MATURATION OF ORGANIC MATTER IN THE PALEOCENE-EOCENE WILCOX GROUP, SOUTH TEXAS: RELATIONSHIP TO CLAY DIAGENESIS AND SANDSTONE CEMENTATION

A number of interactions probably take place between organic matter and mineral matter during the burial and diagenesis of clastic sediments. These may exert a critical influence upon the maturation of organic matter and the migration and accumulation of oil and natural gas. Improved understanding in this area would contribute to the development of rational hydrocarbon exploration strategies.;The samples average 1% TOC, possess type III kerogen, and generally contain less than 150 mg/gOC bitumen. Over the interval sampled, the estimated production potential for gas and light hydrocarbons with the bulk formula "CH(,2)" is 280 mg/gOC. As much as 150 mg/gOC CO(,2) may be liberated by Wilcox organic matter during diagenesis to present-day temperatures of 100(DEGREES)C (0.59% R(,0)). In comparison, the main zone of oil generation occurs between 95(DEGREES)C (0.55% R(,0)) and 125(DEGREES)C (0.82% R(,0)). The timing of these processes suggests that CO(,2) could play an important role in creating secondary sandstone porosity for hydrocarbon migration. As determined from (delta)('13)C measurements, roughly 25% of the carbonate cement present in Wilcox sandstones is derived from the decomposition and diagenesis of organic matter.;Mathematical modelling indicates that giant (> 1 TCF) gas and condensate fields can be produced by primary migration of light hydrocarbons in aqueous solution. Because illitization occurs substantially earlier than the main zone of oil generation, smectite accounts for less than 25% of the total water involved.;A variety of reactions may be responsible for deep-seated production of H(,2)S and CO(,2) gases. These include thermochemical sulfate and ferric iron reduction by organic matter, the possible reaction of organic matter with pyrite, disporportionation of kerogen in the presence of water to form CH(,4) and CO(,2), and reverse-weathering reactions.;To investigate this topic, 33 mudstone core and cuttings samples ranging in depth from 1.6 km to 4.7 km and in temperature from 80(DEGREES)C to 210(DEGREES)C were obtained from wells in South Texas. Bitumen analyses, kerogen analyses, and closed-system sediment pyrolysis experiments were conducted and the results were integrated with available data on clay mineralogy and sandstone cement.