The Ordos Basin, the second largest sedimentary basin in China, contains the broad distribution of natural gas types. So far, several giant gas fields have been discovered in the Upper and Lower Paleozoic in this basin, each having over 1000×10 8 m 3 of proven gas reserves, and several gas pools have also been discovered in the Mesozoic. This paper collected the data of natural gases and elucidated the geochemical characteristics of gases from different reservoirs, and then discussed their origin. For hydrocarbons preserved in the Upper Paleozoic, the elevated δ 13 C values of methane, ethane and propane indicate that the gases would be mainly coal-formed gases; the singular reversal in the stable carbon isotopes of gaseous alkanes suggests the mixed gases from humic sources with different maturity. In the Lower Paleozoic, the δ 13 C 1 values are mostly similar with those in the Upper Paleozoic, but the δ 13 C 2 and δ 13 C 3 values are slightly lighter, suggesting that the gases would be mixing of coal-type gases as a main member and oil-type gases. There are multiple reversals in carbon isotopes for gaseous alkanes, especially abnormal reversal for methane and ethane (i.e. δ 13 C 1 >δ 13 C 2 ), inferring that gases would be mixed between high-mature coal-formed gases and oil-type gases. In the Mesozoic, the δ 13 C values for gaseous alkanes are enriched in 12 C, indicating that the gases are mainly derived from sapropelic sources; the carbon isotopic reversal for propane and butane in the Mesozoic is caused by microbial oxidation and mixing of gases from sapropelic sources with different maturity. In contrast to the Upper Paleozoic gases, the Mesozoic gases are characterized by heavier carbon isotopes of iso-butane than normal butane, which may be caused by gases generated from different kerogen types.Finally, according to δ 13 C 1 -R o relationship and extremely low total organic carbon contents, the Low Paleozoic gases would not be generated from the Ordovician source as a main gas source, bycontrast, the Upper Paleozoic source as a main gas source is contributed to the Lower Paleozoic gases.Ordos Basin, natural gas, chemical composition, carbon isotope, genesisThe Ordos Basin, located in the middle of China, is the second largest sedimentary basin and one of the most tectonically stable basins in China [1] . Several giant gas fields, each containing over 1000×10 8 m 3 (bcm) of proven gas reserves, have been discovered in this basin, e.g. Sulige gas field as the largest gas field in China.The natural gas in the Ordos Basin is characterized by widespread, various types and multiple preservation, so that many scientists have carried on much research into gas sources, gas types and regional distribution. But

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The light hydrocarbon composition of 209 natural gas samples and individual light hydrocarbon carbon isotopes of 53 natural gas samples from typical humic-sourced gas and sapropelic-sourced gas in the four basins of China have been determined and analyzed. Some identification parameters for humic-sourced gas and sapropelic-sourced gas are proposed or corrected. The differences of compound-specific δ 13 C value of individual light hydrocarbon between humic-sourced gas and sapropelic-sourced gas have been founded. The humic-sourced gas has the distribution of δ 13 C benzene > −24‰, δ 13 C toluene >−23‰, δ 13 C cyclohexane > −24‰ and δ 13 C methyl cyclohexane > −24‰, while the sapropelicsourced gas has the distribution of δ 13 C benzene <−24‰, δ 13 C toluene < −24‰, δ 13 C cyclohexane < −24‰ and δ 13 C methyl cyclohexane < −24‰. Among the components of C 7 light hydrocarbon compound, such as normal heptane (nC 7 ), methyl cyclohexane (MCH) and dimethyl cyclopentane (ΣDMCP), etc, relative contents of nC 7 and MCH are influenced mainly by the source organic matter type of natural gas. Therefore, it is suggested that the gas with relative content of nC 7 of more than 30% and relative content of MCH of less than 70% is sapropelic-sourced gas, while gas with relative content of nC 7 of less than 35% and relative content of MCH of more than 50% is humic-sourced gas. Among components of C 5 -7 aliphatics, the gas with relative content of C 5 -7 normal alkane of more than 30% is sapropelic-sourced gas, while the gas with relative content of C 5 -7 normal alkane of less than 30% is humic-sourced gas. These paremeters have been suggested to identify humic-sourced gas and sapropelic-sourced gas.natural gas, humic-sourced gas, sapropelic-sourced gas, carbon isotope, light hydrocarbon, parameters

Shale gas extraction through hydraulic fracturing and horizontal drilling is increasing in China, particularly in Sichuan Basin. Production of unconventional shale gas with minimal environmental effects requires adequate management of wastewater from flowback and produced water (FP water) that is coextracted with natural gas. Here we present, for the first time, inorganic chemistry and multiple isotope (oxygen, hydrogen, boron, strontium, radium) data for FP water from 13 shale gas wells from the Lower Silurian Longmaxi Formation in the Weiyuan gas field, as well as produced waters from 35 conventional gas wells from underlying (Sinian, Cambrian) and overlying (Permian, Triassic) formations in Sichuan Basin. The chemical and isotope data indicate that the formation waters in Sichuan Basin originated from relics of different stages of evaporated seawater modified by water–rock interactions. The FP water from shale gas wells derives from blending of injected hydraulic fracturing water and entrapped saline (Cl ∼ 50,000 mg/L) formation water. Variations in the chemistry, δ18O, δ11B, and 87Sr/86Sr of FP water over time indicate that the mixing between the two sources varies with time, with a contribution of 75% (first 6 months) to 20% (>year) of the injected hydraulic fracturing water in the blend that compose the FP water. Mass-balance calculation suggests that the returned hydraulic fracturing water consisted of 28–49% of the volume of the injected hydraulic fracturing water, about a year after the initial hydraulic fracturing. We show differential mobilization of Na, B, Sr, and Li from the shale rocks during early stages of operation, which resulted in higher Na/Cl, B/Cl, Li/Cl, and 87Sr/86Sr and lower δ11B of the FP water during early stages of FP water formation relative to the original saline formation water recorded in late stages FP water. This study provides a geochemical framework for characterization of formation waters from different geological strata, and thus the ability to distinguish between different sources of oil and gas wastewater in Sichuan Basin.

The characterization of the pore structure and shale gas content provides useful information for shale gas reservoir assessment and evaluation and guides the exploration and development of shale gas. Fresh core samples obtained from three different basin formations in China were analyzed by field-emission scanning electron microscopy, low-pressure CO 2 and N 2 gas adsorption−desorption, high-pressure mercury intrusion, and methane adsorption experiments to clarify the pore structure characteristics of coal-bearing shales and their effects on shale gas content. The inter-and intraparticle pores, organic matter pores, and microfractures were well developed in coal-bearing shales. These pores had different geneses, morphologies, and sizes with main diameters of <6.5 and 80−200 nm and the main shape of slit, taper, and ink bottle. Pores with diameters <10 nm dominated the shale pore networks. Shale gas content was directly influenced by shale pores, and small pores had a large surface area, which resulted in the high adsorption capacity of shale gas. Clay mineral and total organic carbon contents positively controlled the pore structures and shale gas adsorption, whereas brittle minerals were counterproductive. Coalbearing shale gas content was lower than marine shale gas content, with an adsorption gas content percentage of 50−85%. The proportion of adsorbed gas decreased with the increase of pore size diameters, whereas the proportion of free gas increased. When the pore size diameter reached approximately 3.5 nm, the free and adsorption gases reached dynamic balance. The adsorption gas content would be slightly low with pore size ranges of >6.5 nm, whereas the free gas content would be stable and merely increase in the range of 100−300 nm.