Distribution and carbon isotope composition of pregnane in carbonate-evaporitic rocks from the Bonan Sag, Bohai Bay Basin, Eastern China: Insights into sources and associated lake environments

Highlights

• A predominance of pregnane was found in lacustrine rocks.

• Pregnane/C₂₇ ααα20R sterane ratios correlated with bacterial activity.

• The δ¹³C of pregnane reflects an origin from primary producers.

• Unusual enrichment of pregnane indicates stable water column stratification.

Abstract

The distribution and carbon isotope composition of pregnane was determined in eight samples of carbonate-evaporitic rock collected from depths of 2938–3012 m in Well Luojia 14, Bonan Sag of Bohai Bay Basin, Eastern China, as a means of identification of the sources and to draw conclusions about possible formation processes and associated lake environments. The results in four samples from the 2989–3006 m depth range indicated abnormally high abundances of pregnane relative to long-chain (≥C₂₇) homologues, with ratios of pregnane to C₂₇ ααα20R sterane between 5.24 and 21.01 and averaging 13.91. Meanwhile, values of the gammacerane index (gammacerane/C₃₀ αβ hopane) and n-C₂₁₋/n-C₂₂₊ ratios were as high as 3.41–9.29 (mean 5.01) and 21.68–60.76 (mean 33.42) respectively, indicating the presence of stable oxic-anoxic stratification in the water column and blooms of heterotrophic bacteria. The δ¹³C values for pregnane were similar to those of primary producer-derived lipids, such as pristane and phytane, but lower by ∼8‰ than that of 4-methyl pregnane. This is consistent with the reports of ¹³C depletion in steranes (≥C₂₇) relative to their 4-methyl analogues. Taken together, these results suggest the abnormal enrichment of pregnane might result from severe bacterial alteration of algal-derived lipids in response to the stabilization of water column stratification forced by a salinity gradient. In four samples from within the 2938–2948 m band and at 3012 m, similar δ¹³C values for pregnane, pristane and phytane suggest an algal origin for pregnane. However, pregnane/C₂₇ ααα20R sterane ratios, gammacerane index values and n-C₂₁₋/n-C₂₂₊ ratios were all relatively low, with averages at 3.20, 0.43 and 4.60, respectively, suggesting much less bacterial influence on algal lipids and perhaps indicating relatively unstable stratification of the water column, possibly caused by a reduced input of freshwater.

Introduction

C₂₇–C₂₉ steranes with ααα20S, αββ20R, αββ20S and ααα20R configurations are widely distributed in hydrocarbon source rocks and oils. Proxies based on the relative abundances of these compounds are frequently used to assess the maturity of organic matter (OM), to identify rock depositional environments and to establish correlations between oils and source rocks (Mackenzie et al., 1982, Moldowan et al., 1985, Huang et al., 2003, Hao et al., 2011, Furmann et al., 2015). By comparison, short-chain steroids, such as pregnane (C₂₁ αββ sterane), are only occasionally found in great abundance in rock extracts and oils. These compounds differ structurally from C₂₇–C₂₉ steranes primarily in the substitution of a methyl or dimethyl group at C-20, rather than the more usual branched C₅ to C₈ alkyl side chain (Wang et al., 2015). Their occurrence in rocks and sediments is commonly regarded as a result of direct side chain cleavage of C₂₇–C₂₉ steroids because the C-20 position in steroids is weak and easily cleaved in thermal processes during burial (Wingert and Pomerantz, 1986, Huang et al., 1994, Requejo, 1994, Requejo et al., 1997). However, this explanation does not account for high levels of short-chain steroids in immature rocks (Ten Haven et al., 1985, Hughes and Holba, 1988, Requejo et al., 1997, Li and Jiang, 2001, Lu et al., 2009, Wang et al., 2010, Wang et al., 2015). In this instance, the source is suggested to be organisms associated with hypersaline environments (Haven et al., 1985). Unfortunately, there is no evidence to date of a natural product whose carbon skeleton could serve as a precursor for these compounds, leading Li and Jiang (2001) to propose an alternative origin. They argued that microbial activity or photo-oxidation in the water column may promote oxidative cleavage of the precursor steroid side chains and thereby lead to the formation of short-chain steroids. Lu et al. (2009) further suggested that the short-chain steroids in immature evaporitic sediments may be early diagenetic products of microbial metabolic processes. Experimental evidence demonstrated that the microbiological degradation of the cholesterol side chain can produce C₂₂ acids, potential intermediates for formation of pregnane (Sih et al., 1968). However, the sources of pregnane and other short-chain steroids and the formation processes and the environments in which they may occur remain ambiguous, restricting the usefulness of these compounds as paleoenvironmental and paleoecological indicators.

The Bonan Sag is located in the Zhanhua Depression of the Bohai Bay Basin in Eastern China. The fourth member of the Tertiary Eocene Shahejie Formation (Es₄) in this sag comprises deposits of lacustrine evaporite, dolomite, marlstone and calcareous shale. The high abundances of pregnane and homopregnane previously identified in the rocks here were attributed by Wang et al., 2010, Wang et al., 2015) to microbial activity and/or a restricted, clastic starved depositional setting. However, little detailed consideration has been given to the character of the lake environment, or to the biogeochemical process of pregnane formation within it. In this study, the distribution and carbon isotope compositions of pregnane and other aliphatic hydrocarbons, kerogen pyrolysis products and the hydrogen isotope compositions of n-alkanes, were measured in eight carbonate-evaporitic rock samples from the Bonan Sag. The aims were to identify potential sources of pregnane and to reconstruct the lake environment and ecosystems responsible for its marked predominance. The results provide a picture of how environmental changes may influence pregnane distributions in lake systems and thereby enhance the reliability of pregnane as an organic geochemical indicator.