Diagenesis of the Monterey Formation includes the opal-A → opal-CT → microcrystalline-quartz transformation, dolomitization, and in some places the formation of brittle, vitreous cherts. These diagenetic transformations produce a more brittle rock. Diagenesis is thus considered a major control on the fracture potential of Monterey rocks. Once formed, fractures become excellent fluid migration paths. Fluid migration along fractures creates vuggy porosity adjacent to the fractures and may promote the generation of secondary matrix porosity, primarily in carbonate-rich lithologies.
The Monterey is characterized by high matrix porosity but very low matrix permeability. The vast quantities of hydrocarbons produced from this formation require production from matrix porosity in addition to production from fracture porosity.
A model is suggested in which matrix-bound hydrocarbons migrate into fractures that intersect high porosity-low permeability beds. These fractures either intersect other fractures, the borehole, or stratigraphic breccias; any or all of these conditions should enhance productivity. This model diminishes the problems associated with fluids migrating long distances through a low-permeability matrix. Implications of the model include the following: (1) fractured carbonate-rich rocks with matrix porosity should yield the best production; (2) chert-rich lithologies with no matrix porosity will produce primarily from fractures; (3) high-porosity shales with few fractures are candidates for hydrofracture treatments to enhance productivity.