Development and Distribution of Porosity and Permeability in a Distal Chert Reservoir, Devonian Thirtyone Formation, Midland Basin, Upton County, West Texas

Robert Glaser
University of Louisiana, Department of Geology Lafayette, Louisiana. United States of America [email protected]

The Bloxom and Blue Danube fields produce from the Lower Devonian Thirtyone Formation. Reservoir quality rock from these fields consist of porous chert and partially silicified ostracod grainstones and packstones possessing spicule-moldic, fractured, channel, intercrystalline, and intergranular porosity. Prior to diagenesis, these units were spicultic grainstones and ostracod grainstones with minor quantities of monaxon and triaxon sponge spicules. These sediments were deposited as grain flows and turbidites between the proximal slope and distal slope/basin floor interface.
The development and distribution of porosity and permeability of reservoirs within these fields were controlled by the environment of deposition, depositional geometry, and diagenetic processes. Using two 90-foot full diameter cores, thirty-six thin sections, core measurements, and wireline logs, some of the factors affecting the development and distribution of permeability and porosity were determined.
Reservoirs exhibit a degree of laterally continuity due to their apron-like depositional geometry. Within reservoirs, porosity is primarily spicule-moldic, a direct result of silica dissolution and re-mobilization while laminations and microstylolites serve as both permeability barriers and pathways.

AAPG Search and Discovery Article #90070 © 2007 AAPG Foundation Grants in Aid