Mission Canyon Formation Research Articles

Significance of Paleokarst in the Mission Canyon Formation, Southwestern Wyoming Thrust Belt

Regional studies have shown that widespread karst was developed in Mississippian carbonates of the Madison Group (Mission Canyon equivalent) over most of Wyoming during Meramecian to early Morrowan time. The karst surface was subsequently covered by eastward-transgressive Chesterian to Morrowan siliciclastics of the Amsden Formation and equivalents. These terrigenous clastics filled solution cavities in the carbonate bedrock. Cores from Whitney Canyon-Carter Creek field in the southwestern Wyoming thrust belt provide an opportunity to examine the Mission Canyon-Amsden contact in an area of limited surface exposures. The cores contain intensely fractured and stylolitic carbonates, but also have textures that are probable solution breccias of karst origin. These breccias consist of dolostone clasts that are commonly rounded and stained to a pinkish hue. The clasts are locally draped by reddish silt and clay coatings that are in a matrix of very fine- to medium-grained, poorly sorted quartz sandstone that sometimes contains laminae, suggestive of current deposition. The occurrence of carbonate and siliciclastic mixing within brecciated intervals of the uppermost Mission Canyon is significant in a regional sense. Whitney Canyon-Carter Creek field is over 100 mi (160 km) west of the areas most intensively studied by others. These core observations suggest that themore » Mississippian karst plain and associated paleoshoreline were located considerably further west than the current interpretation, especially when thrust belt structural positions are restored. Palinspastic restoration of a published regional cross section places Whitney Canyon-Carter Creek field paleogeographically 28 mi (45.5 km) west of its present location.« less

Whitney Canyon-Carter Creek Field: Gas Production from Carbonate Reservoirs in a Thrust Belt Structural Setting, Western Wyoming, U.S.A.

Located in the Fossil basin area of the Wyoming thrust belt, giant Whitney Canyon-Carter Creek field has in place reserves of approximately 4.5 tcf of gas, 125 MMBO (condensate), and 24 million long tons sulfur. It is the largest gas field in the U.S. Rocky Mountains. Hydrocarbons are trapped in large, reverse faulted anticlinal closures that formed completely within the Absaroka thrust plate during Laramide deformation. These structures are ramp anticlines that developed when the Absaroka plate was thrust eastward over ramps in the underlying fault plane. Production is sour natural gas and condensate mainly from Paleozoic reservoirs. The most significant are dolomitized carbonate reservoirs of the Mississippian Mission Canyon and Lodgepole formations and the Ordovician Big Horn Dolomite. The Pennsylvanian Weber Sandstone and the Triassic Thaynes Formation have minor production. Source rocks are subthrust Cretaceous shales that were placed in the oil generation window after thrusting and subsidence. The economically most important reservoir is the Mission Canyon Formation with 79% of the total gas in place. Intercrystalline and moldic porosity was created by dolomitization and subsequent partial solution of mud-supported sediments during early diagenesis. Structural deformation fractured the reservoir, but also created a diagenetic environment that allowed calcite, anhydrite,more » and dolomite cements to sporadically plug all porosity types. Discovery of Whitney Canyon-Carter Creek field began by identifying a large, potentially productive area within the Fossil basin.« less

Sequence Correlations and Stacking Patterns of Lower Mississippian Strata in Montana and Idaho: Evidence for Eustatic Controls on Sequence Development and Platform Evolution

The Lower Mississippian Mission Canyon Formation and stratigraphic equivalents in southwestern Montana and east-central Idaho consistent of stacked sequences of shallowing-upward platform carbonates that grade westward into correlative slope and basinal sequences. Deeper water facies were deposited in a northern extension of the Antler foreland basin. Detailed biostratigraphic and lithostratigraphic correlations across the platform and into the Antler foredeep document regional stacking patterns of third- to fourth-order depositional sequences across this wide area. Twelve to fourteen (depending on amount of erosion/karstification on top of the Mission Canyon Formation) sequences have been identified and correlated across the platform slope. Major flooding surfaces can be identified and correlated from the Antler foredeep across the Mission Canyon platform and into stratigraphic equivalents in the Williston basin. These regional flooding surfaces, along with biostratigraphic boundaries, allow detailed correlation of individual sequences and sequence sets from the Williston basin to slope environments of the Mission Canyon platform in east-central Idaho, a palinspastic distance of over 1500 km. This study illustrates the application of sequence stratigraphy in a tectonically shortened, carbonate platform-to-basin setting where lateral correlations must be made between isolated and widely separated exposures. The correlation of sequences with very similar stacking patterns from Willistonmore » basin and into the Antler foredeep also suggest that these sequences were caused by eustatic sea-level oscillations.« less

