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Thirty-three mudstone core and cuttings samples ranging in depth of 1.6-4.7 km (5,200-15,400 ft) and in temperature from 80° to 210°C (175-410°F) were obtained from wells in south Texas. The results of closed-system pyrolyses and bitumen and kerogen analyses are related to available data on clay mineralogy and sandstone cement. This study examines the diagenesis of Wilcox organic matter, the migration of hydrocarbons, and the importance of organically derived CO2 in sandstone cementation.
The samples average 1% TOC, contain type III kerogen, and generally show bitumen contents less than or equal to about 150 mg/gC. Modeling indicates that primary migration of methane and light hydrocarbons in aqueous solution is capable of producing giant (>= 1 tcf) gas and condensate fields. Because hydrocarbon generation does not occur until after the main stage of illitization, smectite accounts for less than 25% of the total water involved.
Pyrolysis experiments indicate that as much as 150 mg/gC CO2 may be liberated by Wilcox organic matter during diagenesis to present-day temperatures of 100°C (212°F). The main zone of oil generation occurs at subsurface temperatures of 95°-125°C (203°-257°F). The timing of these processes suggests that CO2 could play an important role in creating secondary sandstone porosity for hydrocarbon migration. The ^dgr13C values indicate that 25% of the carbonate cement present in Wilcox sandstones may originate from decomposition and diagenesis of organic matter.
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