AAPG Bulletin, V. 82 (1998), No. 8 (August 1998), P. 1551-1574.

Fault-Controlled Hydrocarbon Pathways in the Monterey Formation, California¹

Sneha K. Dholakia,² Atilla Aydin,³ David D. Pollard,³ and Mark D. Zoback⁴

©Copyright 1998.  The American Association of Petroleum Geologists.  All Rights Reserved

¹Manuscript received August 20, 1996; revised manuscript received September 2, 1997; final acceptance February 25, 1998.
²Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305-2115. Present address: Amoco Exploration and Production Company, 501 Westlake Park Boulevard, Houston, Texas 77079.
³Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305-2115.
⁴Department of Geophysics, Stanford University, Stanford, California 94305-2551.

This work is supported by Chevron USA, the Department of Energy contract DE-PS22-94BC1114973, the Stanford Rock Fracture Project, the Stanford Borehole Geophysics Laboratory, and the Phillips Fellowship. We thank Bruce Bilodeau of Chevron Production Company for subsurface data, discussion, and review critical for this study. We especially thank Alden Carpenter for an introduction in the field to the Monterey exposures along the coast and at Chico Martinez Creek. We thank Jim Eacmen, Tony Murer, Dalton Lockman, Scott Hornafius, and Steve Graham for showing us additional exposures and discussion. Thanks to Jeannie Barnett and Scott Johnson for data, discussion, and photographs. Colleen Barton at Stanford University, and Michael Ponek at Texaco Inc., assisted with borehole microresistivity image processing and image interpretation. Review by Steve Graham and discussion with Juliet Crider, Michele Cooke, Lisa Koenig, and Caroline Isaacs significantly improved the manuscript. We thank Carlos Fonseca-Rivera for his expertise in siliceous sediments in the field, Carl Twisselman for access to the Chico Martinez Creek field area, and A. J. Field and F. Campbell for their hospitality during field work in the coastal areas. We thank Rich Herrmann, formerly at Chevron and now at GeoGraphix Inc., for initiating, organizing, and supplying data at the start of this study, and Steve Smith for a smooth transition. We also wish to thank our fellow colleagues in the structural geology, active tectonics and geomechanics, and the borehole geophysical research groups for insightful discussion and support. Special thanks to David Campagna, Visiting Scholar at Stanford, for his helpful review of the manuscript and assistance with manuscript preparation. AAPG reviewers A. Brown, M. Clark, and B. Higgs provided insightful comments to improve the manuscript. 

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

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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.