Permian Phosphoria Formation Research Articles

Little Sand Draw Field, Big Horn Basin, Wyoming: A Hybrid Dual-Porosity and Single-Porosity Reservoir in the Phosphoria Formation: DISCUSSION

Little Sand Draw field produces hydrocarbons from an asymmetric anticline located on the southwestern margin of the Big Horn basin. This 479-ac, 32-well field has produced 10.4 million bbl of oil. Most production is from the Ervay Member of the Permian Phosphoria Formation. On the basis of gamma-ray logs and textural evidence, the dolomitized, 90-100 ft (27.5-30.5 m) thick Ervay Member can be subdivided into six reservoir zones. Zones are bounded by marine-flooding surfaces, and each zone is laterally consistent throughout the field. Lower Ervay zones, which were deposited under low-energy open-marine conditions, are predominantly mud-supported rocks. Higher energy, yet more restricted marine conditions occurred during deposition of the upper Ervay and resulted in a domin nce of grain-supported rocks. Moldic and micro-intercrystalline porosity occurs in four zones. Moldic pores dominate total porosity and have significant hydrocarbon storage capacity. Intercrystalline pores control matrix permeability, which is generally less than 20 md. Such permeabilities cannot account for observed production rates of several hundreds to thousands of barrels of fluid per day. Open fractures are present in all zones and contribute significantly to reservoir drainage. Fracture intensity is controlled by lithology and structural position. Silicified strata are the most highly fractured. Structurally, wells with the greatest fracture intensity are aligned along the northwest-trending hingeline of the fold. However, in much of the reservoir, flow behavior is dominated by east-northeast-trending fract res which are recognized on borehole-imaging logs and as directional permeabilities from pressure-interference tests. Most, but not all, wells are at or near virgin reservoir pressure. This, along with high water-production rates, suggests strong aquifer support for reservoir pressure. A comparison of pressure and water-cut data shows that those wells with depleted reservoir pressure also had water cuts that rose relatively slowly. Geographically, these wells define dual-porosity "compartments" in which matrix porosities and permeabilities, as well as fractures, affect flow. Elsewhere in the field, fractures are the dominant control on flow in what appears to be single-porosity behavior.

The influence of pressure on petroleum generation and maturation as suggested by aqueous pyrolysis

Because fluid pressures are transient in sedimentary basins over geologic time, the effect of increasing fluid pressure on organic-matter metamorphism is difficult to determine, and conflicting opinions exist concerning its influence. Properly-performed aqueous-pyrolysis experiments can closely simulate hydrocarbon generation and maturation in nature, and thus offer an excellent way to study the influence of pressure. Such experiments, carried out on the Retort Phosphatic Shale Member of the Lower Permian Phosphoria Formation (type II-S organic matter) at different constant temperatures, demonstrated that increasing pressure significantly retards all aspects of organic matter metamorphism, including hydrocarbon generation, maturation and thermal destruction. This conclusion results from detailed quantitative and qualitative analyses of all products from hydrocarbon generation, from the C 1 to C 4 hydrocarbon gases to the asphaltenes, and also from analyses of the reacted rocks. We have documented that our aqueous-pyrolysis experiments closely simulated natural hydrocarbon generation and maturation. Thus the data taken as a function of pressure have relevance to the influence of normal and abnormal fluid pressures as related to: 1) depths and temperatures of mainstage hydrocarbon generation; 2) the thermal destruction of deposits of gas or light oil, or their preservation to unexpectedly high maturation ranks; and 3) the persistence of measurable to moderate concentrations of C 15+ hydrocarbons in fine-grained rocks even to ultra-high maturation ranks.

Little Sand Draw Field, Big Horn Basin, Wyoming: A Hybrid Dual-Porosity and Single-Porosity Reservoir in the Phosphoria Formation (1)

