Abstract Utah contains unique analogues of microbial hydrocarbon reservoirs in the modern Great Salt Lake and the lacustrine Eocene Green River Formation within the Uinta Basin. Characteristics of both lake environments include shallow-water, ramp margins that are susceptible to rapid widespread shoreline changes, as well as comparable water chemistry and temperature that were ideal for microbial growth and formation/deposition of associated carbonate grains. Thus, microbialites in Great Salt Lake and cores from the Green River Formation exhibit similarities in terms of microbial textures and fabrics. A detailed petrographic analysis provides unique insights into these modern and ancient deposits that can be used to determine reservoir characteristics in other microbial carbonate petroleum plays. Great Salt Lake is a hypersaline lake and carbonate ‘factory’, containing actively forming microbial mats, stromatolites, thrombolites and associated carbonate grains. Open constructional pores are common within a spectrum of microbial structures. Green River Formation cores display excellent examples of stromatolites and thrombolites that contain primary megascopic pore and microporosity, as well as carbonate grainstones composed of ooids, peloids and skeletal material with abundant interparticle and intraparticle porosity. West Willow Creek oil field produces from a Green River microbial buildup/mound, a feature not currently recognized in Great Salt Lake.

Multiple cemented channel-fill deposits from the Late Jurassic and Early Cretaceous, once buried beneath 2400 m of sediment, are now exposed at the surface in arid east-central Utah due to erosion of the less resistant surrounding material. This field guide focuses on examples near the town of Green River where there is public access to several different types of exhumed paleochannels within a small geographic region. We describe the geologic setting of these landforms based on previous work, discuss the relevance to analogous sinuous ridges that are interpreted to be inverted paleochannels on Mars, and present a detailed road log with descriptive stops in Emery County, Utah.

Abstract Flat pebble conglomerates consisting of rounded clasts of bioclastic siltstone, sandstone and grainstone are abundant in a 233 foot (71 m) core from an Upper Cambrian sequence in the northern Powder River Basin. Flat pebble conglomerates form 20 percent of a sequence dominated by ripple-laminated and horizontal parallel-laminated lenticular beds of bioclastic siltstone, sandstone and grainstone. Most flat pebble conglomerate beds are 1 to 7 cm thick, but some multistory units are up to 60 cm thick. Most conglomerates display weak to strong clast imbrication, and several beds exhibit bimodal imbrications. The flat pebbles are derived by segmentation of interbedded thin shale and bioclastic siltstone, sandstone and grainstone beds. The segmentation is produced by subvertical dewatering channels along which relatively fine sediment is injected. In addition to dewatering structures, other features in the finer grained deposits which indicate rapid sedimentation are load casts, piled ripples, ball and pillow structures, microfaults, and climbing ripples. The character of the clastic component in the conglomerate matrix changes abruptly midway through the core, suggesting a major change in the nature of clastic input. Conglomerate beds in the lower part of the core contain subangular silt to very coarse quartz brought to the coastline by fluvial processes. Younger conglomerates contain smaller amounts of silt- to very fine sand-sized quartz, which most likely was introduced by aeolian processes. The conglomerates are interpreted to have been deposited below wave base by tidal currents in a shallow marine basin. Work by Palmer (1971), Aitken (1978) and others has shown this basin was separated from the open ocean to the west by a peritidal bank. Glauconite is a minor to abundant component throughout the Upper Cambrian sequence. Its relative abundance in a sequence of rapidly deposited sediments behind a shelf-edge bank may reflect increased rates of glauconite development in the Cambrian. The increase in rate may been favored by lower temperatures and widespread anoxic conditions in the oceans during the Cambrian. The much greater abundance of flat pebble conglomerates during the Cambrian and Lower Ordovician may be attributable to changes in biologic evolution, as proposed by Sepkoski (1982), or to an increased tidal range during this time period, or to both of these. Available evidence bearing on the tidal range in the Cambrian is sparse and inconclusive.

