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Character and Importance of Pliocene Tectonic Events to Petroleum Accumulation in California

James K. Crouch

It is widely recognized that petroleum within California's Neogene basins comes chiefly from organic-rich Miocene strata, such as the Monterey Formation. Somewhat overlooked, but equally important in terms of petroleum exploration, are subsequent Pliocene tectonic events that provided the majority of structural traps and many of the reservoirs in which the petroleum has accumulated.

Throughout the Miocene, California was characterized by deep and relatively broad basins, in which pelagic-rich sediments were deposited. During the Pliocene, the style of both the structural development and sedimentation of these Neogene basins changed dramatically. Initially, the Miocene basins that had apparently formed and persisted as a result of extension were compressed into a series of smaller, more restricted subbasins. These subbasins were then rapidly filled with terrigenous-rich sediments shed from flanks uplifted along thrust and reverse faults. In general, this thick uppermost Neogene basin-fill sequence is characterized by deep-water turbidites overlain by shallow and nonmarine deposits. These deposits served to cap and bury the underlying organic-rich Miocene sections nd to provide overlying reservoir sections. Flanking thrust and reverse faults formed extensive fault-parallel folds, repeated reservoir and source-rock sections, and accentuated the fracturing of siliceous and dolomitic Monterey reservoirs.

This change, from extension to compression, appears to be related to a change in Pacific-North American relative plate motions. Beginning about 5-6 Ma, the movement of the Pacific plate relative to North America changed to a more northerly direction, which produced convergence across the margin. Surprisingly, this component of convergence appears to be taken up by compressive strain oriented perpendicular to the San Andreas fault. As a result, Pliocene and younger folds and faults are largely parallel with, rather than en echelon to, the San Andreas fault. Hence, even though they formed during a transform tectonic regime, principal Neogene petroleum producing structures, such as the Point Arguello field, are more closely related in structural style to fold and thrust belts than to wre ch tectonics.

AAPG Search and Discovery Article #91035©1988 AAPG-SEPM-SEG Pacific Sections and SPWLA Annual Convention, Santa Barbara, California, 17-19 April 1988.