Well log reversals and pressure data in Cretaceous shales of the northern Green River Basin, Wyoming
Margarita Zyrianova and Philip Nelson
Well logs, seismic data, mud weights, and drillstem test pressures were examined in a 70-mile long study area centered along the Sandy Bend arch in the northern Green River Basin. The northwest part of the study area crosses the uplifted La Barge platform and the southeast part includes the deeper basin. This study focused on the well logs and pressure data within the Hilliard-Baxter shale interval, a marine mudrock sequence 3,000 to 4,000 ft thick. In the deeper part of the study area, the top of the Hilliard-Baxter shale interval is 12,000 ft deep, is located below the 180°F isotherm, and has reached a thermal maturity level of about 0.9 % Ro, according to a burial history model. Resistivity and sonic velocity increase from the surface to the base of the Mesaverde Group. Below this depth the resistivity and sonic velocity trends reverse, becoming less resistive and slower velocity with depth within the Hilliard-Baxter shale interval. Mud weights and drillstem test pressures were converted to pressure-depth ratios (psi/ft) and displayed on a cross section with the top of the well log reversals. Pressure-depth ratios exceed 0.5 psi/ft near the base of the Hilliard-Baxter shale interval as well as within the underlying Frontier Formation and Mowry Shale. In the middle and upper parts of the Hilliard-Baxter shale interval and overlying Mesaverde, Fort Union, and Wasatch Formations, pressure-depth values range from 0.43 to 0.49 psi/ft, indicating normal pressure. In conclusion, the resistivity and sonic logs indicate an abnormal pressure condition whereas the mud weights and drillstem tests indicate a normal pressure condition. The cause of the disparity could be due to one or a combination of the following: 1) failure to detect true pore pressure, 2) lithologic-related effects, 3) presence of gas, and/or 4) a well log signature of early (now dissipated) overpressure. Although no definitive conclusion has been reached regarding the disparity, the pros and cons of these four possible causes are discussed in the poster that is being exhibited.
AAPG Search and Discovery Article #90156©2012 AAPG Rocky Mountain Section Meeting, Grand Junction, Colorado, 9-12 September 2012