--> ABSTRACT: Starved Euxinic Basin Concept for Oil and Gas Genesis in Oklahoma, by Allan P. Bennison; #91039 (2010)

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Starved Euxinic Basin Concept for Oil and Gas Genesis in Oklahoma

Allan P. Bennison

Some black, organic, carbon-rich, Pennsylvanian shales of the Mid-Continent region accumulated in broad, shallow, paleobathymetric troughs at moderate water depth under anoxic conditions during maximum sea level stillstands and sediment starvation. These starved troughs later were filled by northward-prograding wedges and lenses of clastics advancing episodically from southern and eastern highlands. Moreover, biogenic carbonate banks prograded southward along the edges of the trough, resulting in intercalations with the clastics. After several oscillations of sea level, the trough would be nearly filled by the terminal, rapidly deposited sediments of deltas and other shore constructional features. These areas were commonly sites of renewed depression, beginning a new seri s of sedimentation. The thick, organic-rich shales deposited in these troughs may have served as a petroleum source rock if burial was sufficient. Considerable hydrocarbon migration into the adjoining northern shelf carbonate banks was enhanced by their fracture systems, bedding planes, connected vugs, and intergranular porosity modified by diagenetic alteration.

The early episodes of development exhibit stratigraphic thinning of a carbonate and shale sequence into black, sheety, phosphatic shales and thin fossil hash. These black shales are generally on the order of 1-3 m thick, with several episodes having locally coalesced to form an aggregate more than 8 m thick. These black shales probably represent compaction of original anoxic organic muds that were probably 3-10 times thicker.

Loci of several such starved basins have been delineated by detailed stratigraphic mapping in both Oklahoma and Kansas. These basins may contain many cubic kilometers of hydrocarbons source beds, provided levels of thermal maturation were reached. Migration routes were probably provided by clinoform sands interfingering with black shales, by joint and open faults, and by sand-filled channels incised into these source beds by deltaic episodes.

This scenario provides for much more stratigraphically entrapped oil and gas than has been found to date in Oklahoma.

AAPG Search and Discovery Article #91039©1987 AAPG Mid-Continent Section Meeting, Tulsa, Oklahoma, September 27-29, 1987.