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Different opinions on the oil-source correlation and migration of the Moxizhuang-Yongjin block in the Junggar Basin, NW China, have seriously affected oil and gas exploration in this area. This research is based on the results of biomarker analysis and carbon isotope of monomer hydrocarbon on crude oil and source rock samples from different blocks. Two oil families (A1, A2, and B) are classified by cluster analysis on nine typical biomarker ratios. Typically, sub-family A1 oils are distributed in the Zhuang 102 well block of the Moxizhuang block, and their gammacerane index is the highest. The abundance of C

The Fukang Sag in the Junggar Basin is the main exploration block. However, the origin and source of crude oil are still controversial, which seriously affects the well locating and exploration in this area. In the present work, 30 source rocks and 21 crude oils were collected for geochemical analysis to clarify the source of the organic matter, the sedimentary environment, and the evolution degree. Among them, the source rocks of the Pingdiquan Formation are type II1 organic matter with good quality, the source rocks of the Badaowan Formation are type II2-III organic matter with fair–good quality, and the source rocks of the Xishanyao Formation are type II2 organic matter with fair quality. All source rocks are in the mature stage. The results of the biomarker compounds show that the lacustrine mudstone of the Xishanyao Formation and the coal-measure mudstone of the Badaowan Formation were deposited in reducing environments. The former was mainly from lower aquatic organisms, and the latter was from terrestrial higher plants. The mudstone of the Pingdiquan Formation was formed in a weakly oxidizing–weakly reducing depositional environment, and its parent material was of mixed origin. Based on the results of the biomarker compounds and carbon isotopes, the crude oils were divided into three categories. The Family I crude oil has the characteristics of low maturity, low salinity, and more input of low-level aquatic organisms, and the carbon isotope has a good affinity with the lacustrine mudstone of the Xishanyao Formation. The Family II crude oil shows medium maturity, low salinity, mainly higher plant input, and heavy carbon isotope, mainly derived from the Badaowan Formation coal-measure mudstone. The Family III crude oil is characterized by high maturity, high salinity, mixed parent materials, and light carbon isotope and originates from the mudstone of the Pingdiquan Formation. The results provide a reference for oil and gas exploration and development in the eastern area of the Junggar Basin; the future research will focus on well areas with high maturity near the Fukang fault zone.

In this study, the geochemical characteristics of the oil and gas and the related source rocks in the Ningbo Tectonic Zone in the Xihu Sag are analysed to determine the source of the natural gas. The natural gas in the Ningbo Tectonic Zone is dry gas. It has a high content of methylcyclohexane and low contents of n-heptane and dimethylcyclopentane in the C7 light hydrocarbons and a heavy ethane-carbon isotope composition, indicating that the natural gas in the study area was derived from humic parent material, making it a coal-type gas. The ratios of the light hydrocarbon parameters of the NB-A and NB-B natural gas are significantly different from those of crude oil and the Pinghu Formation source rocks, indicating that there is no pronounced relationship between the natural gas and the Pinghu Formation source rocks, and the natural gas may be from the source rocks in the deeper horizons of the Pinghu Formation. The homogenization temperatures of the fluid inclusions in the upper member of the Huagang Formation (NB-A1) are mainly 140–150°C, which correspond to 3.5–0 Ma (Late Cenozoic) according to the paleogeothermal history, indicating that in this area, the natural gas in the upper member of the Huagang Formation has charged since the sedimentary period of the Santan Formation.

Through the regional geological survey, outcrop section measurement and other field geological work, a block of black shale is found at the outcrop of Mengjiawopu Section of Hongshuizhuang Formation in Jixian County, and organic geochemical source rock analysis and evaluation methods are used to study the source rocks in Hongshuizhuang Formation. The set of source rocks is mainly gray-black silty mudstone and gray-black shale, with a cumulative thickness of 78 m; the total organic matter (TOC) is generally high, with an average content of 1.35%, of which good and rich source rock samples account for 86%. A combined analysis of the stable carbon isotope of kerogen, the correlation between Pr/nC17 and Ph/nC18, and the ratio of saturated hydrocarbon to aromatic hydrocarbon identifies that the organic matter is mainly Type Ⅱ kerogen; the equivalent vitrinite reflectance Ro of organic matter is from 1.68% to 1.94%. Rc, the correlation of the methylphenanthrene index (MPI) against the vitrinite reflectance is from 1.63% to 1.81%, with an average of 1.71%, and the Odd/Even Predominance (OEP) value is between 0.87 and 1.11, both indicating that the source rocks of the Hongshuizhuang Formation are at the stage of high to over maturation. In summary, the set of source rocks in Hongshuizhuang Formation is characterized by high organic matter content, good kerogen types, and high to over maturation stage.

