William B. Harrison

Fractured hydrothermal dolomite reservoirs in the Devonian Dundee Formation of the central Michigan Basin

The Middle Devonian Dundee Formation is the most prolific oil-producing unit in the Michigan Basin, with more than 375 million bbl of oil produced to date. Reservoir types in the Dundee Formation can be fracture controlled or facies controlled, and each type may have been diagenetically modified. Although fracture-controlled reservoirs produce more oil than facies-controlled reservoirs, little is known about the process by which they were formed and diagenetically modified. In parts of the Dundee, preexisting sedimentary fabrics have been strongly overprinted by medium- to coarse-grained dolomite. Dolomitized intervals contain planar and saddle dolomite, with minor calcite, anhydrite, pyrite, and uncommon fluorite. Fluid-inclusion analyses of two-phase aqueous inclusions in dolomite and calcite suggest that some water-rock interaction in these rocks occurred at temperatures as high as 120–150C in the presence of dense Na-Ca-Mg-Cl brines. These data, in conjunction with published organic maturity data and burial reconstructions, are not easily explained by a long-term burial model and have important implications for the thermal history of the Michigan Basin. The data are best explained by a model involving short-duration transport of fluids and heat from deeper parts of the basin along major fault and fracture zones connected to structures in the Precambrian basement. These data give new insight into the hydrothermal processes responsible for the formation of these reservoirs.

Devonian Dundee Formation, Crystal Field, Michigan Basin: Recovery of Bypassed Oil Through Horizontal Drilling

Carbonate reservoirs of the Devonian Dundee Formation represent the most productive interval in the Michigan basin, yet have suffered widespread abandonment due to improper early reservoir management and completion practices. Dundee carbonate reservoirs, deposited in marginal-marine and shallow-marine environments, have produced over 351 MMbbl oil from 137 fields, most of which were discovered in the 1930s and 1940s. Initial potentials for many Dundee wells ranged from 2000 to 9000 bbl/day, presumably due to high- permeability fractures and solution-enhanced porosity in some areas and excellent primary interparticle porosity in other areas. Crystal field, discovered in 1935 at a depth of less than 3500 ft (1060 m), was aggressively developed and produced at high rates, causing widespread water coning, such that 95% of the field was drilled and abandoned in less than 5 yr, leaving considerable interwell reserves in the ground. No cores or wireline logs existed for the field prior to 1995, when the field became the subject of a multiyear study aimed at determining the feasibility of employing horizontal drilling to recover bypassed reserves. A vertical test portion of the TOW 1-3 horizontal well, located in a structurally elevated portion of the field, cored and logged the entire reservoir interval, yielding crucial new information. On the basis of relevant data, a horizontal leg was drilled, resulting in an excellent producer with estimated recoverable reserves of 200,000 bbl. Two subsequent horizontal tests with poor results were drilled in downdip locations and oriented perpendicular to the TOW well, suggesting the importance of both structural position and azimuth. Selective use of horizontal drilling is considered an appropriate, cost-effective means for recovering bypassed reserves in Dundee fields where sufficient geologic data exist to adequately characterize the reservoir and oil occurrence. For some fields, this may require new coring and logging operations.

Advanced Characterization of Fractured Reservoirs in Carbonate Rocks: The Michigan Basin

The purpose of the study was to collect and analyze existing data on the Michigan Basin for fracture patterns on scales ranging form thin section to basin. The data acquisition phase has been successfully concluded with the compilation of several large digital databases containing nearly all the existing information on formation tops, lithology and hydrocarbon production over the entire Michigan Basin. These databases represent the cumulative result of over 80 years of drilling and exploration. Plotting and examination of these data show that contrary to most depictions, the Michigan Basin is in fact extensively faulted and fractured, particularly in the central portion of the basin. This is in contrast to most of the existing work on the Michigan Basin, which tends to show relatively simple structure with few or minor faults. It also appears that these fractures and faults control the Paleozoic sediment deposition, the subsequent hydrocarbon traps and very likely the regional dolomitization patterns. Recent work has revealed that a detailed fracture pattern exists in the interior of the Central Michigan Basin, which is related to the mid-continent gravity high. The inference is that early Precambrian, ({approx}1 Ga) rifting events presumed by many to account for the gravity anomaly subsequently controlled Paleozoic sedimentation and later hydrocarbon accumulation. There is a systematic relationship between the faults and a number of gas and oil reservoirs: major hydrocarbon accumulations consistently occur in small anticlines on the upthrown side of the faults. The main tools used in this study to map the fault/fracture patterns are detailed, close-interval (CI = 10 feet) contouring of the formation top picks accompanied by a new way of visualizing the data using a special color spectrum to bring out the third dimension. In addition, recent improvements in visualization and contouring software were instrumental in the study. Dolomitization is common in the Michigan Basin, and it is crucial in developing reservoir quality rocks in some fields. Data on the occurrence of dolomite was extracted from drillers reports for all reported occurrences in Michigan, nearly 50 fields and over 500 wells. A digital database was developed containing the geographic location of all these wells (latitude-longitude) as well as the elevation of the first encounter of dolomite in the field/reservoir. Analysis shows that these dolomite occurrences are largely confined to the center of the basin, but with some exceptions, such as N. Adams Field. Further, some of the dolomite occurrences show a definite relationship to the fracture pattern described above, suggesting a genetic relationship that needs further work. Other accomplishments of this past reporting period include obtaining a complete land grid for the State of Michigan and further processing of the high and medium resolution DEM files. We also have measured new fluid inclusion data on dolomites from several fields that suggest that the dolomitization occurred at temperatures between 100 and 150 C. Finally, we have extracted the lithologic data for about 5000 wells and are in the process of integrating this data into the overall model for the Michigan Basin.

