FAULT-CONTROLLED HYDROCARBON PATHWAYS IN THE MONTEREY FORMATION, CALIFORNIA

Field studies of low-permeability siliceous shale units of the Montere y Formation in the southern San Joaquin Valley and coastal California show evidence for fault control on hydrocarbon transport important for both migration and production. Shearing along preexisting discontinui ties, such as bedding planes and joints, locally increases permeabilit y in the sheared zone and surrounding fractured rock. As the rock is s ubjected to shear, it begins to systematically fragment and subsequent ly to brecciate, thereby creating interconnected voids for hydrocarbon transport. Petroleum-filled breccia zones are exposed in the Antelope Shale at Chico Martinet Creek on the northeastern flank of the Temblo r Range near McKittrick, California, Breccia zones are found predomina ntly parallel to bedding in porcelanite units (4-10 cm thick) and are bounded above and below by siliceous shale beds (2-20 cm thick). Spaci ng between breccia zones is a function of lithology and diagenesis. Th is section of the Antelope Shale exposure originated as alternating la yers of pure and terrigenous-rich diatomaceous sediment, in which thes e compositional variations influence the postdiagenetic material prope rties. Terrigenous-rich diatomite diagenetically alters to an incompet ent siliceous shale (opal CT), whereas the cleaner sediments alter to a more brittle porcelanite (opal CT). Secondary fractures, or splay cr acks, localize in the more brittle porcelanite in response to shearing along both bed-parallel lithologic discontinuities and bed-parallel f ractures,With increased shearing, the fractured porcelanite subsequent ly evolves into brecciated fault zones. In the Chico Martinet Creek ou tcrop, individual breccia zones combine to make a petroleum-filled com pound breccia zone approximately 10 m thick in which the original zone s are partially obliterated by subsequent deformation. This outcrop-ba sed conceptual model for the development of hydrocarbon pathways in th e Monterey Formation is applied to the subsurface using formation micr oscanner (FMS) data and core. Bed-parallel breccia zones are identifie d in the Antelope Shale at Buena Vista Hills oil field, In the borehol e image, the brecciated fault zone appears as unorganized patches of h igh and low resistivity with hints of bedding. At least one breccia zo ne identified in the borehole image correlates directly to hydrocarbon production as indicated by spinner flow-meter data. Although core rec overy from fractured or brecciated zones is typically poor, there appe ars to be an association between fractures related to shearing process es and hydrocarbon occurrence in cores examined for this study. Oil-st ained and brecciated fracture zones associated with slip exist in Buen a Vista Hills and other nearby fields producing from the Antelope Shal e. Our multidisciplinary study, involving both geologic field data and borehole geophysical data, on the Monterey Formation reveals a critic al relationship in which brittle fault zones provide permeable conduit s for hydrocarbon transport and production.