Little Sand Draw field produces hydrocarbons from an asymmetric anticline located on the southwestern margin of the Big Horn basin. This 479-ac, 32-well field has produced 10.4 million bbl of oil. Most production is from the Ervay Member of the Permian Phosphoria Formation. On the basis of gamma-ray logs and textural evidence, the dolomitized, 90-100 ft (27.5-30.5 m) thick Ervay Member can be subdivided into six reservoir zones. Zones are bounded by marine-flooding surfaces, and each zone is laterally consistent throughout the field. Lower Ervay zones, which were deposited under low-energy open-marine conditions, are predominantly mud-supported rocks. Higher energy, yet more restricted marine conditions occurred during deposition of the upper Ervay and resulted in a domin nce of grain-supported rocks. Moldic and micro-intercrystalline porosity occurs in four zones. Moldic pores dominate total porosity and have significant hydrocarbon storage capacity. Intercrystalline pores control matrix permeability, which is generally less than 20 md. Such permeabilities cannot account for observed production rates of several hundreds to thousands of barrels of fluid per day. Open fractures are present in all zones and contribute significantly to reservoir drainage. Fracture intensity is controlled by lithology and structural position. Silicified strata are the most highly fractured. Structurally, wells with the greatest fracture intensity are aligned along the northwest-trending hingeline of the fold. However, in much of the reservoir, flow behavior is dominated by east-northeast-trending fract res which are recognized on borehole-imaging logs and as directional permeabilities from pressure-interference tests. Most, but not all, wells are at or near virgin reservoir pressure. This, along with high water-production rates, suggests strong aquifer support for reservoir pressure. A comparison of pressure and water-cut data shows that those wells with depleted reservoir pressure also had water cuts that rose relatively slowly. Geographically, these wells define dual-porosity compartments in which matrix porosities and permeabilities, as well as fractures, affect flow. Elsewhere in the field, fractures are the dominant control on flow in what appears to be single-porosity behavior.

Depositional Environments of Permian Phosphoria Formation and Related Rocks, Leach Mountains, Northeast Nevada

In the Leach Mountains, northeastern Nevada, the Phosphoria Formation is represented by phosphatic rocks of the late Leonardian Meade Peak Phosphatic Shale Tongue. The Meade Peak is overlain by dolomitic siltstones and chert, including black bedded chert, of the upper Leonardian( ) to lower Guadalupian Murdock Mountain Formation. The black bedded chert is related to the Rex Chert Member of the Phosphoria Formation. There is little consensus regarding depositional environments of the Meade Peak. Also, little work has been presented on the origin of the black bedded cherts such as in the Murdock Mountain and Rex Chert. Locally, a shallow subtidal to perhaps partly intertidal depositional model for the Meade Peak is based upon characteristics of reworked phosphatic clasts, a crowded and mixed shallow-water fauna, oolite beds, and stratigraphic position between shallow subtidal to supratidal rocks. Supratidal deposition of dolomitic siltstones and units of black bedded chert in the Murdock Mountain is based largely on abundant cauliflower-shaped blebs of chalcedony with shapes resembling modern anhydrite nodules. Also, zebraic chalcedony in the black bedded chert is interpreted as a replacement product of evaporite deposits. The setting in this southwestern part of the depositional basin of the Phosphoria Formation, in the latemore » Leonardian, during the deposition of the Meade Peak, is interpreted as a network of shallow to very shallow marine basins and intervening, perhaps periodically merged shoals. Subsequently, during the late Leonardian( ) to early Guadalupian, there was a marine regression and the supratidal environment of the Murdock Mountain Formation was established.« less

Unconformity Related Traps and Production, Lower Cretaceous Through Mississippian Strata, Central and Northern Rocky Mountains: ABSTRACT

Unconformities provide a useful means of equating stratigraphic traps between basins. Systematic mapping can define new concepts through analogy, often from geographically separate areas. Lower Cretaceous through Mississippian surfaces in the central and northern Rockies provide examples. Late Mississippian and Early Pennsylvanian surfaces formed at least four paleodrainage basins separated by the Transcontinental arch. Tyler Formation valley fills (Montana, North Dakota) have produced more than 100 million BOE. Analogous targets in Utah remain untested, but the Mid-Continent Morrow trend continues to yield new reserves. Permian and Triassic paleodrainages filled primarily with seals and form regional traps. A breached Madison trap (Mississippian, Colorado), more than 350 million BOE (Permian Minnelusa, Wyoming), more than 8 billion BOE (from the White Rim Sandstone tar deposits Permian Utah), and eastern Williston basin (Mississippian) are examples. Minor basal valley fill trapping also occurs. Transgressive carbonate facies changes have trapped more than 40 million BOE (Permian Phosphoria Formation, Wyoming). Additional deep gas potential exists. Jurassic unconformities control seal distribution over Nugget Sandstone (Jurassic) reservoirs and partially control Mississippian porosity on the Sweetgrass arch (Montana). Minor paleohill trapping also occurs. Lower Cretaceous surfaces have trapped nearly 2 billion BOE hydrocarbons in 10 paleodrainage networks. Undrilled paleodrainage basinsmore » remain deep gas targets. The systematic examination of Rocky Mountain unconformities has been understudied. New exploration concepts and reserve additions await the creative interpreter.« less

Hydrothermal petroleums from Yellowstone National Park, Wyoming, U.S.A.

