AL-SHAIEB, ZUHAIR, and JIM PUCKETTE
Oklahoma State University, Stillwater, OK

Abstract: Identification of Banded Seals and Compartments in Overpressured Gulf Coast Reservoirs

The Oligocene Lower Vicksburg "9900-ft" interval is a major overpressured gas-producing zone in TCB field, south Texas. This "shaly sand" is a low-resistivity/low-contrast (LR/LC) interval that was often overlooked as an exploration target since its log signatures indicated high water saturations. This interval consists of interbedded and interlaminated shallow marine sandstones and shales associated with a transgressive systems tract. The sandstones are of various lithologies, ranging from sublitharenite to lithic arkose with diverse mineralogical assemblages including quartz, feldspar, chert and volcanic rock fragments. The variety of pore geometries observed reflect the complex relationship between detrital grains, matrix and diagenetic overprints. With the introduction of new logging technology such as Formation Micro-Imaging (FMI), it is now possible to drastically improve interpretation of. LR/LC reservoirs. Detailed core and petrographic analyses were calibrated to the FMI logs. These thinly bedded sandstones, siltstones and shale (0. 15 to 1.2 m thick) exhibit a wide range of cementation patterns, porosities and permeabilities. The white zones on the FMI usually consist of calcite-cemented sandstone with low porosity (<10%) and permeability (av. 008 md). Yellow zones have the highest porosity (20%) and permeability (av. 14 md). Although the orange-colored zones have high porosity (>18%), their permeabilities are relatively low (av. 07 md) due to pore-partitioning authigenic clays. Finally, gray zones are composed of claystones or shales. The cemented white zones represent diagenetic bands with low permeability. These bands act as seals that inhibit vertical and lateral fluid flow. Cementation patterns, porosity and permeability changes show a distinct color pattern on FMI logs and may contribute to reservoir compartmentalization. Therefore, the 9900-ft interval consists of many stacked compartmentalized reservoirs that are separated by diagenetic seals.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas