Methyltrimethyltridecylchromans (MTTCs) are biomarkers that are commonly used to identify immature-low mature source rocks or crude oil. MTTCs are abundant in the mature crude oils found in the Machang area of the southern Dongpu Depression in the Bohai Bay Basin. To explore the mechanism behind the enrichment of MTTCs in mature crude oils, a detailed study was conducted to investigate their distribution characteristics in crude oil samples. The study integrated tectonic evolution history, distribution characteristics, and thermal history of source rocks, molecular fingerprints characteristics of crude oils, and catalytic characteristics and combination features of clay minerals in the wall rock of crude oils. Based on this analysis, two enrichment models were proposed: (1) The first model suggests that crude oils were mixed with a small amount of immature-low mature soluble bitumen containing MTTCs; (2) The second model proposes that the evolutionary mechanism of clay minerals in the reservoir could reduce the decomposition rate and degree of MTTCs, resulting in their relative enrichment. Therefore, this study provides insights into the enrichment mechanism of MTTCs in mature crude oils and highlights the importance of considering various factors, such as tectonic evolution history, source rock characteristics, and catalytic properties of clay minerals, to understand the distribution and enrichment of biomarkers in crude oils.

The Eocene middle number of the Liushagang Formation (LS2) of the Weixinan Sag, Beibuwan Basin, characterized by a thick succession of excellent quality source rocks, is composed of lacustrine organic-rich shales, mudstones, and shales (mudstones/shales). However, the complex and specific depositional environment in the source rocks of LS2 raise questions about the mainly controlling factors of lacustrine organic matter (OM) accumulation. In this study, total organic carbon (TOC) contents, Rock-Eval pyrolysis, as well as biomarker data are used to investigate the nature of the depositional environment and the enrichment mechanism of OM in the source rocks of LS2. The values of Tmax, CPI, C29 steranes αββ/(ααα+αββ), and the 22S/(22S+22R) ratios of the 17α, 21β(H)-C31 hopane together confirm that the OM in the source rocks of LS2 is immature to of low maturity, which suggests that the nature of biomarkers may not be affected by thermal maturity. The hydrocarbon potential was higher in the organic-rich shales (with a mean of 20.99 mg/g) than in the mudstones/shales (with a mean of 7.10 mg/g). The OM in organic-rich shales is type I and II kerogen and that in mudstones/shales is type II kerogen. The C27/C29 regular steranes ratios and 4-methylsterane indices (4MSI) further confirmed the difference in the source of OM between organic-rich shales and mudstones/shales; that is, that the OM of organic-rich shales is mainly derived from the lake algae and aquatic macrophytes and the OM of mudstones/shales is mainly from the higher plants. The values of the gammacerane index and ratios of C21/C23 TT and C24 Tet/C26 TT all indicate that the source rocks from LS2 are deposited in freshwater to a low salinity water column. Moreover, a cross-plot of C21–22/C27–29 sterane versus dia/reg C27 sterane ratios and Pr/Ph ratios suggests that the source rocks from LS2 are recorded to have sub-oxic to oxic conditions. Based on those analyses, two dynamical formation models were proposed: a high-productivity and oxic-suboxic dynamical formation model (Model A) and a low-productivity and oxic-suboxic dynamical formation model (Model B).

Methyltrimethyltridecylchromans (MTTCs), a class of aromatic hydrocarbon molecular marker compounds, provide an assessment of paleosalinity and maturity in sediments and crude oils. However, little is known about the effect of biodegradation on MTTCs. In this study, the saturated and aromatic fractions of a suite of oils from the Neogene Guantao (N1g) and Minghuazhen (N1m) Formation in the Miaoxi Depression, Bohai Bay Basin, were analyzed. Distriubutions of the more bioresistive biomarker show that the oils are derived from similar organic matter (dominantly from higher plants) and depositional environments (low salinity), and have similar maturity (in the middle oil window). Two biodegradation scales, were used to evaluate the level of biodegration. The samples ranged from PM 1 to 8 and MN2 from 140 to 983 (slight to heavy-severe biodegraded). MTTC concentrations decrease with the increasing biodegradation levels and are or almost completely removed at PM = 8. The degree of alkylation affect their relative susceptibilities to biodegradation: trimethyl-MTTC (alpha-MTTC) > dimethyl-MTTCs (including beta-MTTC, gamma-MTTC and-zeta MTTC) > monomethyl-MTTC (delta-MTTC). The relative susceptibilities of dimethyl-MTTC isomers are gamma-MTTC > beta-MTTC >-MTTC. The beta/gamma-MTTC, a maturity indicator, is significantly impacted by biodegradation. Paleosalinity inidcators, alpha/delta-MTTC, alpha/gamma-MTTC, MTTCI and a ternary diagram of the relative abundance in different degree of alkylation of MTTCs, consistently indicate the oils are derived from sources deposited in a low samility environment. Additional research is needed to determine if biodegradation perturbs MTTC distributions in oils from other source environments to influence the inferred paleosalinity assessment.

