Coal-formed Gas Research Articles

Origin and accumulation models of ultra-low permeability-tight sandstone (gravel) gas in Bohai Bay Basin, China

Ultra-low permeability–tight sandstone (gravel) gas reservoir is an important exploration target for natural gas both domestically and internationally. As a crucial oil and gas exploration basin in China, the Bohai Bay Basin has discovered tight sandstone (gravel) gas in every depression. However, the distribution, origin, and accumulation model of tight sandstone (gravel) gas reservoirs were not systematically studied by previous researchers, who instead mainly concentrated on depressions from the basin-wide perspective. The research conclusively demonstrates that ultra-low permeability–tight sandstone (gravel) gas reservoirs are widely distributed in the Bohai Bay Basin, covering various depressions and multiple strata of Paleozoic, Cenozoic, and Upper Paleozoic coal-formed gas as well as Paleogene oil-type gas. The Upper Paleozoic ultra-low permeability–tight sandstone gas reservoirs are mainly distributed in residual Upper Paleozoic strata of the Huanghua, Linqing, and Jiyang depressions. Most of those reservoirs, which are located in uplifted sag zones, are structural gas reservoirs, and because the strata were buried deeply in the past, most of the reservoirs have worse physical properties. This natural gas is typical coal-formed gas originating in Carboniferous and Permian, and others are Paleogene oil-type gas. Paleogene ultra-low permeability–tight gas reservoirs are widely distributed in all depressions, including the multiple strata in the Kongdian Formation and the second, third. and fourth members of the Shahejie Formation (Es2, Es3, and Es4); The gas reservoirs are located in the uplift, slope, and steep slope zones of the sags, including a variety of trap types, such as lithology, lithology-structure, structure reservoirs, and others. Sandstone reservoirs often found in the Es2 and Es3 are distributed in gentle slopes, whereas sandstone and glutenite reservoirs can be found in the Es4 and Kongdian Formation (Ek), with the glutenites mainly distributed in steep slope area. Since the Paleogene is a continuous deposit, burial depth appears to be a controlling factor in the reservoirs physical properties. Densification threshold depths for distinct depressions range from 3200 m to 4000 m. The majority of the natural gas is gas formed from Paleogene oil-type, while some of it is gas formed from Upper Paleozoic coal. Four different hydrocarbon accumulation models have been developed in this area in accordance with the relationship between gas sources and reservoirs. These models include migration along fault with old source rock and new reservoir, near-source accumulation with new source rock and new reservoir, distant-source accumulation with old source rock and old reservoir, and migration along fault with new source rock and old reservoir, of which the first two are the main hydrocarbon accumulation models. Two sets of Carboniferous–Permian and Paleogene strata are developed, and the remaining natural gas resources have great potential in the Bohai Bay Basin. The Carboniferous–Permian beneficial zone of secondary hydrocarbon generation, the structures adjacent to the margin of the Paleogene gas generation center, and the sweet spot in lithology reservoirs would all be promising areas for further exploration. The research is of instructive significance for tight sandstone (gravel) gas exploration in the Bohai Bay Basin.

Geochemical Characteristics and Gas Source Contributions of Noble Gases of the Sulige Large Tight Gas Field of Upper Paleozoic in Ordos Basin, China