Little Knife Field--U.S. Williston Basin

Little Knife field is a combination structural and stratigraphic trap located near the structural center of the Williston basin, North Dakota. The field is approximately 12 mi (19.3 km) long and 2.5 to 5.5 mi (4 to 8.9 km) wide. Little Knife was discovered by Gulf Oil in 1976 as part of a regional exploration play involving a transition from impermeable to porous carbonate rocks. In 1987, ultimate recovery from the Mission Canyon (Mississippian) reservoir was estimated to be 97.5 MMBO. This included 57.5 MMBO primary, 27 MMBO secondary, and 13 MMBO tertiary (CO{sub 2}) oil. At present the field is still under primary recovery, since utilization efforts have not been successful. Approximately one-third of Little Knife's 130 ft (39.6 m) oil column is trapped by structural closure beneath a regional anhydrite seal in a north-south-trending anticline. The remaining two-thirds of the oil column is trapped where the reservoir beds change facies from porous dolostones and dolomitic limestones to nonporous limestones. Structural entrapment accounts for approximately 50% (127 MMBO) of the OOIP, but covers only 30% of the producing area. Production is from the upper portions of the Mission Canyon Formation, a regressive, shoaling-upward carbonate-anhydrite sequence deposited in a slowlymore » shrinking epeiric sea. The Mission Canyon in the Little Knife area is divided into six zones that record predominantly cyclic, subtidal deposition. These are overlain by prograding lagoonal, tidal flat, and sabkha beds. The source of Mission Canyon oil is thought to be the Bakken Formation, an organic-rich shale at the base of the Mississippian.« less

Can Seismic Character Identify Lithologies Associated with Bluell and Sherwood Shorelines and Shoals?

Several new oil fields have been discovered in the Bluell and Sherwood members of the Mission Canyon Formation, reviving the Mississippian shoreline play in the eastern Williston basin. During periodic regressions, carbonates and evaporites were deposited along the shoreline of a shallow sea. Embayments in the shoreline created traps that contain 3 - 8 million bbl of oil. Carbonate shoals developed offshore. Surrounded by impermeable carbonate mud, they trap 1 - 3 million bbl of oil. The ability of seismic character to distinguish lithologies associated with shorelines and shoals was investigated using sonic logs, models, and seismic data. Shorelines: The gross thickness of the Bluell zone can range between 40 and 70 ft, the Sherwood zone between 35 and 80 ft. Changing thicknesses on geologic models had a distinct effect on seismic character. Also, seismic character varied in response to changing stratigraphy above, within, and below the Bluell and Sherwood zones. Carbonate and anhydrite bulk densities and velocities differ by about 10%. Modeling this difference demonstrated a minimal change in seismic character. Seismic character cannot delineate the shoreline transition from carbonate to anhydrite. Other stratigraphic variations alter seismic character more than this lithologic change. Shoals: Velocity and density variations between more » shoal carbonates and intershoal mud can differ by 30%. Sonic log seismograms and seismic data show a distinct character change between these lithologies. Seismic character can be used to delineate carbonate shoals and mud. However, the shoal/mud character change can be modified by the seismic response to other stratigraphic variations. Calibration with nearby wells can reduce interpretational uncertainty. « less

