Using Presence of Calcite Cap Rock in Shales to Predict Occurrence of Reservoirs Composed of Leached Secondary Porosity in the Geopressured Zone

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Kaiser, W. R.
Magara, Kinji
Milliken, Kitty L.
Richmann, D. L.

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The distribution of high-resistivity shale in the Frio Formation between hydropressured and geopressured strata has been mapped along the Texas Gulf Coast. Two high-resistivity intervals more than 1,000 ft thick have been mapped, one in Brazoria and Galveston Counties and the other in Kenedy County. They coincide with Frio delta systems and may be related to extraordinary quantities of CO2 produced by deltaic sediments rich in woody and herbaceous matter. Beyond being calcareous, the nature of the high-resistivity interval is enigmatic, and its relationship to deep secondary porosity is problematic.

Most of the contained carbonate is microscopically and isotopically skeletal in origin, revealing no evidence of diagenetic modification. Minor rhombs of iron-bearing carbonate tens of microns in size were identified. Detrital feldspar compositions are being established to test subsequent changes in feldspar composition resulting from progressive burial and albitization. Hydrolysis reactions for authigenic minerals and reactions between key pairs of minerals have been written. Thermodynamic functions for complex phyllosilicates at temperatures up to 200°C have been calculated. From thermodynamic calculations, it was predicted that ferroan calcite would be the favored authigenic carbonate in shales.


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