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Study of the occurrence and formation of sedimentary organic matter in the Sichuan Basin, southern China, provides valuable insights into the biological sources and compositions of Neoproterozoic to Lower Cambrian shales. Rock samples from two sets of shales in the Sichuan Basin and adjacent areas, including the Ediacaran Doushantuo shale and the Lower Cambrian Qiongzhusi shale in the Gaoshiti-Moxi and Ziyang-Weiyuan areas were collected and geochemically analyzed in this study. The distribution of biomarkers, stable carbon isotope compositions and microfossil characteristics reveal that eukaryotic algae including chlorophytes, pelagophyceae, dinoflagellates, diatoms (or their ancestors) and sponges proliferated widely during the Late Neoproterozoic to Early Cambrian in the Sichuan Basin. Cyanobacteria, some α-Proteobacteria, rhizobiales and aerobic methanotrophs also made a contribution to organic matter in the studied shales. Aerobic methanotrophs that utilize 13C-depleted substrates such as CH4 as carbon sources may have been the cause of anomalous 13C-depleted kerogen in the study area.

Pyrobitumen has been discovered in the dolostone reservoir of the Central Sichuan Basin, southwest China, accumulates coevally with hydrothermal minerals in pores. The pyrobitumen has strong anisotropy with an evolutionary sequence from mosaic to a fibrous texture. A similar sequence of anisotropy has also been observed in mesophase pitch during the process of coking coal. Reflectance of the pyrobitumen shows a positive correlation with anisotropic evolution and increasing depth. Hopane and sterane biomarkers are abundant in extracts of highly anisotropic pyrobitumen with smooth, rough, hyperplastic surface and honeycomb micropores. Methylpyrene and methylchrysene maturity indicators and the reflectance values of pyrobitumen show a positive linear correlation in Cambrian samples, but show random correlation in Sinian samples. Fluid inclusion microthermometry of hydrothermal minerals, together with the burial and heat flow history of the reservoir indicate an abrupt thermal event that may have impacted the generation and evolution of pyrobitumen. This appears to be confirmed by the reflectance values of pyrobitumen in the source rock and reservoir. The pyrobitumen in source rock was less heated by the hydrothermal fluid due to its poor porosity and have lower reflectance values. However, the pyrobitumen in weathering crust reservoir was more heated by the hydrothermal fluid and have higher reflectance values, because the reservoir rocks have higher porosity. The occurrence and characteristics of pyrobitumen indicate an in-situ pyrolysis of oil, which was triggered by an abrupt thermal event in the reservoir. The reconstruction of burial history and heat flow, as well as the hydrothermal fluid minerals of the dolomite reservoir all indicate the thermal event of the Sichuan Basin might correlate with the Emei mantle plume in Late Permian.

The lower Cambrian sediments including the Qiongzhusi and Maidiping formations in the Sichuan Basin are generally considered as the main source rocks of Precambrian-Cambrian petroleum system. A comprehensive study integrated with multiple geochemical methods was conducted on 119 lower Cambrian drillcore samples collected from the upper Yangtze Platform. The results show that the fluctuation of sea level and climate during the deposition of the Qiongzhusi and Maidiping formations act as the critical factors controlling the watermass conditions and thus OM accumulation. During the deposition of upper Maidiping Formation (Є1m), the paleoclimate was arid and sea level was relatively low in the study area. The depositional environment indexes (UEF-MoEF, gammacerane index, β-carotane and VEF) suggested that the upper Є1m was deposited in a strong restricted marine setting with high water salinity and anoxic water conditions in Ziyang-Weiyuan area. However, during the early Qiongzhusi Formation period, the paleoclimate became humid and sea level rose in the study area. High paleoproductiviy caused by high concentrations of nutrients brought by seawater and freshwater plays an important role in OM accumulation in the basal part of the Qiongzhusi Formation (Є1q). Furthermore, the model of paleoproductivity promotion caused by the coordinated control of climate and extensive transgression in the lower part of Є1q may also be applicable to interpret the origin of high organic carbon abundance in the strata deposited in the same period with the Є1q (such as Shiyantou, Shuijingtuo and Niutitang formations) in other parts of the upper Yangtze Platform.

Dibenzofuran and its methyl substituted homologues are the important oxygen-containing polycyclic aromatic compounds in crude oils. The ratio of 1-/4-methyldibenzofuran has been successfully used in sandstone reservoir tracing oil migration orientation. Here we focus on the geo-chromatographic fractionation effect of methyldibenzofuran (MDBF) in dolomite reservoirs. We calculated the charge density differences between 1-methyldibenzofuran (1-MDBF) and 4-methyldibenzofuran (4-MDBF) based on density functional theory. The E I n between MDBF isomers and dolomite (104) surface was also calculated according to molecular dynamics simulation. The result show that comparing with the 4-MDBF, there is strong interaction between 1-MDBF and dolomite (104) surface leading the ratio of 1-MDBF and 4-MDBF increase with the migration distance. There is no significant correlation between bitumen reflectance and the ratio of 1-MDBF and 4-MDBF in our samples. In addition, molecular simulation results show that the MDBF isomers are thermodynamically stable and dissociation of Ph-CH3 is difficult to occur under geological condition. All these demonstrate that 1-MDBF and 4-MDBF can be survived after oil-cracking to gas and pyrobitumen. Therefore, in theory, the ratio of 1-/4-methyldibenzofuran is also regarded as an index for oil migration orientation in dolomite reservoir. Gas chromatography-mass spectrometry results show that MDBFs occur at relatively higher abundance in the bitumens of the Ediacaran Dengying Formation (Z2dn4) and Cambrian Longwangmiao Formation (Є1l) in the Sichuan Basin. The ratio of 1-/4-methyldibenzofuran has been successfully used to trace the crude oil filling pathway in the Z2dn4 and Є1l dolomite reservoir in the Sichuan Basin in this study. Two preferential filling orientations (i.e. from west to east and from north to south) for paleo-oil have been identified in the contour maps of 1-/4-methyldibenzofuran in the Z2dn4 and Є1l, which is consistent with the geological background.

