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 wit hin calcite-cemented reservoirs comprised of late Miocene Stevens sand stone. We undertook ion microprobe carbon and oxygen isotope ratio mea surements on calcite cements to assess both the source of the carbon a nd the temperature of cementation. By combining thermal history result s from Ar-40/Ar-39 analyses (reported in the companion study) with cal culated cementation temperatures based on our oxygen isotope measureme nts, a model cementation history is derived which indicates that carbo nate precipitation occurred primarily between 4 and 6.5 Ma. Convention al oxygen isotopic measurements yield a more restricted range of isoto pic compositions reflecting the averaging properties of that method. T he associated carbon isotopic measurements suggest that most of the ea rly cements were derived from a marine carbonate source or a mixture o f marine carbonate and lighter carbon from maturing hydrocarbons. Carb onates precipitated most recently (and thus at the highest temperature s) contain light carbon, interpreted to result from thermal decomposit ion of kerogen in the interbedded shales. Based on the light carbon va lues (< - 10%(PDB)) and low range of temperatures over which the bulk of the cement formed, the maturation of petroleum in the interbedded s hales likely postdates cementation. (C) 1998 Elsevier Science B.V. All rights reserved.