Chappel (Mississippian) Biohermal Reservoirs in Hardeman Basin, Texas: ABSTRACT

Abstract

Carbonate mud mounds with flanking crinoidal sands are commonly associated with oil production in the Hardeman basin. These buildups are similar to the Waulsortian facies in Europe and to the Mississippian bioherms in the Sacramento Mountains of New Mexico. Cores and logs from Sun Oil Co.'s Quanah field provided subsurface control, enabling us to delineate six principal lithofacies that portray the depositional history of the Quanah buildup. The facies, in ascending order, are: (1) the mudstone core facies, (2) the skeletal wackestone upper core facies, (3) the skeletal veneer facies, (4) the flanking facies, (5) the intermound facies, and (6) the oolite shoal facies. The facies relations indicate an overall shoaling-upward sequence with two regressive-transgressive cycle superimposed. Early regression subjected parts of the core facies to subaerial exposure. Early dolomitization and solution brecciation resulted in intercrystalline and vuggy porosity. Subsequent marine sedimentation blanketed the karst surface. The dolomitized, vuggy core facies was subjected to several episodes of fracturing in the subsurface. One early fracture set was partly filled with large saddle dolomite crystals. Equivalent limestone strata were not comparably fractured and do not contain the saddle dolomite. The Quanah biohermal reservoir exhibits three kinds of porosity: moldic-vuggy, intercrystalline, and fractures. However, the porous zones are only marginally productive unless they are fractured because the vuggy porosity has low permeability. End_of_Article - Last_Page 1423------------