Generation and migration of petroleum in the Miocene Monterey Formation, southern San Joaquin Basin, California

Evaluation of petroleum generation in the Monterey Formation by conventional maturity indicators is difficult due to the compositional characteristics of Monterey kerogen. Kerogen in the Monterey mainly consists of amorphous material which is thought to be derived from marine algae. Maturity indicators based on the fluorescence spectra of algal matter and bitumen have been established and used to identify the maturation stage of kerogen and to determine the origin of associated bitumens in the Monterey. For the Monterey Formation in the southern San Joaquin basin, the threshold of petroleum generation is at 9000 ft (2743 m), or ;Kerogen lamellae, stylolites formed by pressure solution, and microfractures induced by the volume expansion during petroleum generation are pathways for primary migration of petroleum, in the Monterey Formation. Pressure solution can concentrate organic matter along stylolites. The relationship between stylolites and tectonic structures, which can be incorporated into Anderson's theory of faulting, is used as a framework to discuss the direction of primary migration. In the southern San Joaquin basin, horizontal and vertical stylolites provide avenues for petroleum migration in siliceous shales. In the Monterey Formation, kerogen lamellae and stylolites are well developed in siliceous shales and horizontal microfractures are common in siliciclastic shales. Pervasive horizontal microfractures, which are only observed in siliciclastic shales, may be induced by bedding slip and/or petroleum generation. Siliceous shales seem to have a poorer sealing capacity and a higher efficiency of oil expulsion than siliciclastic shales.;Oil migration in the Monterey Formation is probably mainly in a separate oil-phase or single-phase fluid. Water released by silica phase transformation, or water expelled during pressure solution, seems to be unimportant in oil migration.