In Yellowstone National Park, oil is actively discharging with thermal waters at several widely separated thermal areas. Samples from two locations were studied. Petroleum from Calcite Springs occurs as vapor condensates in steam vents. Detailed geochemical analyses show a complex origin for the petroleum fluid. Deeply buried, hydrothermally altered sedimentary rocks are believed to have contributed a high temperature pyrolysate consisting largely of polynuclear aromatic hydrocarbons and heterocyclic compounds. Shallow sedimentary rocks, which are also hydrothermally altered, have contributed large amounts of aromatic and polar compounds, as well as minor amounts of hydrocarbons. The Permian Phosphoria Formation is suggested as a likely source rock for the bulk of the oils. Autochthonous pyrolysates and lipids from thermally altered surface and near surface bacterial and land plant debris have also been introduced to the vents. The latter may have been introduced by downward percolating meteoric water. The oil from Rainbow Springs is a highly paraffinic, low-S crude oil, depleted in light hydrocarbons. Unlike the Calcite Springs samples, the oil from Rainbow Springs is believed to have originated from a single sedimentary source rock with little or no hydrocarbon contributions from surface biota. The oil contains a unique molecular marker assemblage that suggests both gymnosperm and angiosperm input and places the age of the source unit in the Tertiary. Consideration of the local stratigraphy and water temperatures limits the probable source zone to the Eocene Aycross Formation, an intra-volcanic lacustrine mudstone. Calculation of deep water temperatures indicates that this unit is exposed to thermal waters in an extensive near surface aquifer. Such conditions are consistent with the widespread surface manifestations and biomarker kinetic models. Consideration of the petroleum chemistry and regional hydrodynamics indicates that the Yellowstone Park oils did not migrate into the Park from distant sources. The chemical data are also inconsistent with the oils being generated and trapped prior to Tertiary volcanism. At both Calcite Springs and Rainbow Springs, oil generation appears to be contemporaneous and is attributed to thermal alteration of relatively shallow sedimentary rocks by hydrothermal fluids.

Sedimentology and Petroleum Occurrence, Schoolhouse Member, Maroon Formation (Lower Permian), Northwestern Colorado (1)

The Lower Permian Schoolhouse Member of the Maroon Formation (formerly considered the Schoolhouse Tongue of the Weber Sandstone) forms a partly exhumed petroleum reservoir in the Eagle basin of northwestern Colorado. The Schoolhouse consists mainly of yellowish gray to gray, low-angle to parallel bedded, very fine to fine-grained sandstone of eolian sand-sheet origin; interbedded fluvial deposits are present in most sections. The sand-sheet deposits of the Schoolhouse Member are sedimentologically and petrologically similar to those in the underlying red beds of the main body of the Maroon Formation, and the Schoolhouse is considered the uppermost sand sheet in the Maroon depositional sequence. The bleached and oil-stained Schoolhouse Member is distinguished from the unde lying Maroon red beds on the basis of its diagenetic history, which is related to regional hydrocarbon migration and development of secondary porosity. Geological and geochemical data suggest that Schoolhouse Member oils have upper Paleozoic sources, including the intrabasinal Belden Formation. Late Paleozoic faults have served as local conduits for vertical petroleum migration. Large-scale (>200 km) lateral migration from sources in the Permian Phosphoria Formation is also possible but less likely. Belden oil was generated and migrated before about 75 Ma. Subsequently, the Schoolhouse Member reservoir was uplifted, then partly exhumed on the monoclinal flank of the Laramide (latest Cretaceous-Paleogene) White River uplift. Based on this study, exploration models for Maroon Formation and Weber Sandstone reservoirs in northwestern Colorado should be expanded to more fully consider Belden source rocks and the controls of late Paleoz ic structures on hydrocarbon migration and trapping. Stratigraphic units of diagenetic origin comparable to the Schoolhouse Member are likely in other basin provinces, and their proper interpretation is critical for reconstructing the histories of associated petroleum systems.

