Abstract: Controls on the Sulphur Content of Source Rocks and Oils as Assessed from Biomarkers

Alan F. M. Hill

To assess the factors controlling the sulphur levels within oil, a number of source shales from Great Britain and the North Sea were analyzed in terms of their biomarkers and other geochemical parameters.

A high-sulphur oil (associated with H₂S) was found to be sourced from a mature oil (Ro = approx. 0.95). The depositional environment was highly anoxic with abundant C35 homohopanes, bisnorhopane and trisnorhopane whilst limited fluvial influence was inferred from low pristane/phytane ratios (<0.85). Thermal maturity affected many biomarkers but a low abundance of diterpanes and high C27/C29 sterane ratios suggest a labile organic matrix. These factors, combined with a high, uniform degree of pyritisation (>0.96) indicate deposition within a euxinic basin. Under such conditions the organic matter becomes saturated with sulphur (kerogen Carbon/Sulphur = approx. 20) even though the abundance of organic matter is relatively low (<6%) and iron is abundant. The most abun ant sulphur aromatic compounds were the methyldibenzothiophenes. Absence of alkylated thiophenes and benzothiophenes can be attributed to maturation, and it is the thermal alteration and destruction of these compounds that gave rise to the H₂S.

In non-euxinic environments, increasing abundances of organic matter enable sulphate reduction to be sustained for longer periods. This caused an increase in the relative amount of organic sulphur until a point of saturation was attained which is dependent on anoxia and the reactivity of the organic matrix. Such saturation was only attained at TOC>12% and was paralleled by a levelling off in the abundance of most organosulphur compounds (relative to total aromatics).

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994