Abstract Cores have been examined in detail from two wells located six miles apart in northwest South Dakota. They exhibit different degrees of dolomitization in the Upper Ordovician Yeoman (Upper Red River) Formation. Depositional characteristics and early diagenetic alteration of these carbonates is similar to that of the Yeoman of the northern Williston Basin. Early diagenetic cementation as previously proposed (Kendall, 1977) is now questioned. In addition to dolomite-mottled carbonates that arose by preferential dolomitization of burrow networks, the Yeoman sequence contains carbonates that exhibit nodular structures (produced by preferential lithification of burrows) and those that lack conspicuous burrow structures. These different sediment types have markedly affected the pathways taken by later diagenesis. Variations in skeletal carbonate abundance are primary and have not been significantly affected by later dolomitization. Stratigraphic units composed of skeletal-poor wackestones and mudstones parallel thin euxinic organic rich beds and are thus considered isochronous. One unit (the “D” porosity zone) has a basinwide distribution and has been particularly prone to being dolomitized. It may even correlate with the Cat Head Member of the Manitoba outcrop. Variations in Yeoman lithology are ascribed to variations in water oxygenation during deposition and many Yeoman carbonates were deposited under dysaerobic conditions. Yeoman deposition is believed to have occurred in deep shelf environments. Chemical compaction has had a profound effect and is responsible for the removal of matrix from many dolomites and the creation of diagenetic grainstone textures by concentrating the more resistant components in the matrix. Breccias composed of broken and reoriented dolomite mottles closely resemble depositional features but also result from matrix elimination by pressure solution. Fractures associated with pressure solution seams and stylolites are identified: previously they had been regarded as being early diagenetic. Relations between dolomitization and chemical compaction are ambiguous and require further study.

Abstract Data from the “C” interval of the Ordovician Red River Formation in Divide and Sheridan counties show depositional environments of the “C” burrowed carbonate, “C” laminated carbonate, and “C” anhydrite are remarkably persistent. Diagenetic events, however, are highly localized, and dolomitic porosity is discontinuous. Environmental conditions changed during deposition of the Red River “C” interval. At the base of the interval is a burrowed carbonate deposited in a well-circulated, subtidal, normal marine environment. Gradually a restricted subtidal, relatively quiet water environment developed and the “C” laminated carbonate was deposited. Intertidal conditions may have existed intermittently. The sequence is capped by the “C” anhydrite deposited in a hypersaline, subtidal restricted environment. Red River paleotopographic highs coincide with “C” laminated carbonate thins and contain vertical microfractures healed with anhydrite. In contrast to laterally persistent dolomite of the “C” laminated mudstone, dolomite of the “C” burrowed carbonate is discontinuous. Anhydrite crystals within the “C” burrowed carbonate are always associated with dolomite and may have been derived from downward percolating sulphate-rich brines from the overlying “C” laminated dolomite, or “C” anhydrite.

Abstract The Winnipegosis carbonates were deposited in three episodes during the initial transgressive-regressive cycle of the Kaskaskia sequence. The first episode is represented by a normal, open marine environment. During the second episode, the basin differentiated into two regions: the shallower shelves and a bathymetrically “deeper” basin. This deeper basin was comprised of a restricted environment with large pinnacle reefs. Shallow marine, patch reef, lagoon, and tidal flat environments became established in the shelf region. During the third episode, supratidal stromatolites and lime mudstones were deposited on top of the shelf and pinnacle reefs while stromotolites, dolomites, and starved-basin anhydrites accumulated in the inter-reef areas. The pinnacle reefs contain four Hthofacies: the Stromatoporoid, Tabulate Coral Boundstone; the Codiacean Algae, Calcisphere, Peloid Packstone; the Porous Dolomite; and the Pisolite Dolomite Lithofacies. The patch reef is composed of the Stromatoporoid, Tabulate Coral Boundstone Lithofacies. The pinnacle reefs exhibit a three-stage successional biofacies development: (I) pelmatozoan packstones which represent an intrinsic preparation of the substratum for reef growth; (II) establishment of reef-building domal and dendroidal tabulate corals; and (III) alternating zones of more massive thamnoporid corals and stromatoporoids. Stromatoporoid-algal boundstones are also common. During Stage III a lateral zonation also developed with higher energy environments on an outer reef rim and a lower energy backreef lagoon. The backreef deposits are composed of codiacean algae, calcispheres, peloids and Amphipora . Patch reefs show a four-stage, shallowing-upward succession: (I) pelmatozoan skeletal sand shoals; (II) growth of domal and branching tabulate corals; (III) increasing size, number of genera, and growth forms of the reef building taxa, with growth shapes indicative of a higher energy, above mean wave-base environment predominating; and (IV) large massive, hemispherical stromatoporoids which dominate in turbulent high energy environments. Diagenetic fabrics in the pinnacle reefs include partial to extensive dolomitization. The extent of dolomitization and the destruction of the allochems and textures decreases downward within the reef core. The diagenesis of the patch reef is minor in comparison with that of the pinnacle reef. Diagenetic events include isopachous calcite and equant calcite spar cements, stylolites, local dolomitization of allochems and the mudstone matrix and rare bladed or blocky anhydrite.

