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ABSTRACT: Calculating Sandstone-Shale Ratios from Shapes of Growth Normal Faults

R. E. Bischke, John Suppe

We present a method for calculating sandstone-shale ratios and depth to reservoir utilizing dips of growth normal faults that exhibit small displacements on seismic sections. Comparison of our theoretical results to sample wells indicates a close correspondence between observed sandstone-shale ratios and the ratios predicted by theory. As compaction is dependent on solid volume, shale intervals that have higher initial porosities will compact more than sandstones that have lower initial porosities. Thus, growth normal faults in shale horizons will compact more than growth normal faults in sand horizons, resulting in lower fault dips in shale intervals relative to sand intervals. This observation is used to calculate sandstone-shale ratios from fine arrays of small antithe ic growth faults by digitizing fault dips on seismic sections and then depth correcting the fault dips. Our study area is located along the Brazos Ridge offshore Texas, where we have ten high-quality seismic lines. Growth normal faults in this region are at first listric in the shale section, but then become antilistric between about the 4000 to 6000 ft level. Calculations of the sandstone-shale ratios from SP logs in nearby wells show that the sandstone-shale ratios average between 0.5 and 1.0 over this depth interval. If an array of normal faults exist in an area, then 3-D sandstone-shale ratio maps can be constructed to better select well locations. Also this information can be utilized by geologists and engineers to improve the calculations of potential reserves prior to drilling. Th usefulness of the method increases as well control becomes limited and the distance between wells increases.

AAPG Search and Discovery Article #90999©1990 GCAGS and Gulf Coast Section SEPM Meeting, Lafayette, Louisiana, October 17-19, 1990