A predictive model of diagenesis, involving hydrocarbon migration, for the Monterey Formation in the Pismo syncline, Coast Ranges of California, includes (1) diagenetic history, (2) lithofacies relations, and (3) tectonic setting. The diagenetic reactions control mineralogy,

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mass transfer, and provide the hydrodynamics for hydrocarbon migration. Facies control the distribution of significant chemical components, including hydrocarbon sources. The tectonic setting controls fracture porosity, thermal gradients, and timing of hydrocarbon migration.

The important diagenetic reactions are interpreted on the basis of five phases: (1) silica, (2) carbonate, (3) clay, (4) organics, and (5) seawater. From the reactions characterizing these five phases, paragenetic sequences can be derived for each of the four solid components (opal, carbonate, clay, and organics) in the presence of seawater as temperature increases. Integration of paragenetic sequences results in an interpretive model of the diagenetic history of the Monterey Formation.

Three significant Monterey facies are: (1) sandstone-diatomite, (2) diatomite, and (3) carbonate-diatomite. The carbonate-diatomite facies in the Monterey has outstanding source rock potential; some of the mud and siltstone contain 18 wt. % organic carbon.

Hydrocarbons evolve when source rocks are buried to or below 6,500 ft (1,981 m). The hydrocarbon migration path is controlled by fracture porosity along active tectonic zones, particularly near fault systems at the margin of the fold (basin). The reaction, organic matter ^rarr hydrocarbons, provides pressure buildup and hydrocarbon expulsion, whereas the reactions, opal A ^rarr opal CT, and opal CT ^rarr quartz, provide the fluid drive for the hydrocarbon migration.

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