The chemical and carbon isotopic compositions of C1–C3 and C6–C7 light hydrocarbons were analyzed in 14 natural gas samples from the Kela 2, Dabei and Dina 2 gas fields in the Kuqa depression of the Tarim Basin. The relative content of light aromatic compounds (benzene and toluene) in C6–C7 light hydrocarbons is very high, with benzene as the main peak in the gas chromatogram. The relative content of aromatic compounds generally amounts to over 30% of the C6–C7 light hydrocarbons, especially in the Kela 2 gas field where light aromatics comprise more than 70% of the C6–C7 light hydrocarbons. Compared with 255 coal derived gas samples in many other sedimentary basins in China, an abnormal distribution of hydrocarbon composition is observed in the study area. In order to investigate this abnormal distribution, light hydrocarbon yields generated from low maturity coals at different pyrolysis temperatures were simulated quantitatively. The results showed that the yield of light aromatic compounds (benzene and toluene) gradually increases with increasing pyrolysis temperature. Furthermore, the relative content of aromatic hydrocarbons generated from coal samples at high temperatures, similar to an instantaneous accumulation mode under geologic conditions, can amount to 90% of C6–C7 light hydrocarbons, while that of aromatic hydrocarbons generated cumulatively at high temperatures, similar to a cumulative accumulation mode in natural geologic bodies, and is no more than 46.6%. These phenomena have not yet been reported in the literature. By combining these data with carbon isotopic data for natural gas components, it is proposed that an abnormal distribution of aromatic hydrocarbons in gases from the Kuqa depression may imply that the Kela 2 gas field was mainly formed by instantaneous gas accumulation, while the Dabei and Dina 2 gas fields were largely formed by cumulative gas accumulations.

After analyzing the molecular and stable isotope compositions of 45 natural gas samples in the Changxing and Feixianguan Formations in northern Sichuan basin, significant differences have been observed between the samples in the northwest Kaijiang-Liangping trough and those in the northeast. The δ13C2 values of the gas in the northeast of Kaijiang-Liangping trough (δ13C2=−25.0‰∼−32.3‰ with an average of −28.2‰)are lighter than those in the northwest of Kaijiang-Liangping trough (δ13C2=−21.3‰∼−28.9‰ with an average of −24.8‰). The δD1 values of the gas in the northeast (δD1=−114‰∼−129‰ with an average of −121‰) are also lighter than those in the northwest (δD1=−107‰∼−115‰ with an average of −113‰). By analyzing the kinetic fractionation of carbon isotope in methane and ethane, we find that gas in the northwest of Kaijiang-Liangping trough originated from the P2l coal measures while those in the northeast were sourced from the thermal cracking of oil, which are most likely generated from the marine source rocks in the Upper Permian Longtan formation(P2l) and the Silurian. The CO2 content varies from 0 to 30.20% with the δ13CCO2 values ranging from −2.9‰ to 3.3‰, which suggests that the CO2 may possibly originate from the carbonate reservoirs dissolved by acidic fluids. Thermochemical Sulfate Reduction (TSR) has resulted in the richness of deuterium in the methane.

Two sets of coal-bearing measures, the Upper Triassic Xujiahe Formation (T 3 x) and the Upper Permian Longtan Formation (P 2 l), have been extensively developed in the Sichuan Basin, SW China and some large gas fields related to these coal-bearing measures have been discovered. The stable carbon and hydrogen isotopic composition of alkane gases sourced from the two sets of coal-bearing measures (T 3 x and P 2 l gases) have been investigated in this study. The main controls on the gas composition and isotopic abundances of gases in the Sichuan Basin have been identified and distinguished on the basis of their molecular and isotopic composition. The carbon and hydrogen isotopic compositions of methane ( δ 13 C C1 : − 42.5‰ to − 26.8‰; δ D C1 : − 173‰ to − 107‰) vary widely and show a positive correlation with the thermal maturity of the source rocks. The δ 13 C C2 value of the T 3 x and P 2 l gases in the Sichuan Basin ranged from − 28.2‰ to − 21.3‰, and showed less variations with increasing maturity from R o values of 1.2% to 2.8%. However, these δ 13 C C2 values differ greatly from that of oil-associated gas in the Sichuan Basin. Thus we provide further evidence that the value of δ 13 C C2 is a useful parameter to distinguish coal-derived and oil associated gases. The hydrocarbon gases in the Xujiahe and Jurassic reservoirs in contrast showed little isotopic difference indicating that secondary migration has little influence on their carbon and hydrogen isotope compositions. In addition, the δ 13 C values of P 2 l CO 2 (− 2.6− to 3.3‰) may reflect an origin from dissolution of the carbonates in the reservoir by acidic fluids containing H 2 S.

An experimental study on the performance of an adjustable ejector is presented. An axial spindle extending into the nozzle is used to change the nozzle throat area to make the ejector adjustable. The variation of operation parameters, such as pressure, temperature, and entrainment ratio, was investigated for several typical spindle positions. The tests observe that when the nozzle throat cross section area of the adjustable ejector was reduced, the entrainment ratio was raised at the expense of the falling of discharge pressure. These characteristics of the adjustable ejector provide potential for a more economical design and operation.