Preliminary report on petroleum source potential and thermal maturity of the Lospe Formation (lower Miocene) near Point Sal, onshore Santa Maria Basin, California

Abstract

The Lospe Formation is an 830-m-thick sequence of sedimentary and minor volcanic rocks at the base of the onshore Neogene Santa Maria basin of central California. Eighteen outcrop samples (14 from lacustrine and shallow-marine mudstones of the Lospe Formation, and 4 from bathyal marine shales of the overlying Point Sal Formation) were collected from a measured stratigraphic section at North Beach (informal name) near Point Sal, and analyzed using Rock-Eval pyrolysis and vitrinite reflectance. The Rock-Eval data indicate that mudstones of the Lospe are low in organic carbon (range 0.18 to 0.80 weight percent, mean about 0.35 percent), and therefore are generally poor potential source rocks of petroleum. In contrast, shales of the Point Sal Formation exhibit much higher total organic carbon (range 1.47 to 3.63 weight percent, mean about 2.4 percent), and therefore are good to very good potential source rocks. These results should be regarded as preliminary because only a small number of samples were analyzed, and because interpretation of the data is complicated by weathering effects, relatively high thermal maturity, and evidence of migrated bitumen in some samples.Vitrinite reflectance values (range 0.68 to 1.56 percent RQ, mean 1.29 percent RO) and calculated maximum burial temperatures (range 106 °C to 192 °C, mean 172 °C) of the Lospe and Point Sal Formations in the North Beach section are the highest ever reported for Neogene rocks of the onshore Santa Maria basin. These high values can be explained by a combination of burial heating plus a local heat source such as a nearby gabbro sill and (or) a high-temperature hydrothermal system. The local heating event is poorly dated but probably late early or early middle Miocene, and may have stimulated thermal generation of oil and gas from organic-rich strata of the Point Sal Formation.1 Above base of Lospe Formation in the measured stratigraphic section at North Beach. Numerous normal(?) faults of unknown displacement probably have removed parts of the section (Johnson and Stanley, 1994).