend stretches in a
southwest-northeast direction from Mason County, on the
shore of Lake Michigan, to Presque Isle County, on the shore
of Lake Huron. Drilling for pinnacles along this trend has
dominated Michigan exploration activity for the last 14

years.

AREA AND PURPOSE OF STUDY

 

The area of study includes Townships 21 North through
27 North and Ranges 9 West through 17 West encompassing all
of Grand Traverse and Manistee Counties, and parts of Benzie
and Wexford Counties of northwestern Michigan.

The major intent of this study was to make
stratigraphic and structural analyses of Niagaran-Salina
units in a localized area. Lithologic differences in the
Brown Niagaran, A-1, and A-2 Evaporites were noted to
determine if facies changes were related to production (i.e.
salt-plugged, gas, oil, or water producing reefs). Looking
at structural, lithologic, and isopachous trends, it was
anticipated these could be related to petroleum occurrence,
or to indications of pinnacle reef location. Information for

analysis was limited to dry holes, as pinnacle related data

is anomalous to off-reef regional data. Directionally
drilled, non—reef wells were used when drilling density for
an area was low. Data was obtained from approximately 475

wells.

METHOD OF INVESTIGATION

 

Subsurface data were collected primarily from
geophysical well logs on file at the Department of Natural
Resources, Lansing, Michigan. The log most often used to
determine formation tops was the Gamma Ray log in
conjunction with the Borehole Compensated Sonic, and
Compensated Neutron-Formation Density log. Lithologic
determinations were made primarily from Compensated
Neutron-Formation Density and Sidewall Neutron Porosity
logs. When lithologic determination was not possible from
geophysical logs, sample descriptions from drillers logs
were used.

Subsurface information was used to construct isopachous
maps of the Brown Niagaran, A-l Evaporite, A-l Carbonate,
A-2 Evaporite, A-2 Carbonate, and B-Unit. Structural contour
maps of the Brown Niagaran and A-2 Carbonate were
constructed to observe rates of subsidence between periods
of deposition in the Niagaran-Salina sequence. Lithofacies
maps of the Brown Niagaran, A-1, and A-2 Evaporites were
constructed to determine whether or not lithologic changes

in these units could be related to petroleum occurrence, or

could be used as pinnacle indicators. Two cross-sections
perpendicular to the reef trend (one through Grand Traverse
County, the other through Manistee County) were drawn
illustrating subsurface changes encountered in going from
the Niagaran massive reef complex, basinward.

The Clinton carbonate was not included in the study due
to the scarcity of wells penetrating the subsurface to that

depth.

PREVIOUS WORK

Numerous outcrop and subsurface studies have been done
on the Niagaran-Salina sequence of the Michigan basin.
Faunal studies were done by Cummings and Shrock (1928) and
Lowenstam (1950, 1957). Faunal assembledges of individual
reefs were studied by Sharma (1966) and Gill (1977).
Lithologic subdivision and classification of Niagaran units
has been done by Landes (1945), Evans (1950), Ells (1967),
Gill (1973), and Budros and Briggs (1977).

Regional studies, encompassing the lower peninsula of
Michigan, which aid in the understanding of structure,
stratigraphy, sedimentation, reef growth, and petroleum
occurrence in the reef areas have been done by Alling and
Briggs (1961), Autra (1977), Briggs and Briggs (1974, 1975),
Cohee (1948), Ehlers and Kesling (1962), Ells (1963, 1967,
1969), Fisher (1969,1973), Gill (1975), Lilienthal (1978),
Mantek (1973), Melhorn (1958), Mesolella gt a; (1974, 1975),

5
Nurmi (1974), and Potter (1975).

Studies restricted to local areas in southeastern
Michigan include: Bates (1970), Budros and Briggs (1977),
Felber (1964), Gill (1973, 1977), Jodry (1969), Johnson
(1971), and Sharma (1966); in southern and south central
Michigan: Ells (1962), Fincham (1975), and Walles (1980); in
western Michigan: Hartsell (1982); and in northern Michigan:
Caughlin gt gt (1976), Fisher (1973), Gill (1979), Huh
(1973), Huh, Gill, gt gt (1977), Meloy (1974), Mesolella gt
gt (1974, 1975), and Sears and Lucia (1979, 1980).

One controversial topic is the growth history of
Niagaran reefs. Three basic models have been proposed. Gill
(1975), after studying the Belle River Mills field, proposed
that reef growth was entirely of Niagaran age and terminated
prior to the deposition of the A-l Evaporite and Carbonate.
This model was based on A-l Carbonate being present above
A-l Evaporite in inter-reef areas, but not on top of the
reef. Therefore, Gill placed the Niagaran-Salina contact at
the base of the A-2 Evaporite which caps the pinnacles. Work
done by Huh (1973, 1977), Mantek (1973), and Sears and Lucia
(1979), 1980) supported Gill's proposal that reef growth
entirely predated deposition of units surrounding and
capping the reefs.

Jodry (1969), in the second model, proposed reef growth
as being contemporaneous with inter-reef cyclic carbonates
and evaporites. The reef material represented a facies

change of the evaporites and carbonates being deposited. The

6

units are not laterally equivalent now, due to differential
compaction of the sediments after deposition.