Abstract The vertical fades sequence comprising the Upper Devonian Duperow and Birdbear (Nisku) formations in the Williston Basin reveals three major shoaling-upward depositional cycles, punctuated by more frequent salinity cycles expressed as remarkably widespread anhydrite and argillaceous marker beds. Identification of syndepositional structures, many coincident with structures, was gained through isopach mapping of intervals bounded by these marker beds. Excellent examples of reservoir development under paleostructural control are illustrated for Duperow Unit 4 in the Billing Nose area by the Tenneco Gawryluk #1-30 and Federal #2-30 cores and for the Birdbear (Nisku) in the Wolf Creek Nose area by the Murphy Sethre #1-B and Sletvold #1-B cores. In both examples, the paleostuctures represented relative paleotopographic highs and, as such, influenced the local development of favorable reservoir fades. Duperow Unit 4 skeletal banks, dominated by globular stromatoporoid floatstone, formed over the crest of the Billings Nose paleostructure. Birdbear (Nisku) skeletal banks, comprised of Amphipora wackestone, and packstone and platy stromatoporoid boundstone, formed along the flanks of the Wolf Creek Nose paleostructure, while the crest of the structure sustained peritidal deposition unsuited for bank development. Dolomitization of skeletal bank facies and portions of adjacent fades was the mechanism for reservoir development. Mg ++ -enriched brines expelled from overlying evaporites during burial compaction provided the dolomitizing fluids in both examples, although the style and magnitude of dolomitization was regulated by the facies distribution and early burial history peculiar to each. Migration of dolomitizing fluids and hydrocarbons was facilitated by fracturing of intervening lithified strata. Sources for both fluids are believed to have been within readily defined stratigraphic intervals.

Abstract Rival, North and South Black Slough, Foothills, and Lignite fields in northeastern portions of the Williston Basin had produced 24.2 million barrels of oil through mid 1982 from the Mississippian Lower Charles Formation. These fields were discovered in the late 1950’s and unitized in the early to late 1960’s. In 1984 all, except Rival, were deunitized. The Rival subinterval at the base of the Charles Formation serves as the reservoir in Rival, North and South Black Slough, and Foothills fields. It is 30 to 60 feet thick. This subinterval was named at Rival Field but is commonly called the “Nesson”. The overlying Midale subinterval serves as the reservoir in Lignite Field and extends into southeastern Rival Field. It is 30 to 50 feet thick. Most oil production is from the Rival “Nesson” (21.3 MMSTBO, as of 1982). The study area is tilted basinward 2/3’s of one degree, with three subtle anticlinal noses trending through the fields. The Rival “Nesson” represents a rapidly prograding shoreline and coastal sabkha which ceased building basinward and began slowly retreating. Barrier island and intertidal buildups of sparsely skeletal to skeletal, oolitic, pisolitic, intraclastic packstones formed along the shoreline. Most particles were micritized and neomorphosed to microspar. Inner portions of the shoreline were tightly cemented by anhydrite, while outer portions were partially cemented. Later, anhydrite was leached in outer portions of the shoreline to enhance reservoir porosity. Inner portions of the shoreline remained tight and along with anhydrite beds provide the updip stratigraphic trap. Some pores were later partially to completely filled with dolomite and calcite cement, drastically reducing permeability. The Midale records a transgression which flooded the study area. Restricted marine to tidal flat, sparsely anhydritic, spiculitic, pelletal wackestone/packstones were dolomitized and serve as the reservoir. Porous dolostone is aphanocrystalline to very finely crystalline. Leaching of sponge spicule monaxons and some anhydrite further enhanced porosity. The Rival “Nesson” pore system is composed of: (1) moldic and solution enlarged pores (10 to 1000 microns in width); (2) interparticle pores (5 to 25 microns); (3) intraparticle pores (5 to 10 microns); and (4) intercrystal pores (approximately 1 to 3 microns in width). The Midale pore system is composed of: (1) moldic pores (5 to 500 microns in width); and (2) intercrystalline pores which are, (a) polyhedral pores (3 to 10 microns), (b) tetrahedral pores (3 microns), and (c) interboundary-sheet pores (1 micron in width).

