A close correlation between lithofacies and organofacies in meter-scale high-order cycles composed of lacustrine sediments enables comparison and refinement of lithofacies-defined cyclostratigraphy. Four lithofacies and four organofacies have been identified in fluctuating profundal high-order cycles in the lower-Permian Lucaogou Formation, southern Bogda Mountains, NW China. The four lithofacies include interbedded and interlaminated coarse siltstone and very fine sandstone, black shale, wackestone and dolostone, and calcareous and dolomitic shales. Four distinctive organofacies have been identified, on the basis of geochemical composition of organic matter and specific biomarker proxies related to organic matter types, rather than to depositional conditions and thermal maturity. The four organofacies are associated with the four lithofacies in the meter-scale high-order cycles, suggesting litho- and organo-facies may be genetically linked and may have been controlled by lake contraction and extension. The study shows that the lithofacies-derived and environment-defined high-order cycles can be delineated and substantiated by geochemical proxies-defined organofacies. This study also demonstrates that a holistic approach combining litho- and organic geochemical data is useful in reconstruction of meter-scale lacustrine cycles in a half-graben.

The determination of short-chain alkylbenzenes (C0-4) in source rocks has always been a challenge due to potential losses during pretreatment processes such as solvent extraction of the soluble organic matter. This work proposes a method for analyzing short-chain alkylbenzenes in solid powder samples from source rocks by combining headspace (HS)-solid phase microextraction (SPME)-Arrow and gas chromatography-mass spectrometry (GC-MS). The recommended optimal conditions of HS-SPME-Arrow are: use of a polydimethylsiloxane (PDMS) phase with a 250 mu m thickness coated fibre, an extraction temperature of 90 degree celsius, exposure time of 13 min, and desorption time of 8 min. The biomarkers and short-chain alkylbenzenes of 37 lacustrine source rocks with varying degrees of thermal maturation from different sedimentary basins in China were analyzed using this method. Three parameters based on short-chain alkylbenzenes isomers were then proposed, including the dimethyl-4-ethylbenzene ratio (DM4EBR), the 1,2-dimethylethylbenzene ratio (1,2-DMEBR), and the trime-thylbenzene ratio (TMBR), which showed significant linear correlation with vitrinite reflectance of the source rocks. These can be used to evaluate the maturity or hydrocarbon generation stage of such source rocks, ranging from immature-low mature to postmature stages. An additional three parameters, including the dimethylethylbenzene ratios (DMEBR-1 and DMEBR-2) and the 1-methylisopropylbenzene ratio (1-MiPBR), were also proposed; these related to the organic matter inputs and depositional environments of the source rocks and can be used to identify the organic matter input and depositional environment conditions of source rock samples prior to the late stage of oil generation (Ro < 1.2 %). The usefulness of short-chain alkylbenzene parameters in indicating organic matter inputs, depositional environment conditions, and thermal maturity levels makes them valuable tools for geochemical studies of highly matured light oils and condensates.

Steroids with alkyl groups at C-2 and C-3 are not known yet in any living organism, while 4-methylsteranes are previously thought specific to 4-methylsteroids bio-synthesized by limited types of organisms, such as dinoflagellates, microalgae, and certain bacteria. Here we investigate the occurrence of C30 2α-, 3β-, 4α-methyl-24-ethylcholestanes, C31 3β,24-diethylcholestanes, and 3,4-dimethyl-24-ethylcholestane, which are tentatively identified in fourteen fossil conifer Protophyllocladoxylon trunk samples from the upper Permian Wutonggou and Guodikeng formations in northwest China. In contrast to the paucity of alkyl steranes in the background sediments, abundant C30 4α-methyl-24-ethylcholestanes in fossil conifers from diverse fluvial–lacustrine environments suggest that conifers were the most likely biological origin for the 4-methyl steranes in conifer trunk samples, although origins from associated fungi might also be possible. Alternatively, these C-4 methyl steranes might have possible algal or prokaryotic origins. It is noteworthy that diagenetic methylation or bacterial alkylation at early stages of diagenesis during burial may have caused a re-arrangement of classical 24-ethylsterols that were bio-synthesized by plants to generate C-2 and C-3 alkylated steranes. This study reports the presence of 4-methylsteranes in conifer fossils for the first time, pointing toward a likely origin from higher plants. This study also provides the first report of C-2 and C-3 alkylated steranes in plant fossils, a.llowing for the possibility that plants may produce functionalised sterol precursors of orphan alkylsteranes that are abundant in geological sediments but whose functionalised precursors are not known from any living organisms.

