Christopher J. Schenk

Sedimentology and petroleum occurrence, Schoolhouse Member, Maroon Formation (Lower Permian), northwestern Colorado

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.

Recognition of Interstitial Anhydrite Dissolution: A Cause of Secondary Porosity, San Andres Limestone, New Mexico, and Upper Minnelusa Formation, Wyoming

Rectangular and stair-step pore reentrants in carbonate mudstones have been recognized previously as indirect evidence for anhydrite dissolution. In this study, direct evidence for subsurface dissolution of interstitial anhydrite in both dolomite grainstones and quartz sandstones includes: (1) cleavage-related dissolution fringe on anhydrite crystal surfaces, and (2) isolated remnants of optically continuous (formerly poikilotopic) anhydrite. Influenced by the prominent cleavages, the dissolution fringe on the surfaces of the anhydrite crystals consists of a series of sharp, right-angled projections and reentrants. Experimentally etched anhydrite surfaces exhibit features that directly compare to the dissolution fringe, whereas experimentally grown anhydrite does not. We deduced the following sequence of anhydrite dissolution within dolomite grainstones and quartz sandstones. Slow incipient dissolution began along the boundaries between anhydrite and adjacent minerals. From these intercrystalline boundaries, solutions penetrated anhydrite cleavages, leading to more rapid preferential dissolution perpendicular to the more prominent cleavage planes. The widened cleavage planes, together with intercrystalline boundaries, acted as conduits for the removal of dissolved ions. In the final stage, as dissolving anhydrite borders retreated toward pore throats, dissolution slowed and was, again, restricted to intercrystalline boundaries. This process was then repeated in adjacent interstices.

Porosity and Textural Characteristics of Eolian Stratification: ABSTRACT

An analysis of experimentally formed eolian stratification has shown that processes of deposition are the major factors controlling the primary porosity and texture in the avalanche, grainfall, and ripple-produced types. Avalanche stratification generally exhibits the highest primary porosity (avg 47%) because of loose packing and grain arrangements that occur as a mass of sand shears downslope. Wind ripple-produced stratification has the lowest primary porosity (avg 39%) due to the relatively close packing and the presence of finer grained sublayers developed near the base of each inversely graded ripple stratum during ripple migration. Primary porosity of grainfall stratification (avg 43%) is usually intermediate between that of avalanche and ripple-produced deposits be ause the packing is between the avalanche and ripple produced types. Avalanching forms the high-angle, porous, lee-slope stratification commonly observed in eolian deposits. Ripple-produced stratification is formed in two principal localities--on the low-angle windward slope where potential for preservation is poor, and on the lower part of the lee slope where preservation potential is excellent. The ripple deposits, produced by winds moving across the lee slope, form the tangential foreset-to-bottomset stratification common to eolian dunes. The relatively low porosity observed in these deposits is due to the stratification being composed of ripple-produced deposits. Grainfall stratification has a poor preservation potential because of redeposition by avalanching on the upper part of the lee slope. Grainfall on the lower lee slope usually occurs on a r ppled surface, and these grains are incorporated into the ripple-produced stratification being formed at that site. End_of_Article - Last_Page 986------------

Assessment of undiscovered oil and gas resources in the Eagle Ford Group and associated Cenomanian–Turonian Strata, U.S. Gulf Coast, Texas, 2018

The U.S. Geological Survey (USGS) assessed undiscovered, technically recoverable hydrocarbon resources in self-sourced continuous reservoirs of the Upper Cretaceous Eagle Ford Group and associated Cenomanian–Turonian strata, which are present in the subsurface across the U.S. Gulf Coast region, Texas (fig. 1). The USGS completes geologybased assessments using the elements of the total petroleum system (TPS), which include source rock thickness, organic richness, and thermal maturity for self-sourced continuous accumulations. Assessment units (AUs) within a TPS are defined by strata that share similar structural and petroleum-charge histories along with lithology and stratigraphy.

