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Abstract

Grainstone deposits are important reservoirs in the Rodessa limestones (Lower Cretaceous) in the East Texas basin. Three conventional cores from producing wells in West Purt and Bois D’Arc fields located in Anderson County, Texas, illustrate facies and porosity types typifying the grainstones. These cores from the Gloyd “A” (middle) productive zone demonstrate that even in a relatively small geographic area, reservoir grainstones may differ widely in grain types, depositional environments, diagenetic history, and reservoir properties.

One core from Bois D’Arc field contains well-sorted, bedded oolite grainstone overlain by less sorted, coarser ooid-skeletal grainstone. A second core above the ooid-skeletal grainstone zone consists of an upward coarsening foram-skeletal grainstone. Environmental interpretation suggests that the Gloyd “A” zone in Bois D’Arc field was deposited as a barrier-bar beach and shoaling complex on the flank of what is interpreted from additional data as a “turtle structure.”

Facies identified in the West Purt field core, in depositional order are: 1) a set of cycles containing fine-grained mollusk-echinoid grainstone and packstone overlain by coral-stromatoporoid rudstone and 2) a capping facies of fine to coarse-grained, mollusk-peloid grainstone. Environmental interpretation of West Purt field indicates that facies in the Gloyd “A” zone were deposited in a skeletal sand shoal off the face of a biological buildup, and that the initial topography which controlled the deposition of these sediments was the result of a salt pillow structure.

All of the grainstones found in these reservoirs share three common physical characteristics: 1) initial good primary interparticle porosity; 2) development of secondary moldic porosity; and 3) compaction. The secondary porosity development and extent of compaction vary widely between facies. The effects of both early and late diagenetic processes have greatly altered the original reservoir characteristics of these grainstones.

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