Climate Controls on Petroleum Systems: The Pennsylvanian of Eastern Wyoming, U.S.A., and the “Sinking Reservoir” Model
T. S. Ahlbrandt, J. L. Clayton, C. J. Schenk, 2003. "Climate Controls on Petroleum Systems: The Pennsylvanian of Eastern Wyoming, U.S.A., and the “Sinking Reservoir” Model", Climate Controls on Stratigraphy, C. Blaine Cecil, N. Terence Edgar
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Outcrop and subsurface examples of vertically stacked, multicyclic Pennsylvanian petroleum systems in the Powder River Basin of Wyoming are discussed. In the Rocky Mountain region, different source rock, reservoir quality, oil and gas composition, and seals among cycles reflect increasingly arid conditions from Desmoinesian to Virgilian time as the North American plate migrated from an equatorial position in early Carboniferous time into the arid trade-wind belt (∼ 20° N) in late Carboniferous time with associated eolian-sand-sea deposits. Here, Pennsylvanian rocks contain prolific hydrocarbon-producing, stratigraphically isolated reservoirs (commonly eolianites or phylloid algal mounds) and closely associated organic-rich black shale source beds. The black shales, containing as much as 80% total organic carbon (TOC), are the sources of Paleozoic oils in the Powder River and Paradox basins. The source beds are in a cyclic sequence composed ideally of black shale overlain by carbonate, clastic, and evaporite sediments grading back to carbonate and culminating in the next younger shale. Differences among the black shales, however, reflect in part climatic fluctuations in the marginal-marine settings where these shales were deposited. Kerogen in the black shale and carbonate marlstone of the Leo Formation in the Powder River Basin range from type II to type IV and are predominantly type III. Different proportions of terrestrial organic matter consistently are present in individual shale beds and most likely correlate with climatic variations; i.e., more humid in the early Pennsylvanian and more arid in the late Pennsylvanian. Transported organic matter may reflect cyclic variations in runoff and (or) terrestrial vegetative cover. A model of wetter climate would explain the combination of high TOC, high proportion of type III kerogen, high carbonate content, and carbon isotope composition (i.e., 12C-depleted kerogen and 12C-enriched bitumen) in some Desmoinesian shales and the resulting high associated H2S gas composition and higher sulfur content of derived oils. Desmoinesian reservoirs have dominantly carbonate cements. Wet/dry cycles are suggested for Missourian cycles where eolian dune sandstones cemented by both anhydrite and carbonate cement intercalate with very organic-rich, oil-prone (sapropelic) lacustrine black shales. Virgilian cycles in the Powder River Basin reflect even more arid conditions dominated by carbonates and evaporites with relatively minor clastics and a predominance of anhydrite cement in eolian reservoirs. The mechanisms controlling the deposition of the cyclothemic black shales are of great interest because source-rock quality is affected. These results suggest that differences among black-shale cycles should be considered as resulting from climatic fluctuations; a eustatic mechanism alone offers little utility in differentiating or understanding oil/rock differences among various Pennsylvanian cycles. Such variable effects are critical, as demonstrated by completion and production concerns relating to production of toxic associated gases and or formation damage, which can be related to specific cycles. Associated gases are highly variable among cycles, ranging from some with very high sulfur content (e.g., 48% in some Desmoinesian cycles) to others with high nitrogen and low sulfur content. Some of these differences are related to different regions with differing depth of burial and thermal maturity; however, where different oils are produced within a single field, as at Red Bird field, the differences are due to differences in source-rock composition that reflect variations in climate.
A “sinking reservoir” accommodation model in an uncompensated basin is proposed to explain ancient coastal eolian reservoirs in the Pennsylvanian of eastern Wyoming. This model contrasts significantly with a “buried topography” model, which requires tectonic subsidence or eustatic rise to accommodate and preserve reservoirs. The “sinking reservoir” model may also be applicable to isolated carbonate (e.g., phylloid algal, thrombolitic, or stromatolitic) reservoirs in the Pennsylvanian of the Rocky Mountains, the Cambrian of Russia, and elsewhere. The repeated exposure of sediments to climatic influences in uncompensated basins provides conditions whereby the progressive loading of eolian, fluvial, deltaic, and carbonate sediments (i.e., potential reservoirs) on an unconsolidated substrate may preserve these potential reservoirs via the “sinking reservoir” model. The concept of subsidence moats around such reservoirs preserving a variety of nearby diachronous sediments, together with so called “disjunctive” (growth) faults created during subsidence, has important implications for interpreting sediment sequences.