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High resolution carbon isotope analyses of carbonate and organic carbon from Meishan, South China showed that the variation of δC is marked by three large positive excursions during the Changhsingian (end-Permian). Carbon isotope stratigraphy during this stage shows three cyclic intervals in δC, with two cycles corresponding to the lower (Paleofusulinid minima Zone) and one corresponding to the upper Changhsingian (P. sinensis Zone). The large positive δC excursions indicate episodes of enhanced burial of isotopically light organic carbon, presumably in response to deep-water anoxia episodically extending into shallow water with the rise of sea level. The organic carbon during the Changhsingian is distinguished into two groups, and the δC of each group parallels (separately) the more detailed profile of δC, strongly showing that the values of fractionation ΔC remain relatively constant, with only two intervals with anomaly. The enhanced fractionation ΔC with large negative δC excursions apparently indicates significant inputs from sulfide-oxidizing bacteria and green sulfur bacteria, notably at bed 24 just predating mass extinction. Our evidence appears to support that the extended euxinic water is possible for the main pulse of mass extinction at the end-Permian. [ABSTRACT FROM AUTHOR]

The age-accumulation effect of Ar in hydrocarbon source rocks was discussed in accordance with the decay law of radioactive elements. In terms of the mean values of Ar/Ar, the old Sinian gas reservoirs (mean values of Ar/Ar: 7009) were definitely distinguished from the Permian gas reservoirs (mean values of Ar/Ar: 1017) in Weiyuan, Sichuan Province, and the gas source of the Permian gas reservoir (mean values of Ar/Ar: 5222) in well Wei-7 with the Weiyuan structure is defined as the Sinian system. Based on the values of Ar/Ar, the coal-type gases (The source rocks are of the C-P system; mean values of Ar/Ar: 1125) are definitely distinguished from the oil-type gases (The source rocks are of the Tertiary system; mean values of Ar/Ar: 590) in the Tertiary reservoirs of the Zhongyuan Oilfield. Besides, Ar/Ar values also have a positive effect on the oil-source correlation of oil reservoirs in ancient hidden mountains. According to the crust-mantle interchange information reflected by He/He values, petroliferous provinces in China can be divided into three major tectonic regions. (1) The eastern active region: The crust-mantle volatile matter exchanges actively, and the He/He values are mainly around 10, partly around 10. (2) The central stable region: The He/He values are all around 10. (3) The western sub-stable region: The He/He values are mainly around 10, and around 10 on the edges of the basins. Helium contents of some gas wells in China's eastern petroliferous region reach the industrial abundance (He≈0.05%-0.1%), the He/He values reach 10, and the equivalent values for the mantle-source components in helium gas can reach 30%-50%. As viewed from this, a new type of crust-mantle composite helium resources has been proposed. Geneses of some CO gas reservoirs in the east of China and some issues concerning mantle-source methane were discussed in the light of the helium and carbon isotopes of CO and CH in natural gases. In the discussion on helium isotopic characteristics of inclusions in the reservoirs, it was discovered that the He/He values are close to those in natural gases. That is to say, this phenomenon is related to regional tectonism. The He/He, CO/He and CH/He data were used to discuss the tectonic activities of fault zones in a certain number of regions in China. [ABSTRACT FROM AUTHOR]

Ordovician natural gases in the Tahe Oilfield are composed predominantly of hydrocarbon gases dominated by methane with a significant amount of heavy hydrocarbon gas component. The non-hydrocarbon gases include N2, CO2 and minor H2S. The Ordovician natural gases are believed to have originated from the same source rocks, and are composite of gases differing in thermal maturity. Carbon dioxide was derived from thermal metamorphism of Ordovician carbonate rocks. The generation of natural gases involves multiple stages from mature normal oil and condensate-associated gas to thermally cracked gas at the maturity to over-maturity stages. In the main part of the Tahe Oilfield, the Ordovician natural gases appear to be filled in two major phases with a typical petroleum-associated gas from southeast to northwest and from east to west in the early stage; and a thermally cracked gas from east to west in the late stage. At the same time, the oil/gas filling boundary has been primarily established between the two stages. [ABSTRACT FROM AUTHOR]

