Relationships Among Secondary Porosity, Pore-Fluid Chemistry and Carbon Dioxide, Texas Gulf Coast
Stephen G. Franks, Richard W. Forester, 1984. "Relationships Among Secondary Porosity, Pore-Fluid Chemistry and Carbon Dioxide, Texas Gulf Coast", Clastic Diagenesis, David A. McDonald, Ronald C. Surdam
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Sequences of diagenetic minerals associated with secondary porosity show striking similarities. The formation of quartz overgrowths on detrital quartz grains is generally followed by carbonate cementation. The dissolution of this carbonate is the main secondary porosity-forming event, which commonly precedes kaolinite precipitation and iron-rich carbonate cementation. In the Texas Gulf Coast, oxygen isotopic data provide temperature estimates of authigenic phases that predate and postdate secondary porosity development: quartz, ⩾ 80° C; kaolinite, ⩾ 70° C; albite, 100–150° C; late carbonate, > 100° C. These data suggest that secondary porosity in the Tertiary Gulf Coast forms at temperatures of about 100 ± 25° C.
Correlations among calcite saturation indices in pore fluids, abnormally high permeabilities, and mole percent CO2 in natural gases of the Eocene Wilcox Group imply a strong interrelationship between carbon dioxide and secondary porosity development in clastic reservoirs. The CO2 content of gases varies systematically with both the reservoir age and temperature, which suggests a kinetic control on generation. The amount of CO2 in natural gases increases rapidly at approximately 100° C; this coincides with a rapid increase in the ratio of secondary to total porosity in associated sandstones. Stable isotopic analyses of carbonate cements indicate a strong component of organically derived carbon and therefore cycling of carbon between inorganic and organic systems. The type, amount and distribution of organic matter, and early carbonate in both shales and sandstones control the quantity of CO2 available for generating secondary porosity.
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Clastic diagenesis has evolved from a very descriptive science to a much more process-oriented study. This evolution has been driven by the realization that many hydrocarbon reservoirs have significant diagenetic compotents directly affecting porosity and permeability characteristics. The prediction in time and space of reservoir characteristics affected by diagenesis can greatly reduce the risk in the search for hydrocarbon accumulations, particularly in subtle targets lacking pronounced structural expression. This publication contains three sections designed to increase understanding in the processes controlling clastic diagenesis: Conepts and Principles; Aspects of Porosity Modification; and Applications of Clastic Diagenesis in Exploration and Production. The first two sections deal with processes controlling various aspects of clastic diagenesis, and the third section applies these principles and observations to specific examples. Altogether, the three sections contain 22 chapters.