The thermal and cementation histories of a sandstone petroleum reservoir, Elk Hills, California: Part 1: 40Ar/39Ar thermal history results

Recent developments in K-feldspar 40Ar/39Ar thermochronometry have provided new tools to deal with interpretive impediments that previously limited its application to investigating sedimentary basin thermal histories. These new methodologies were applied to detrital K-feldspars recovered at various depths from a deep well drilled through a carbonate-cemented sandstone petroleum reservoir in the southern San Joaquin Valley, California. Thermal histories obtained from both thermochronological analysis and burial history constraints were used to constrain a conductive heat flow model of basin evolution. The basin appears to have experienced a broadly linear heating history due to burial throughout the early and middle Miocene followed by a significant increase in heating rate between 9 and 6 Ma. In a companion paper [Mahon, K.I., Harrison, T.M., McKeegan, K.D., 1998. The thermal and cementation histories of a sandstone petroleum reservoir, Elk Hills, California. Part 2: In situ oxygen and carbon isotopic results. Chem. Geol. 152, 257–271], we apply the thermal model to predict the temperature evolution of the much shallower late Miocene Stevens sands, a prolific petroleum producer. Ratios of excess 40Ar to chlorine calculated from the differential release of radiogenic and nucleogenic argon during isothermal heating steps indicate a uniform value of 5.4±0.9×10−5 over the 2.5 km vertical distance separating the samples from well 934-29R suggesting that the basin receives a high basal flux of radiogenic argon.