ABSTRACT

As part of an effort funded by the Gas Research Institute and the U. S. Department of Energy, and with the cooperation of Shell Oil Company, sandstones in the Vicksburg "S" (Oligocene) reservoir were cored in the McAllen Ranch gas field in the A. A. McAllen B-18 well. Detailed correlation of the cores with petrophysical data illustrates the controls of depositional features and diagenesis on reservoir quality.

The cores consist of prodelta and delta-front facies. Distal delta-front deposits are shales, siltstones containing climbing ripples, and siltstones showing evidence of slumping and liquefaction. Burrows are relatively rare; plant debris is common.

The mid-delta-front deposits coarsen upward overall and consist predominantly of thin, 3-inch- to 5-ft-thick (7.5-cm- to 1.6-m-) upward-fining sequences that have abrupt, scoured bases. Cross-stratification, evident in oriented core and dipmeter logs, indicates northwesterly dispersal. Microresistivity curves from the high-resolution dipmeter log match individual upward-fining sequences within the core.

The proximal delta-front deposits are upward-coarsening intervals interpreted to be either delta mouth-bar crest or similar upper-shoreface deposits. Poorly defined laminae are the most common sedimentary structure. In one core, trough cross-strata and subhorizontal laminae alternate. Rare scours and rip-up clast beds indicate that periodic storms or floods influenced deposition. Northerly sediment dispersal is indicated by the dips of trough cross-strata in dipmeter and oriented core.

Permeability and porosity generally increase as grain size increases and are greatest in 1- to 2-ft-thick (0.3-0.6 m) zones within massive and laminated beds in the uppermost delta front. Porous intervals increase in abundance upward within the delta-front sandstones. Permeability variation of more than 2 orders of magnitude within the reservoir sandstones corresponds to diagenetic facies within the core. High permeability occurs only within thin bands. Trough cross-stratified sandstone is commonly porous only near the tops of the foresets.

Differences in the character of the microresistivity curve of the high-resolution dipmeter log correlate with differences in cementation and with different depositional facies within the cores. Comparison of microresistivity logs and cores allows extrapolation of facies and cement characteristics and resulting reservoir properties to uncored intervals.