As an important component of biological markers, steranes (cyclopentane perhydro phenanthrene), particularly regular steranes, distribute in hydrocarbon source rocks and crude oils, referring to a class of tetracylic compound (also called steroid nucleu) with an alkyl side chain, in which an anglur methyl exists both in A/B ring and C/D rings. Although steranes are diverse in type, regular steranes are named according to their structures that contain a methyl at C-10 and C-13 position and five asymmetric carbon atoms (C-5, C-14, C-17, C-20 and C-24). Regular steranes widely distributed in the geosphere mainly consist of C27, C28, and C29 ααα (20S + 20R) and αββ (20S + 20R).
C27, C28 and C29 steranes were first identified in the American green river and Pre-Cambrian shale by
Meanwhile, the presence of abnormal regular sterane distributions in source rocks and crude oils has also attracted the attention of organic geochemists. Even though abnormal regular sterane distributions in hydrocarbon source rocks and crude oils have hitherto been documented in numerous literatures, their origin and genetic mechanism still remain a puzzle. Based on the previous literatures and researches, this paper systematically summarizes the researches status of abnormal regular sterane distributions to discuss the development directions of abnormal regular streane distributions in the future, which may provide some new information for a better understanding of origins and genetic mechanisms of regular steranes with abnormal distributions in the future. This is not only one of the most interesting scientific issues in organic geochemistry field, but also a breakthrough for the new theories and applications in oil and gas exploration.
2. Research Status
2.1. Abnormally High Abundance of C29 Regular Steranes
Grantham (1986)  discovered the unusual regular sterane distributions in the study on geochemical characteristics of ancient crude oils from Amanan. This kind of crude oil was dominated by C29 regular steranes, while its C27 regular sterane relative content was lower. Obviously, the predominance of C29 regular steranes in this kind of crude oils is not indicative of terrestrial organic matter input, because the terrestrial vascular plants begin to flourish in the Devonian. Henceforth, some scholars have also identified the abnormally high abundance of C29 regular steranes in the Proterozoic and Paleozoic marine source rocks and crude oils   . Hua and Huang (1989)  noted that the superiority of C29 regular steranes existed in the Lower Paleozoic and Proterozoic source rocks from Northern China, which was attributed to the input of cyanobacteria. Some species of cyanobacteria, most commonly found in Lower Paleozoic with an advantage of C29 regular steranes, has been reported by Volkman (1986)  . Through the research on the geochemical characteristic of Cambrian-Ordovician source rocks in Tarim Basin, Chen et al. (2001)  found that the regular sterane distributions during the Cambrian-Lower Ordovician differed from that of the Middle-Upper Ordovician source rocks in Tarim Basin, and demonstrated that the former had an inverted “L” pattern distribution of C29 > C28 > C27, while the latter was characterized by a “V” pattern distribution of C29 > C27 > C28. They suggested that the variation in regular sterane distributions during the Cambrian-Lower Ordovician and the Middle-Upper Ordovician might illustrate that the organic chemical structure components obviously changed during the evolutionary process of organisms. It is interesting that, Zhang et al. (2001)  detected the abnormally high concentration of dinosteranes, 4-methylsteranes, 24-norcholestanes and C28 regular steranes according to the study on Cambrian and Sinian source rocks in Tarim Basin, indicating the inputs of dinoflagellates and diatoms probably originated in the Mesozoic period or earlier. It suggested that some of the phytoplankton, such as dinoflagellates and diatoms, may have more ancient species.
2.2. Thermal Effect on Regular Sterane Distribution
In fact, the relative abundance and distribution of C27, C28 and C29 regular steranes can also be affected by thermal maturation. The relative abundance of C29 regular steranes decreases with increasing thermal maturity, while the relative abundance of C27 regular steranes has an increasing trend with thermal maturity. The thermal stability of C29 and C28 regular steranes worse than C27 regular steranes can be responsible for that. Moldowan et al. (1992)  found that C27, C28 and C29 regular steranes displayed an inverted “L” type distribution dominated by C29 regular steranes in the hydrous pyrolysis experiment of immature Triassic source rocks. There was a pronounced shift towards the C27 and C28 regular steranes with increasing maturity until the C27, C28 and C29 regular steranes showing an asymmetric “V” type distribution. The increase in C27 regular steranes indicated the formation of secondary C27 steranes from the demethylation of C29 steranes. However, through the study on absolute concentrations of biomarkers in Carboniferous coals from western Pennsylvania and eastern Ohio, Dou et al. (1995)  found that the absolute amount of C29 regular sterane was initially high in coals of low maturity, but decreased rapidly in the 0.6% - 1.0% Ro range and then remained relatively constant at high maturities, while the absolute concentration of C27 was uniform through the entire maturity range, resulting in C29/C27 sterane ratios that essentially followed the trend of decreasing C29 steranes. Subsequently, Zhang et al. (2002)  discovered that the content of C27 regular steranes increased with increasing thermal maturity through the analysis of geochemical characteristics of the Jurassic and Triassic cores and coal samples of outcrops in Kuche Depression of Tarim Basin, especially at late mature stage. Sun and Duan (2011)  conducted a simulation experiment on peat and coal, which indicated that the relative content of C29 regular steranes increased with simulated temperature increase and then remained constant after simulated temperature up to 350˚C, while that of C27 and C28 regular steranes had an increasing trend through the entire maturity range. Nevertheless, it should be noted that according to Yang et al. (2017)  , as shown in Figure 1, the C27/C29
Figure 1. The plot of C27/C29 sterane vs. Ro and tmax of coal from Kuga sag and Ordos Basin (Yang et al., 2017).
