Clay Mineralogy and Porosity Estimates of the Lower Permian Wolfcamp Shale

Diagenetic controls (pressure, temperature, and time) altered petrophysical properties in black shale petroleum source rocks. Several samples of the lower Permian Wolfcamp Shale, a prominent tight gas oil play from the mid-continent US, were collected from a range of thermal maturity values (0.8-1.07% VRo) to test this hypothesized relation among the formation of diagenetic illite, over-pressuring, and porosity generation. Microporosity was observed between clay aggregates, organic material, and cementing materials. Moldic porosity was seen within POM. Microporosity measured 5.5% for the most thermally mature sample and 7.1% for the least mature sample. Based on preliminary measurements on partially reduced small angle scattering data (SAXS), packets of particles 30 Å, consistent with fundamental particles were visible. These preliminary results suggest that both micro- and possibly nano-porosity decreased with thermal maturity. Lithification, diagenetic mineral formation/pore fill, as well as organic matter migration, have been attributed to disaggregating porosity at increased thermal maturities