Abstract: Rock-Log Evaluation of Fluvial-lacustrine Seals

W. T. Shea, J.R. Schwalbach, D. M. Allard

We have evaluated seal potential in a sand-rich package of fluvio-lacustrine sediments using capillary pressure curves, petrographic and XRD analyses, and log data from selected wells. Traps hold maximum oil columns of a few hundred feet, and early exploration suggested column heights might be limited by capacities of relatively inefficient, silt-rich capillary seals. However, our results show that most seal rocks have pore systems capable of trapping very large (>1000 ft) columns, and suggest that hydrocarbon distributions are more likely controlled by fault-plane leak points or lateral discontinuities in effective seal packages.

Seal-prone intervals consist of two principal depositional mudrock facies: (1) mottled, gray-to-red mudstones (interpreted as floodplain or floodplain lake deposits) and (2) gray, wavy-to-planar laminated silty mudstones (interpreted as either shallow lacustrine or overbank deposits). Entry pressures of most samples, as derived from mercury injection capillary pressure (MICP) tests, exceed 1500 psi (air/Hg). For typical basin column heights (0-500 ft), significant hydrocarbon flow is only expected through pore throats of the "cleanest" (<30% clay) laminated siltstones, which have Hg/air entry pressures of 500 psi or less. Stain distributions in cores are in qualitative agreement with the MICP results. However, interpretation of MICP data for smectite-bearing samples is complicated y the presence of pervasive fine-scale fractures.

Sample-to-log calibration was carried out using discriminant analysis techniques. The facies model successfully differentiates floodplain from lacustrine and overbank facies. We have used the results to predict seal-rock stacking patterns in uncored wells, and to assess lateral facies variations within chronostratigraphic packages.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994