Mesolella gt gt (1974), offering paleontological
evidence for an unconformity within the reef, proposed a
third model for reef growth. He placed upper parts of the
pinnacle reefs as being stratigraphically equivalent to the
inter-reef A-l Carbonate. Therefore, A-l Carbonate
deposition recorded a rejuvenation of growth on former
Niagaran pinnacles following a hiatus in reef development
associated with A-l Evaporite deposition. This
interpretation places the Niagaran-Salina contact within the
pinnacle reef.

Sears and Lucia (1980) contested Mesolella's proposal,
claiming in over 40 cored wells there was no supporting
evidence for any unconformity within the pinnacles. They
also questioned whether or not the paleontological evidence
offered by Mesolella was sufficient to place the upper parts
of the reef equivalent to the Salina units surrounding the
reefs. They cited further evidence to support Gill in that
clasts of calichified algal stromatolite, normally found on
the reef crests, had been discovered beneath the A-l
Evaporite in flank wells. This implied reef growth entirely
predated deposition of the Salina units.

- Huh (1977) concluded that definite A-l Carbonate
equivalents overlaid the reefs, but that they represented
tidal flat sediments, not reef regrowth. Sears and Lucia

(1980) agreed and felt the carbonate deposited on the reef

7

tops represented a restricted marine sequence. This
over-reef restricted marine unit interfingered on the reef
flanks with an inter—reef "poker chip" facies, establishing
contemporaneous deposition of the two units. These
researchers concluded that A-l Carbonate is present over the
tops of reefs and represents a restricted marine deposit,
not a rejuvenation of reef growth as Mesolella proposed.

Models one and three have the support of most
researchers. Further study of reef models is needed to
establish their validity.

A second area of debate centers on deep vs. shallow
water origin of the Salina evaporites. Dellwig and Evans
(1969), on evidence obtained in an underground salt mine in
Detroit, concluded water depth needed to be ”deep” to
prevent disturbance of primary bedding structures by waves
or currents. Other researchers favored shallow water origin
for deposition of Salina evaporites. Nurmi (1974) felt
evidence of ripple marks and observable unconformities in
salts in an underground Ontario mine, suggested a
sabkha-type depositional environment. Sears and Lucia (1980)
cited evidence of sylvite, nodular anhydrite, halite
hoppers, and brecciation of A-l Evaporite as being
indicative of shallow water deposition. A gradual change
from laminated anhydritic dolomite in the A-l Carbonate, to
interbedded halite and anhydrite in the A—2 Evaporite also
indicated shallow water deposition.

Evidence used to interpret water depths in the

8

formation of basin margin evaporites may help researchers
conclude water depths for central basin evaporite deposits.
Further thin section analysis of cores is needed before
definite conclusions about water depth in the central basin

can be drawn.

GENERAL STRATIGRAPHY AND SEDIMENTATION

 

The Michigan Basin is comprised of sediments ranging
from Precambrian to Mesozoic in age. Except for the Permian,
Paleozoic sediments of all ages are present. Of sediments
Mesozoic in age, only Jurassic are present. Most of the
basin is covered by Pleistocene glacial deposits (Fig. l).

Silurian rocks account for close to. 30 percent of the
total sediment in the basin. Total Silurian thickness has
been estimated at 4,000 feet in the central part of the
basin. One-third to one-half of those sediments are pure
salt deposits. Sediments of Middle and Upper Silurian age
are highlighted in this study, and include units of the
Niagara and Lower Salina Groups.

The Niagara has been divided into the following
formations: "Clinton“ (Burnt Bluff and Manistique
equivalents), "White and Gray” Niagaran (Lockport
equivalent), and "Brown" Niagaran (Guelph equivalent). Terms
in parentheses represent formal formational usage, whereas
"Clinton", "White", "Gray", and "Brown” Niagaran are

correlative terms used by the oil industry, based on the

  
 

STRATIGRAPHIC SUCCESSION IN MICHIGAN

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FIGURE 1

10
stratigraphic position and color of drill cuttings of these

units (Ells, 1967). The "Brown", or Guelph, is the reef
facies of the Niagara Group. Pinnacle reefs, within the
"Brown", are the principal oil and gas reservoirs of
Silurian age that interest the oil industry. Units older
than the "Brown" Niagaran are not analyzed in this study due
to the scarcity of wells that completely penetrate the
entire thickness of the Niagara Group.

Salina Group rocks were divided by Landes (1945) into
units A through H, with A being deposited directly above the
”Brown” Niagaran. The H-Unit is presently referred to as the
Bass Islands Group. Evans (1950) further divided the
alternating evaporites and carbonates of Landes' A-Unit into
the A-l Evaporite, A-l Carbonate, A-2 Evaporite, and A-2
Carbonate. Budros and Briggs (1977) formally named Evans'
A-l Carbonate Unit the ”Ruff" Formation. Ells (1967) divided
the B-Unit into the B-Carbonate and the B-Salt on the basis
of geophysical log response, but for this study the two
units were combined. Gill (1973) recognized an interreef
carbonate unit, above the ”Brown” Niagaran and beneath the
A-l Evaporite, and designated it the A-O Carbonate. The A-O
Carbonate was not distinguishable on Gamma Ray logs and was
essentially ignored for this study. This investigation was
limited to the lower units of the Salina Group. The units in
ascending order are: the A-l Evaporite, A-l Carbonate, A-2
Evaporite, A-2 Carbonate, and the B-Unit.