The Influence of Pressure on Petroleum Generation and Maturation as Suggested by Hydrous Pyrolysis: ABSTRACT

Because fluid pressures in sedimentary basins fluctuate over geologic time, their effect on hydrocarbon (HC) generation and maturation is difficult to determine and is a source of much controversy. Assuming that hydrous pyrolysis reactions simulate HC generation and maturation in nature, then laboratory experiments of such reactions offer an excellent way to study any possible influence of pressure. Such experiments carried out on shale of the Permian Phosphoria Formation (type II-S organic matter) at different constant temperatures demonstrate that increasing pressure significantly retards both HC generation and maturation reactions. This conclusion is derived from quantitative analysis of the generated C{sub 1}-C{sub 4} HC gases, C{sub 4}-C{sub 7} gasoline-range HCs, and C{sup 8+} bitumen; pyrolysis-gas chromatography of asphaltenes; Rock-Eval and pyrolysis-gas chromatography of the Soxhlet-extracted, reacted rock, and many detailed, qualitative gas chromatographic and mass spectrometric analyses of the various generated products. If hydrous pyrolysis does mimic natural HC generation and maturation, then these data have relevance to HC generation, accumulation, and destruction in nature, specifically regarding the influence of abnormal fluid pressures and the factors that create and disrupt such pressures.

Effects of weathering on biological marker and aromatic hydrocarbon composition of organic matter in Phosphoria shale outcrop

GC-MS analyses were performed on core samples collected from a shale outcrop of the Permian Phosphoria Formation in Utah, U.S.A., to study effects of weathering on selected biological marker and aromatic (phenanthrene) hydrocarbon compounds. Among the biological markers, the most important weathering effects are a decrease in the 20S 20R diastereomer ratio of the C 29 steranes and loss of low molecular weight triaromatic steroids. A decrease in the C 19 through C 22 tricylcic terpanes occurs relative to the total C 19-C 26 tricyclic fraction. Pronounced loss of methyl-substituted phenanthrenes occurs relative to phenanthrene. No major effect on the overall distribution of pentacyclic terpanes is evident.

Stable isotopic characterisation of francolite formation

Stable isotopic data are presented for 112 samples of francolite from 18 separate phosphate deposits. Values of δ 13C and δ 34S in most offshore deposits suggest formation within oxic or suboxic environments either by carbonate replacement or direct precipitation of francolite from water of normal marine compositions. The exceptions are concretionary francolite from Namibia, which has an isotopic composition in keeping with its formation within organic-rich sediments, and that from offshore Morocco, which has an isotopic signature of the anoxic/suboxic interface. Onshore deposits from Jordan, Mexico, South Africa and, possibly, the Permian Phosphoria Formation in the western U.S.A., are substantially depleted in 18O: they appear to be too altered for deductions to be made about their environments of formation. In other onshore deposits which are unaltered, or minimally altered, the isotopic composition suggests that some formed within sulphate-reducing sediments (Sedhura, Morocco) whilst francolite from the Georgina Basin of Australia formed at the oxic/anoxic boundary, where oxidation of biogenic H 2S decreases the δ 34S of pore water. In general, pelletal samples show non-oxic isotopic signatures, whilst non-pelletal samples show oxic isotopic signatures, but samples from Namibia, Peru (Ica Plateau) and the Californian and Moroccan margins are exceptions to this rule. Morphology may therefore be a misleading indicator of francolite genesis as no definitive relation exists between phosphorite type and isotopic signature.

Geologic Setting of Petroleum Source Rocks in Permian Phosphoria Formation: ABSTRACT

The Permian Phosphoria Formation in the northwestern interior United States contains two phosphatic and organic-carbon-rich shale members--the Meade Peak Phosphatic Shale Member and the Retort Phosphatic Shale Member. These rocks were formed at the periphery of a foreland basin between the Paleozoic continental margin and the North American cratonic shelf. The concentration, distribution, and coincidence of phosphorite, organic carbon, and many trace elements within these shale members probably were coincident with areas of optimum trophism and biologic productivity related to areas of upwelling. Upwelling is indicated to have occurred in the Phosphoria sea by the presence of sapropel that was deposited adjacent to shoals near the east flank of the depositional basin. Maximum organic-carbon concentration is as much as 30 wt. % in the organically richest beds in the shale members and the maximum average in each member is about 10 wt. %. A close association occurs in the distribution of the organic carbon, silver, chromium, molybdenum, nickel, titanium, vanadium, and zinc. Phosphorous differs slightly from the distribution of organic carbon and by contrast seems typically associated with copper, lanthanum, neodymium, strontium, yttrium, and ytterbium. Burial of the sapropelic muds by Triassic and younger sediments and the consequent rise in ambient temperature has led to catagenesis of hydrocarbons from the kerogen in these rocks. In some areas of southwestern Montana, hydrocarbons have not been generated; however, burial has been minimal and temperatures have remained low. Consequently, these rocks remain organic-rich shales that have the potential for producing synthetic oil and gas. End_of_Article - Last_Page 942------------