Lucky Mound Field: A New Mississippian Sherwood Shoreline Field

Lucky Mound field produces oil and gas from the Sherwood interval of the Mississippian Mission Canyon Formation. Presently, eight wells are producing with development ongoing. Extensive coring, testing, logging, and petrographic evaluations throughout the field have allowed for detailed analysis of reservoir characteristics and paleoenvironmental interpretation. Sherwood shoreline fields typically produce from reservoir-quality packstones and grainstones trapped by a lateral facies changes into impermeable dolomite and anhydrite. At Lucky Mound, packstones, grainstones, and a productive dolomite facies all contribute to the producing interval. The productive dolomite facies is generally found in the upper portion of the Sherwood along the eastern margin of the field. Porosity as high as 22% and permeability values up to 16 md are present in the dolomite facies. These dolomites are the result of complete to partial replacement of micrite. In addition, the dolomitization process has enhanced intercrystalline and intraparticle porosity throughout the Sherwood interval. Pore types present include vuggy, intergranular, intraparticle, and intercrystalline. Pore occluding and replacive cements include fibrous calcite, prismatic calcite spar, baroque dolomite, anhydrite, celestite, pyrite, and chert. An understanding of carbonate depositional environments, diagenetic processes, Williston basin structural development, and Sherwood reservoir behavior is essential in the exploration for new Sherwoodmore » fields.« less

The Stratigraphy of Selected Mission Canyon Wireline Log Markers, U.S. Portion of the Williston Basin, North Dakota

The Mission Canyon Formation along the northeast flank of the US Williston basin has been informally subdivided into intervals (members) based on wireline log markers. Wireline log responses of the markers are produced by both lithologic changes and radioactive elements present within these thin stratigraphic intervals. The wireline markers were originally described as transgressive events. Detailed stratigraphic analyses of the Sherwood and State A markers indicate they were deposited during progradation and sea level stillstand. A typical facies tract from east to west within the Sherwood marker contains anhydrites and anhydritic dolomites deposited in sabkha environments; patterned dolomudstones along shoreline trends (the Sherwood argillaceous marker); and limestones in shoaling environments along the Mission Canyon shelf (Sherwood gamma marker). During stillstand, brines produced in sabkha environments (east of the Sherwood shoreline) were enriched in magnesium and potassium. These brines migrated basinward first, dolomitizing mudstones. These brines were magnesium depleted by the time they reached shoals along the shelf. Potassium, however, remained in the system and is present within the marker along the shelf, as shown by a slight increase in API units on Spectrologs.

Wabek and Plaza Fields: Carbonate Shoreline Traps in the Williston Basin of North Dakota

Wabek and Plaza fields in Mountrail and Ware counties, North Dakota, will ultimately produce 8 million and 3 million bbl of oil, respectively, from reservoirs in the Sherwood and Bluell intervals of the Mississippian Mission Canyon Formation. Both fields produce from porous, oolitic, and pisolitic lime packstones and grainstones deposited as shoals along a low-energy shoreline. A facies change to impermeable dolomitic and salina/sabkha environments to the east provides the updip trap. The Sherwood at Wabek has more than 100 ft of oil column driven by solution gas and water influx. Effective porosity consists of interparticle, vuggy, and minor dolomitic intercrystalline porosity. Log porosities range from 6 to 26%, averaging about 10%, and net pay averages about 26 ft. One mile west of Wabek, Plaza field produces from the Bluell, stratigraphically overlying the Sherwood. Log porosities range from 6 to 16%, averaging about 9%. Net pay averages about 6 ft. An oil-water contact is not yet defined, but at least 120 ft of oil column are present. Regional depositional slope and local depositional topography were major controls on Mission Canyon shoreline trends and the development of reservoir facies. In the Wabek-Plaza complex, the position and trend of the Sherwood andmore » Bluell shorelines can be related to structural trends indentified in the crystalline basement from aeromagnetic data. Locally, thickness variations in the underlying Mohall interval amplified relief on the Wabek-Plaza structure and influenced the deposition of shoreline reservoirs.« less

Exploration for Stratigraphic Traps in a Mature Hydrodynamic Setting, Williston Basin, North Dakota: ABSTRACT

Hydrodynamic effects on oil accumulations generally can be recognized at an early stage of exploration, but become of critical importance with increased drilling and discoveries. At the mature stage, hydrodynamic concepts readily can be applied in exploration and development to reduce risk and to increase success ratios. The south flank of the Williston basin is an example of a mature area with significant hydrodynamic effects on accumulation. Early exploration was aimed at stratigraphic traps in the Mission Canyon Formation but the development of major fields showed that all are strongly influenced by hydrodynamic flow and some may be largely independent of porosity pinchouts. Examples of hydrodynamic effects are illustrated by the Billings Nose fields, and the Elkhorn Ranch and Knutson fields. These accumulations have hydrodynamic gradients on the order of 20 ft/mi (4 m/km) or more; tilted oil-water contacts with gradient of 30 to 50 ft/mi (6 to 10 m/km); displacement of oil downdip to the northeast; and variable formation water salinities that range from nearly fresh to highly saline. Some producing zones have been described as purely hydrodynamic traps, lacking both structural and stratigraphic closure. Future success will depend on applying hydrodynamic concepts in exploration and development, and predictionmore » methods are illustrated by possible extensions to existing one-well fields. Simple graphic techniques can estimate the limits of production before drilling, but a knowledge of local structure is most important to the interpretation.« less