Abstract Lustre Field, the westernmost commercial Mississippian field in the Williston Basin, produces mainly from dolomites in the Charles Formation. Stratigraphic terminology is still debated, but the major reservoir occurs in the Charles “C” or Ratcliffe Zone just above the Richey Shale. Depositional facies indicate that the Charles “C” interval represents a generally shallowing-upward depositional sequence. Fossiliferous open shelf facies predominate near the base, and grade upward through restricted shelf mudstones into peloidal, oolitic, and oncolitic grainstones near the middle of the zone. The upper part of the Charles “C” consists of anhydritic, laminated, dolomitic mudstones deposited in a sabkha to hypersaline lagoon environment. The major shallowing upward cycle can be subdivided into five smaller cycles separated by minor periods of transgression. Each of these subcycles is incomplete, i.e., only a few of the facies occur in each, but isopach maps and facies distribution within these subcycles suggest that a northwest-southeast trending structural nose extended through the Lustre Field area and exerted an influence on deposition. The depositional facies can be traced far beyond the field’s productive limits, but the degree of dolomitization in the lower part of the Charles “C” correlates directly with production. The best wells in the field contain up to 35 feet (10 m) of dolomite with more than 14% intercrystalline porosity and 0.5 md permeability. In the best reservoir zones, porosity reaches 30% and permeability (in unfractured rocks) is typically 1 to 10 md. Sparse vertical fractures enhance production. The porous dolomites formed when brines generated in the sabkha to hypersaline lagoonal facies of the upper Charles “C” seeped down into the burrowed mudstones and wackestones of the lower Charles “C”. Subtle paleotopography, permeability of the lower Charles “C” rocks, and original composition of the Charles sediments interacted to determine porosity development.

