Abstract

Field studies of low-permeability siliceous shale units of the Monterey 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 discontinuities, such as bedding planes and joints, locally increases permeability in the sheared zone and surrounding fractured rock. As the rock is subjected to shear, it begins to systematically fragment and subsequently to brecciate, thereby creating interconnected voids for hydrocarbon transport. Petroleum-filled breccia zones are exposed in the Antelope Shale at Chico Martinez Creek on the northeastern flank of the Temblor Range near McKittrick, California. Breccia zones are found predominantly parallel to bedding in porcelanite units (4-10 cm thick) and are bounded above and below by siliceous shale beds (2-20 cm thick). Spacing between breccia zones is a function of lithology and diagenesis. This section of the Antelope Shale exposure originated as alternating layers of pure and terrigenous-rich diatomaceous sediment, in which these compositional variations influence the postdiagenetic material properties. Terrigenous-rich diatomite diagenetically alters to an incompetent siliceous shale (opal CT), whereas the cleaner sediments alter to a more brittle porcelanite (opal CT). Secondary fractures, or splay cracks, localize in the more brittle porcelanite in response to shearing along both bed-parallel lithologic discontinuities and bed-parallel fractures. With increased shearing, the fractured porcelanite subsequently evolves into brecciated fault zones. In the Chico Martinez Creek outcrop, individual breccia zones combine to make a petroleum-filled compound breccia zone approximately 10 m thick in which the original zones are partially obliterated by subsequent deformation. This outcrop-based conceptual model for the development of hydrocarbon pathways in the Monterey Formation is applied to the subsurface using formation microscanner (FMS) data and core. Bed-parallel breccia zones are identified in the Antelope Shale at Buena Vista Hills oil field. In the borehole image, the brecciated fault zone appears as unorganized patches of high and low resistivity with hints of bedding. At least one breccia zone identified in the borehole image correlates directly to hydrocarbon production as indicated by spinner flow-meter data. Although core recovery from fractured or brecciated zones is typically poor, there appears to be an association between fractures related to shearing processes and hydrocarbon occurrence in cores examined for this study. Oil-stained and brecciated fracture zones associated with slip exist in Buena Vista Hills and other nearby fields producing from the Antelope Shale. Our multidisciplinary study, involving both geologic field data and borehole geophysical data, on the Monterey Formation reveals a critical relationship in which brittle fault zones provide permeable conduits for hydrocarbon transport and production.

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