SHORT-CHAIN (C-21 AND C-22) DIASTERANES IN PETROLEUM AND SOURCE ROCKSAS INDICATORS OF MATURITY AND DEPOSITIONAL ENVIRONMENT

Geochemical analysis of steranes in source rocks from the Western Cana da Basin has allowed the identification of several C-21 and C-22 short -chain sterane isomers. The principal compounds in carbonates from the U. Jurassic Nordegg Formation have been identified as diginane and ho modiginane, more thermodynamically-stable isomers of the compounds pre gnane and homopregnane that have 5 alpha,14 beta,17 beta(H) stereochem istry. The Triassic Doig formation also contains two compounds tentati vely identified on the basis of geologic and geochemical data as diapr egnane and diahomopregnane, rearranged isomers of pregnane and homopre gnane with methyl substitution at the 5- and 14-carbon positions. The distribution of these compounds parallels that of the higher molecular weight diasteranes. A well-defined relationship is evident between th e ratio Fe/S and short-chain diasterane content: samples with Fe/S app roximately <0.90 exhibit low diasterane contents, while those with Fe/ S approximately >0.90 exhibit high diasterane content. This threshold Fe/S value is nearly equal to the value associated with stoichiometric pyrite (0.87) and suggests that the abundance of short-chain diastera nes is related to excess iron associated with detrital clays. This is consistent with the accepted mechanism of sterane rearrangement, which is thought to proceed via catalysis of clay minerals. In contrast to the higher molecular-weight diasteranes, which increase in abundance r elative to other sterane isomers with increasing thermal maturation, t he abundance of the rearranged short-chain compounds varies little rel ative to diginane or homodiginane in a maturity suite from the U. Devo nian Duvernay Formation. Predicted stabilities based on computational chemistry agree with the observed distributions; diginane/diapregnane and homodiginane/homodiapregnane represent compound pairs with similar thermal stabilities. The relative invariance of short-chain sterane d istributions with maturity suggest that these compounds may be better suited for estimating the elastic content (i.e., Fe/S ratio) of source rocks than the higher molecular-weight steranes, which are both matur ity and facies dependent. Copyright (C) 1997 Elsevier Science Ltd.