Guaymas Basin in the central Gulf of California receives a high influx of sedimentary detritus, including immature organic matter derived from both marine and terrigenous sources, which constitutes approximately 2% of the sediment dry weight. In the shallow sections the organic matter undergoes normal diagenesis, but regional high heat flow from magmatic influences results in petroleum genesis at depth.

The effects of intrusive and advective thermal stress have been studied on a number of Pleistocene sediment samples obtained from Sites 477, 478, and 481 of Leg 64 of the DSDP-IPOD program in the Gulf. The molecular data show that lipids of the thermally unaltered samples are derived from microbial and terrestrial higher plant detritus. Samples near sills contain thermally generated distillates, whereas those closest to sills contain only traces of bitumen. Pyrolysis GC and GC-MS show that kerogens from the unaltered samples reflect their predominantly autochthonous microbial origin. Pyrograms of altered kerogens are much less complex than those from unaltered samples, confirming the thermal effects. Kerogens adjacent to sills produce little or no pyrolysis products because igneous in rusions into unconsolidated, wet sediments have caused in situ hydrous pyrolysis of the organic matter. Differences in maturity, based on stable carbon isotope composition and electron spin resonance data, between Sites 477 and 481 and Site 478 are attributed to dissimilarities in thermal stress and to chemical and isotopic heterogeneity of Guaymas Basin protokerogen.

Hydrothermal activity and seabed mounds have been further explored in Guaymas Basin by piston coring, dredging, and diving with the D.S.V. Alvin. The immature sedimentary organic matter is easily converted to petroleum under the hydrothermal regimes. These

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petroleums are relatively mature and migrate as fluids and by diffusion to the seabed. The fluid migration is aided by near-critical aqueous solution and supercritical hydrocarbons (such as CH₄) and carbon dioxide (minor component). Petroleum compositions vary from condensates to naphthenic to waxy, all with significant amounts of asphaltenes and hydrothermal products such as sulfur and nitrogen compounds and polynuclear aromatic hydrocarbons (PAH). The heavy ends condense at the seabed, where they are deposited mainly as a cement with sulfides and other minerals, and to a lesser extent as entrapped oil and crystalline wax in vugs and conduits in the mounds. The volatile components are vented into the ambient seawater and rise as plumes. The PAH, which are high-temperature esynthesis-aromatization products from residual organic matter, are present in all oils, but are also deposited as discrete trace fractions in the hottest regions of the vent systems. Preliminary estimates of total hydrocarbon generation during hydrothermal alteration indicate that this process could result in significant petroleum accumulations if adequate traps and seals are available.