The thermal and cementation histories of a sandstone petroleum reservoir, Elk Hills, California: Part 2: in situ oxygen and carbon isotopic results

Liquid hydrocarbon accumulations within the Elk Hills and North Coles Levee oil fields, southern San Joaquin basin, are largely isolated within calcite-cemented reservoirs comprised of late Miocene Stevens sandstone. We undertook ion microprobe carbon and oxygen isotope ratio measurements on calcite cements to assess both the source of the carbon and the temperature of cementation. By combining thermal history results from 40Ar/39Ar analyses (reported in the companion study) with calculated cementation temperatures based on our oxygen isotope measurements, a model cementation history is derived which indicates that carbonate precipitation occurred primarily between 4 and 6.5 Ma. Conventional oxygen isotopic measurements yield a more restricted range of isotopic compositions reflecting the averaging properties of that method. The associated carbon isotopic measurements suggest that most of the early cements were derived from a marine carbonate source or a mixture of marine carbonate and lighter carbon from maturing hydrocarbons. Carbonates precipitated most recently (and thus at the highest temperatures) contain light carbon, interpreted to result from thermal decomposition of kerogen in the interbedded shales. Based on the light carbon values (<−10‰PDB) and low range of temperatures over which the bulk of the cement formed, the maturation of petroleum in the interbedded shales likely postdates cementation.