Print this pageFracture Permeability and Cement Distributions in Anomalous Pressured Gas Reservoirs
DUNN, THOMAS L., JOHN D. HUMPHREYS, and RONALD C. SURDAM
Thin sections of natural fractures from cores of the Almond Formation, Washakie Basin, Wyoming were examined for cementation history and to estimate in situ permeability for the open fractures. The core characterization included close examination of the fracture surfaces, the cements present, and measurements of in situ fracture aperture. The measurements were made using scanning electron microscopy on pressure-impregnated polished thin sections prepared at insitu effective stress conditions. Open fractures are always confined to smaller apertures, averaging roughly 100 microns. Fractures that were completely mineralized occurred over a greater range of larger apertures. The cements found are quartz, kaolinite, dolomite, barite and calcite. Their distribution and geochemical modeling suggest their formation was controlled by lithology, extent of burial, involvement of organic-inorganic reactions, and pressure.
Late forming calcite fracture fill indicates the pressure compartment was breached at Standard Draw gas field. The calcite precipitated after a sequence of mineralization interpreted to be the result of progressive burial diagenesis. Geochemical modeling indicates that in the center of the Washakie Basin, the peak gas generation and formation of anomalous pressure was initiated 42 to 35 m.y.o. This overlaps with the formation of the Wamsutter Arch and related fracturing. Recurrent opening of relatively early-formed fractures associated with the Arch allowed for the progressive burial diagenetic sequence observed and the breach-related calcite mineralization. The mineral morphology suggests compartment breach was a gradual leakage rather than a catastrophic rupture.