Meteoric Alteration of Early Formed Dolomite During Long-Term Subaerial Exposure: Mississippian Mission Canyon Formation: ABSTRACT

Dolomite from the Mississippian Mission Canyon Formation in south central Montana has been petrographically and geochemically analyzed to constrain the effects of long-term subaerial exposure on the formation of well-ordered and stoichiometric dolomite. Samples from five measured stratigraphic sections were analyzed for stoichiometry and for carbon and oxygen isotope composition. Dolomite {delta}{sup 18}O values from all sections vary from {minus}11.2 to +7.5{per thousand}. {delta}{sup 13}C ratios vary from +0.4 to +5.5{per thousand} but are relatively consistent within individual sections. Stoichiometry of the dolomite varies from 49.9 to 55.7 mole % CaCO{sub 3}. Measured sections from the northern portion of the study area dominantly contain {sup 18}O-depleted dolomite (+0.8 to {minus}11 2{per thousand} {delta}{sup 18}O). The most {sup 18}O-depleted dolomite, however. typically occurs in the upper 100 m of these sections. These {sup 18}O-depleted dolomites have homogeneous {delta}{sup 13}C ratios and are stoichiometric. Dolomite from the upper 100 m of these sections predates early calcite cement and locally is calcitized or partially replaced by Fe-oxides. These data suggest that early formed dolomite may have been recrystallized by meteoric water under very high water-rock ratio conditions. Decreasing amounts of water-rock interaction with depth is suggested by more heterogeneous {delta}{sup 18}O and {delta}{supmore » 13}C ratios downsection. Measured sections in the southern portion of the field area contain greater amounts of {sup 18}O-enriched dolomite (+7.5 to -7.1{per thousand} {delta}{sup 18}O). Dolomite from these sections also has more heterogeneous {delta}{sup 13}C values and variable CaCO{sup 3} content. These data suggest that the dolomite probably has not been extensively altered and may more closely represent the original geochemistry of the early formed dolomite.« less

Structural History and Reservoir Characteristics (Mississippian) of Nesson Anticline, North Dakota: ABSTRACT

Nesson anticline is the largest hydrocarbon productive structure in the North Dakota portion of the Williston basin. Nesson anticline was discovered in 1951, just a few months after discovery of oil in the Williston basin. Fifty-four fields, producing from 14 lower to middle Paleozoic formations, are scattered along the north-south length of the anticline. Nesson anticline fields have produced a total of 377 million bbl of oil, with the Madison Group accounting for two-thirds of the total production. Central southern parts of the anticline were subdivided into nine areas that revealed episodic and independent structural movement since the late Precambrian. All Phanerozoic periods are present within the stratigraphic section. Unconformity-bound, major tectonic-eustatic sequences were mapped along the length of the Nesson anticline, and sedimentary tectonics were documented for the entire Phanerozoic. Greatest amounts of tectonic development of the anticline were during the Devonian to Early Mississippian. Post-Greenhorn, Laramide tectonism was responsible for the last major structural deformation of the anticline. Selected oil fields, productive from the Madison Group, were studied where they are productive from the Mission Canyon Formation and the Rival (Nesson) subinterval. These intervals record sediment infill of a slowly shrinking epeiric sea, as a series of shorelines more » separated by brief transgressions prograded toward the center of the basin. The Mission Canyon Formation can be characterized as a major shallowing-upward sequence, which upsection is (1) shallow open marine, (2) transitional open to restricted marine, (3) restricted marine, and (4) fringing marginal marine. Barrier island and shoreline buildup complexes developed along the shoreline, with bedded evaporites located away from the anticline toward the east and south. « less

