Recognition of Fifth-Order Cycles in a Biodestratified Shelf Sandstone Parasequence: Olmos Sandstone, South Texas: ABSTRACT

Abstract

This investigation of the Olmos Sandstone (Upper Cretaceous), A.W.P. field, McMullen County, Texas, demonstrates the utility of using genetic units and sequence stratigraphy to discern the depositional architecture of these enigmatic sand bodies. The Olmos represents a single parasequence within a transgressive systems tract. The parasequence is dated as Maastrichtian UC 15, based on planktonic foraminifera. A transgressive surface marked by submarine erosion caps the Olmos. The overlying glauconitic shale and marl is a regional marker horizon recognizable on seismic profiles and wireline logs and represents the rapid vertical deepening of facies. The Olmos is interpreted as a low-relief sand shoal that accumulated on the middle to outer shelf under low-energy conditions and slow rates of deposition. Internally, the Olmos parasequence is comprised of eight subtle cleaning-upward genetic subunits 10-20 ft thick which formed in response to incremental shallowing/deepening events associated with fifth-order sea level cycles. These in turn constitute the overall shoaling upward Olmos parasequence. Each subunit is comprised of lithofacies A (a relatively clean biodestratified reservoir-quality sandstone with a trace fossil assemblage of robust U-shaped and vertical burrows), lithofacies B (a shaly, biodestratified nonreservoir sandstone and siltstone with small, predominantly horizontal burrows); or a stacked A/B sequence. Subunitmore » contacts are gradational and burrowed; the upper contact is sharp and burrowed. The subunits correlate for 10 to 15 mi in both strike and dip directions except where shaled out or truncated by postdepositional erosion. These depositional concepts were used in conjunction with structure mapping and seismic inversion techniques to drill a successful outpost to A.W.P. field.« less