sterane ratios in the colas from Kuga sag and Ordos Basin also changed significantly with thermal maturity on the research of the thermal effect on distributions of regular steranes, increasing with enhanced Ro values or tmax values. This phenomenon further illustrates that the thermal maturation has some connections with the distributions of regular steranes.
2.3. Abnormally High Abundance of ββ Steranes
Generally speaking, the isomerization degree of regular steranes is related to thermal maturity    . However, Ten Haven et al. (1986)  suggested early diagenetic formation of steranes in hypersaline environments. Abundant ββ steranes in these sediments could be due to reaction with sulfur. Rullkǒtter and Mariz (1988)  noted higher ββ/(ββ + αα) and 20S/(20S + 20R) for extracts from hypersaline rocks. Peakman et al. (1989)  identified the series of (20R)- and 20S-∆8(14) and ∆145α(H)-sterenes in an immature marl from a hypersaline deposit (northern Apennines, Italy) by co-injection studies with authentic compounds. These alkenes co-occurred with ∆13(17)5α(H)-spirosterenes and transient (20R)- and (20S)-∆145α(H), 17β(H)-sterenes. They indicated that the unusually high degree of isomerisation may result from diagenetic reduction of ∆7(14) and ∆8(14), 5α(H)-sterols via the ∆145α(H), 17β(H)-sterenes. This diagenetic pathway was supported by laboratory rearrangenment/hydrogenation experiments. And through the comparison of (20R)/(20S) sterane values in different carbon number, 5α(H), 14β(H), 17β(H)-steranes were confirmed to be actually related to ∆13(17)5α(H)-spiral sterenes and ∆8(14) and ∆145α(H)-sterenes rather than 5α(H), 14α(H), 17α(H)-steranes. In addition, the regular steranes distributions with unusually high isomerization were detected in immature-low mature Paleogene source rocks of Bohai Bay Basin   , namely, the abundances of 5α(H),14β(H),17β(H)-sterane configuration and ααα-20S configuration were higher in these sediments. The crude oil from central region of Tarim Basin was divided into three types according to the unusual regular sterane distributions of crude oils. It should be noted that the distributions of steroids in three types of crude oils were similar to each other. They were all featured by the relative content of C27 regular steranes lower than that of C28 and C29 steranes and the relative abundance of C27, C28 and C29-ββ isomers higher than that of αα isomers, but their origins were inconsistent. The first and third types of oils were mainly affected by thermal maturity and migrated fractionation, while the biodegradation beside thermal maturity and migrated fractionation had an effect on the second types of oil  .
It is interesting that, the author recently carried out a study on the National Natural Science Foundation of China (Genesis mechanism of rearranged hopanes with high abundance in geological bodies and its significance in petroleum geology), finding that the regular sterane αββ isomers in the Carboniferous- Permian coal measures of Ordos Basin and the Lower Cretaceous lacustrine source rocks and crude oils from the Lishu Fault Depression of Songliao Basin present the excellent correlations with rearranged hopanes including the 18α(H)-neohopane series, 17α(H)-diahopane series and early-eluting rearranged hopane series. This phenomenon appears to be not reported in previous literature.
