ABSTRACT: Basin Evolution Along Active Transform Margin: Los Angeles Basin

Kevin T. Biddle

The Los Angeles basin, for its size, is the richest hydrocarbon-producing basin in the world, with estimated ultimate reserves of 10.4 billion oil-equivalent bbl. In addition, the basin occupies a key position in the tectonics and stratigraphy of California. An understanding of this basin's evolution is important to those involved in oil and gas exploration and to those interested in the development of transform plate boundaries.

The present, structurally defined basin has formed over the last 3 m.y. by shortening and limited strike-slip deformation. The early history of the basin, however, is one of strike-slip associated extension. Extension began in the Los Angeles basin area in the early to middle Miocene. The basin itself opened in the late Miocene and early Pliocene by extension along the Santa Monica, Palos Verdes, and Whittier fault systems.

Rapid late Miocene and Pliocene subsidence formed a deep basin with water depths reaching 2 km or more. This deep basin was the depositional site of three major submarine fan systems and of the Monterey Formation and its equivalents. Most of the oil in the Los Angeles basin is produced from the sandstones and conglomerates of these submarine fans. The Monterey Formation and its equivalents are locally rich in sapropelic kerogen and are the source of the basin's oil.

Oil quality varies both geographically and with depth throughout the basin. Higher quality oils (higher API gravity, low sulfur content) characterize the northeastern part of the basin; lower quality oils characterize the western and southern parts of the basin. Sterane biomarker and carbon isotope data suggest that these variations are not caused by changes in kerogen type. Different degrees of maturity, longer migration pathways, and differing amounts of kerogen oxidation during deposition are possible explanations for the variability of oil quality. Depth-related changes in oil quality and in CO₂ content of associated gases can be caused by biodegradation.

Each phase of basin evolution has had an effect on the hydrocarbon systems of the basin. The early extensional/strike-slip phases created space for deposition and formed initial trap geometries. The later shortening phase aided maturation by increased burial, but distorted rather than enhanced many traps.

AAPG Search and Discovery Article #91001©1989-1990 AAPG Distinguished Lecture Tours 1989-1990