Tight gas is the fastest developing unconventional natural gas resource, becoming the principal part for gas reserves and production growth in China. The Sulige gas field is the largest gas field and also the typical low porosity and low permeability tight sandstone gas field discovered in China, with an annual natural gas output exceeding 300 billion and cumulative output exceeding 290 billion, playing an important role in ensuring national energy provision, helping China’s energy transformation, and promoting green, low-carbon, environmental protection and high-quality development. Based on sample collection and laboratory analysis, natural gas compositions including hydrocarbons, non-hydrocarbons, light hydrocarbons, and noble gases of the Sulige gas field are systematically analyzed, their genetic identifications are identified, and finally gas source originations and contribution proportions are comprehensively discussed from the perspectives of noble gases and hydrocarbon gases. The main achievements are as follows: 1) natural gases in the Sulige gas field are mainly alkane gases, with high methane content, high drying coefficient, low heavy hydrocarbon contents, low non-hydrocarbon gas contents of CO2 and N2, and relatively low noble gas contents. The helium content is relative 2 order of magnitude higher than the atmospheric value, while neon, argon, krypton, and xenon are relatively about 1–2 orders of magnitudes lower than the atmospheric values. 2) The carbon and hydrogen isotopes of alkanes are generally positive sequence distributions with some part inversion. The 3He/4He values are mainly distributed in magnitude of 10−8, the 40Ar/36Ar is ranged from 506 to 1940, the 129Xe is relative loss, and the 132Xe is relative surplus. 3) Natural gases in the Sulige gas field are typical coal-formed gases generated from a humic organic mother material with maturity from high mature to over mature according to C7 and C8 light hydrocarbons and alkane carbon isotopes. Noble gases are typical crustal genesis, mainly originating from the radioactive decay of crustal source materials. 4) The gas source correlations of noble gases and alkane gases and their quantitative evaluations on source contributions show that natural gases in the Sulige gas field are originated from Carboniferous-Permian coal measure source rocks in Ordos Basin, mainly contributed by coals and supplemented by mudstones, accounting for 55–60% and 40–45%, respectively.

Geochemical characteristics and genesis of the Upper Paleozoic natural gas in Longdong area, Ordos Basin, NW China

The Upper Paleozoic gases from the Longdong areas in the Ordos Basin are characterized by low contents of heavy hydrocarbon gases, the mean value of wetness as 2%, relatively heavy δ13C1(δ13C1 value -28.3‰), and large variations in δ13C2 +. The distribution pattern of carbon isotopic sequence of natural gas is mainly characterized by the abnormal light δ13C2 + values and carbon isotopic reversal. According to the organic geochemical characteristics of natural gas and the development of gas-source rocks, the upper Paleozoic natural gas in Longdong area belongs to high mature and over-mature “coal-formed gas”. In theory, only the mixture of highly mature gas with extremely low contents of heavy hydrocarbon gases and a small amount of natural gas with higher contents and lighter carbon isotopic composition of heavy hydrocarbon gases can lead to the occurrence of abnormally light δ13C2 + and the carbon isotope reversal between methane and ethane. The mixed gas is probably derived from the thermal cracking of a small amount of residual soluble organic matter in the gas-source rocks and reservoirs during the initial stage of the uplift movement at the end of Early Cretaceous. Key words: Ethane carbon isotope; Inverted sequence of carbon isotopic composition; Natural gas of high maturity; natural gas genesis; Ordos Basin.

Geochemical characteristics and genesis of tight gas in Shahezi Formation, Xujiaweizi Fault Depression, North Songliao Basin, China

Along with exploration of unconventional natural gas in North Songliao Basin, the tight gas in Shahezi Formation has become a new hotspot, followed by continual breakthroughs in terms of gas exploration. In order to understand the generation and distribution of tight gas in deep-layer of North Songliao Basin, this study focuses on the geochemical characteristics and accumulation period of tight gas, based on analysis of geochemical data such as gas compositions, carbon isotopes and homogenization temperature. According to the result of these analysis, gas in Shahezi Formation is dominated by methane with a light portion of heavy hydrocarbons and generally non-hydrocarbon, such as carbon dioxide and nitrogen. There is a widely distribution of δ13C1 which is ranges from −33.2‰ to −27.8‰, while δ13C2 is higher and have a relatively intensive distribution from −29.2‰ to −22.4‰. Carbon isotopic distribution of tight gas is characterised by a normal pattern, some partial reversal of carbon are observed. The identification of tight gas shows that the main type of tight gas in Shaheizi Formation are coal-formed gas and there are two accumulation periods including Nengjiang period and Qingshankou period. The mix of high maturity coal-formed gas in Nengjiang period and relatively low maturity coal-formed gas in Qingshankou Period is the major cause of carbon isotope reversal. In addition, it is notable that carbon dioxide in study area are inorganic origins, resulted from magma activity in Yingcheng Period.

