Preliminary assessment of high-resistivity cap-rock shale in the Frio Formation of the Texas Gulf Coast. Annual report

Abstract

Mapping of high resistivity cap rock shales in the Frio Formation of the Texas Gulf Coast shows that few areas of thin cap rock occur in the upper Texas Gulf Coast, and more extensive, thicker cap rock occurs in the lower Texas Gulf Coast. Increases in (1) maximum shale resistivity, (2) unstable minerals (volcanic rock fragments, detrital carbonate grains), and (3) authigenic cementation parallel the increase in cap rock from the upper to the lower Gulf Coast. Similarity in cap rock distribution in two major Frio deltaic depocenters is not evident. Facies analysis of regional cross sections in the lower Texas Gulf Coast and of cross sections in Sarita East field, Kenedy County, shows preferential development of cap rock in the delta-front/slope facies of the Norias delta system. Sand content of the cap rock interval varies from 23 to 41 percent in part of Sarita East field, suggesting that if cap rock is due to authigenic cementation, such sands may act as fluid conduits during mineralization. Cap rock is rarely developed in the shale-rich prodelta and distal delta-front facies. High resistivity cap rock shales have been considered a result of authigenic calcite cementation, but definite evidence for this origin is more » lacking. Preliminary mineralogic analyses of well cuttings have not yielded satisfactory results. Analysis of core through cap rock and non-cap rock intervals will be required to determine the mineralogic variability within each interval and to accurately assess any mineralogic control of the high resistivity log response. « less