Abstract A joint surface-subsurface study of the Devonian Hare Indian-Ramparts (Kee Scarp) Formation examined the nature of basin-fill and platform-reef development in the Mackenzie Mountains and Norman Wells area, Northwest Territories. The Givetian-Frasnian (?) Hare Indian and Ramparts (Kee Scarp) strata consist of repeated shoaling-upward sequences. These sequences or cycles of sedimentation were initiated in response to accelerated rates of relative sea-level rise. Two major first-order cycles (each greater than two hundred metres thick) were discerned. The lower cycle consists of prograding shale banks of the Hare Indian Formation and the immediately overlying “shale ramp” sequence of the Ramparts Formation. The upper cycle commenced with deposition of the widely correlatable, dark argillaceous, carbonaceous limestones of the “Carcajou Marker”. Overlying platform and reefal limestones of the Ramparts (Kee Scarp) Formation, off-reef and fondothem shales of the Canol Formation and interbedded clinothem shales and sandstones of the Imperial Formation make up the remainder of the upper cycle. The first-order cycles consist of a number of smaller second-order cycles. These cycles are best defined in shallow-water platform and reef complexes where they can be traced across the entire complex, but also are recognized in the Hare Indian shale clinothem. In platforms and reefs, these second-order cycles contain a wade range of facies, varying from shallow reef and platform to deeper foreslope limestones that can be traced, in certain locations, even further out into adjacent basinal limestones. In reef interiors, these second—order cycles are made up of even smaller third-order cycles consisting of subtidal and tidal-flat limestones. This cyclic arrangement of strata is well-defined in both the Mackenzie Mountains and the subsurface Norman Wells Field, enabling correlation of time-equivalent growth stages between both complexes over a distance in excess of one hundred kilometres. This and the small-scale periodicity of the second and third-order cycles argue for eustatic sea-level control. The following eight major growth stages are recognized in both complexes: (1) drowning of the Hume limestone, (2) progradation of Hare Indian shale banks, (3) drowning of Hare Indian shale banks, and deposition of the “Carcajou Marker”, (4) carbonate platform inception, initial growth and subsequent localized upbuilding, (5) reef inception, progradation and subsequent aggradation, (6) reef backstepping and subsequent aggradation, (7) drowning of the Norman Wells reef complex and backstepping of the Mackenzie Mountain reef complex and (8) drowning of the Mackenzie Mountain reef complex. Evidence to support the presence of the previously interpreted pre-Canol unconformity above the Ramparts and underlying Formation is lacking. Instead, the Canol shale is observed to intertongue with, onlap onto and drape over (at Norman Wells) the Ramparts and Kee Scarp reef complexes.

Abstract The Duvernay Formation is an organic-rich basinal carbonate succession that has provided most of the petroleum presently found in Leduc-age reefs of east central Alberta. It accumulated under deep-water anoxic bottom conditions that favoured the preferential preservation of organic material. This, together with slow sedimentation rates resulted in a rich (up to 17 wt. percent TOO source rock. Rich source intervals occur as dark black laminites interbedded on a fine scale with leaner bioturbated lime mudstones. Intervals sampled had very low amounts of insolubles, rendering them true carbonate source rocks. Comparison of reservoir oil Level of Organic Metamorphism (LOM) and source rock LOM suggests that long distance secondary migration has taken place within the basin with distances on the order of 100 kms (60 miles) being indicated. Migration took place through a dolomitized aquifer underlying the Leduc reefs, with the amount of petroleum entering each reef being dependant on the nature of vertical permeability barriers separating the two. A portion of the reef-platform system conforms to a classical example of spill-point updip displacement of petroleum, but other portions do not, and an understanding of the mode of migration into these buildups is important. Furthermore, it seems conceivable that the Duvernay Formation could have contributed significantly to hydrocarbons presently found in the giant Athabasca and associated tar sands. The multidisciplinary approach embodied in this study has resulted in a better understanding of carbonate source rocks in general, as well as the apprection of a more fully integrated exploration model.

Abstract The relationship between deformation-induced pressure solution and fracturing in carbonate rocks can be complex and may be interactive. Multiple episodes of pressure solution and fracturing are common in strongly deformed carbonate units such as the Mississippian Madison Group of the western Overthrust Belt. Paragenetic relationships in these rocks suggest that fractures both modify and enhance continued pressure solution by opening the host rock to fluid migration. The composition of vein and stylolite mineralization may be used to evaluate the history of fluid migration during deformation. In the Madison carbonates, the earliest veins were filled by dolomite or calcite, while all subsequent veins were filled with calcite. Host limestone and dolomite are non-luminescent while filled veins are variably luminescent. The isotopic compositions of vein-filling calcite and dolomite are distinct from host rock compositions and document changes in fluid chemistry during burial and deformation. Taken together, the temporal change in mineralogy, luminescence and isotopic compositions of various vein-filling carbonate cements vs. host rock carbonates are strongly suggestive of rapid allochthonous fluid migration during deformation of the Madison Group in the western Overthrust Belt.

