A Preliminary Assessment of High Resistivity Cap Rock Shale in the Frio Formation of the Texas Gulf Coast

The mapping of high resistivity cap rock shales in the Frio Formation along the Texas Gulf Coast reveals variations in thickness and distribution between the upper and lower regions. Thinner cap rock is observed in the upper Texas Gulf Coast, while thicker and more extensive cap rock is present in the lower Texas Gulf Coast. This variation correlates with increases in maximum shale resistivity, the presence of unstable minerals such as volcanic rock fragments and detrital carbonate grains, and authigenic cementation, all of which are more pronounced in areas with thicker cap rock in the lower Gulf Coast. However, the distribution of cap rock in two major Frio deltaic depocenters does not show clear similarity.

Facies analysis of regional cross sections in the lower Texas Gulf Coast, particularly in the Sarita East field of Kenedy County, indicates the preferential development of cap rock in the delta-front/slope facies of the Norias delta system. The sand content within the cap rock interval varies, suggesting that sands within the cap rock may serve as fluid conduits during mineralization processes. Cap rock is less commonly developed in the shale-rich prodelta and distal delta-front facies.

While high resistivity cap rock shales have been attributed to authigenic calcite cementation, conclusive evidence for this origin is lacking. Preliminary mineralogic analyses of well cuttings have not provided satisfactory results, highlighting the need for core analysis through cap rock and non-cap rock intervals to determine mineralogic variability and accurately assess any mineralogic control over the high resistivity log response. Further investigation is necessary to better understand the origin and characteristics of high resistivity cap rock shales in the Frio Formation.