Discovery and geological knowledge of the large deep coal-formed Qingyang Gas Field, Ordos Basin, NW China

Abstract After 50 years of oil and gas exploration in Longdong area of southwest Ordos Basin, NW China, a deep coal-formed gas field was discovered for the first time in Qingyang area. Through observation of field outcrops and core, analysis of common, cast and cathode thin sections, Ro and other geochemistry indexes, carbon isotope, electron microscope and other supporting tests and analyses, the hydrocarbon generation, reserves formation and reservoir formation characteristics of gas reservoirs at different buried depths in Yishaan slope were examined and compared. The deep gas reservoir has an average buried depth of more than 4 200 m, and the main gas-bearing formation is the Member 1 of Lower Permian Shanxi Formation, which is characterized by low porosity, low permeability, low pressure and low abundance. It is believed that hydrocarbon generation in thin seam coal source rocks with high thermal evolution can form large gas fields, high-quality sandstone reservoirs with dissolved pores, intergranular pores and intercrystalline pores can still develop in late diagenetic stage under deep burial depth and high thermal evolution, and fractures improve the permeability of reservoirs. High drying coefficient of natural gas and negative carbon isotope series are typical geochemical characteristics of deep coal-formed gas. The integrated exploration and development method has been innovated, and the economic and effective development mode of gas fields of “dissecting sand body by framework vertical wells, centralized development by horizontal wells” has been formed. The discovery and successful exploration of the large gas field has provided geological basis and technical support for the construction of natural gas fields of 100 billion cubic meter scale in the southwest of the basin, and has important guidance for exploration of coal-derived gas with deep buried depth and high thermal evolution widely distributed in China.

Geochemical characteristics and exploration potential of Jurassic coal-formed gas in northwest China

Jurassic coal-measure strata are widely developed in Northwest China and mainly distributed in the Tarim Basin, Junggar Basin, Qaidam Basin, and Tuha Basin. The Jurassic coal-formed gas resources are abundant. Although the Jurassic strata in Northwest China have similar tectonic and sedimentary environments, the geochemical characteristics of the coal-formed gas formed in Northwest China have both similarities and differences. Therefore, analyzing and summarizing the geochemical characteristics and exploration potential of the coal-formed gas in the Jurassic petroleum systems in different basins are of great significance for searching for different types of coal-formed gases. Through comprehensive comparison and analysis, it is believed that the drying coefficient of Jurassic coal-formed gases in western China present three types: namely the single peak type of wet gas, the double peak type wet gas-dry gas and the single peak type dry gas; the distribution range of δ13C1 is large, corresponding to the sequence of source rocks from low maturity to over-mature; the partial isotopic inversion of C2 and C3 is mainly caused by the mixed source and biological actions; source rock maturity is the main reason causing the geochemical differences of natural gases in different regions. The Jurassic coal-formed gas in Northwest China has large exploration potential, and the exploration degree of structural gas reservoirs is relatively high. The structural-lithological and lithological gas reservoirs, as well as the main exploration reservoir types for Jurassic coal-formed gas in Northwest China; the Jurassic system in the Kuqa Depression of Tarim Basin, Cretaceous-Jurassic system in the southern margin of the Junggar Basin, the Jurassic high maturity coal-formed gas in the Tuha Basin and the Jurassic high maturity–post-mature coal-formed gas in the Qaidam Basin have great exploration potential, which are important exploration fields for Jurassic coal-formed gas in Northwest China in the future.