Carbazole (CA) and alkylated carbazoles are common nitrogen-containing heterocyclic aromatic compounds widely found in crude oils and sedimentary rocks extracts. Their geochemical significances in petroleum exploration have been widely investigated in previous studies. Using density functional theory (DFT) and molecular dynamics (MD) simulation, we calculated the polarity of alkylated carbazole isomers/homologues and their adsorption energy with water and α-quartz, respectively. The calculated results show that there are significant differences in migration fractionation effects for alkylated carbazole isomers/homologues. This study confirmed the validity of previously proposed carbazole migration indicators, i.e. 4-/1-MCA (the relative abundance of 4-methylcarbazole to 1-methylcarbazole), 2,5-/1,8-DMCA (2,5-dimethylcarbazole to 1,8-dimethylcarbazole) and 2,4-/1,8-DMCA (2,4-dimethylcarbazole to 1,8-dimethylcarbazole) on the basis of quantum chemistry and molecular simulation. All these three parameters have been successfully applied in a lacustrine sandstone reservoir in the Fushan Depression, Beibuwan Basin (South China Sea). The calculations of Gibbs free energy show that the differences in thermal stability between the isomers of alkylcarbazoles can be negligible. The source input and depositional environment have no significant effect on these indicators for oils from a same oil family. Hence, the values of these parameters are mainly controlled by subsurface petroleum migration processes. Therefore, the molecular parameters relative to alkylated carbazoles can be used as practically effective geochemical markers to trace the oil filling orientation and pathways for subsurface oil migration.

In this study, n-alkanes, isoprenoids and n-alkyl-ketones were detected in lacustrine sediments in the Maogou section of the Linxia Basin, NE Tibetan Plateau, NW China. The distribution characteristics of these compounds correspond to arid climate change in inland Asia from the upper Oligocene to the Pliocene. The characteristic bimodal distribution of n-alkanes, which was observed in the investigated samples, is centered on n-C17–n-C20 and has maximum values at n-C18 in all samples; n-C27–n-C31 has maximum values at n-C29 in some of the samples. The front mode shows a weak even carbon number predominance of short-chain n-alkanes (CPI17–21 0.55–0.89); in contrast, the back mode has a strong odd carbon number predominance of long-chain n-alkanes (CPI25–31 1.41–2.42). Changes in the nC27/nC31 ratio (woody plants/grassy plants) along the entire section corresponded to three climate stages: an arid to humid climate stage from ~ 22.5 to 18.4 Ma; 6.25 to 5.5 Ma; and two major humid stages at ~ 18.4 Ma and ~ 5.5 Ma. A warm-humid climate was identified for the ~ 10 Ma to ~ 9 Ma period, which turned to an arid-cold climate from ~ 8 Ma to ~ 7.5 Ma, by assessing changes in the (n-C17–n-C21)/(n-C27–n-C31) ratios. Warm-humid conditions gradually increased from ~ 7.5 Ma to ~ 5.5 Ma, as has been confirmed by multiple geochemical climate indicators. A distinctive climate change toward arid-cold conditions at ~ 8 Ma corresponded to a striking increase in the high-carbon-numbered n-alkanes and a rapid decrease in the (n-C17–n-C21)/(n-C27–n-C31) and n-C27/n-C31 ratios of n-alkanes, which suggests a large input of higher plants. Our observation from the biomarker analysis is in agreement with previous reports that used palynofloras to determine that climate change in the investigated region responded to the uplift of the Tibetan Plateau at ~ 8 Ma.

This study reports the first observation of predominant even C-numbered n-alkanes from sediments in the continuous lacustrine-sedimentary section (Maogou) from the Late Miocene to the Early Pliocene (13–4.4 Ma) in the Linxia Basin, NE Tibetan Plateau. The n-alkanes showed a bimodal distribution that is characterised by a centre at n-C16–n-C20 with maximum values at n-C18 and n-C27–n-C31 as well as at n-C29. The first mode shows a strong even C-number predominance (OEP16–20 0.34–0.66). In contrast, the second mode has a strong odd C-number predominance (OEP27–31 1.20–2.45). Certain types of special autochthonous bacteria are a possible source for this distribution of even C-numbered n-alkanes in lacustrine sediments. These bacteria may have a high production rate in weak oxic–anoxic and arid depositional environments, in which a variety of geochemical parameters have recorded palaeoclimate change.