Regional Lithostratigraphy of Permian Phosphoria Formation, Western Overthrust Belt: ABSTRACT

A regional synthesis of the lithostratigraphic relationships for the Permian Phosphoria Formation illustrates the cyclic lithofacies types, carbonate reservoir, and potential organic facies for the western Overthrust belt of Wyoming, Idaho, and Utah. Palinspastically restored subsurface control sections (70) and measured outcrop sections (35) indicate a complex, intertonguing, transgressive-regressive sequence of shallow-water carbonates, clastics, and highly organic phosphatic mudstone deposited in a shallow marine shelf-margin environment. Distribution of facies types was affected by paleostructural growth of late Paleozoic structural elements including the Sublette basin of Utah and Bannock highs of western Wyoming-southeastern Idaho. These highs helped define the Wyom ng shelf-margin slope. Identification of paleostructural growth is based on interpretation of depositional environments and thickness patterns of the Permian deposits. The Phosphoria Formation and the equivalent beds of the Park City Formation are divisible into two main depositional cycles--the Meade Peak-Franson Members and the Retort-Ervay Members--based on the regional correlation network generated for the Overthrust belt. Carbonate deposition and reservoir development occurred during maximum transgressive stages, whereas deposition of phosphorite and organic shales occurred during maximum regressive stages. The reservoir facies is closely associated with early diagenetic dolomitization of carbonate shelf deposits concentrated along paleostructural high areas of the shelf-margin environment. End_of_Article - Last_Page 952------------

Geology of North End of East Pioneer Mountains, Beaverhead County, Montana: ABSTRACT

The north end of the Pioneer Mountains is underlain by the following rocks: lower Proterozoic gneiss and amphibolite; middle proterozoic clastic rocks of the Missoula Group (Mount Shields Formation?); basal Cambrian clastic unit; Cambrian-Devonian-Carboniferous shelf sequence; Permian Phosphoria Formation; and Lower Triassic Dinwoody Formation. Jurassic rocks are missing. The Lower Cretaceous Kootenai Formation is of lagoonal to fluviatile facies, overlain by a thick (^approx 2 km, 1 mi) section of fluviatile Colorado Group. An upper member of the Colorado Group has yielded a Campanian-to-Maestrichtian pollen flora. The sedimentary rocks are cut by calc-alkalic plutonic rocks (80 to 65 m.y.B.P.), the oldest of which are synchronous with upper Colorado sedimentation. The y ungest pre-Quaternary rocks are Eocene and Oligocene calc-alkalic lavas and Oligocene pumiceous tuff. The Missoula Group is entirely in thrust sheets that postdate the Colorado, so the thrusting is no older than Campanian, but the thrusts are cut by 72 to 74 m.y.B.P. plutons. The Johnson thrust of Fraser and Waldrop, on the 1972 U.S. Geological Survey Geologic Quadrangle Map 988, is part of this thrust system. A klippe of Missoula Group on Morrison Hill is an erosional remnant, but most of the overthrust rocks are west of the Wise River valley and probably overlie Phanerozoic strata. In addition to the thrust sheets, two families of high-angle faults dominate. One family trends west-northwest and displacements along these faults can be measured in kilometers. The Johnson thrust is interpreted to have been displaced by a sinistral fault of this system along the straight valley of Big Hole River between Seymour Creek and Dewey. The eastward projection of the mountain front at Maiden Rock, just south of Divide, resulted from block displacement along two strands of this fault. This particular fault is of major significance because it marks an abrupt jump in the initial strontium ratios of intrusive rocks, from values typical of the Boulder batholith (0.706 to 0.709) to those of the Pioneer batholith (0.711 to 0.716), indicating that different crustal blocks were juxta osed. The second family of high-angle faults trends north-northeast. Wise River valley is interpreted to be a graben in this system. One important fault in the system is the Fourth of July fault, with downdrop (to west) of several kilometers; the fault may be continuous with the Comet Mountain fault. The west-northwest fault system began at least before the Eocene lava flows, but complex field relations between the two high-angle fault systems indicate their growth must have overlapped in age, possibly through the late Tertiary. End_of_Article - Last_Page 1362------------