Isotopic Composition of Abiogenic Gas Produced in Closed-System Fischer-Tropsch Synthesis: Implications for the Origins of the Deep Songliao Basin Gases in China

In this study, closed-system Fischer-Tropsch synthesis was conducted at 380°C and 30 MPa for 72 h with magnetite as a catalyst. The isotopic composition of the closed-system Fischer-Tropsch synthesis gas and the composition of known abiogenic gas were systematically studied, and the deep Songliao Basin gas was also investigated. The results show that closed-system Fischer-Tropsch synthesis of gaseous hydrocarbon isotopes exhibits a partial reverse order, which includes the reverse order of methane and ethane such as δ13C-C1>δ13C-C2<δ13C-C3 and δ2H-C1>δ2H-C2<δ2H-C3. Furthermore, experimental data on the control of NaBH4 content indicates that the carbon isotopes demonstrate a reverse order on condition that the H2/CO2 (mole ratio) is equal to or greater than 4.0; meanwhile, the hydrogen isotopes show a normal order. The deep Songliao Basin hydrocarbon gas component is similar to thermogenic gas and has a trend of a transition to oceanic hydrothermal system abiogenic gas. In addition, the deep Songliao Basin gas isotopic pattern is different from both Lost City and Kidd Creek where the deep Basin gas carbon isotopic pattern has a reverse order, and the hydrogen isotopic pattern has a normal order. Therefore, the deep Basin gas might be a mixture of the oil-type gas and the coal-formed gas, which could be the cause of the isotopic reverse.

Characteristics of coal-measure source rock and gas accumulation belts in marine-continental transitional facies fault basins: A case study of the Oligocene deposits in the Qiongdongnan Basin located in the northern region of the South China Sea

This study carried out the development and distribution prediction research on coal-formed gas accumulation belts from the perspectives of the development and distribution of the coal measure source rock and the thermal evolution degrees of those rock masses in marine-continental transitional facies fault basins. The purpose of the current study was to improve the accuracy of gas reservoir explorations in the sea areas. The methods used in this study were based on the systematic analyses of the drilling, logging, geochemistry, and seismic data of the Oligocene deposits in the Qiongdongnan Basin of the northern region of the South China Sea. It had been previously established that the sedimentary processes of the fault basin were mainly controlled by faulting, and (fan) deltas had often developed in the fault bending sections, tectonic transformation sections, and fault ends. The distribution along the fault zone was observed to be beaded. During the early Oligocene period of the Qiongdongnan Basin, there were strong extensional actions, and the marine and continental transitional facies strata had developed forming the northern fault depression and the central fault depression. Three (fan) delta belts were formed in the slope belt in the northern fault depression and also in the slope belts located in the north and south of the central fault depression. Due to the superposition of the humid and hot climate during that period, the majority of the coal measure source rocks were developed in each (fan) delta. Also, three macroscopic coal measure source rock belts were formed. The types of source rock had mainly included autochthonous coal, allochthonous coal, and terrigenous marine mudstone, with terrigenous higher plant organic matter accounting for more than 50%. After the deposition of the lower Oligocene period, the basin continued to sink, and the coal measure source rock masses continued to be heated. During that period, the maturity of the organic matter had continued to rise. In the northern slope belt of the northern fault depression, the maturity of the organic matter in the coal measure source rock belt were low and the gas generation abilities were weak, which was advantageous to the formation of potential gas accumulation belts. The coal measure source rock belt on the northern side of the central fault depression had a high maturity of organic matter and strong gas generation ability, forming a macroscopic gas accumulation belt. In addition, the Yacheng 13–1 large coal-formed gas field had been discovered. The maturity of the organic matter had varied greatly in the coal measure source rock belt on the southern side of the central fault depression. The coal measure source rock masses with high maturity had strong gas generation abilities and had also formed a macroscopic gas accumulation belt, and three large and medium-sized coal-formed gas fields (such as Lingshui 17–2) had been discovered. The gas accumulation belts on the southern and northern sides of the central fault depression were favorable areas for coal-formed gas explorations. Furthermore, the trap zones in and near the coal measure source rock belts with mature thermal evolution were determined to be the most favorable areas for coal-formed gas explorations.

