Diagenetic History, Porosity Development, and Permeability Pathways in Mississippian Greenbrier Oolites of West Virginia

Abstract

In the Rhodell gas field of southern West Virginia, grainstones of the union and Pickaway oolite members have initial potentials that exceed 2.0 MMCF GPD and reserves of up to 1 bcf per well, Thirty-two wells in the field are productive from the Greenbrier. Porosity is fair with a mean value of 9.5%, and permeability is variable but generally poor, averaging only 0.15 md. The production rate for these wells is directly related to the oolites' diagenetic history. Pores are intragranular and 1-5 {mu}m in size. Ooids underwent recrystallization during meteoric diagenesis, and they now consist of aphanocrystalline calcite. Micropores with relatively large throats and numerous interconnections exist between the very fine calcite rhombs. The pores are arranged in alternating laminae of the ooids' cortex, hinting that the ooids were originally bimineralic. Intergranular pore space has been largely occluded by successive generations of cement, including isopachous calcite rims that precipitated in the marine environment. Following mineral stabilization, some grainstones were subjected to chemical compaction, which produced a fitted fabric. The compacted limestones possess large contact areas between neighboring ooids and, consequently, and enhanced permeability and exceptional production. In contrast, limestones with an early marine cement had the strength to resistmore » compaction, and the porous ooids have small point-to-point contacts. Because the pathways for gas migration are narrow, tortuous, and discontinuous, permeability in these rocks is low. The best reservoirs, therefore, are ooid grainstones that escaped early marine cementation but underwent meteoric diagenesis and chemical compaction.« less