ABSTRACT: Significance and Application of Genetic Unit Mapping of Permian-Pennsylvanian Clastics on the Eastern Shelf of the Midland Basin

John Thoma

Late Pennsylvanian to middle Permian strata on the Eastern shelf of the Midland basin reflect an overall period of marine transgression. Persistent terrigenous influx produced extensive regressive clastic prisms, and periodic flooding of shelf areas during marine transgressions terminated regressive phases and produced conditions favorable for deposition of carbonates and black shale. The resulting sequence argues favorably for extrabasinal or eustatic controls on transgression and a combination of eustatic and tectonic controls on regression.

Bounded by transgressive carbonate and black shale, each regressive clastic prism represents a genetic sequence of strata (GSS). Within each GSS are locally definable bodies of sediment, typically sandstone packages, which can be correlated as genetic increments of strata (GIS). Extensive subsurface control within the study area allows for definition of four GSSs and numerous associated GISs.

Each GSS exhibits a shelf and a basin component. Sediments in the shelf component result primarily from a fluvial depositional environment, whereas sediments in the basin component are the product of marine, deltaic, and submarine depositional environments.

The basin component of any given GSS exhibits an upward-coarsening sequence reflecting four primary depositional environments which can be localized on a GSS isopach map. They are the basin floor, toe of slope, slope, and deltaic environments. Reservoir geometries associated therewith can be identified through field modeling.

Use of the genetic approach to mapping and interpretation of Eastern shelf Permian-Pennsylvanian clastics enables early identification of areas of interest along with an ordered, reliable means of localizing and evaluating associated prospects.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990