CLAY MINERALOGY AND POROSITY ESTIMATES OF THE DEVONIAN WOODFORD AND LOWER PERMIAN WOLFCAMPIAN SHALE

Samples of the Woodford Shale in the Anadarko Basin (OK) and the Wolfcamp Shale in the Midland Basin (TX) were examined to assess porosity development at different scales and related to thermal maturity. Techniques used included x-ray diffraction, petrography of thin sections, scanning electron microscopy, and small-angle scattering (SAXS)/ultra-small angle scattering (USAXS). The samples were collected both from core and outcrop that ranged from 0.55 to 1.5% VRo. The mineralogy of the clay fractions in both units was composed of illite (I-S with high % of illite layers), smectite, quartz, kaolinite, and small amounts of albite. Authigenic chlorite was found in some of the Wolfcamp samples.

The Woodford contains meso-scale porosity which occurs within pyrite framboids, vugs in fractures and allochems, and between clay sheets. The Woodford contains minerals in fractures interpreted as hydrothermal in origin, including witherite, norsethite, magnesite, saddle dolomite, gorceixite, potassium feldspar, chalcopyrite, and sphalerite. Porosity was destroyed by the fluids which precipitated minerals in fractures and migrated into the matrix. The presence of hydrothermal minerals suggests the Woodford was an open system at some time in its diagenetic history.

In the Wolfcamp Shale, early diagenetic phases including calcite/phosphate concretions, sphalerite, barite, quartz, dolomite, and ferroan dolomite destroyed primary porosity. Porosity is present in pyrite framboids and between sheets of chlorite in mudstones. Intercrystalline and moldic porosity is present in dolomitized intervals. Mineralized fractures are abundant, contain hydrocarbons, calcite, and celestine-barite, and some contain porosity. Fluid inclusion analysis suggests that mineralized fractures were conduits for warm, high salinity brines that were externally derived, suggesting the shale was an open system during part of its burial history. Preliminary SAXS and USAXS analysis of Wolfcamp with the highest thermal maturity (%VRo = 1.07) showed a single mode of the microporosity distribution rather than the expected biomodal distribution of microporosity. Porosity is present at different scales in the units and comparison of the results are currently underway to assess the role of maturity in porosity development.