Chemometrics has been widely used to cope with the problems of oil-oil and oil-source correlations because of its unique advantages in the comprehensive consideration of multiple parameters and the classification of samples or variables. In this paper, three chemometric methods, especially multidimensional scaling, were used to revisit the genetic oil family and the relationship between the crude oil and the source rock, because the oil source in the Wushi Sag, a significant petroliferous sag in the Beibu Gulf Basin of South China Sea, is still controversial. Two genetic families of crude oils, namely group A and group B, have been identified based on chemometric results. Group A oils are characterized by relatively higher Pr/Ph ratios and a high abundance of C27 ααα 20R steranes and C30-methylsteranes than those of group B oils, suggesting that this group of oils was deposited under a more oxic condition with more contribution of algae organic matter. Group A oils have been interpreted to be a mixture derived from the member 2 and member 3 of the Liushagang Formation (LS-2 and LS-3), whereas group B oils can be ascribed to the LS-2 member. The contribution of LS-3 mudstone member to the Wushi oils in previous studies may have been underestimated to some extent, which was inferred from the chemometric oil-source correlation results. The results of oil-source rock correlation may be used to guide future petroleum exploration activities with the incorporation of geological evidence. The spatial distribution of oil and gas reservoirs varies with burial depth. Taking into consideration other geological evidence, we may infer that the reservoir in eastern Wushi Sag was mainly distributed in the deep of Liushagang Formation, whereas the reservoir in southeast Wushi Sag was likely located in the shallow Liushagang Formation.

Calcareous shale is a favorable target for shale oil exploration, because shale oil moves more easily in calcareous shale than in clay-rich shale and the high carbonate content is beneficial to shale fracturing. Although shale oil exploration has been undertaken successfully in marine carbonate-rich shale (e.g., the Eagle Ford shale) in the USA, no major breakthrough has yet been made considering lacustrine calcareous shale. The lack of understanding pore evolution in lacustrine calcareous shale increases exploration risk. Thus, an integrated analysis (Rock-Eval pyrolysis, X-ray diffraction (XRD1), low-pressure gas adsorption, field emission scanning electron microscopy (FE-SEM2), energy dispersive X-ray spectroscopy (EDS3), and helium pycnometry) of lacustrine calcareous shale from the Lucaogou Formation in the Santanghu Basin and of pyrolyzed shale was undertaken to document the hydrocarbon generation and expulsion, oil bearing capacity, and evolution of pores in the oil window. The thermal stress was calculated as the vitrinite reflectance equivalent (%Re4) based on the Easy%Ro method. The experimental results showed that the residual oil content and oil saturation index (OSI5) increased with increasing maturity at 0.6–1.29 %Re. The clogging of pores by generated oil and intense compaction caused a decrease in the volume of mesopores and macropores at 0.79–1.2 %Re, while abundant organic matter (OM6) pores formed due to hydrocarbon generation and expulsion at 1.29 %Re. Micron-scale dissolution pores and shrinkage OM pores contributed considerably to the total porosity, which improved the performance of the lacustrine calcareous shale reservoir at 1.0–1.29 %Re. Accordingly, the maturity range of 1.2–1.29 %Re is favorable for shale oil exploration in lacustrine calcareous shale due to the high OSI and the widely developed dissolution pores and shrinkage OM pores.

In recent years, the natural gas has displayed a growing significance in oil and gas exploration in the northwestern Junggar Basin (NWJB), although oil has been the main focus of exploration in the basin. Here, we systematically discuss the classification and origin of the natural gases from the NWJB based on the natural gas geochemistry and chemometric methods. The natural gases collected from the NWJB were chemometrically classified into three groups. Group A gases, defined as coal-derived gases, were likely generated from the mixing of the Jiamuhe Formation and Carboniferous strata. Group B gases, defined as the mixing of coal-derived and oil-associated gases, were restricted to the source rocks of group A and C gases. Group C gases, defined as oil-associated gases, were likely derived from both the Fengcheng and Wuerhe Formations, with a higher contribution from the latter strata. The result of this study suggests that the potential of oil generation in the Wuerhe Formation has been underestimated in the past. This is in accordance with geochemical and geological evidence. This study provides an effective chemometric method of natural gas classification and evaluation of hydrocarbon generation potential. This contributes to a better understanding of the origin of gases and distribution of oil and gas, assisting in exploration deployment in the basin.

Geochemical studies of crude oil and source rock play an important role in future exploration in Zhanhua Depression. In this study, thirty-one oil samples collected from Shahejie Formation in Zhanhua Depression, Bohai Bay Basin, NE China have been geochemically analyzed and their organic geochemical characteristics have been applied to differentiate groups of oils. These oil samples can be classified into two families based on multiple biomarker proxies and stable carbon isotopic values. Family I is characterized by a low ratio of pristane over phytane (Pr/Ph < 0.7), a relatively high ratio of phytane over n-C18 (Ph/n-C18), varying ratios of gammacerane over C30 hopane (Ga/C30H) and C22/C21 tricyclic terpane, and a low ratio of C19/C23 tricyclic terpane. Family II is marked by a relatively high Pr/Ph ratio (0.7–1.6), relative low ratios of Ph/n-C18 and C22/C21 tricyclic terpane, and avarying ratio of C19/C23 tricyclic terpane. Both families I and II within these crude oils can be subdivided into two families based on different values of stable carbon isotopic composition of individual n-alkanes. Moreover, the potential source rocks of oil samples in Family I and Family II were likely derived from the upper Es4 member and Es3 member, respectively, based on the correlation of organic geochemical characteristics of the oils and source rocks. The results of oil–source rock correlation provide insight into the process from oil generation to migration and to final accumulation, providing a better understanding of factors controlling oil–gas distribution for prediction of sweet spots.