Mesolella gt gt (1974) offered the following

11

depositional history for the Niagaran—Lower Salina units.
Carbonates were forming during Niagaran time and a system of
organic, platform reefs developed around the margins of the
basin. Basinward from this massive reef complex, pinnacles
grew vertically as much as 100-200 meters. Niagaran
carbonate was thickest adjacent to the reefs and thinner
immediately basinward, where sedimentation rates were lower
and the generation of carbonate was minimal (Fig. 2).
Restriction, increasing salinity, and a lowering of sea
level may have terminated Niagaran reef growth and initiated
the deposition of the A-l Evaporite.

The A-l Evaporite is thickest in the center of the
basin where it is predominantly halite. At the basin
margins, the halite grades laterally into anhydrite. The
anhydrite pinches out on the flanks of pinnacles and against
the front of the Niagaran massive reef complex. This unit
contains a sylvite lens (potassium chloride) which increases
in thickness and purity basinward. The significance of
sylvite as a possible near-reef indicator was summed up by
Elowski (1980). He observed that sylvite occurred in the
center of embayments between Niagaran reefs in the northern
trend. These embayments surrounded reefs and groups of reefs
along the basinward edge of the trend. The disappearance of
sylvite in these areas may indicate proximity to pinnacle
reefs. Because these embayments of sylvite are restricted in
areal extent to the basinward half of the pinnacle trend,

the usefulness of potash salts in the A-l Evaporite as an

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exploration tool is limited.

A rise in sea level initiated the deposition of A-l
Carbonate. This is where researchers disagreed. Mesolella gt
gt (1974) felt that A—l Carbonate deposition reflected a
resurgence in reef growth on the tops of Niagaran pinnacles.
Huh (1977) and Sears and Lucia (1980) felt A-l Carbonate
deposition represented an over-reef restricted, tidal flat,
marine carbonate. The A-l Carbonate, thickest near the
massive reef complex and thinnest toward the basin interior,
contains thin lenses of anhydrite proximal to pinnacles.
These thin lenses of anhydrite ("rabbit ears") were
deposited as halos on supratidal flats around the pinnacle
reefs. The supratidal flats are limited in areal extent and
resulted from regressions in sea level.

The two lenses of anhydrite are separated by a thin
layer of carbonate. This separation indicates the
regressions were cyclic in nature. Because of the limited
areal extent of these anhydrite lenses and their proximity
to pinnacle reefs, "rabbit ears" are used as near-reef
indicators.

Sears and Lucia (1980), after studying dolomitization
patterns in the A-l Carbonate, concluded the depositional
history of this unit involved periods of tidal flat
conditions which were believed to be important to
dolomitization of the underlying Niagaran reefs. Following
deposition of the restricted marine carbonate, a tidal flat

facies developed on top of the pinnacles. These tidal flat

14

sediments (later dolomitized) extended from the Niagaran
shelf basinward, about half way through the reef belt. Reefs
underlying this tidal flat environment had been entirely or
partially dolomitized. Reflux of hypersaline brines through
tidal flat sediments was given as the mechanism for
dolomitization. Beyond this reef belt midpoint, basinward
pinnacles were found to be less dolomitized and
predominantly composed of limestone. If Sears and Lucia's
theory is correct, then dolomitization of underlying reefs
followed dolomitization patterns of the A-l Carbonate.

Since dolomitized reefs are known to be more productive
(due to better porosity) than limestone reefs, the
conclusion drawn by Sears and Lucia is significant. Reefs
which were beneath A-l Carbonate tidal flat sediments should
be more productive.

A lowering of sea level terminated A-l Carbonate
deposition and initiated the deposition of the A-2 Evaporite
(Mesolella gt gt 1974). The A-2 Evaporite is thickest in the
interior of the basin and thins toward the margins. It thins
rapidly over the pinnacle and barrier reef complex and
undergoes a facies change from halite to anhydrite, similar
to the A-l Evaporite. Anhydrite seals the tops of the
pinnacles and the Niagaran massive reef complex.

A rise in sea level initiated depoisition of the A-2
Carbonate. This unit was not characterized by reef building.
There was also a reversal in carbonate thickening trends.

The A-2 Carbonate thickened basinward and thinned toward the

15
basin margins. This suggested physical, not biological,

processes were dominant during deposition. Lowering of sea
level began another phase of evaporite deposition.

The B-Unit (B-Carbonate and B-Salt combined) is
thickest in the basin interior and thins toward the margins.
The B-Unit basin covers more area laterally than basins of
earlier periods of deposition. Along the margins, the unit
undergoes a facies change from halite to anhydritic
dolomites and shales. Because this study area was located
basinward of the margin, the facies change was not observed.

In the Northern trend the B-Unit is anomalously thick
overlying the pinnacle reef belt. Mesolella gt gt (1974)
believes this thickening coincides with A-2 Evaporite
thinning caused by solution lower in the sequence. The

B-Unit is the uppermost unit analyzed in this research.