Whitney Canyon Sour Gas Well Completion Techniques

Kilstrom, Kevin J., Amoco Production Co. Summary Sour gas wells require special considerations for successful production. Corrosion, safety considerations, prolific gas rates, the casing program, and hydrate problems affect the final completion design for wells in the Whitney Canyon sour gas field. A completion technique that addresses all these constraints yet maintains a degree of simplicity and therefore presents less risk is being used in Whitney Canyon well completions. Additionally, problems that do not exist in Whitney Canyon, such as salt or sulfur deposition, can be handled with this same basic system. Introduction The economics associated with exploration of hydrocarbons in the U.S. have improved during the past 8 years, resulting in more deep drilling and frequent discovery of sour gas. One such discovery is the Whitney Canyon field located in the Overthrust Belt of southwest Wyoming, about 15 miles (24 km) north-northeast of Evanston (Fig. 1). The field was discovered in 1977 with the drilling of the Amoco/Chevron/Gulf Working Interest Unit Well No. 1, which reached a total depth of 10,691 ft (3258.6 m) in the Permian Phosphoria formation. Sour gas was recovered in the drilling fluid, and subsequent failure of tubular goods resulted in plugging back the well and completing the Triassic Thaynes formation in the 9,178- to 9,266-ft (2797.5- to 2824.3-m) interval for 4.3 MMcf/D (121.8 × 10(3) m3/d) of sweet gas and 96 B/D (15.3 m3/d) of condensate. The field is on the Absaroka thrust plate, which has resulted in placement of Ordovician Big Horn above Cretaceous rock in this vicinity (Fig. 2). The Ryckman Creek field, which produces oil and gas from the Jurassic Nugget and sweet gas from the Thaynes formation, lies 5 to 6 miles (8.0 to 9.7 km) to the east. The Carter Creek field has been shown to be a continuation of the Whitney Canyon field to the north (Fig. 3). A second well was spudded to test the deeper formations in the Whitney Canyon play and reached a total depth of 16,393 ft (4996.6 m) in the subthrust Cretaceous. Testing in this well verified substantial quantities of sour gas in the Ordovician Big Horn and Mississippian Mission Canyon carbonates and indicated sour gas in the Pennsylvanian Weber sandstone. Subsequent field development has indicated commercial quantities of gas in four of the Paleozoic Era formations. These include the Devonian Darby, Mission Canyon, Mississippian Lodgepole, and Big Horn formations. Limited quantities of sweet gas have been tested in the subthrust Cretaceous, and noncommercial sour gas production was indicated from Triassic Dinwoody and Permian Phosphoria. The approximately 4,500-ft (1372-m) thick Paleozoic interval lies at a depth range of 10,000 to 16,000 ft (4877 to 3048 m) in the Whitney Canyon area and exhibits normal pressure gradients of 0.45 to 0.50 psi/ft (3.1 to 3.4 kPa/m) and a maximum temperature of 250 degrees F (121 degrees C). Drilling has taken place for 13 miles (21 km) along the Whitney Canyon/Carter Creek trend, indicating a sizeable productive area (Fig. 3). The reserves of sour gas are trapped in the folding associated with the thrust faulting in the area and are held in primary and secondary porosity (matrix porosity in the 5 to 10% range). Permeability is generally low [matrix permeability less than 0.1 md (9.869 × 10(-5) mum2)] and production is highly dependent on extensive natural fracturing. The gas composition in the Whitney Canyon field varies from formation to formation, with all of the Paleozoics indicating H2S content from 1 to 15 mol%. The Mississippian Age Mission Canyon formation has indicated the largest reserves in the field and one 30-ft (9.1-m) interval in the Champlin 457 A Amoco No. 1 well tested at 33 MMcf/D (934.5 × 10(3) m3/d) (Table 1). The Mission Canyon formation is a 1,000-ft (304.8-m) thick carbonate section with a gas composition of approximately 15% H2S, 5% CO2, and 70% methane, and the balance is ethane and heavier hydrocarbons. To develop a successful completion design for the Whitney Canyon field, we first defined all factors that would be detrimental to production. JPT P. 40^

