Chapter 6 Geochemistry of Los Angeles Basin Oil and Gas Systems
The Los Angeles basin is one of the most prolific petroleum- producing provinces in the world. The basin has produced in excess of 6 billion barrels of oil (GBO) and over 7 trillion cubic feet (tcf) of gas and includes one of the worlds largest single accumulations, the Wilmington field. Oil gravities are highly variable, ranging from less than 10° API in shallow producing zones to condensate (>50° API) in a few deep fields. However, much of the oil produced in the basin is rather heavy 25° API) with an appreciable sulfur content(>1%).
The Los Angeles basin contains abundant organic-rich source rocks containing kerogen rich in sapropelic material. Maturity estimates of the source rocks based on vitrinite reflectance values are low. This appears to be related to the sapropel-rich kerogen, which may generate oil at lower maturities than is conventionally accepted or may cause suppression of vitrinite reflectivity in the kerogen. Maturity estimates based on bitumen production indicate that upper Miocene rocks in deeper parts of the basin are the source of the oil and gas accumulations. Vertical migration of this oil and gas into shallower reservoirs is a feature of many fields, especially the giant Wilmington and Huntington Beach fields.
Oil quality within the basin varies geographically, with higher quality (high API gravity, low sulfur content) oils in the northeast of the basin and lower quality oils in the west and south of the basin. The variation in oil quality does not appear to be caused by differences in kerogen type, because the sterane biomarker ratios and carbon isotope ratios of the oils are very similar and are consistent with a marine-derived kerogen as the source. Ratios of biomarkers and lower molecular weight hydrocarbons suggest that the superior quality of the oil in the northeast of the basin may be caused by higher maturity, greater migration, a more oxidizing depositional environment, or a combination of all three. Oils appear to be genetically related to the associated gases in a reservoir, except for some cases of localized mixing of gases with microbial methane.
Depth-related variations in oil quality and in carbon dioxide content in the associated gas appear to be caused by biodegra- dation. A suite of oils and associated gases from the Salt Lake area in the northwestern basin shows that a variety of interrelated physical and chemical changes in many shallow oil and gas deposits results from microbial oxidation of liquid and gaseous hydrocar- bons to carbon dioxide. This is an important process in the giant Wilmington and Huntington Beach fields, where deep reservoirs contain a medium quality oil; whereas shallower reservoirs contain a genetically similar oil that has been microbially transformed to a lower gravity, higher sulfur crude.