STRUCTURAL HISTORY

 

The Michigan Basin has been classified as an
intracratonic basin. Located on the southern edge of the
highly fractured and faulted granites of the Canadian
Shield, the basin is bordered on the west by the Wisconsin
Arch and Wisconsin Dome. To the east and southeast lie the
Algonquin and Findlay Arches, the two being separated by the
Chatham Sag. To the south and southwest lie the Cincinnati
Arch and the Kankakee Platform (Fig. 3).

These structural features on the southeast, south, and

16

 

 

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FIGURE 3. MICHIGAN BASIN AND SURROUNDING

STRUCTURAL ELEMENTS (MODIFIED AFTER ELLS,
I969). -

17

southwest separate the Michigan Basin from the Illinois and
Appalachian Basins. During Niagaran and Lower Salina time,
these features were believed to have had little structural
influence on the basin due to their low relief. However,
these features may have played an important role in the
formation and growth of the Niagaran massive reef complex.

Structural trends within the Michigan Basin have a
strong northwest - southeast alignment. The Albion - Scipio
trend, Howell and Northville Anticlines, and many Devonian
oil fields conform to this pattern. Pirtle (1932) believed
this alignment reflected structural patterns of the
Precambrian basement. Anticlinal structures and draping of
Paleozoic sediments over these structures may have indicated
reactivation of basement blocks along pre-existing fault and
fracture systems (vertical tectonics). Although it seemed
that basement tectonics controlled structural patterns in
the Michigan Basin, is was not until Middle to Late
Ordovician time that the present shape of the basin became
apparent (Fisher, 1969 and Haxby gt gt, 1976).

Numerous theories have surfaced dealing with the
creation and formation of the Michigan Basin. Newcombe
(1933) proposed a downwarping of the basin resulting from
forces imposed from the northeast. Hinze (1963), based on
gravity work, proposed that the addition of dense,
Keweenawan basalt flows to the Precambrian basement resulted
in subsidence. Subsidence restored isostatic equilibrium

between the Earth's crust and mantle. Haxby gt gt (1976)

18

proposed that mantle diapirs penetrating the lower crust
resulted in density changes in the crustal rocks. Subsidence
was needed to restore isostatic equilibrium. Whatever the
cause of formation of the basin, Ells (1969) believed the
Appalachian Orogeny caused further structural deformation of
basin sediments by imposing a force from the southeast.
Cohee and Landes (1958) believed the basin underwent
structural deformation throughout the Paleozoic. The most
intense deformation occurred during the Mississippian
period. Cohee and Landes also proposed that the basin
underwent the greatest amount of subsidence during Late
Silurian and Middle Devonian time. This conclusion was drawn
from the vast thicknesses of salt present in units of these
two periods. Assumptions about water depth at the time of
salt formation become critical when thicknesses of salt
deposits are used to determine rates of subsidence. Because
researchers cannot unanimously conclude whether halite forms
in deep or shallow water, the timing of subsidence in the
Michigan basin by Cohee and Landes may be questionable.
Fisher (1973), after iSOpaching the entire Salina
Group, concluded that the northern rim of the Michigan Basin
subsided more rapidly than the southern during Salina time.
Sediments over the crests of the massive reef complex were
1000 feet thicker in the north than in the south. This
probably accounted for differences in depths to pinnacles,
heights of pinnacles, and facies relationships of overlying

A-1 and A—2 Evaporites to pinnacle positions between the

19

northern and southern rims.

Sears and Lucia (1979) believed the pinnacle reefs in
the north grew on a ramp that was tectonically stable. The
ramp subsided at a uniform rate, not at differential rates.
This would imply the basinward pinnacle reefs grew at a
faster rate in order to keep up with a rising sea level.
Whatever the case, lithologic and stratigraphic
relationships of Lower Salina units to Niagaran barrier and
pinnacle reef complexes are not identical along the northern

and southern rims of the Michigan Basin.

FACTORS CONTROLLING PETROLEUM OCCURRENCE

 

Niagaran pinnacle reefs are found along a "fairway"
located basinward from the massive reef complex. This
"fairway“ is approximately 8 miles wide in the study area.
Basinward pinnacles tend to be taller in height and smaller
in areal extent. Shelfward pinnacles are generally shorter
and brOader.

A definite segregation of hydrocarbons is observable
along the northern pinnacle trend (Gill, 1979) (Plate 12).
Pinnacles lying farthest basinward are usually void of
hydrocarbons and plugged with salt. Updip lie reefs that
produce ”sour" gas and, to a lesser degree, “sour" oil.
"Sour" production denotes high concentrations of hydrogen
sulfide present in the hydrocarbon. These ”sour” reefs are

generally composed of limestone and have limited pore space.

20

This makes production from these reefs difficult because the
hydrocarbons cannot migrate easily through the reef to the
well bore.

The pinnacle reefs in the center portion of the trend
produce oil, gas, or a combination of both. The majority of
the production is ”sweet", although there are exceptions to
this rule. Reefs in this portion of the trend are dolomitic,
have better porosity, and tend to produce hydrocarbons more
easily than the reefs down-dip. Pinnacles closest to the
massive reef complex are often filled with anhydrite, or
they produce water and limited amounts of low gravity oil.

