Pacific Coastal and Marine Science Center
Natural Oil & Gas Seeps in California
Saturated versus Aromatic Hydrocarbons
Isotopic Composition of Methane
The two upper plots show the interpreted source rock ages for selected oil samples collected from this quadrangle. Data from seeps are shown on the left, data from wells on the right. The chemistry is plotted in terms of the carbon isotopic compostion (δ13C) of saturated and aromatic hydrocarbons. Units are parts per thousand (‰). Samples that plot in or adjacent to the red polygon are considered to be from Miocene source rocks; those plotting in or adjacent to the green polygon, Cretaceous; and those plotting in or adjacent to the blue polygon, Eocene. The age interpretations are based on the chemistry of oils derived from source rocks of known age. "No Data" indicates that no oil from this quadrangle was analyzed.
The two lower plots show the interpreted origin of gas samples collected from this quadrangle. Data from seeps are shown on the left, data from wells on the right. The chemistry of the gas is plotted in terms of its methane constituents: isotopes of carbon (δ13C) and hydrogen (deuterium, δD). Depending on where the sample data plots, the origin of the gas is interpreted to be thermogenic (from burial and heating of hydrocarbon source rocks), bacterial (from activities associated with bacteria), geothermal (related to hot springs), or some combination of those sources. "No Data" indicates that no gas from this quadrangle was analyzed.