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The AAPG/Datapages Combined Publications Database
Oklahoma City Geological Society
Abstract
Geochemical Characterization Of The Lower Pennsylvanian Morrow Shale In The Anadarko Basin Of Oklahoma
Abstract
The Morrow Formation is one of the most widespread units of Early Pennsylvanian age in the Anadarko Basin and has been one of the most productive oil and gas reservoirs in the Basin since the 1950s. Rock-Eval pyrolysis indicated Morrow source rocks are fair to good source rocks, typically characterized as a thermally mature Type-III kerogen with gas generating capacity and limited oil potential. Biomarker data show the Morrow is composed of mixed organic matter with pronounced terrestrial input deposited in a near-shore/transitional environment subjected to periodic terrigenous influx. The abundance of 17α(H)-norhopane in some of the rock extracts indicated increasing carbonate content, or evaporitic conditions. Relatively high concentrations of rearranged hopanes suggest the Morrow was deposited under clay-rich, brackish/freshwater conditions in a suboxic environment. The presence of 18α(H)-oleanane and 1,2,7-trimethylnaphthalene (TMN) suggests a contribution from angiosperms that evolved long before the Cretaceous or a sister-group of angiosperms that shared common characteristics with contemporary angiosperms. The presence of 25-norhopanes in multiple source rock samples was interpreted as paleo-seepage from an ancient heavily biodegraded reservoir or in-situ biodegradation of oil in fissures and fractures of the shale. Polycyclic aromatic hydrocarbons (PAHs) such as fluoranthene, benzopyrenes, benzofluoranthenes, benzo(a)anthracene, triphenylene and chrysenes in the Morrow source rocks indicate paleo-wildfires were part of the terrestrial ecosystem during Early Pennsylvanian time.
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