Depositional Environments, Diagenetic History, and Reservoir Characteristics of Upper Smackover at Eustace Field, Henderson County, Texas

Jose J. Sequeira, Jr., Wayne M. Ahr

Hydrocarbon reservoirs associated with ooid shoals are common in the downdip Smackover of the East Texas basin. Eustace field, located within the salt anticlinal trend of the East Texas basin, is such a reservoir.

Three shoaling-upward cycles were identified in the upper Smackover at Eustace field. Shoaling cycles ended during minor regressive stages. Each successive cycle was more widespread than the previous. Four depositional environments were defined: (1) shoal crest, (2) shoal flank, (3) back shoal, and (4) open marine. The shoal crest and flank environments are represented by ooid grainstones and ooid pisolite grainstones, respectively. Ooid-pellet grainstones and packstones were deposited in the back-shoal environment, whereas skeletal-pellet packstones were deposited in the open marine environment.

The burial diagenetic history can be traced from early to late by the occurrences of the following events: micritization, formation of nodular anhydrite, isopachous rim cement, seepage reflux dolomitization, selective leaching, coarse sparry calcite cement, stylolites, saddle dolomite, pyrobitumen, quartz overgrowths, replacive anhydrite, and late leaching.

Two porosity types are present at Eustace field: intercrystalline dolomite and oomoldic. Dolomitization occurred early and is the principal reason for the development of reservoir-quality porosity and permeability; it was selective for ooid-pellet grainstones and packstones of the back-shoal environment. Oomoldic porosity, which formed concurrently or slightly later than dolomitization, is not as extensively developed. Several agencies have reduced porosities and permeabilities in the field; they include saddle dolomite cement, sparry calcite cement, and pore-filling pyrobitumen (dead oil).

AAPG Search and Discovery Article #91042©1987 GCAGS and GC-SEPM Section Meeting, San Antonio, Texas, October 28-31, 1987.