Abstract Tin Cup Mesa field in the Utah portion of the Paradox Basin produces from the Upper Ismay Zone of the Pennsylvanian Paradox Formation. The principal depositional and diagenetic facies within the reservoir are visible in cores from two wells. The Tin Cup Mesa #3-26 (NW NE Sec. 26, T28S-R25E) was drilled approximately 1160 feet northeast of, and updip from, the #3-26 and encountered only 30 feet of carbonate with a commensurate increase in anhydrite to a thickness of 90 feet. The five major lithofacies within the field area include: (1) the phylloid-algal mound; (2) mound cap; (3) mound flank; (4) intermound; and (5) an evaporitic facies. All of these are present in the Marathon #3-26 producer, while in the Marathon #1-23 dry hole, facies numbers 4 and 5 above predominate. All lithofacies are potentially productive with the exception of the intermound and evaporitic facies found in the #1-23. Diagenetic processes which have enhanced the reservoir include dolomitization and carbonate dissolution. These two processes, along with primary shelter porosity, are responsible for all of the reservoir present in the field. Cementation was the primary process by which porosity was destroyed, and at least six different types of cement are present. Compaction, bioturbation, and internal sedimentation were additional agents of reservoir destruction. It should be emphasized that repeated early exposure of the reservoir at Tin Cup Mesa field was of paramount importance in the creation of secondary porosity and in the enhancement of primary porosity.

Abstract The Niobrara Formation of the Western Interior consists of 100 to 250 m of interbedded chalks and calcareous shales. The formation is divided into two members: (1) a thin, basal, dominantly chalk-bearing sequence, the Fort Hays Limestone Member, and (2) an upper, thick, calcareous shale unit, the Smoky Hill Chalk Member. Both units represent widespread pelagic to hemipelagic sedimentation in an epicontinental seaway during a relative highstand of sea level. During this highstand, the shoreline migrated westward and large areas of the Western Interior were covered by relatively deep water and received little terrigenous influx. Productivity of nanno- and microfossils was sufficiently great to yield moderately thick sequences of regionally homogeneous chalk and marl. These strata are very fine grained, have high primary porosity, and contain few macrofosstls other than bivalves. Sedimentary structures in the Niobrara consist mainly of laminations, fecal pellets, and burrows. Chalk-shale depositional cycles are found at scales ranging from millimeters to tens of meters, Cyclic sedimentation may have been related to variations in climatic patterns which, in turn, influenced terrigenous sediment influx and (or) biological productivity in the region. Climatic fluctuations probably influenced the salinity of the surface waters in the Western Interior seaway by altering circulation patterns and basinal water turnover. This resulted in periods of dysoxic and even anoxic bottom-water conditions within the seaway. Stagnation events are reflected by intervals with virtually no benthic fauna or burrows and with good preservation of millimeter-scale laminations and abundant organic matter. The primary properties of the chalks and calcareous shales of the Niobrara have been greatly modified by burial diagenesis. Increased burial brought about rapid reduction of porosity by a combination of mechanical and chemical compaction and associated calcite overgrowth cementation. Authigenic pyrite is widespread in association with organic matter. Clay minerals of detritai origin and altered voicanogenic material originally underwent transformation from disordered, predominantly smectitic mixed-layer clays to ordered, predominantly illitic mixed-layer clays. During diagenetic alteration, brittle chalks and calcareous shales were deformed and significant fracturing occurred. Finally, thermal maturation associated with burial led to hydrocarbon generation from the organic-carbon-rich chalks and calcareous shales of the Niobrara. In areas of shallow burial, formation of biogenic methane was widespread and has proven to be economically important.

Abstract This core workshop was organized to give geologists from across the country and around the world the opportunity to see a wide variety of carbonate reservoirs as well as some carbonate source rocks from the Rocky Mountain region. Cores displayed at the workshop range in age from Cambrian to Cretaceous and come from a number of the major oil-producing basins in the Rocky Mountains. Depositional facies represented in the cores range from sabkhas and tidal flats through algal and coral buildups to relatively deep water chalks. Dolomite and evaporite minerals are important in approximately half the cores described; the others are dominantly limestone. Porosity of many different types is discussed. Diagenesis, or lack of it, has played a major role in forming virtually all the reservoirs. Thus, the workshop offers the chance to observe and study a wide variety of depositional and diagenetic textures in a number of economically important rock units.