The Great American Carbonate Bank in the Central Michigan Basin

Upper Cambrian–Middle Ordovician carbonates and mixed siliciclastic rocks in the Michigan Basin form a shallow-water marine sedimentary package that thickens greatly into the basin center to more than 2000 ft (600 m). These sedimentary rocks, representing the upper part of the Sauk megasequence, are truncated or completely absent at the edges of the basin, whereas sedimentation may have been continuous across the major interregional Sauk-Tippecanoe unconformity in the basin center. The Michigan Basin strata have been correlated with the Prairie du Chien Group and to the underlying Trempealeau Formation, both of which crop out farther west in the upper Mississippi Valley, especially in southwestern Wisconsin. Current stratigraphic nomenclature assigns the upper interval in the central part of the Michigan Basin to the Foster Formation, whereas the middle interval is unassigned and the base of the package is the Trempealeau Formation. More than 250 wells penetrate most of this entire sedimentary package that reaches a thickness exceeding 2300 ft (700 m) in the center of the Michigan Basin. Using this well data, along with the few available cores and drill cuttings, mostly from the Foster Formation, a regional stratigraphic and lithologic framework can be established. The package around the basin margins is pervasively dolomitized but is generally pure carbonate except for some cherty horizons. In the basin center, strata are also mostly dolomite, but fabrics and facies characteristics are better preserved. Local, thin, discontinuous layers of quartz sandstone and shale are interbedded with the carbonates. Some of the carbonate strata contain abundant siliciclastic silt, and diagenetic nodular anhydrite is found sporadically throughout the Foster Formation. A single continuous core of more than 1200 ft (360 m) in length through the Foster Formation in the north-central basin shows multiple meter-scale shallowing-upward packages dominated by shallow subtidal to peritidal carbonate facies. It appears that shallow-marine depositional environments existed throughout the Michigan Basin during the latest Cambrian and Early Ordovician. Although the stratigraphic sequence is affected by a major unconformity at the basin margins, sedimentation may have kept up with significant subsidence in the basin center. These sediments, preserved in the center of the Michigan Basin, represent some of the youngest deposits of these rocks anywhere in the midwest.

Using Recent Advances in 2D Seismic Technology and Surface Geochemistry to Economically Redevelop a Shallow Shelf Carbonate Reservoir: Vernon Field, Isabella County, Class Revisit

Continued the fault study to find more faults and develop new techniques to visualize them. Data from the Dundee Formation was used to document 11 major faults in the Michigan Basin which have now been verified using data from other horizons. These faults control the locations of many of the large anticlinal structures in the Michigan Basin and likely controlled fluid movements as well.

Using Recent Advances in 2D Seismic Technology and Surface Geochemistry to Economically Redevelop a Shallow Shelf Carbonate Reservoir: Vernon Field, Isabella County, M, Class III

In this project a consortium consisting of Cronus Exploration (Traverse City, MI), Michigan Technological University (Houghton, MI) and Western Michigan University (Kalamazoo, MI) proposed to develop and execute an economical and environmentally sensitive plan for recovery of hydrocarbons from an abandoned shallow-shelf carbonate field that is typical of many fields in the U.S. Midwest. This is a 5-year project that will use surface geochemistry as a tool to reduce risk in locating and producing hydrocarbons in Class II fields. The project will develop new techniques for measuring hydrocarbon gases in the soil horizon to locate new and bypassed oil in the shallow-shelf carbonate environments typified by the Dundee and Trenton Formations of the Michigan Basin (Fisher et. al., 1988). In Phase I of the project, the consortium proposes to re-develop the Vernon Oil field located in Vernon Twp, Isabella County, Michigan and produce both bypassed hydrocarbons from the original field and to locate and produce extensions of the original field.