This ,updip segregation of fluid types (from gas and
light condensate, to high gravity oil, to low gravity oil
and water, to water producing reefs) has been deemed a
classic example of Gussow's theory on differential
entrapment of oil and gas (Gill, 1979). According to this
theory, if hydrocarbons are sourced from down-dip areas, a
partitioning of fluids of different gravities would occur.
Basinward pinnacles would fill up with higher gravity fluids
(gas and high gravity oil) because less viscous hydrocarbons
would migrate faster into reservoirs. Shelfward pinnacles
would then fill with a mixture of low gravity oil and water,
or just water.

Assuming constant slope along the length of the trend,
these bands of production should be continous across the
study area and parallel to one another. As noted, this is

not always the case. Pinnacles in Cleon Township of Manistee

21

County tend to produce "sour" gas. This "sour" production is
embayed further up into the pinnacle trend than it should
be. The width of the band of producing pinnacles narrows in
Cleon Township. No production has been found to date in the
northern-half of the township. This thinning is anomalous
when looking at townships located to either side of Cleon.
With that in mind, an attempt was made to discover why these
production trends are not parallel to one another. By
examining relationships of units surrounding and overlying

the pinnacles in off-reef wells, determination was possible.

NIAGARAN STRUCTURE MAP

 

To determine whether production trends were
structurally related, a contour map was drawn using the top
of the Brown Niagaran as a datum (Plate 1). In looking at
the Brown Niagaran structure map, the contours in the
fairway trend essentially north-south through Township 21
north. From Township 22 north, the contours swing in a
general direction of N 300 E until they reach a point in
Cleon Township (T24N - R13W) where I have indicated a fault.
Northeast of the fault, the contours follow a trend in the
direction N 550 E. Assuming the fault is correctly placed,
this abrupt change in direction could be related to vertical
displacement of basement blocks along already existing
northwest - southeast fault patterns. Another possible fault

is located in Paradise Township (T25N - RlOW) exhibiting the

22

same northwest - southeast trend. Since sufficient well
control deeper than the Clinton is not available, structural
contour trends attributed to fault mechanisms can only be
hypothesized. Linear trends observed in pinnacle locations

generally parallel Brown Niagaran structural trends.

BROWN NIAGARAN ISOPACH AND LITHOFACIES MAP

Niagaran reef growth is a reflection of biologic
activity. It is assumed wherever concentrations of pinnacle
reefs are fairly high, off-reef wells will reflect biologic
activity by containing thicker sections of carbonate (Plate
2). Wells with 15 feet or less of Brown Niagaran were
considered to be ”regional" in nature. Wells with more than
15 feet were considered to represent areas of debris either
adjacent to pinnacles (flank buildup) or debris fans off of
the massive reef bank.

The interpretation of the Brown Niagaran isopach map
may be questionable. Areas inside the 15 foot contours may
not actually contain thicker sections of carbonate. Two or
three flank wells contoured together would display the same
result. Additional well control will confirm or deny the
interpretation of this study.

The lithology of the Brown Niagaran in the study area
is both limestone and dolomite (Plate 3). The break-point
between the two lithologies parallels the massive reef bank

and is located at about the mid-point of the trend. The

23

massive reef bank and the Shelfward half of the pinnacle
trend is composed of dolomite, while the basinward half of
the pinnacle trend is composed of limestone. Two noticeable
embayments of limestone into the dolomite region occur in
Cleon Township (T24N - R13W) of Manistee County and East Bay
Township (T26N - RlOW) of Grand Traverse County. If
limestone formation reflects a deeper water environment,
then these embayments could be attributed to the faulting
mentioned earlier. The limestone is present on the
downthrown side of the fault in both areas. Water depth
would be expected to be greater there. Around the margins,
the Niagaran massive reef is 500 - 600 feet thick and
pinnacles average 300 - 600 feet in thickness. In the center
of the basin, the Niagaran averages 60 feet in thickness

(Fisher, 1973).

A-l EVAPORITE ISOPACH AND LITHOFACIES MAP

 

In examining A—l Evaporite isopach trends (Plate 4), it
is apparent that after evaporative drawdown of sea level,
A-l Evaporite deposition filled in existing Niagaran
topography. In areas where Niagaran biologic activity was
sparse, isopach lines reflect steep slopes and parallel
Niagaran structure. In areas where Niagaran pinnacle growth
was concentrated, the contour spacing widens.

In evaluating the lithology of the A-l Evaporite (Plate.

5), wells on the basinward side of the pinnacle trend

24
contain a clean salt section. Moving toward the bank, the

evaporite section becomes a mixture of halite and anhydrite
with the anhydrite being at the base. Off-reef wells
throughout the entire pinnacle reef complex of the study
area had an anhydrite cap over the top of the Brown
Niagaran. Next to the massive reef complex, the evaporite
becomes all anhydrite and pinches out against the face of
the massive reef. As flanks of individual pinnacles are
encountered, the anhydrite at the base of the A-l Evaporite
thickens. The evaporite pinches out against the sides of the
pinnacles. The A-l Evaporite thickness is generally less
than 200 feet within the pinnacle trend. In the central
basin, it thickens to about 400 feet (Mesolella gt gt,

1974).

