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The AAPG/Datapages Combined Publications Database

GCAGS Transactions


Gulf Coast Association of Geological Societies Transactions
Vol. 40 (1990), Pages 39-49

Calculating Sandstone-Shale Ratios from Growth Normal Fault Dips on Seismic Profiles

R. E. Bischke (1), J. Suppe (1)


We present a method for calculating sandstone-shale ratios as a function of Previous HitdepthNext Hit utilizing dips of growth normal faults that exhibit small displacements on seismic sections. The basic source of information required is that the degree of compaction exhibited by growth normal faults is directly related to sandstone-shale ratios (Xiao and Suppe,1989). Our predictions based on seismic show a good agreement with sandstone-shale ratios observed in nearby wells. The accuracy of the method depends on the accuracy of the Previous HitvelocityNext Hit and Previous HitdepthNext Hit determinations; however our results show that even routine interval velocities give reasonable results. Furthermore, the resolution on sand-shale sections is limited to the resolution of the fault shape on seismic sections. Our study area is located along the Brazos Ridge offshore Texas. Growth normal faults in this region are at first listric in a shale section, but then become antilistric between about the 4500^prime to 7000^prime level. Previous HitCalculationsNext Hit of the sandstone-shale ratios from SP logs in nearby wells show that the sandstone-shale ratios average between 0.5 to 1.0 over this Previous HitdepthNext Hit interval. As compaction is dependent on solid volume, shale intervals, which have higher initial porosities, will compact more than sandstones which have lower initial porosities. Thus growth normal faults in shale horizons will flatten 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 antithetic growth faults by digitizing fault dips on seismic sections and then Previous HitdepthNext Hit correcting the fault dips. If an array of growth 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-engineers to improve the Previous HitcalculationsTop of potential reserves prior to drilling. The usefulness of the method increases as well control becomes limited and the distance between wells increases, and is particularly helpful in frontier exploration.

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