Nuclear Magnetic Resonance: A Technique for Direct Nondestructive Evaluation of Source-Rock Potential

A direct nondestructive method for source-rock evaluation based on solid-state nuclear magnetic resonance (NMR) techniques is demonstrated. Direct measurement of the aliphatic and aromatic carbon structures of the total organic matter in whole rocks is provided by NMR techniques known as cross polarization with magic-angle spinning. The proportion of organic carbon present in aliphatic structures correlates with the oil-generating potential of a source rock. In addition, whether the organic matter in the rock has been altered during catagenesis can be evaluated. These evaluations are demonstrated for carbonaceous shales from the Retort and Meade Peak Phosphatic Shale Members of the Permian Phosphoria Formation. The NMR results show that the Retort shales have a high poten ial to generate oil, but the Meade Peak shales, having already undergone thermal alteration, have a low potential to generate additional oil.

Phosphorite, Organic Carbon, and Hydrocarbons in Permian Phosphoria Formation, Western United States: ABSTRACT

The Permian Phosphoria Formation in the northwestern interior United States contains two phosphatic and organic carbon-rich shale members that include both phosphorite and petroleum source beds. The association suggests an intimate relation between factors which generate phosphorite deposits and hydrocarbon source beds. The two members, the Meade Peak Phosphatic Shale Member and the Retort Phosphatic Shale Member, were deposited at the periphery of a foreland basin between the Cordilleran geosyncline and the North American craton. The concentration, distribution, and coincidence of phosphorite, organic carbon, End_Page 746------------------------------ and many of the trace elements within these shale members probably were coincident with areas of optimum trophism and biologic productivity. The paleogeographic relations suggest that the trophic regions were areas of upwelling in the Phosphoria sea adjacent to shoals near the eastern flank of the basin. The richest phosphorite beds, mostly pelletoidal, are near the base and the top of each shale member, and the organic carbon and the residual hydrocarbon contents of the phosphorite beds are low in comparison to the other strata. The beds richest in organic carbon are near the middle part of both members, where phosphorite content is low and residual hydrocarbon content is high. The deposition of both phosphorite and organic matter that is precursor to petroleum seems to be directly r lated to areas of inferred upwelling and biomass concentration, but the sediments have been differentiated into dominantly phosphorite beds and dominantly petroleum source beds by chemical or mechanical factors. End_of_Article - Last_Page 747------------

Upper Paleozoic Paleogeography of Idaho: ABSTRACT

In south-central Idaho, Mississippian rocks were deposited in flysch-trough and carbonate-bank environments. Lower Mississippian detritus shed from the Antler highland accumulated to a thickness of more than 3,000 m in an adjacent elongate flysch-trough and End_Page 785------------------------------ spread eastward to the inner craton margin. In southeastern Idaho, less detritus was available from the Antler highland in Early Mississippian time, and a starved basin developed between the foreland basin on the west and the inner craton margin on the east. Carbonate-bank deposition followed on both sides of the Snake River plain in Late Mississippian time, as the northern flysch basin ceased subsidence. By Middle Pennsylvanian time, the flysch deposits rose to form a highland from which coarse detritus was shed west, east, and south to interfinger with the fine-grained craton-derived, in part subarkosic, sands of the Wood River and Sublett basins, which gradually deepened through Late Pennsylvanian into Early Permian (pre-Phosphoria) time. Carbonate-bank deposition continued into Early Permian time east of the Copper Basin highland. The Lower and Upper(?) Permian Phosphoria Formation is recognized throughout southeastern Idaho and in south-central Idaho as far west as the Lemhi Range. West of the Lemhi Range, mollusk-rich fine-grained Phosphoria-equivalent rocks are known from one locality in the White Knob Mountains and from two localities in the Pioneer Mountains. Fine-grained, banded and graded siltites of Phosphoria age, which resemble continental-rise contourites, are present in the Boulder Mountains. Phosphoria-equivalent andesitic and dacitic volcanic rocks in western Idaho record a contemporaneous volcanic arc west of the continent. End_of_Article - Last_Page 786------------