Economic Analysis of Mission Canyon Production, 1980-1987, Burke County, North Dakota: ABSTRACT

Mission Canyon formation oil pools in central Burke County, North Dakota, were the exploration objective for operators from 1980 to 1987. Area exploration led to the drilling of 309 tests, with successful Mission Canyon completions achieved in 134 of these wells. Spectacular initial potentials, as high as 600 BOPD, were recorded for several wells. Twenty-two Mission Canyon oil fields were discovered in the study area, all characterized by high initial potentials, rapid production decline, and subsequent stabilization at or below economic limit. Of all completion attempts, 33% were commercial ventures and 67% were economic failures. Only three Mission Canyon oil fields were total economic successes - Coteau, South Coteau, and Rival. These fields are neither the largest in well count nor aerial extent. All the producing Mission Canyon formation wells in central Burke County were studied to determine oil reserves required to reach present and historic investment hurdle rates of 25% BFIT. Several economic simulations were constructed to examine value profiles for Mission Canyon development. It was observed that profitability is associated with those wells demonstrating the greatest amount of structural relief across well-defined subsurface noses. High structural relief helped the development of favorable reservoir characteristics. Shallow Charles Formation production,more » present over many of the Mission Canyon oil fields, did not convert uneconomic Mission Canyon development into profitable ventures. The authors recommend that operators involved in central Burke County Mission Canyon exploration use structural relief across subsurface noses as the discriminating factor when selecting prospects, and that management abstain from the development of discoveries demonstrating low structural relief.« less

Presence and Control of Evaporite Top Seals on Occurrence and Distribution of Hydrocarbon Traps: Main Fairway, Central Overthrust Belt, Wyoming and Utah: ABSTRACT

In the Central Overthrust belt fairway, there are 18 known productive formations ranging from Cambrian to Cretaceous in age. Four units - the Ordovician Big Horn formation, Mississippian Mission Canyon formation, Jurassic Nugget formation, and Jurassic Twin Creek Formation - account for 99% of a reported 1.4 billion bbl of oil estimated recoverable reserves discovered to date. A critical factor necessary for significant hydrocarbon accumulations is the presence of effective evaporite seals above these reservoirs. Major accumulations exist in anticlines associated with Paleozoic or Jurassic cutoffs in the hanging wall of the Absaroka thrust with recognized source rocks occurring in the Cretaceous footwall section. Vertical migration is required to source the overlying reservoirs. Migration pathways are via hanging wall cutoffs of these reservoirs or through fractures in the cores of the structures. Hanging wall reservoirs either lacking top seals or having top seals disrupted by extensive fracturing due to folding or faulting, permit vertical migration into higher, more effectively sealed reservoirs. In the Overthrust producing fairway, anhydrites of the Mission Canyon formation and Gypsum Spring Member of the Twin Creek formation as well as the Jurassic Preuss salt are the three effective seal units. Analysis of Overthrust belt traps suggestsmore » that the presence and ductile nature of these evaporite seals are important controlling factors for the significant hydrocarbon accumulations in this highly deformed terrain.« less

Alternative Hypothesis for Development of Vugular Porosity in Mission Canyon Formation: ABSTRACT

Several writers have discussed the development of vugular porosity in carbonates of the Mission Canyon Formation along the eastern margin of the Williston basin. Suggestions have included dissolution of carbonates by fresh water in the mesogenetic and/or telogenetic zones, or dissolution by meteoric waters in a vadose setting. An alternate model in which dissolution occurred in an eogenetic, subaqueous, marine setting is proposed here. Apparently, the most pervasive vugular or enlarged-fenestral porosity is located near the contact between limestone and penecontemporaneously deposited gypsum bodies. During deposition, gypsum (now anhydrite) precipitation occurred in less oxygenated water than that from which the limestone formed. Microbial processes within a less oxygenated, gypsum-precipitating area can reduce sulfate ions to sulfides. This process, in the study area, produced sulfide-rich brines that flowed seaward into the more oxygenated, limestone-forming areas. The introduction of sulfides into a more oxygenated environment may produce an acid corrosive to limestone by the reaction HS/sup -/ + 2O/sub 2/ ..-->.. SO/sub 4//sup 2 -/ + H/sup +/. Dissolution of limestone by this acid probably took place near the sediment-water interface and to a shallow depth in the underlying sediments. Vugs formed by this process would be morphologically similar to those formedmore » by vadose processes. Implications of this model to hydrocarbon exploration include the possibility of extensive dissolution porosity occurring near depositional-topographic lows and near limestone-anhydrite contacts. The occurrence of stacked porosity zones can also be explained by this model.« less

