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Geological Society, London, Special Publications; 1987; v. 26; p. 217-228;
DOI: 10.1144/GSL.SP.1987.026.01.14
© 1987 Geological Society of London

Part II: Depositional Processes and Environments

Hydrothermal petroleum from diatomites in the Gulf of California

B. R. T. Simoneit & O. E. Kawka

School of Oceanography, Oregon State University, Corvallis, Oregon 97331 U.S.A.

The plate boundary zone within Guaymas Basin, Central Gulf of California, is a set of spreading axes offset by a central transform fault and covered by a thick sedimentary sequence. The high heat flow and concurrent hydrothermal circulation cause extensive thermal alteration and leaching of the organic and mineral matter covering the rift graben. The rapid vertical migration of the leach and subsequent quenching by colder seawater results in the precipitation of hydrothermal deposits rich in sulphides and organic pyrolysates at the sediment surface. Deposits from the northeastern half of the Basin’s Southern Trough have been sampled using the D.S.R.V. Alvin, to examine the organic condensate character and regional patterns, from which fractionation processes during hydrothermal fluid migration may be inferred.

With the characterization of the bitumen, three major patterns are resolved:

(1) Hydrocarbons with n-alkane predominance and no carbon number preference are the most common pattern;
(2) branched and/or unsaturated hydrocarbon predominance relative to n-alkanes mainly within the kerosene-diesel range;
(3) biodegraded material, dominated by a naphthenic hump, with a minor contribution of resolved compounds. Samples with predominant n-alkanes are further defined by their molecular weight range into distinct subgroups. Waxy condensates versus kerosene-range n-alkanes are found at different localities and may reflect regional differences in hydrothermal fluid exit temperatures.

It is postulated that variable pyrolytic temperatures and selective fractionation of pyrolysates during migration, with differential condensation and precipitation under varied thermal regimes, induce the observed general patterns. In addition, post-depositional processes such as biodegradation and removal of low molecular weight components by water solubilization can affect the bitumen character.