Comparison on Rare Gas Geochemical Characteristics and Gas Originations of Kuche and Southwestern Depressions in Tarim Basin, China

The Kuche Depression is considered as the most important gas resource potential and gas exploring area with great gas resource potential and prospect in the Tarim Basin. Based on geochemical experimental analyses and comprehensive geological studies, the general geochemical characteristics of molecular and isotope compositions of rare gases as well as hydrocarbon gases and nonhydrocarbon gases are comparatively studied in the Kuche and Southwestern Depressions. Then, their genetic types are separately identified and gas originations are comprehensively discussed. The main results are as follows. (1) Gas fields in the Kuche Depression have a higher methane abundance, accompanied with low N2and CO2abundances, but the Akemomu gas field in the Southwestern Depression has a relatively lower average methane abundance, accompanied with high average N2and CO2abundances. The helium abundance of natural gases in gas fields from the Kuche Depression general has 1 order of magnitude higher than the air value. Comparatively, it has more than 2 orders of magnitude higher than the atmospheric value in the Akemomu gas field from the Southwestern Depression. The neon, argon, krypton, and xenon abundances in both Kuche and Southwestern Depressions are lower than the corresponding air values. (2) Natural gases from gas fields in the Kuche Depression and the Southwestern Depressions are generally typical coal-formed gases. The rare gases in the Kuche Depression have typical crustal genesis, mainly deriving from the radioactive decay of elements in the crust, while in the Akemomu gas field from the Southwestern Depression, the rare gases have main crustal genesis with a proportion of 92.5%, probably accompanied with a little mantled genetic contribution. (3) Natural gases in the Kuche Depression are generally derived from coal measure source rocks of Jurassic and Triassic, which principally originated from Jurassic in strata period and coals in source rock types. The Jurassic source rocks account for 55%-75% and the Triassic source rocks account for 25%-45% approximately, while coals occupy 68% and mudstones occupy 32% separately. Natural gases from the Akemomu gas field in the Southwestern Depression mainly originated from humic mudstones of marine and continental transitional source rocks of Carboniferous to Permian.

Affecting factors and application of the stable hydrogen isotopes of alkane gases

Abstract To study the composition, affecting factors of the stable hydrogen isotopes of alkane gases and their application to identification of the natural gas origin and maturities, the chemical and isotopic compositions of 118 gas samples of Carboniferous- Permian in the Ordos Basin, and of Triassic in the Sichuan Basin, combined with 68 gas samples from the Sinian and Cambrian reservoirs in the Sichuan Basin, and Ordovician and Siliurian reservoirs of Tarim Basin, are analyzed comprehensively. The following conclusions are obtained: (1) Natural gases in the study area and strata of the Ordos and Sichuan basins are dominated by alkane gases, and the dryness coefficients and maturities of the Carboniferous-Permian gases in the Ordos Basin are higher than the gases in the Triassic Xujiahe Formation of the Sichuan Basin, while the hydrogen isotopes of the latter ones are much enriched in 2 H than the former. (2) The δ 2 H CH4 -C 1 /C 2+3 genetic identification diagram of natural gas was drawn, and the diagrams of hydrogen isotopic differences between the heavy alkane gases and methane vs. hydrogen isotopes of alkane gases can also be used in natural gas genetic identification. (3) The δ 2 H CH4 -R o formulas of coal-formed gas in different areas of the two basins are given, and the δ 2 H C2H6 - δ 2 H CH4 is a new index for maturity, and the ( δ 2 H C2H6 — δ 2 H CH4 )-R o formula of the coal-formed gas can be used to calculate the maturity of the natural gas. (4) The stable hydrogen isotopes of alkane gases are affected by parent materials in source rocks, maturity, mixing and the aqueous medium conditions, among which the aqueous paleo-salinity is the key factor. To sum up, the hydrogen isotopes of alkane gases are affected by multiple factors, and they are significant to the identification of the origin, and maturity of natural gas, and the water environment during the deposition of source rocks.