A-l CARBONATE ISOPACH MAP

Upon examining this map (Plate 6), a definite
thickening trend is observed parallel to the massive reef
complex. Areas inside the 100 foot contour interval are
areas of isopachous thickening. Whether this thickening is
attributable to a resurgence in reef growth, or whether it
reflects areas of tidal flat sedimentation, the map shows
that thick areas of A-l Carbonate do exist. Using the 100
foot contour interval, support for either tidal flat
sedimentation or a rejuvenation of biologic growth on

existing Niagaran sites is plausible because thick sections

25

of A-l Carbonate mask thick sections of Niagaran deposits.

If the 120 foot contour is used on the A-l Carbonate
map a different pattern develops. Areas inside the 120 foot
interval tend to be located close to the massive reef
complex and toward the back side of the pinnacle reef
complex. When these areas are compared to areas of intense
Niagaran biologic activity, the two do not generally
coincide. The 120 foot contour isopach would support the
theory that A-l Carbonate deposition did not reflect a
rejuvenation of reef growth. If it did, one would expect the
areas of thicker sections on both maps to overlap. This
researcher prefers the explanation that areas inside the 120
foot contour reflect areas of tidal flat sedimentation. They
are located up against the massive reef complex where one
would expect them to be.

At this point, Sears and Lucia's (1980) research is
pertinent. Dolomitization of the A-l Carbonate occured in
tidal flat areas. Refluxing of salt saturated brines down
through the sediments is the mechanism they offer for
accomplishing this lithologic change. They assume the
process that dolomitized the A-l Carbonate also dolomitized
Niagaran reefs below the tidal flats. If so, there should be
a general correlation observed between the tidal flat
boundaries and the dolomite/limestone breakpoint on the
Niagaran lithofacies map. If the 100 foot isopach contour is
used on the basinward side of the pinnacle complex, the two

boundaries are generally correlative. The exception to this

26

is in southern Whitewater (T27N-R9W) and northern Union
(T26N-R9W) Townships of Grand Traverse County. Here the
limestone boundary swings farther into the inter-reef area
and cuts inside the 100 foot contour that approximates the
basinward edge of the tidal flats.

On flanks of pinnacle reefs, the upper section of the
A-l Carbonate in the study area contains a "rabbit ears"
anhydrite. This lithologic change has been noted by Gill
(1973) and Huh (1973). The "rabbit ears" are described as
being a nodular, supratidal anhydrite which suggests it
formed under sabkha-type conditions. Since the A—l Carbonate
thins over the tops of pinnacles, the presence of "rabbit
ears" anhydrite supports the existence of shallow water,
tidal flat environments during the latter part of A-l
Carbonate deposition. Assuming the "rabbit ears" anhydrite
is deposited around the entire periphery of the pinnacle
reef, this lithologic change can be used to indicate
proximity to pinnacles. Data from this study is supportive
of Sears and Lucia's tidal flat model of A-l Carbonate
deposition. Along the margin of the study area, the A-l
Carbonate thickness averages between 80 - 120 feet In the
basin center the average thickness is 50 feet (Mesolella gt

gt, 1974).

A-2 EVAPORITE ISOPACH AND LITHOFACIES MAPS

Following deposition of the A—l Carbonate, evaporative
drawdown of sea level and restriction of the basin allowed
for deposition of the A-2 Evaporite. On this map (Plate 7),
isopach lines parallel the front of the massive reef
complex. A general thickening of the unit is noted
basinward.

Lithofacies changes in the A-2 Evaporite lead to some
exciting conclusions about depositional environments of that
time period (Plate 8). Basinward of the pinnacle trend, the
A-2 Evaporite is composed of a clean salt throughout the
entire section. As the tidal flat environments of the A-l
Carbonate are approached, the base of the A-2 Evaporite
becomes anhydritic. It is a "ratty" anhydrite intermixed
with halite. This indicates a shallowing effect in water
depth (i.e. slope environments off of the tidal flats). On
top of the tidal flats, the base of the A-2 Evaporite
becomes a clean anhydrite. It forms a cap over the
underlying A-l Carbonate. Passes through the pinnacle trend
become very apparent, as the water is deeper there and the
A-2 Evaporite remains a clean salt (halite).

One such pass is located in Cleon Township (T24N-R13W),
southwest of the fault noted on the Niagaran structure map.
Another is located in Paradise Township (T25N-R10W) near the
fault indicated there. A third possible pass is located

between Mayfield (T25N-R11W) and Grant (T25N-R12W) Townships

27

28

but this one may or may not be fault related. The pass in
Cleon Township and the pass in Paradise coincide with
limestone embayments in the Niagaran. This supports deeper
water conditions in a localized area, as well as the
presence of a clean halite section in the A-2 Evaporite. The
above conditions line up near the downthrown side of the
fault where the water depth would be greater.

It is interesting that behind these passes lie possible
lagoon or back-reef areas in front of the massive reef
complex that contain a clean halite section. Currents
through the passes may have scoured out tidal flat sediments
enough to create a regime where water depth inhibited
anhydrite deposition. It is interesting that where the
possibility of lagoon environments exist, the width of the
pinnacle trend narrows. It seems odd that few or no
producing pinnacles have been found in northern Cleon and
northwestern Grant Townships. The environment of deposition
determined from using the lithologic changes at the base of
the A-2 Evaporite could possibly explain the lack of
producing pinnacles in those areas.