Assessment of undiscovered oil and gas resources in sandstone reservoirs of the Cotton Valley Group, U.S. Gulf Coast, 2015

Using a geology-based assessment methodology, the U.S. Geological Survey estimated undiscovered mean volumes of 14 million barrels of conventional oil, 430 billion cubic feet of conventional gas, 34,028 billion cubic feet of continuous gas, and a mean total of 391 million barrels of natural gas liquids in sandstone reservoirs of the Upper Jurassic–Lower Cretaceous Cotton Valley Group in onshore lands and State waters of the U.S. Gulf Coast region.

Assessment of undiscovered continuous oil and gas resources in the Monterey Formation, Los Angeles Basin Province, California, 2015

U.S. Department of the Interior U.S. Geological Survey Fact Sheet 2016–3036 July 2016 Printed on recycled paper Oil AU. Conventionally trapped oil and gas fields with oil-water contacts were excluded from the areas of the AUs. Assessment unit boundaries outline the areas in which generation of oil began as the base of the organic-rich Monterey Formation rocks reached burial depths of about 9,000 to 12,000 ft. The Central and Eastern Los Angeles Monterey Continuous Oil AU is defined by the -12,000-foot structure contour near the base of the Monterey Formation, from the map of Wright (1991, fig. 10). The Western Shelf Los Angeles Monterey Continuous Oil AU includes the area west of the Newport-Inglewood fault zone where the base of the Monterey Formation is deeper than 9,000 feet, interpolated from Wright’s (1991) map. Geochemical evidence suggests that the top of this zone may be about 12,000 ft deep in the eastern part of the basin but is likely to be somewhat shallower, about 9,000 ft deep, west of the Newport-Inglewood fault zone where the organic matter in the rocks contains more sulfur, allowing it to generate oil with less heating. Figure 1. Map of the Los Angeles Basin Province in California with boundaries of the two assessments defined in this study. Introduction

Assessment of Undiscovered Oil and Gas Resources of the Timan-Pechora Basin Province, Russia, 2008

The U.S. Geological Survey (USGS) recently assessed the undiscovered oil and gas potential of the Timan-Pechora Basin Province in Russia as part of the USGS Circum-Arctic Oil and Gas Resource Appraisal program. Geologically, the Timan-Pechora Basin Province is a triangular-shaped cratonic block bounded by the northeast-southwest trending Ural Mountains and the northwest-southeast trending Timan Ridge. The northern boundary is shared with the South Barents Sea Province (fig. 1). The Timan-Pechora Basin Province has a long history of oil and gas exploration and production. The first field was discovered in 1930 and, after 75 years of exploration, more than 230 fields have been discovered and more than 5,400 wells have been drilled. This has resulted in the discovery of more than 16 billion barrels of oil and 40 trillion cubic feet of gas. Several studies have presented geological summaries of the Timan-Pechora Basin Province and the potential for its remaining oil and gas resources (for example, Ulmishek, 1982; Lindquist, 1999; Ulmishek, 2000). This report summarizes a reassessment of the undiscovered oil and gas potential of the province, as the last assessment was completed in 2000 (Ulmishek, 2000). The total petroleum system and three assessment units defined by the USGS for the assessment in 2000 were adopted for this reassessment.

Assessment of undiscovered biogenic gas resources, North-central Montana Province

In 2000 the U.S. Geological Survey (USGS) assessed the undiscovered biogenic (also known as microbial) continuous gas resource potential of the North-Central Montana Province in eastern Montana (fig. 1) as part of a national oil and gas assessment project. The assessment was based on the general geologic elements used to define a total petroleum system (TPS), including hydrocarbon source rocks (hydrocarbon generation and migra tion), reservoir rocks (sequence stratigraphy and petrophysical properties), and hydrocarbon traps (trap formation and timing). Using this geologic framework, the USGS defined the Cretaceous Judith River through Belle Fourche Biogenic Gas TPS and seven assessment units (AUs) within it, and quantitatively estimated the undiscovered continuous gas resources within each AU. Resource Summary