Re-Exploration of Cratonic Basins Using Passive-Margin Sequence-Stratigraphic Concepts: Examples from Upper Paleozoic Rocks, Eastern Margin, Midland Basin

L. F. Brown, Jr.

Worldwide studies of passive-margin basins during the past two decades have resulted in relatively sophisticated cyclic sequence models for basins with relatively simple subsidence histories. These concepts also appear to be readily applicable to cratonic and even some foreland basins. Limited seismic resolution of thin cratonic cyclic sequences requires the use of well and outcrop data. Identification and mapping of systems tracts, however, offer new strategic tools for stratigraphic prospecting in basins that have had intensive structural-trap exploration. Lowstand siliciclastic systems tracts represent primary targets.

Use of 5,000 well logs and extensive outcrop information with a 22,000-mi² test region on the eastern margin of the Midland basin permitted delineation of 16 probably third-order type 1 depositional sequences. Sandstone-isolith maps of siliciclastic highstand and lowstand systems tracts show that most structural traps produce from highstand fluvial-deltaic reservoirs, but most stratigraphic traps discovered to date occur within lowstand depositional systems, principally incised valley fills and basin-floor fans. Hydrocarbons are rarely trapped in retrogradational (transgressive) systems tracts.

Maps of lowstand tracts refocus attention on reservoirs that can be predicted to exist basinward of preexisting shelf edges. A basinward shift of exploration emphasis from incised valley-fill reservoirs to other lowstand elements--such as basin-floor fans, canyon and leveed-channel fills, and lowstand progradational deltaic wedges--could lead to plays where lenticular reservoir sandstones and marine-condensed source and seal shales exhibit the optimum conditions for pinch-out traps.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.