This study analyzed the developing environments of hydrocarbon source rocks in the Ordos Basin and evaluated carbonate rocks as hydrocarbon source rocks and their distributions on account of the fact that China’s marine carbonate rocks as hydrocarbon source rocks are characterized by the intensive thermal evolution and relatively low abundance of organic matter, by taking the Lower Paleozoic of the Ordos Basin for example and in light of the calculated enrichment coefficients of trace elements, the profile analysis of trace element contents, ratios of relevant elements, and stable isotopic compositions and their three-dimensional diagrammatization in combination with the necessary organic parameters. As for the Ordos Basin, TOC=0.2% is an important boundary value. Studies have shown that in the strata TOC>0.2%, V/(V+Ni)>0.50, Zr/Rb<2, Rb/K(× 104)>30, Z<122, Th/U>0.80, Zn and Mo are enriched with a positive δ13Ccarb excursion. All these indicated a stagnant and stratified sedimentary environment that has low energy, anoxia and high salinity in bottom water. In these strata the geological conditions are good for the preservation of organic matter, hence favoring the development of hydrocarbon source rocks. These strata have δ13Corg <−28‰ (I–II type) and high hydrocarbon-generated potential. The Klimory and Wulalik formations show certain regularities in those aspects, therefore, they can be regarded as the potential effective hydrocarbon source rocks. In the strata TOC⩽0.2%, Zr/Rb>1, V/(V+Ni)<0.50, Rb/K<30, Th/U<0.80, Cu, Zn, etc are depleted, and δ13Corg values range from −24‰ to −28‰. All these facts showed that most of the carbonate rocks or mudstones were formed in high-energy oxidizing environments, thus unfavorable to the development of hydrocarbon source rocks. It is feasible to make use of the geochemical method to comprehensively assess the highly evolved marine carbonates rocks as potential hydrocarbon source rocks and their distributions. [ABSTRACT FROM AUTHOR]

Natural gases of shallow reservoirs with the carbon isotopic compositions of methane ranging from -50‰ to -60‰ (PDB) were considered as mixed gases of biogenic and thermogenic origins previously and some of them were considered as low-mature (or low temperature thermogenic) gases lately. In this paper natural gases with the carbon isotopic compositions of methane in the above range were identified using the molecular and stable carbon isotopic compositions of methane, ethane and propane. The mixed gases of biogenic and mature thermogenic origins display the characteristics of δ 13C1 ranging from -50‰ to -60‰, δ 13C2 > -35‰, Δ values (δ 13C3 - δ 13C3) < 5‰ and C1/ΣC2+ ratios < 40. Immature to low-mature gases display the characteristics of δ 13C1 ranging from - 50‰ to - 60‰, δ 13C1 < -40‰, Δ values (δ 13C3 - δ 13C3) > 7‰, and C1/ΣC2+ ratios >60. [ABSTRACT FROM AUTHOR]

The Jiyang Sag and the Liaohe Basin are the two important areas where immature nil resources are distributed in China. From these two areas immature-low mature to mature oil samples were collected for carbon isotopic analysis. The extracts of source rocks are dominant in the Jiyang Sag while crude oils are dominant in the Liaohe Basin. The maturity index, Ro, for source rocks varies from 0.25% (immature) to 0.65% (mature). Studies have shown that within this range of Ro values the extracts of source rocks and crude oils, as well as their fraction components, have experienced observable carbon isotope fractionation. The carbon isotopic values tend to increase with burial depth, the oils become from immature-low mature to mature, and the rules of evolution of oils show a three-stage evolution pattern, i. e. , light → heavy → light → heavy oils. Such variation trend seems to be related to the occurrence of two hydrocarbon-generating processes and the main hydrocarbon-forming materials being correspondingly non-hydrocarbons and possessing MAB characteristics, lower thermodynamic effects and other factors. In the process towards the mature stage, with increasing thermodynamic effects, the thermal degradation of kerogens into oil has become the leading factor, and correspondingly the bund-breaking ratio of 12C-13C also increases, making the relatively 12C-rich materials at the low mature stage evolve again towards 13C enrichment. [ABSTRACT FROM AUTHOR]

A low-mature coal (R0 = 0.4%, from the Manjia'er depression, Tarim Basin, China) was subjected to closed system pyrolysis, in sealed gold tubes, under isothermal temperature conditions. The carbon isotopic compositions of the pyrolyst fractions (hydrocarbon, CO2, CO, etc.) at two temperature points (350°C and 550°C) were measured. The results showed that δ 13CCH4 value is generally heavier at 350°C than that at 550°C, because the high abundance of CO generated at low temperature would greatly influence δ 13CCH4 value, and the retention time of CO in gas chromatograph is close to that of CH4. But CO is formed through chemical reaction of the oxygen-containing functional group -C = O, e.g. lactones, ketones, ether, etc. at low temperature, while CO2 comes mainly from decarboxylization. The carbon isotopic composition of coal gas from Lanzhou Coal Gas Works as definitely different from that of thermally pyrolysed products from coal. The δ 13CCH4 value of coal gas was abnormally heavier than δ 13CCO. At the same time, the reversed sequence (δ 13C1 > δ 13C2) of δ 13C1 and δ 13C2 happened. The bond energy of free ions generally decides the sequence of generation of hydrocarbon fractions according to the chemical structure, whereas the stability of pyrolysate fractions and their carbon isotope fractionation are affected by the C-C bond energy. [ABSTRACT FROM AUTHOR]