Types and Origin of Porosity in St. Peter Sandstone of Michigan Basin: ABSTRACT

Recent deep drilling for natural gas has demonstrated the widespread existence of a quartzarenite sand body stratigraphically below the Glenwood Formation in the central and northern parts of the Michigan basin. This unit has been called the Prairie du Chien, Jordan, massive sandstone, and Bruggers. Based on stratigraphic position and mineralogy, they believe the proper formation name is the St. Peter sandstone. Analysis of resin-impregnated thin sections of core and cuttings samples reveals the major porosity types. Scanning electron microscopy also details the pore and grain geometries. Because of burial depth and chemistry of migrating pore fluids, most of the porosity is secondary. Types of secondary porosity include: (1) dissolution of carbonate cement, (2) dissolution of framework grains, and (3) fracturing. The porosity distribution is mostly related to the original depositional facies, which controls the grain size, sorting, and clay content. Porous zones are vertically interbedded with tight zones as a result of repetition of facies sequences. Porosity-occluding processes are dominated by lithostatic compaction and quartz overgrowth cements.

Petrology, Stratigraphy, and Depositional Environments of Burnt Bluff Group in Michigan: ABSTRACT

Recent discoveries of sizable natural gas deposits (production at least 1 MMCFGD/well) in 3 widely separated areas of Michigan have touched off exploration interest in the lower Middle Silurian Burnt Bluff Group. The Burnt Bluff and Manistique Groups are stratigraphically equivalent to the better known Clinton Group. Analyses of core samples, outcrop samples, and wireline logs allow for a preliminary reconstruction of facies relationships and depositional environments. Where thickest, in northeastern Michigan, the Burnt Bluff Group can be divided into 3 formations: Lime Island Dolomite, Bryon Dolomite and Hendricks Dolomite. To the southwest the group thins dramatically to a single lithologic unit. The Lime Island and the Hendricks Dolomites represent shallow subtidal facies with abundant large, whole bioclasts of corals, stromatoporoids, and brachiopods. The Bryon Dolomite is a thinly laminated intertidal and supratidal carbonate with desiccation cracks, algal laminae, and anhydrite nodules. Presently, natural gas production is found only in slightly dolomitized portions of the subtidal bioclastic facies in the Burnt Bluff. Porosity development is the key to production and is primarily solution-enlarged interparticle porosity restricted to the Hendricks and Lime Island Dolomites. End_of_Article - Last_Page 263------------

Petrographic and Trace Element Analysis of Nonreef Silurian Carbonates, Northern Lower Michigan: ABSTRACT

Carbonate sediments of the northern reef trend in Michigan have been the subject of serious scientific scrutiny since the discovery of significant hydrocarbon reservoirs in pinnacle reefs in the late 1960s and early 1970s. The reef trend is a complex of reef and nonreef carbonate facies capped by an evaporite/carbonate sequence. The reef and the evaporite/carbonate sequence have been extensively described and discussed over the last decade. However, the nonreef (interreef) facies has received little more than brief descriptions throughout this time. As these nonreef carbonates are virtually barren of hydrocarbons, their analysis has been neglected. Most exploration for pinnacle reefs centers around geophysical techniques; however, it should be considered that patterns of textural or geochemical gradients may occur between the reef masses and surrounding nonreef deposits. This study examines, in detail, the petrographic and geochemical character of these nonreef (interreef) sediments. These characteristics can provide additional data and a potential exploration tool for determining the spatial facies relationship between pinnacle reefs and interreef carbonates in the Michigan northern reef trend. End_of_Article - Last_Page 518------------

Paleoecologic Interpretation of Environmental Stability--A Different Approach: ABSTRACT

Evolutionary patterns, taxonomic diversity, and genetic variability commonly have been used to interpret environmental stability in ancient communities. Paleoecologists have lamented the imprecision of these interpretations. Much of the imprecision was due to inadequately defined species distribution within units. The problem here is that adult organisms, particularly marine benthos, occur in isolated patches. By contrast, larval and juvenile forms, commonly ignored in paleoecologic studies, are more widely distributed and much greater in number. They also are more sensitive to subtle environmental fluctuations. This greater distribution and larger population size of younger organisms is well exemplified by the fossils recovered from acid residues of limestone units from he 240-ft (72 m) thick Kope Formation (Cincinnatian Series). In each of the five sections studied, at least half of the individual layers contained large populations of larval and juvenile organisms. Adults of the same species of these varied taxa are comparatively rare, however. Taxa represented include gastropods, pelecypods, and brachiopods. Any paleoecologic interpretations based on adult forms only would be drawn from much more restricted information than one drawn from all stages of the life cycle represented in these samples. It is suggested that much more precise interpretations of environmental factors such as stability and diversity can be drawn from analyzing remains of the entire life cycle, rather than just adult forms. End_of_Article - Last_Page 463------------