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Structural Block Theory for Origin of Oil-Producing Trends Within Los Angeles Basin

Louis A. Orlowski

Oil entrapment within the Los Angeles basin is confined to several unique structural trends which developed in a recurring pattern across the basin. These trends are the result of the interaction between eleven discrete fault-bounded basement blocks which controlled the structural evolution of the basin.

Block tectonics occurred primarily under a transpressional stress regime of oblique convergence and vertical offset that resulted in discrete patterns of internal and external deformation to the blocks. Oblique convergence and vertical downthrow resulted in the internal deformation of the block and the superjacent sedimentary section into an en echelon pattern of structures concentrated within the block. Convergence and vertical upthrow resulted in the external deformation of the block and the superjacent section which is concentrated along the block margins, with little internal deformation, along which a pattern of en echelon structures was also produced. The magnitude of the structures formed is directly attributable to the degree of oblique convergence between adjacent blocks.

The onset of oil generation and migration occurred early in the development of the basin while it was still evolving structurally. As a result, structural traps predominate along the distinct trends which were developing concurrently. All of the major trends and fields in the Los Angeles basin have been found. The remaining unexplored areas lack sufficient development of similar structural trends and adequate seals to have entrapped economic quantities of oil. Intense urban development and continued environmental concerns will preclude further geophysical and geological exploration of the deeper untested portions of the basin.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.