ABSTRACT

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 one such reservoir, where an ooid grainstone shoal developed over a bathymetric high formed by a Louann Salt swell. Four depositional environments were present in the vicinity of the shoal. They are: 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. Skeletal pellet packstones were deposited in the open marine environment. Three shoaling-upward cycles were identified in the upper Smackover at Eustace Field; shoaling cycles ended during minor regressions. Each younger cycle was more widespread geographically than its predecessor.

The burial diagenetic history can be traced from early to late by the occurrences of the following events (from youngest to oldest): micritization, formation of nodular anhydrite, isopachous rim cementation, dolomitization, formation of moldic porosity, coarse sparry calcite cementation, stylolitization, saddle dolomite cementation, oil migration, quartz overgrowth formation, replacement by anhydrite, and late leaching.

Two porosity types are present at Eustace Field: intercrystalline (dolomite) and oomoldic. Dolomitization occurred early and led to the development of extensive porosity. Dolomitization was facies selective for ooid pellet grainstones and packstones of the back shoal environment. Oomoldic porosity, which occurred during or slightly later than dolomitization, is not as extensively developed. Events which have reduced porosity and permeability include saddle dolomite pore-filling cementation, sparry calcite cementation, and emplacement of pore-filling dead oil.