New indexes and charts for genesis identification of multiple natural gases

Abstract Identification of natural gas genesis and source for high-matured multiple natural gases is a great challenge in the exploration of deep–ultra deep and unconventional natural gases. In this paper, the genesis identification method system of multiple natural gases is enriched through new experimental techniques and comprehensive analysis of geological data. New indexes and charts of genesis identification for multiple natural gases were determined to distinguish the sapropelic kerogen degraded gas and oil cracking gas, accumulated and scattered liquid hydrocarbon cracking gas in different evolution stages, nitrogen, carbon dioxide of organic and inorganic origins, inert gases of crustal and mantled origins, coal-formed gas and oil-typed gas by helium, nitrogen, carbon dioxide and mercury content in natural gas. These indexes and charts have been successfully applied in the Sichuan, Tarim and Songliao basins to identify the natural gas genesis and source for complicated gas reservoirs. The research results have provided effective supports for the natural gas exploration in the Sinian–Cambrian ancient carbonate formations in the Sichuan Basin, deep formations in the Kuqa depression of the Tarim Basin, and deep volcanic formations in the Songliao Basin.

The origin of gas in the Changxing–Feixianguan gas pools in the Longgang gas field in the Sichuan Basin, China

In this paper, the origin of natural gas in the formations of the Changxing–Feixianguan within the Longgang gas field was studied in detail using geochemical methods. The gas discovered has a very high dryness coefficient, yet low ethane and other less heavy hydrocarbons content. Apart from a small amount of N2 and CO2 gasses it generally contains H2S. In the field location, the Changxing–Feixianguan formations itself does not have a hydrocarbon generation potential. Nearing the edge of the Kaijiang-Liangping Trough, there developed the Dalong Formation. However, it also has a very low TOC content in the area of the Longgang gas field, and it cannot act as an effective source rock. The geochemistry of natural gas is much different from the gasses generated by the Silurian and Cambrian source rocks. Therefore, it is impossible that the gas in the Longgang gas field is from the Silurian and Cambrian source rocks. Gas reservoirs generally contain bitumen which is considered a product of crude oil cracking. The carbon isotope fractionation between the bitumen and methane is not distinct, and it indicates that the gas is not directly from oil cracking. The carbon of methane and ethane has isotopically less negative value, which is considered to be in a high-overmature coal-formed gas, mainly from the Longtan Formation coal measures. In comparison to the gas from high overmature stage obtained from the Xujiahe coal measure source rock in the Western Sichuan Depression. The methane in the Longgang gas field has abnormal less negative carbon isotopic value. It is due to the superposition of these two factors together: higher evolution of source rocks and mixing of gas degassing from the water. It is not caused by TSR that most researchers believed at present because the methane carbon isotopic values have no relationship with H2S content.

Rare gases geochemical characteristics and gas source correlation for Dabei gas field in Kuche depression, Tarim Basin

The total components and isotopes of rare gases as well as hydrocarbon gases for Dabei gas field are analyzed firstly, then their geochemical characteristics and genesis are discussed respectively, finally gas source correlation and their contributions are evaluated. The research results show that: (1) In Dabei gas field, methane is the main component, accompanying with a few N2 and CO2, as well as extremely low rare gases. In total component contents of rare gases, the He content is from 56.0 to 60.4 ppm with an average of 57.8 ppm and generally 1 order of magnitude higher than the air content value, while the Ne, Ar, Kr, and Xe contents are generally 1–3 orders of magnitude lower than the corresponding air content values. (2) The δ13C1 value in Dabei gas field usually ranges from −31.9‰ to −29.4‰. The δ13C2 value generally ranges from −24.2‰ to −19.4‰, which is obviously heavier than −28.5‰, The natural gases in Dabei gas field can be identified as typical coal-formed gas. (3) The 3He/4He value in natural gases is generally from (6.31–11.24) × 10−8 with an average value of 8.42 × 10−8. The 20Ne/22Ne value is from 9.563 to 9.734, and the 21Ne/22Ne value is from 0.0298 to 0.0307. The 40Ar/36Ar value is usually from 390 to 858 with an average value of 552, and the 38Ar/36Ar value is from 0.1955 to 0.2035. The 129Xe is relative loss, while the 132Xe is relative surplus. The comprehensive genesis identifications of He, Ne, Ar, and Xe indicate that rare gases are crustal derived genesis, mainly originating from the decay of related radioactive elements in crust. (4) The gas source correlation indicates that the natural gases in Dabei gas field are originated from humic coal series source rocks of Triassic-Jurassic, mainly contributed by Jurassic in vertical strata and coal rocks in type, coal rocks accounting for 63% and mudstones accounting for 37%.