As the massive reef complex is approached from behind
the pinnacle reef complex, the A-2 Evaporite becomes pure
anhydrite. This is true except in areas that are behind or
near passes in the pinnacle complex. In many of those areas,
a clean halite section is present right up against the front
of the massive reef complex. The evaporite section thins

abruptly at the front of the massive reef, and continues up

29

over the top and caps it.

In studying initial production of producing pinnacles
of the study area, it is interesting that the most
productive pinnacles underlie areas that are capped by
anhydrite at the base of the A-2 Evaporite or areas where
the base of the A-2 Evaporite is becoming anhydritic (the
”ratty” zone where the halite and anhydrite are
interbedded). If these two areas best represent proximity to
tidal flat environments of the underlying A-l Carbonate,
then this would support conclusions drawn from Sears and
Lucia's work. Niagaran pinnacles beneath A-l Carbonate tidal
flats were dolomitized by the same processes that
dolomitized the A-l Carbonate. In areas where Niagaran reefs
were dolomitized, greater initial production would be
expected. In conclusion, the lithology of the salt at the
base of the A-2 Evaporite may be used as a mapping tool for
delineating areas of more productive Niagaran reefs.

On top of the Niagaran massive reef complex, the A-2
Evaporite thickness averages less than 40 feet. In the
pinnacle belt, it is generally less than 250 feet thick. In
the central basin, the A-2 Evaporite averages thicknesses

between 450 - 500 feet (Mesolella gt gt, 1974).

A-2 CARBONATE STRUCTURE MAP

Structural contours indicate the slope into the basin

is gentler than it was during Niagaran time. The contours

30

are wider apart and evenly spaced. The shape is no longer
controlled by the Niagaran massive reef complex. Contours
trend N 200 E until they reach Cleon Township. There they
take a turn and trend in a direction of N 450 E. The turn
here may be related to the fault indicated on the Niagaran
structure map. Two structural noses into the basin are
apparent. They are located approximately where faults are

indicated on the Niagaran map.

A-2 CARBONATE ISOPACH MAP

The A-2 Carbonate is thinnest on top of the Niagaran
massive reef complex and thickens basinward (Plate 10).
Thick areas of sedimentation occur along the front edge of
the massive reef complex where draping of carbonate might be
expected. Thick sections of carbonate are also present
within the boundaries of the pinnacle reef complex. These
areas may represent topograghic lows where A-2 Evaporite was
scoured away by currents or wave action. Most of the thicker
sections of A-2 Carbonate occupy lepe positions off of the
A-l Carbonate tidal flats either on the front or back sides
of the pinnacle reef complex. In the study area, the A-2
Carbonate is generally less than 120 feet thick. In the
central basin, this carbonate unit averages 50 feet in

thickness (Mesolella gt gt, 1974).

B-UNIT ISOPACH MAP

 

In general, the B-Unit thickens basinward, but thickens
locally in the study area (Plate 11). Localized thickening
is found inside the 400 foot contour interval. Mesolella gt
gt, (1974) attributes the thickening to A-2 salt solution
lower in the stratigraphic section. Upon examination of the
A-2 Evaporite Isopach map no thinning of this unit is
observed over the top of the pinnacle reef fairway. It may
be possible the Michigan Basin was actively undergoing
subsidence along old Niagaran structural boundaries, but at
this point, an explanation for localized thickening of the
B-Unit seems elusive. In the study area, the B—Unit is 300 -
350 feet thick except over the top of the pinnacle fairway.
There it thickens to between 350 and 450 feet. In the
central basin, the B-Unit averages 400 feet in thickness

(Mesolella gt gt, 1974).

PETROLEUM PRODUCTION

 

In the northern pinnacle belt, a definite updip
partitioning of petroleum products exists. Basinward
pinnacles produce gas (unless salt plugging has occurred)
while pinnacles updip produce oil and water (close to the
massive reef complex). Oil downdip is less viscous and of
higher degree than oil updip. Production along the basinward

edge contains appreciable amounts of hydrogen sulfide

31

32
contaminants. These observations concur with those of Gill

(1979) and present further questions that need to be
answered.

If the oil migrated into place before the pinnacle
reefs were sealed with A-2 Evaporite, what part did
refluxation of brines during A-l Carbonate time play in
contaminating production ? If the oil was generated in place
after the reefs were capped, did contamination result from
organisms within the reefs, or does the presence of hydrogen
sulfide depend on hydrocarbon maturation and depth of
burial? If carbonates from a central basin reducing
environment sourced the oil for the pinnacle reefs updip,
what limited the hydrogen sulfide components from
contaminating all of the pinnacles in the northern pinnacle
belt ? These unanswered questions need to be investigated
for a better understanding of production trends observed

within the northern pinnacle belt.

CONCLUSIONS

 

A stratigraphic, structural, and lithological analysis
was done on Niagaran-Lower Salina units in northwestern
Michigan. From the analysis, it was determined that
lithologic changes in the Brown Niagaran and the A-1 and A-2
Evaporite units could be used as pinnacle reef indicators,
and could be related to petroleum type and occurence.

