ABSTRACT: Controls on Porosity Evolution in Devonian Cherts of West Texas

Susan D. Hovorka, Stephen C. Ruppel

Porous chert reservoirs in the Devonian Thirtyone Formation constitute a major hydrocarbon play in west Texas. Cumulative oil production from these reservoirs totals approximately 700 million bbl. As much as 500 million bbl of mobile oil will remain in these high-porosity, low-permeability reservoirs at abandonment given current development strategies.

Moldic porosity and intergranular microporosity in the Thirtyone chert originated during diagenetic transformation of biogenic opal to quartz. Molds of opal spicules measure 10s of microns in diameter and are lined and partly filled with 2-10-µm, euhedral, microquartz crystals and chalcedony cements. Intergranular micropores fractions of microns in diameter are developed between loosely packed, 0.5-µm microquartz ellipsoids that compose the matrix between the spicules.

The distribution of these pore types is a function of depositional processes. Porosity formation and modification are related to the development of major hiatuses in the Middle Devonian and Pennsylvanian. Formation of porous chert is interpreted as the result of processes that enhanced the rate of conversion of biogenic opal to quartz, such as the introduction of meteoric water and the addition of Mg to pore waters because of mineralogic stabilization of magnesian calcite beneath the MIddle Devonian unconformity. Porosity was modified during Pennsylvanian uplift and deformation by fracturing, calcite cement precipitation, and karstification of carbonate and by the concurrent enhancement of porosity in chert. Porosity development and hydrocarbon recovery efficiency in chert reservoirs re variable depending on original texture and type of diagenetic overprint.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990