Geochemical characteristics of the shallow soil above the Muli gas hydrate reservoir in the permafrost region of the Qilian Mountains, China

In this study, the Muli gas hydrate reservoir in the Qilian Mountains was chosen as a test area for the geochemical exploration of gas hydrates in mid-latitude regions. Soil headspace gases, acid-extracted hydrocarbons and stable carbon isotopes of methane, and soil magnetic susceptibility as well as microbes were tested. The results show that the distribution of geochemical anomalies can be well correlated with the underlying gas hydrate reservoir. Acid-extracted hydrocarbons, soil headspace gases, and the stable carbon isotopes of methane can be considered as major indicators for geochemical exploration of gas hydrates, whereas magnetic susceptibility and microbes served as complementary indicators. The stable carbon isotopes of methane and the hydrocarbon composition of the surface geochemical anomalies indicated a thermogenic origin, which shows that the gas source of the potential gas hydrate reservoir in this area may be contributed by deep oil and coal-formed gases. An accumulation model for the gas hydrate reservoirs was also developed and an integrated exploration project for gas hydrate, oil and coal bed methane is also proposed for the Muli area.

Research on Coal-Formed Gas Reservoir Combined Qualitative and Quantitative Methods

At present, coal is still the primacy of China's energy structure, and this situation will not change in a short time. Coal-formed Gas Reservoir quality and the predicting method have also become one of the urgent needs to address the issue of energy exploration, but the lateral variation of the coal-formed gas reservoir is a common geological phenomenon in the coal mine, if the prediction error is too large, it will greatly impact on the production of coal mine. Especially in the area where has complex structure characteristics, the reservoirs are impacted by structural changes, the reservoir predicting become the most important job and the most difficult job. This paper take the example of DaCheng, the research carried out the sedimentary reservoir characteristics, the coal body structure, the coal thickness, the heterogeneity of the coal seams and collapse columns of coal reservoir, and then reservoir fine evaluation was done combined Micro and Macro, Qualitative and Quantitative methods.

Characteristics of the Permian coal-formed gas sandstone reservoirs in Bohai Bay Basin and the adjacent areas, North China

This paper focuses on the physical properties and spatial distributions of the Permian sandstone reservoirs in Bohai Bay Basin and the adjacent areas in the northeast China. The coal-formed gas sandstone reservoirs occur in the Permian Shihezi Formation and the maturity of the sandstone in the lower part is gradually higher than that in the upper part. The high-maturity sandstones are mainly the channel lag deposits and widely distributed in the study area. Due to the low porosity and permeability, however, the sandstones formed only the medium-poor reservoirs. The sandstones contain clay matrix and commonly a small portion of calcareous, siliceous, and authigenic clay cement. On the other hand, severe compaction and cementation caused nearly complete destruction of the primary porosity during diagenesis. Therefore, the main space of the sandstone reservoirs comes from the secondary pores. The distributions of reservoirs are affected by several periods of faulting and denudation. The sandstones of the Lower Shihezi Formation are thicker in the Jizhong and Bozhong depressions, and Luxi areas, and those of the Upper Shihezi Formation are thicker in Jizhong and Jiyang depressions and thinner in Luxi areas.

