Diagenesis and Migration of Hydrocarbons in the Monterey Formation, Pismo Syncline, California

Abstract Kerogen in the Miocene Monterey Formation in the Pismo syncline consists of amorphous material of algal origin. Most of the rocks presently contain 1-5 wt. % total organic carbon. Thermal alteration index determinations and pyrolysis (thermal extract) data suggest that the organic material at the center of the fold is mature, whereas the organic material on the limbs of the fold is immature. The Monterey Formation in this region entered the liquid hydrocarbon window at a present day depth of approximately 1700 meters. The liquid window appears to be coincident with the opal CT to quartz reaction (80 ± 10°C). Hydrocarbons were expelled from the Monterey rocks deep in the center of the fold and began migrating as a result of microfracturing. The hydrocarbons migrated up into the southwest limb of the fold through macrofractures. This limb of the fold is characterized by brittle dolomitic and opal CT rich rocks that were intensely fractured prior to hydrocarbon migration. The potential reservoirs on this limb of the fold are in fractured Monterey. In contrast, on the northeast limb of the fold Monterey rocks consist of silty and sandy siliceous rocks that tend to be more resistant to fracturing; hydrocarbons migrate up into this limb of the fold through relatively low angle faults and spread into structural traps along the fault in the adjacent Pliocene Pismo Formation through conjugate shears. The potential reservoirs on this limb are in structural traps in the Pismo Formation. A third potential hydrocarbon target is the basal sand in the Pismo Formation; almost everywhere in the fold this unit is highly bituminous.