Permian Trace Fossils of Western Wyoming and Adjacent Areas: ABSTRACT

Burrows are common to abundant in much of the Permian Phosphoria Formation and correlative units in western Wyoming and adjacent states. Surface traces are rare. Coarse phosphorite units are characterized by straight, full-relief traces with fine-grained fillings. They are commonly horizontal but some are vertical. Sandstones contain burrows varying greatly in orientation and regularity. Carbonate rocks and certain cherty units typically contain burrows similar to those of modern crustaceans; the burrows are filled with skeletal material, fine-grained carbonate rock, phosphorite, or chert. End_Page 669------------------------------ Absence of primary sedimentary structures in certain units is probably due to bioturbation. Units with high concentrations of burrows near the top suggest rapid deposition. Straight vertical burrows in certain marine sandstones may be insect burrows in barrier-island environments. Cylindrical, irregular chert nodules are characteristic of most carbonate units. Some nodules are isolated but others coalesce into beds. The carbonate matrix in many places has flow structures around the chert nodules suggesting relatively early lithification of the chert. Some cherty beds contain irregularly cylindrical carbonate bodies. The cylindrical bodies, both chert and carbonate rock, are interpreted as burrow fillings. Burrows filled with material coarser and more permeable than the host sediments were sites of chertification. In beds in which the opposite conditions prevailed, the interburrow material was silicified. Because large burrows with coarse-grained fillings are abundant and in many places penetrate less permeable sediments, they may have functioned as important fl id conduits during early diagenesis. End_of_Article - Last_Page 670------------

Whitney Canyon Field--Potential Gas Giant in Wyoming Thrust Belt: ABSTRACT

Recent drilling in the Absaroka plate of the Wyoming thrust belt has confirmed the presence of a major gas-condensate accumulation in the Whitney Canyon area of Uinta County, Wyoming. Reserves are primarily in porous and/or fractured Paleozoic carbonate formations. Triassic carbonate rocks also appear to be commercially productive. The discovery well, which was scheduled as a 13,400-ft (4,084 m) test, was spudded in October 1976. Mechanical problems were encountered at 10,691 ft (3,259 m) in the Permian Phosphoria Formation and the well was subsequently completed in the Triassic Thaynes Formation. Paleozoic gas production was established in 1978 by the Amoco-Chevron-Gulf No. 2 well, which was drilled into a nearly normal stratigraphic section of Jurassic Twin Creek Limestone through Ordovician Bighorn Dolomite before crossing the Absaroka thrust at a true vertical depth (TVD) of 15,516 ft (4,729 m). Cretaceous sandstones and shales were drilled to a total depth of 16,224 ft (4,945 m) or 15,894 ft (4,845 m) TVD. A development well located approximately one mile north of the No. 2 well was drilled into a similar s ratigraphic sequence. The Whitney Canyon structure is a north-trending geophysical anomaly with little or no surface expression. Its general shape can be defined well with reflection seismic data. At the Phosphoria level, the structure is approximately 10 mi (16 km) long and 2 mi (3.2 km) wide with 2,500 ft (762 m) of closure. Gas tested to date from the Triassic Thaynes Formation is sweet, whereas the Paleozoic gas is sour with a maximum H2S content of 18%. Environmental considerations and gas treatment plant construction will delay Paleozoic gas production until late 1981. Although reserve estimates for the Whitney Canyon structure are quite speculative at this time, it appears to be a giant field. End_of_Article - Last_Page 730------------

Subaerial weathering of sedimentary organic matter

Small diameter core samples were taken from outcrops of the Permian Phosphoria Formation and the Cretaceous Pierre Shale of the Western United States to determine the effects of weathering on organic matter in shale outcrops. While the Pierre Shale core showed no evidence of weathering, the Phosphoria Formation showed significant reduction of overall organic content and pronounced changes in organic composition over the near-surface interval of the core. Total organic carbon is lower by as much as 60% over the upper 2 ft of the core. Chloroform-soluble organic matter and total hydrocarbon (C 15+) concentrations are 50% lower over this same interval. The ratio of saturated to aromatic hydrocarbons decreases steadily with core depth over the upper 2.6 ft of the core. Aromatic hydrocarbons are enriched in the stable carbon-13 isotope by an average of 1.7%. over this same interval. Shallow core samples also show a loss of n-paraffins relative to branched/cyclic compounds in the saturated C 15+ fraction. Although the extent of weathering is variable, certain characteristic effects are recognizable and can be applied to the interpretation of outcrop data in organic geochemical studies.