Gas Hydrate Resource Potential and its Exploration and Development Prospect of the Muli Coalfield in the Northeast Tibetan Plateau

The Muli coalfield of Qinghai province is rich in gas hydrate, which is an important alternative energy in the post-petroleum era. The results of the preliminary exploration and estimation show that the amount of gas hydrate reaches approximately 300 million m3 in the coalfield and surpasses 500 billion m3 in its surrounding regions according to the calculation method used in Messoyakha gas field. The results indicate that Qilian Mountain permafrost area is extremely abundant in gas hydrate resources. Therefore, it will be rewardful to research on how to explore and exploit its gas hydrate resources in this area. In the present paper, the characteristics of structure, fault and the coal seams in Muli coalfield were analyzed; the formation condition and the stability zone of its gas hydrate were investigated; a model of “coal-formed gas hydrate” was identified and the reserves of gas hydrate in the Muli coalfield and the potential reserves of gas hydrate in its surrounding area were estimated.

Geochemical Characteristics of Carboniferous-Permian Coal-Formed Gas in Bohai Bay Basin

Coal-formed gas reservoirs have been found in several depressions in Bohai Bay Basin. The gas was mainly generated by the Carboniferous-Permian coal measures, which are good source rocks. The exploration of coal-formed gas has a broad prospect. The main reservoirs of the coal-formed gas are Ordovician, Carboniferous-Permian, and Paleogene stratum. Coal-formed gas in the Bohai Bay Basin is chiefly composed of hydrocarbon gases. The percentage content of carbon dioxide is more than that of the nitrogen gas. The stable carbon isotope values of the hydrocarbon gases of different depressions and different reservoirs usually reversed. The reversed values of gas samples account for 52.1% of all the samples. Reversion values of the carbon isotope are mainly because of the mixing of gases from same source rocks but with different maturity. Among the three main reservoirs, coal-formed gas preserved in Paleogene stratum has the heaviest carbon isotope, the second is the gas in Carboniferous-Permian stratum, and the Ordovician gas possesses the lightest carbon isotope. Based on the analysis of the characteristics of carbon isotope of hydrocarbon gases in well Qishen-1 and the distribution of the Carboniferous-Permian coal measures, the gas of the well is derived from the high-matured Carboniferous-Permian coal measures.

Geochemical Characteristics of Natural Gas in the Yinggehai-Qiongdongnan Basin, South China Sea

Geochemical characteristics of natural gas in the Yinggehai-Qiongdongnan Basin were studied and analyzed based on gas component properties and carbon isotopic characteristics which were tested and collected of more than 100 samples. The gas in the Yinggehai-Qiongdongnan Basin was generally alkane gas, and all the area showed dry gas characteristics except Yanan Depression and its periphery area. The gas in the Basin is mainly high mature -over mature coal formed gas except Songnan and eastern of the second fault belt. There are three genetic types of the natural gas in the Yinggehai-Qiongdongnan Basin, such as microbial gas, coal-formed gas and the mixture of them. The coal-formed gas is the main genetic type.

Carbon Isotopic Characteristics of Jurassic Alkane Gases in the Sichuan Basin, China

There are plentiful natural gas resources in Jurassic in the Sichuan basin, where a lot of gas fields (reservoirs) have been discovered, and the natural gas is mainly alkane gas. Comprehensive study on carbon isotopic characteristics indicated that Jurassic alkane gas in Sichuan Basin can be divided into two types, coal-formed gas and oil-type gas. As the majority of the natural gas, coal-formed gas distributed in Central and Northwest gas districts and was mainly from the underlying coal-measure hydrocarbon source rocks in Xujiahe Formation. The carbon isotopic series were mainly normal, while a minority were reversed. The reversal ones in Northwest gas district were caused by mixing of gases from the same source rocks with different maturity, while those in Central gas district were caused by mixing of oil-type and coal-formed gases. Oil-type gas was only discovered in Bajiaochang gas field in Central oil and gas district, and derived from self-generation and self-accumulation in Lower Jurassic, with normal carbon isotopic series and without any reversal. Comprehensive analysis suggested that the carbon isotopic characteristics of Jurassic alkane gas were different according to different regions and layers. The δ13C1 and δ13C2 values of Jurassic alkane gas in central area were significantly lower than those in northwestern area for the influence by oil-type gas generated from Lower Jurassic Ziliujing Formation.