Monterey Fractured Reservoir, Santa Barbara Channel, California: ABSTRACT

The South Elwood field in the Santa Barbara Channel is a faulted anticline with cumulative production of 14.5 million bbl from the Monterey Formation as of September 1, 1982. The distributions of pressure, flow rates, and oil-water contacts and the low average matrix permeability of 0.2 md require a fractured reservoir. Within the field, the relation of oil productivity with respect to typical geological parameters of structure or gross lithology is random. However, the present study reveals that the best production is obtained from the wells that penetrate the complete Monterey section, are extensively perforated, and are deviated strongly along structural strike. Observed variations in production as a function of borehole geometry can be quantitatively related to the ge metry of fracturing and brecciation. Core and outcrop studies show a dominant fracture set characterized by vertical, lithologically controlled fractures oriented across strike, and breccias controlled by lithology and structure. Generally, the fracture intensity is unaffected by structural position or bed curvature but is controlled by lithology and bed thickness. Chert, porcelanite, and carbonate rocks are the most intensely fractured lithologies, but the widest fracture openings occur in carbonates. Other varieties of fracturing in the Monterey are related to a protracted history of diagenesis, deformation, and fluid injection. Three types of tar-bearing breccias occur in the Monterey Formation: stratigraphic breccia, coalescent-fracture breccia, and fault-related breccia. Stratigraphic breccias in porcelanite and chert are attributed to volume decrease owing to silica diagenesis. Coalescent-fracture breccias occur where tar intrudes fractures that lie at high angles to bedding, disaggregating rock adjacent to the fracture. Fault-related breccias commonly are found in conjunction with large-displacement normal faults and rotational strike-slip faults. Formation of breccias probably involves high End_Page 421------------------------------ pore pressures. Because of their polygenetic origin, breccia masses have diverse orientations paralleling bedding or fracture/fault systems. In conclusion, fracturing and brecciation of the Monterey Formation reflect the interplay between processes of diagenesis, deformation, and fluid dynamics. The most important features of the reservoir in the area of the present study are: (1) vertical fractures oriented normal to the structural trends and inferred to be favorably oriented (to remain open) with respect to the regional minimum horizontal stress; and (2) breccias that are both stratigraphically and structurally controlled and inferred to be related to the interaction of rock stress and fluid dynamics. End_of_Article - Last_Page 422------------