Chapter 7 Stratigraphic Controls of Oil Fields in the Los Angeles Basin: A Guide to Migration History
Unmetamorphosed strata of Turonian to middle Miocene age exposed around the margins of the Los Angeles basin (LAB) predate the formation of the basin and respond to a different structural framework. Highly organic middle and late Miocene (Luisian and Mohnian) strata also predate the present LAB framework: the Puente Hills received a thicker sequence than did the area southwest of the Whittier fault, and the Santa Monica Mountains were at the distal end of Mohnian turbidites derived from farther north. The central trough became a major depocenter during deposition of the upper Mohnian (after 8 Ma), and it achieved its present northwest-southeast trend about 4 Ma. The central trough filled with “Delmontian” and Repettian turbidites shallowing upsection to Pliocene and Pleistocene deposits. Miocene and early Pliocene source rocks were buried beneath the oil-generating thermal threshold so that oil migrated to stratigraphic and broad structural traps formed during deposition. Many oil fields contain a thick stack of reservoir turbidites; the boundary between highly productive turbidites and overlying water-bearing turbidites, also in trapping position, is abrupt. Late Quaternary deformation after basin filling distorted rather than enhanced oil traps, and some oil accumulations were breached by erosion. New oil prospects could result from a better understanding of (1) the Mohnian basin framework in contrast to the far different post- Mohnian framework and (2) the post-Mohnian fold and thrust belt tectonicaUy loaded by the southern margin of the Transverse Ranges onshore and offshore.
Figures & Tables
“The most distinctive characteristic of the Los Angeles basin“The most distinctive characteristic of the Los Angeles basin is its structural relief and complexity in relation to its age and size” (Yerkes et aI., 1965, p. AI6); however, its very complexity caused no small amount of discussion in designing and naming this volume of the AAPG World Petroleum Basin Memoirs. (See the Foreword for a discussion of the scope of these memoirs.) The series coordinators decided early that the Los Angeles basin should be included in the World Petroleum Basins project because of its interesting geology and importance as a hydrocarbon producer. Initially, the Los Angeles basin was considered for a convergent-margin volume, presumably in recognition of the late-stage shortening that has taken place in the Los Angeles region of southern California. There is little doubt, however, that the Los Angeles basin has formed and deformed within the evolving San Andreas transform system (Atwater, 1970, 1989; Campbell and Yerkes, 1976; Blake et al., 1978; Engebretson et al., 1985; Wright, this volume). There is also little doubt among those who have worked in the area that the initial subsidence of the Neogene Los Angeles basin was caused by extension (Yeats, 1968; Crowell, 1974, 1976, 1987; Wright, this volume). The series coordinators decided, therefore, that to portray the Los Angeles basin as a model for basins formed in convergent-margin settings would be misleading.
The title of this volume, Active Margin Basins, is a compromise, but, like many compromises, this title falls short of completely describing its subject