Microporosity in Ooids: Mesozoic and Palaeozoic of Texas, USA

Porosity within micritized ooids may exceed 15 percent with permeabilities up to about 1 millidarcy. The most porous and permeable micritized ooids all exhibit a characteristic texture: they consist of euhedral calcite rhombs 1 to 5 microns in longest dimension. Massive Cotton Valley gas wells interpreted to produce from this type of microporosity may initially yield as much as 300,000 cubic feet of gas per day or more. Intraooid porosity may constitute a significant reservoir for gas, but not for oil. Micritized ooids from 14 samples ranging in age from Ordovician .to Jurassic exhibit a common fabric and may have been altered by a common process. Micritization of these ooids may have occurred during mineralogical stabilization from aragonite to calcite. The micritized texture of the ooids described in this paper is not a product of leaching, for this type of micritized ooid is common in strata unaffected by leaching. Micritization of ooids may be of great significance to the development of gas fields in grainy rocks which have not been dolomitized or leached. Micritized ooids may also be important in many oil reservoirs with binodal pore sizes.