Brown Niagaran reef growth was determined by Gill

33
(1975) and Sears and Lucia (1980) to be entirely of Niagaran

age. Brown Niagaran buildup is thickest in the massive reef
complex and in individual pinnacles. It thins basinward
where carbonate generation was minimal and sedimentation
rates were lower. The pinnacles are found in a "fairway"
located basinward of the massive reef complex. The linear
trends observed for pinnacle locations generally parallel
regional structure that existed at the time. Lithology of
the Brown Niagaran in the study area is both limestone and
dolomite. The dividing line between the two lithologies
parallels the massive reef complex and is located at the
approximate mid-point of the pinnacle reef trend. The Brown
Niagaran is dolomite north of this line and limestone south
of it. Embayments of limestone into the dolomite region have
been attributed to faulting and reflect a deeper water
environment.

A definite segregation of hydrocarbons within the
pinnacle reefs has been observed in the study area. Most
pinnacles lying farthest basinward are plugged with salt and
are void of hydrocarbons. Updip, the pinnacles are generally
composed of limestone and have pore space. The limestone
composition and taller heights of these pinnacles reflect a
deeper water environment that the pinnacles located farther
updip. Production rates are lower for these basinward
pinnacles and production has been contaminated with hydrogen
sulfide. Pinnacles located in the center and upper portion

of the pinnacle trend have been dolomitized and have

34

appreciable pore space. Rates of production are higher than
pinnacles located downdip and "sweet" oil is produced. Near
the massive reef complex, many pinnacles are filled with
anhydrite, or they produce limited amounts of low gravity
oil and water, or just water.

A-l Evaporite deposition filled in existing Niagaran
topography. This evaporite is composed of halite in the
center of the basin where it is the thickest. From the
basinward edge of the pinnacle trend, to the front edge of
the Niagaran massive reef complex, the A-l Evaporite becomes
a mixture of halite and anhydrite with the anhydrite at the
base. When approaching the flanks of pinnacle reefs, the
anhydrite at the base increases in thickness. The A-l
Evaporite pinches out on the flanks of pinnacles and against
the front of the Niagaran massive reef complex.

Data from this study support the.restricted marine
tidal flat model for A-l Carbonate deposition proposed by
Huh (1977) and Sears and Lucia (1980). Tidal flat sediments
extend from the massive reef complex approximately half way
through the pinnacle reef belt. Within the pinnacle belt,
and proximal to pinnacle reefs, the "rabbit ears" anhydrite
appears in the upper portion of the A-l Carbonate section.
The tidal flat sediments were dolomitized by refluxing of
salt saturated brines during A-2 Evaporite time. The process
that dolomitized the A-l Carbonate was believed to have
dolomitized the Niagaran pinnacle reefs located beneath

these tidal flat environments. In conclusion, pinnacles

35
beneath tidal flat environments that had been fully or

partially dolomitized would be expected to be more
productive. Upon studying initial production rates of
producing pinnacles in the study area, it was found that the
most productive reefs were located beneath A—l Carbonate
tidal flat environments, which can be detected by lithologic
changes at the base of the A-2 Evaporite.

The A-2 Evaporite is thickest in the interior of the
basin and thins towards the margins. Over the tops of
pinnacles this evaporite thins and becomes anhydritic at the
base. Lithologic changes in this unit are similar to those
of the A-l Evaporite. Basinward of the pinnacle trend the
A-2 Evaporite is composed of halite. As tidal flat
environments of the underlying A-l Carbonate are approached,
the base of the evaporite becomes a ratty mixture of halite
and anhydrite. On top of the underlying tidal flats, a clean
anhydrite lens is present at the base of the section. These
lithologic changes at the base of the A-2 Evaporite simply
reflect a change in water depth. Passes through the pinnacle
trend become apparent, as the deeper water depth is
reflected by the presence of a clean halite section. As the
massive reef complex is approached, the A-2 Evaporite
becomes anhydrite, thins abruptly, and continues up over the
top of the massive reef complex and caps it. Exceptions are
noted in areas directly behind, or near passes in the
pinnacle reef complex. In those two areas, a clean halite

section is present in contact with the front of the massive

36
reef complex.

Combining results of the inital production rates study
with lithologic changes observed at the base of the A-2
Evaporite has produced a mapping tool for delineating
general areas where Niagaran reefs should be more
productive. Areas where the base of the A-2 Evaporite is a
ratty mixture of halite and anhydrite, or where a clean
anhydrite lens is present, best represent tidal flat
environments of the underlying A-l Carbonate. Dolomitized
pinnacles located beneath these areas have the highest rates
of initial production.

The A-l Carbonate is thinnest on top of the massive
reef complex and thickens basinward. This suggests physical,
not biological, processes were dominant during deposition of
this unit.

Localized thickening of the B-Unit was observed in the
study area, but an explanation for this phenomenon seems
elusive.

No conclusions were drawn as to how and when the
hydrocarbons along the basinward edge of the pinnacle reef
trend were contaminated with hydrogen sulfide, or as to what
limited the areal extent of the contamination. Further
examination of questions such as these is needed to gain a
better understanding of production trends in the northern

Michigan pinnacle reef belt.

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4O

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41

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University.

 

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