Stratigraphic Controls of Oil Fields in Los Angeles Basin--A Guide to Migration History
Robert S. Yeats
Unmetamorphosed strata of Turonian to middle Miocene age exposed around 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 adjacent Richfield-Coyote Hills trend, and the Santa Monica Mountains were at the distal end of Mohnian turbidites derived from farther north. The present LAB formed after the Mohnian (after 6 Ma), and the central trough filled with "Demontian" and Repettian turbidites shallowing upsection to younger Pliocene and Pleistocene deposits. Miocene organic deposits 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 may have been breached by erosion. High productivity of the LAB results from (1) preservation of Miocene organic source beds, (2) preservation of thick accumulations of reservoir-quality coarse-grained sandstones derived from nearby granitoid highlands, alternating with hemipelagic mudstones inhibiting vertical migration, (3) deep burial of source beds and thermally generated expulsion of oil to san stone reservoirs formed at the same time, and (4) only moderate post-basin deformation, preserving most early formed oil fields.