Lower Wilcox Shelf Edge in Texas--Relation to Growth Faulting and Geothermal Reservoir Quality: ABSTRACT

Abstract

Most geopressured sandstone reservoirs in the lower Wilcox (Eocene) of Texas occur along a narrow trend associated with the ancient shelf margin. Traps for geopressured fluids were created by early, rapid growth of down-to-the-basin faults as part of a large-scale instability of the continental slope. Basinward translation and rotation of upper-slope fault blocks over a decollement zone of geopressured shale (south Texas) or salt (east Texas) initiated fault movement near the shelf break. After the shelf edge had prograded farther basinward, continued movement of some of these faults at a much reduced rate created normally-pressured traps in post-lower Wilcox formations. In east Texas, the shelf-edge structural style has been overprinted by growth of salt dome. Faults tha originate near the restricts the volume of potential geothermal reservoirs. Lower Wilcox deposition was dominated by the Rockdale delta system in east Texas, similar in scale to delta system of the Quaternary Mississippi depocenter. Sand distribution reached its maximum extent when deltas prograded to the shelf edge. The thickest geopressured sands occur in De Witt County at the southern edge of the Rockdale system. Maximum permeabilities occur in distributary sandstones; these are laterally continuous with extensive delta-front sandstones of lower permeability. A proposed test-well site for the Cuero geothermal prospect in De Witt County has been located to intersect the greatest total thickness of distributary sandstones. End_of_Article - Last_Page 804------------