Diagenesis and Hydrocarbon Generation in the Monterey Formation, Huasna Basin, California

Abstract The Huasna Basin, and adjacent basins developed along the San Luis Obispo transform, a major branch of the San Andreas fault system. The cessation of Gorda Plate subduction and the initiation of transform fault motion between the Pacific and North American plates at the triple junction greatly influenced the thermal history of coastal California. As a consequence of this tectonic framework, abnormally high heat flow persisted for several million years in the Huasna Basin, creating a thermal regime capable of producing hydrocarbons. The diagenesis and hydrocarbon generation in the Monterey Formation in the context of this tectonic and thermal history are the subject of this paper. Four main diagenetic relationships are observed: 1) the transformation of biogenic opal to quartz has occurred in the basin center, but not at the margins; 2) mixed layer expandable clays decrease with increasing depth of burial; 3) authogenic kaolinite and migrated hydrocarbons have nearly a 1:1 correlation; 4) feldspar grains are etched at all depths in the subsurface, whereas the feldspar in surface sections shows intense dissolution. Conventional organic geochemical techniques such as pyrolysis, thermal alteration indices and elemental analysis indicate immature to low levels of thermal maturity. However, thermal models developed for the basin demonstrate that the Monterey Formation has been in the liquid window for several million years. This apparent irreconcilable difference can be explained if either 1) the early hydrocarbons generated are mainly long chained (“heavy”) hydrocarbons and/or 2) the light hydrocarbon fraction has escaped. Asphaltic seeps at the margins of the basin and oil shows encountered during drilling suggest that hydrocarbons have been and probably still are being generated in the Huasna Basin.