在川中地区大安寨段烃源岩抽提物中检测出丰富的重排藿烷系列、重排补身烷系列、重排甾烷系列及C26+长链三环萜烷系列的分子标志物组合.结合烃源岩的有机质来源、沉积环境、热演化程度特征,探讨上述甾烷、萜烷分布特征指示的地质地球化学意义.结果表明:重排类化合物与C26+长链三环萜烷系列存在着某些相同的富集途径,重排藿烷系列富集途径多于重排补身烷系列、重排甾烷系列及C26+长链三环萜烷系列;特殊的甾烷、萜烷分布特征指示,烃源岩中有机质以湖泊内源(藻类和细菌等)为主,形成于范围局限的弱还原环境,高丰度的重排化合物指示,有机质热演化处于成熟阶段.此外,特殊的甾烷、萜烷分布特征还可能指示了特定的酸性粘土矿物、钙质沉积催化和形成环境中发育的特殊生物族群.综上,研究结果对分子标志物异常组合的地球化学应用具有重要的示范意义.

川中油气区金华地区侏罗系油气勘探开发取得较大进展,侏罗系发育多套油气供烃层位.采用饱和烃气相色谱、色谱-质谱等技术,细致对比并揭示了研究区自流井组大安寨段和须家河组五段(简称须五段)烃源岩的地球化学特征,探讨了 2套主要供烃层位烃源岩的沉积环境和生源构成特征.结果表明,大安寨段烃源岩的分子地化特征与须五段烃源岩差异较大:大安寨段烃源岩中等姥植比(Pr/Ph),中等三环萜烷参数(C19+20TT/C23TT),中等降藿烷/藿烷系列化合物(C30D/C30H)、早洗脱重排藿烷/藿烷系列化合物(C30E/C30H),高降藿烷/升藿烷(C27D/C27R),相对低三芳甾烷参数(C27/C28TAS),生源构成以菌藻生源为主,兼有丰富的高等植物生源的混源输入;须五段烃源岩呈现低-中等Pr/Ph,低C19+20TT/C23TT、C30D/C30H、C30E/C30H,中等C27D/C27R,相对高C27/C28TAS,生源构成以高等植物生源为主.

Gas chromatography-mass spectrometry (GC-MS) analysis has revealed extremely high abundances of rearranged hopanes in Jurassic source rocks and related crude oils in the center of the Sichuan Basin. The detected rearranged hopanes include 17α(H)-diahopanes (C27D and C29–35D), early-eluting rearranged hopanes (C27E and C29–33E), and 18α(H)-neohopanes (C29Ts and Ts). Both the 17α(H)-diahopanes and the early-eluting rearranged hopanes exhibit a distribution pattern similar to that of the 17α(H)-hopane series, with a predominance of the C30 member and the presence of 22S and 22R epimers of hopanes in the extended series (>C30). The results of this study show that the relatively high abundance of rearranged hopanes in Jurassic source rocks in the study area is associated with their depositional environments and with clay-mediated acidic catalysis rather than, as was previously thought, thermal maturity. Shallow lacustrine facies with brackish water and a suboxic to weak reducing sedimentary environment have contributed to the enrichment of rearranged hopanes, and clay-mediated acidic catalysis may also have had a positive influence on their abundance. The distribution patterns of the diahopane series indicate that the oils from Jurassic reservoirs in the Gongshanmiao Oilfield are sourced from Jurassic source rocks. Rearranged hopanes are therefore considered to be effective biomarkers for oil-source correlation in the center of the Sichuan Basin.

The GC-MS analyses of a suite of 24 Jurassic source rock samples from the center Sichuan Basin (SW China) show that they contain variable amounts of rearranged hopanes, including 17α(H)-diahopanes (D series), 18α(H)-neohopanes (Ts and C 29 Ts) and the early-eluting rearranged hopanes (E series). D series have similar distribution patterns to 17α(H)-hopane series, including a range from C 29 to C 35 for carbon numbers and presence of 22S and 22R epimers for C 31 –C 35 homologues. In particular, the 17α(H)-hopanes were not detected in several samples, which have extremely relatively higher abundance of D series. E series extend from C 29 to C 31 , and also have 22S and 22R epimers for C 31 homologues. The relative abundance of E series has a strongly linear relationship with that of D series. The slopes and intercepts for tender line in both C 30 E/C 30 H vs C 30 D/C 30 H and C 29 E/C 29 H vs C 29 D/C 29 H plots are less than 0.7 and 0.2 respectively, indicating that E series have similar forming conditions with D series, but probably following different mechanisms. In contrast, the abundance of C 29 Ts also has a significant linear correlation with that of C 29 D.

The Woodford–Mississippian “Commingled Production” is a prolific unconventional hydrocarbon play in Oklahoma, USA. The tight reservoirs feature variations in produced fluid chemistry usually explained by different possible source rocks. Such chemical variations are regularly obtained from bulk, molecular, and isotopic characteristics. In this study, we present a new geochemical investigation of gasoline range hydrocarbons, biomarkers, and diamondoids in oils from Mississippian carbonate and Woodford Shale. A set of oil/condensate samples were examined using high-performance gas chromatography and mass spectrometry. The result of the condensates from the Anadarko Basin shows a distinct geochemical fingerprint reflected in light hydrocarbon characterized by heptane star diagrams, convinced by biomarker characteristics and diamantane isomeric distributions. Two possible source rocks were identified, the Woodford Shale and Mississippian mudrocks, with a variable degree of mixing. Thermal maturity based on light hydrocarbon parameters indicates that condensates from the Anadarko Basin are of the highest maturity, followed by “Old” Woodford-sourced oils and central Oklahoma tight oils. These geochemical parameters shed light on petroleum migration within Devonian–Mississippian petroleum systems and mitigate geological risk in exploring and developing petroleum reservoirs.