Distribution of Petroleum Reservoirs Relative to Allocycles and Autocycles, Upper Portion of the Cherokee Group (Middle Pennsylvanian, Desmoinesian), Mid-Continent Region, U.S.A.: ABSTRACT

Abstract

Sequences of mud rocks, lenticular sandstones, coals, and thin carbonates form autocycles and allocycles in the upper portion of the Cherokee Group. Autocycles delineated in eastern Kansas and northern Oklahoma are relatively local in extent, while allocycles are traceable over the entire region. All autocycles delineated in this study are embedded within the regressive portions of allocycles. Petroleum-bearing sandstones consist of shoestring-shaped and thin sheetlike units in thicker sedimentary lobes. These lobes were deposited as deltaic complexes, which included fluvial and distributary channel sands, interdistributary muds, crevasse splay sands and muds, flood-basin muds, delta-front sands, and predeltaic muds. Delta lobes prograded across the margins of the Middle Pennsylvanian epeiric sea during times of eustatic stillstand or regression. When lobes were abandoned, waves and currents winnowed their upper portions, leaving thin sheetlike lenses of sand. These reworked sands along with marine muds above regressive deltaic sequences form the transgressive parts of autocyclothems. The transgressive parts of allocyclothems, generally consisting of marine shale, resulted from sea level rises that rapidly shifted shorelines far northeastward, moving siliciclastic sources away from the study area. The positions of reservoir-containing deltaic complexes were determined by strandline positions at various sea levels. Extent of eustatic sea levelmore » changes appears to have been the major mechanism that controlled the distribution of petroleum reservoir and source units. In addition, sea level changes probably were a significant factor in the nature of diagenetic alterations that affected reservoir properties.« less