ABSTRACT: Sources of Calcite Cement Components in the Central San Joaquin Basin: Implications for Mass Transfer and Oil/water Interaction

BOLES, JAMES R.

Calcite cements in the central San Joaquin basin, including North and South Coles Levee, Paloma, Canal, Landslide, Yowlumne, San Emidio Nose, and Rio Viejo fields have formed between burial depths of about 1.5 kilometer to greater than 4 kilometers. Cement precipitation temperatures are estimated from cement volumes and oxygen isotopic compositions. The calcites typically have significantly lower {87/86}Sr than the depositional marine water, have progressively lower ratios with increasing temperature of crystallization and are sourced from plagioclase feldspar. Calcite cements formed at intermediate burial depths have carbon isotopic composition sourced in part from thermogenic-derived carbon and in some cases show marked fluctuation in carbon isotopic values, which has been interpreted as evidence for pulses of deep-basin fluid mixing with fluids at shallow levels.

During deep burial, carbon isotopic values of cements are, in many cases, near zero (PDB). Strontium isotopic values preclude dissolution of shell tests as the primary calcium source in these cements and suggests that carbon is also not derived from shell tests. The cement carbon may be indirectly sourced from organic acids in the formation waters, which may be related to oils in the reservoir. Recent studies of others suggest organic acids may have carbon isotopic compositions near zero, similar to that in the cements. Late cements appear to have formed in a relatively closed system, with cement components being derived locally from within the reservoir as a result of the presence of the hydrocarbons.

Search and Discovery Article #90945©1997 AAPG Pacific Section Meeting, Bakersfield, California