Petroleum Geology of MC-3 Member, Mississippian Mission Canyon Formation, Pierson Area, Southwestern Manitoba: ABSTRACT

Mississippian beds in the Manitoba portion of the Williston basin produce oil from a series of stratigraphic units where porous cyclic carbonates are truncated by pre-Mesozoic erosion and sealed by Amaranth (Watrous/Spearfish) strata. In the Pierson area of southwest Manitoba, oil is trapped within the MC-3 member of the Mission Canyon Formation, correlative with the Alida beds of Saskatchewan. Production was first obtained in 1954 and subsequent exploration has led to the discovery of 11 pools of various sizes. As of December 31, 1987, 2.4 million bbl (375,510 m/sup 3/) of 36/degrees/ API gravity oil had been produced from the area, and currently eight pools are producing. In Manitoba, the Mission Canyon is subdivided into three members: MC-1, MC-2, and MC-3, in ascending order. The MC-3 is further subdivided by the MC-3 marker into lower MC-3a and upper MC-3b units. Oil, in the Pierson area, occurs in these two units. The MC-3 member consists of a cyclic sequence of lithofacies deposited in a shallow-water, moderate to high-energy, carbonate-dominated inner shelf environment that was present over much of southwest Manitoba. Five lithofacies have been recognized in core, but oil is found largely within the more porous shoal and backshoal facies (averagemore » porosity 14% and permeability 12 md). The cap rock, in most places, is a secondary, dense dolomite and anhydrite zone (altered zone) directly below the pre-Mesozoic erosion surface. Where the altered zone is thin or absent, lower Amaranth shales (red beds) form the cap rock. Entrapment is primarily stratigraphic, resulting from regional truncation of the MC-3 beds with accumulation localized and controlled by (1) local paleotopographic highs, (2) porosity and permeability pinch-out due to primary lithofacies variations, and (3) porosity closure resulting from variation in the thickness of the altered zone.« less

Genetic Stratigraphic Analysis of Upper Mission Canyon Formation (Mississippian), Williston Basin, North Dakota: ABSTRACT

Genetic Stratigraphic Analysis of Upper Mission Canyon Formation (Mississippian), Williston Basin, North Dakota: ABSTRACT

Case History of Acid Stimulation in the Fryburg Zone, Western North Dakota

Summary Acid stimulation has been used successfully in the Mystery Creek leases in the TR field, Fryburg zone of the Mission Canyon formation in western North Dakota. The Fryburg zone is a carbonate reservoir consisting of alternating layers of limestone and dolomite. Since 1982, the Fryburg wells have undergone an acid-stimulation program that has resulted in substantial increases in oil and gas production.

Anatomy of a Dolomitized Carbonate Reservoir--Mission Canyon Formation at Little Knife Field, North Dakota: ABSTRACT

The Mission Canyon Formation is a regressive, shoaling-upward carbonate to anhydrite sequence deposited in a slowly shrinking epeiric sea. From its base upsection, the formation is mostly subtidal in origin and emergent at its top, and consists of (1) deeper water carbonates, (2) major cycles of open shallow-marine mudstones grading up into skeletal packstone or grainstone, (3) minor cycles of dolomitized transitional open to restricted marine mudstone grading up into skeletal wackestone, (4) dolomitized restricted marine pelletal wackestone or packstone, (5) partially dolomitized marginal marine skeletal wackestone, (6) slightly skeletal, oolitic-pisolitic wackestone or grainstone barrier-island buildups with storm washover aprons, (7) thin lagoonal limestones, (8) tidal flat anhydrite, and (9) sabkha anhydrite. The oil is structurally trapped on the north, east, and west, within the northward plunging Little Knife anticline. Facies changes entrap the oil southward; the vertical seal is the overlying anhydrite beds. Closure is less than 100 ft (30 m). Porous, hydrocarbon-bearing beds were deposited as transitional open-to-restricted marine, restricted marine, and marginal marine lime muds. These became porous dolomitic reservoir rock by undergoing three diagenetic changes: (1) anhydrite replacement of skeletal fragments, (2) dolomitization of the muddy matrix, and (3) later, leaching of the anhydrite to create moldic porosity. The reservoir's pore system is composed of moldic pores and three types of dolomite intercrystalline pores--polyhedral, tetrahedr l, and interboundary-sheet pores. Pore throats in productive beds are of two general sizes (1.2-1.6 µ and 5.2 µ). End_of_Article - Last_Page 854------------

