Origin and Geochemical Implications of Hopanoids in Saline Lacustrine Crude Oils from Huanghekou East Sag and Laizhouwan Northeastern Sag, Bohai Bay Basin.

Abstract

A suite of low-mature crude oils (five high-sulfur oils and six low-sulfur oils) from the Huanghekou and the Laizhouwan Sags, Bohai Bay Basin, are analyzed to investigate the fate of the hopanoids. Abundant hopanes, such as secohopanes, 25-norhopanes, benzohopanes, aromatized secohopanes, and sulfide hopanes, are identified, and their carbon isotope compositions are determined. Varying 13C isotope values of C31 hopane (−38.7–34.0‰) and C29-30 hopanes (−38.5–31.5‰) suggest different bacterial sources of these compounds. The presence of 25-norhopanes with enriched heavy carbon isotopes in severely biodegraded oils suggests that they are microbially mediated products. The detection of the isotopically depleted C29 and C30 D-ring-8,14-secohopanes (−45.6–41.2‰) indicates that secohopanes are from methane-oxidizing bacteria (methanotrophs). The presence of isorenieratane, lower aryl isoprenoid ratios, and a good correlation between the sulfur content and the gammacerane index indicate the presence of green sulfur bacteria (Chlorobiaceae) under photic zone euxinic conditions. Water column stratification results in good preservation of the organic matter, and it is in favor of diversity of aquatic microorganisms. The ratios of C35/C34 sulfide hopane, C35 sulfide hopane-2/C35 sulfide hopane-1, and C35/C34 benzohopane are influenced by the reducing environments in this region. In addition, the D-ring monoaromatized 8,14-secohopanoid/(D-ring monoaromatized 8,14-secohopanoid + benzohopanes) and C31–C35 secomoretanes/secohopanes are affected by the maturity. We hypothesize that the reducing environments and thermal effects are important markers for the hopanoid transformation, including the incorporation of inorganic sulfur in substituting functional groups, cyclizing, aromatizing, and opening ring C of the hopanoids.