3. Development Trend
Regarding the origins of abnormal regular steranes distributions, there is still no specific comment on it at present. One hypothesis is that the origin of C29 steranes in the Proterozoic and Paleozoic marine source rocks and crude oils is considered to be marine algae because the terrestrial vascular plants increase in population from the Devonian  . These marine algae are initially presumed to be cyanobacteria   , but later some scholars speculated that they may be macroscopic algae   . However, the Proterozoic and Paleozoic C29 steranes are more likely to derive from the modern planktonic chlorella according to the data of micropaleontology, thermal simulation experiments and the stable carbon isotope of sterol in modern algae    . The formation of abnormally high abundance of 5α, 14β (H), 17β (H) 20R and 20S steranes in immature-mature sediments and crude oils is more likely to be related to sedimentary environment rather than thermal effect     . Ten Haven et al. (1986)  suggested that the reaction with sulfur can account for the presence of abundant ββ steranes in hypersaline environments, whereas Peakman et al. (1989)  proposed that the abnormally high degree of isomerisation may ascribe to diagenetic reduction of ∆7(14) and ∆8(14), 5α(H)-sterols. Nevertheless, thermal simulation experiment showed that the bacterial action, which occurred in low-evolutionary source rocks of salt lacustrine facies during the early stage of sedimentation and diagenesis, was favorable for the conversion of ααα configuration to αββ configuration due to the reduction of activation energy of organic matters  . Furthermore, thermal maturity may contribute to abnormal regular sterane distributions      . Generally, the relative concentration of C27 regular steranes increases with the increasing maturity, whilst C29 regular sterane abundance shows a decreasing trend with maturity     . In summary, multiple controlling factors on the distribution and genetic mechanism of abnormal regular steranes are still controversial and being investigated.
Methods and techniques in previous literature and research, which are used in abnormal regular sterane distribution studies, include three aspects. First, according to the distributions of modern marine algae sterol and combination of modern biology, organic petrology and micropaleontology are used to reveal the organic matter source of regular steranes with unusual distributions  . Also, geochemical properties of unusual regular sterane distributions are investigated on the basis of studying the relative abundance and distribution of biology markers in source rock extracts and crude oils using GC-MS and GC-MS-MS analysis techniques   . In addition, thermal simulation experiments were carried out to study thermal simulation products of mudstones and coals derived from different sedimentary environments, and then the researchers discussed the thermal effect on the isomerization degree of regular steranes   . However, considering the abnormal distributions of the source, genetic mechanism and geochemical property of regular steranes, which are still under active investigation, it is urgent to exploit some new approaches for the assessment of the geochemical characteristics of hydrocarbon source rocks and crude oils.
With the increasing perfection of gas-chromatography-isotope ratio-mass spectrometry (GC-IRMS), especially the development of compound-specific carbon isotope, it therefore provides a powerful tool for the determination of compound-specific carbon isotope composition of biological makers     . There are four main methods for separation, purification and enrichment of saturated hydrocarbon biomarkers, including silicalite, molecular sieve adsorption, urea treatment and sulfur urine complex. However, these methods are complicated and the repeatable analysis results are unsatisfactory. In order to meet the requirements for GC-IRMS analysis, Ryoshi et al. (2013)  used gel permeation chromatography (GPC) with a high performance liquid chromatography (HPLC) system for a more convenient and effective separation and enrichment for steroid and hopanoids of hydrocarbon source rocks and crude oils, which is an important milestone in the genesis and evolution studies of steroids. Different organism species have different carbon isotopes, which has been proved by the compound-specific carbon isotope composition of steroids derived from hydrocarbon source rocks and crude oils in geological bodies as well as in modern organisms    . However, there have hitherto been few reports on the distribution, variation, key controlling factors of compound-specific carbon isotope of abnormal regular steranes of hydrocarbon source rocks, crude oils originating from diverse sedimentary environment, genetic types and thermal maturity.
As mentioned above, comprehensive and systematic study on macroscopic composition, biomarker combination characteristic and compound-specific carbon isotope composition of hydrocarbon source rocks and crude oils containing abnormal regular sterane distributions can not only reveal the origin and genetic mechanism of unusual regular steranes, but also enrich and improve the methods and theories on biomarker study. In the future, we hope to roundly investigate the compound-specific carbon isotope compositions of regular steranes in sediments and crude oils using speration, enrichment, and purification of regular steranes in sediments and crude oils, intergrated with organic prteological, micropaleontological, and sedimentary geochemical characteristics of hydrocarbon source rocks with abnormal regular sterane distributions to discuss the occurrence, variation and main controlling factors of these compounds. This information may contribute to a better understanding of origins and genetic mechanisms of regular steranes with abnormal distributions and further exploit the new applications on the genesis of hydrocarbons, oil and source correlation, quantitative assessment of crude oil mixing degree and secondary alteration to expand the new research orientation in petroleum geology and organic geochemistry.
This research is financially supported by the National Natural Science Foundation of China (Grant No. 41772124).