Paleo-temperatures from Fluid Inclusions--Advances in Theory and Technique: ABSTRACT

Recent studies of fluid inclusions in diagenetic cements have attempted to determine paleosubsurface temperatures. Three sets of observations are necessary to make accurate interpretations: (1) detailed petrography to establish the relative time of formation of the inclusions, (2) careful analysis of the burial and tectonic history of the host rocks to relate the diagenetic paragenesis to the geologic history of the basin, and finally, (3) analysis of individual inclusions for homogenization and final melting temperatures, and for chemical composition to define the PVT properties of the trapped fluids. Once these observations are complete, 2 major limitations on the temperature interpretation remain. First is the assumption that the inclusions have not altered in composition or volume since entrapment. Recently published work shows that inclusions can re-equilibrate, but the extent that this affects most observations in sediments is unknown. Second, we must independently determine a during inclusion formation, and we must know whether this pressure was hydrostatic or approached lithostatic. Data from both hydrocarbon and aqueous fluid inclusions in core samples from the Mission Canyon formation, Williston basin, North Dakota, illustrate a method for independently determining both paleotemperature and paleopressure from a single set of fluid inclusion measurements. Th technique requires petrographic evidence for End_Page 241------------------------------ simultaneous trapping of 2 immiscible fluids. Theoretical analysis of the PVT properties of coexisting immiscible fluids demonstrates that the isochores for the 2 different fluids must intersect at the temperature and pressure of entrapment of the inclusions. Calculation of the PVT properties of each fluid requires detailed chemical analyses of both fluids. Recent results from new analytical techniques, especially capillary column gas chromatography to analyze hydrocarbon inclusions and laser Raman spectroscopy to analyze gases in aqueous inclusions, demonstrate that this approach to paleotemperature studies can be widely applicable in sedimentary environments. End_of_Article - Last_Page 242------------

Facies Control of Mississippian Porosity, Whitney Canyon-Carter Creek Field, Wyoming Overthrust Belt: ABSTRACT

Mississippian Mission Canyon carbonates are the most prolific Paleozoic reservoir in the Wyoming Overthrust belt. At Whitney Canyon-Carter Creek field, the Mission Canyon Formation holds recoverable reserves of 240 million bbl of oil equivalent. Production comes from a 350-ft (107 m) gross interval of shallow-water shelf carbonates. Capping the reservoir interval is a 300-ft (91 m) section of anhydrite and tight dolomite that represents sabkha deposits that prograded seaward (westward) over the shelf carbonates. Production from the shelf sequence comes exclusively from sucrosic dolomites that are interbedded with tight limestones and tight crystalline dolomites. Examination of cores spanning the entire reservoir interval reveals that it is composed of a series of nine shallowing-upward sequences, reflecting a history of progradational events across the Mississippian Wyoming shelf. A single complete sequence averages 40 ft (12 m) in thickness and grades upward from open-marine through restricted-marine to intertidal and/or supratidal environments. Open-marine units are predominantly fossiliferous grainstones and packstones--rocks containing little or no carbonate mud. The overlying restricted-marine and intertidal and/or supratidal units are primarily mud-supported carbonates. Petrographic evi ence indicates that carbonate mud was dolomitized preferentially relative to grains. The best reservoir-quality dolomite, therefore, usually occurs in the mud-rich, upper portions of the shallowing-upward sequences, and tight intervals separating the porous zones generally represent grain-supported, open-marine units. Such facies characteristically are continuous for great distances along depositional strike. At Whitney Canyon-Carter Creek field, individual porous zones can be correlated for more than 12 mi (19 km) across the End_Page 842------------------------------ field. This understanding of facies-controlled porosity development has application both in regional exploration and in field development. End_of_Article - Last_Page 843------------