The origin and geochemical significance of the rearranged hopanes in hydrocarbon source rocks or crude oil have attracted extensive attention. Despite numerous studies, there is not yet a proper conclusion. Therefore, this paper discusses the formation conditions of such compounds and points out their geochemical significance in more detail using a remarkably broad range of source rocks and crude oils from four basins in China. Varying content of rearranged hopanes was found in a total of 19 source rocks and oils from the Ordos, Sichuan, and Tarim basins and the North China Block. Gas chromatography-mass spectrometry (GC-MS) in combination with X-ray diffraction (XRD) and conventional geochemical parameters was used for Pearson correlation analysis to reveal the enrichment mechanisms of rearranged hopanes in the studied rock and oil samples. The GC-MS and XRD results showed that the studied source rocks with high rearranged hopane contents are closely associated with the high abundance of quartz rather than that of clay. Furthermore, the present study reveals that anoxic lacustrine conditions are the primary controlling factors of relatively high abundance of rearranged hopanes in the studied rocks and oils, whereas thermal maturity and terrigenous organic matter input are the secondary factors.

The oil–oil and oil–source rock correlations, also termed as geochemical correlations, play an essential role in the construction of petroleum systems, guidance of petroleum exploration, and definition of reservoir compartments. In this study, the problems arising from oil–oil and oil–source rock correlations were investigated using chemometric methods on oil and source rock samples from the WZ12 oil field in the Weixinan sag in the Beibuwan Basin. Crude oil from the WZ12 oil field can be classified into two genetic families: group A and B, using multidimensional scaling and principal component analysis. Similarly, source rocks of the Liushagang Formation, including its first, second, and third members, can be classified into group I and II, corresponding to group B and A crude oils, respectively. The principle geochemical parameters in the geochemical correlation for the characterisation and classification of crude oils and source rocks were 4MSI, C 27 Dia/C 27 S, and C 24 Tet/C 26 TT. This study provides insights into the selection of appropriate geochemical parameters for oil–oil and oil–source rock correlations, which can also be applied to other sedimentary basins.

In this study, we rediscuss genetic types of natural gases from the Turpan-Hami Basin using classic diagrams and chemometric methods. In comparison with classic diagrams, chemometrics is an improved method based on multi-parameter for natural gas classification. The natural gases were divided into three groups by the classic diagrams, but chemometric results show that they can be divided into four groups and two sub-groups. Furthermore, natural gases can be characterized genetically based on three characteristic geochemical parameters. Chemometrics improves the classification of natural gas in terms of optimizing the proportion of characteristic geochemical proxies genetically with the natural gases.

Organic-rich shales from Lower Silurian are widely distributed in the Middle Yangtze region, central China. However, the lack of fundamental data for shale gas reservoirs increases the difficulty.of gas exploration. In this study, 34 core samples were collected to characterize the shale pore structure and conduct a preliminary evaluation of the shale gas reservoir. The TOC (total organic carbon) content of the successively-deposited black shales range from 1.6% to 5.9%, while the total porosity range from 0.5% to 4.2%. The positive correlation between TOC and porosity indicates that TOC is the key factor determining porosity. The major component of the mineral matrix is quartz (content of 21.4%-69.2%), followed by clay minerals (content of 16.7%-44.5%). Field-emission scanning electron microscopy (FE-SEM) and energy dispersive X-ray spectroscopy (EDS) results illustrate that organic matter, mixed with clay minerals, can form an organo-clay complex containing many nanopores. Furthermore, larger organic pores are developed in organo-clay complexes with higher clay content than in those with lower clay content. Correlational analyses between pore volume (or pore surface area) and TOC (or clay content) demonstrate that micropores are associated with organic matter, while mesopores and macropores are probably associated with clay minerals. Many of the clay-related nanopores are organic in nature and are developed in organo-clay complexes containing both organic matter and clay minerals. Overall, the TOC content controls development of nanopores in the shale pore structure, followed by clay content. The DFT-derived PSD indicates that the pore volume is comprised primarily of pores having widths larger than 10 nm, while the surface area is comprised primarily of micropores. When considering the gas in place model and mechanisms of shale gas storage, further shale gas exploration in central China should aim to the deep (>1000 m) and well preserved Longmaxi Shales. (C) 2017 Elsevier Ltd. All rights reserved.