Coal-formed Gas Research Articles

Progress of coal-formed gas geological theory and its status in natural gas industry in China

Abstract The concept of coal measures forming natural gas was proposed by German scholars in the 1940s. By the late 1970s, coal-formed gas accounted for almost 70% of the proved gas reserves in the world. Geological theory of coal-formed gas in China has gone through a process of introduction, absorption, innovation, development and forming into a system. The theoretical system, which covers coal-formed gas (conventional resource) and coalbed gas (non-conventional resource), includes gas source identification, coal-formed gas accumulation and distribution, as well as evaluation and prediction method. It has given good directions for the establishment and development of natural gas industry in China and accelerated the growth of reserves and production of natural gas in the past 30 years. It is also a successful case in satisfying the national need through combining major scientific and technological issues with practice closely.

Geochemical characteristics of coal-measure source rocks and coal-derived gas in Junggar Basin, NW China

Carboniferous and Jurassic coal-measure rocks with their relative coal-formed gas are developed in Junggar Basin. Carboniferous source rocks have high organic material abundance, Type III organic material, and high maturity. Jurassic source rocks are widely distributed with big thickness and great maturity variety. The Middle and Lower Jurassic are the predominant sources of natural gas, and the potential is large. Carboniferous coal-formed gas is dry gas, its carbon isotope is heavy, the methane carbon isotope composition is about −30‰, the ethane carbon isotope composition is above −28‰, the carbon isotope series is positive (δ13C1δ13C2δ13C3δ13C4). Gas reservoir is self-generating and self-preserving. Gas from the Jurassic source rocks has wide distribution and is wet. Methane carbon isotope composition varies greatly, and the carbon isotope reversal of butane and propane is common, reflecting that natural gas has different maturity and the mixing of different-maturity natural gas originated from the same source results in the light isotopic values of heavy hydrocarbons. The potential of the Carboniferous rocks is huge, and is the major exploration area. Gas from the Jurassic source rocks is distributed widely and the southern margin of the basin is a favorable area.

Major developments of coal-formed gas exploration in the last 30 years in China

Abstract Since its formation in China 30 years ago, the theory of coal-derived gas has not only opened a new field of coal-derived gas exploration, but also increased the proved reserves of coal-derived gas from 1/10 to 7/10 in the total gas of China; the nation's proved gas reserves rise sharply from 2 264×10 8 m 3 to 6.4×10 12 m 3 ; the annual production of gas rises from 137.34×10 8 m 3 to 760×10 8 m 3 ; and the nation's coal-derived gas fields (reservoirs) increases from less than 10 (excluding Taiwan) to 124. After briefing the major developments of China's coal-derived gas exploration, this paper concentrates on the exploration development of coal-derived gas in Ordos Basin, Tarim Basin and Sichuan Basin where large numbers of coal-derived gas fields have been discovered. With carbon isotopic data of alkane gases, it also demonstrates that some gas fields of controversial origin (Jingbian, Kekeya and Akemomu gas fields) are of, or predominantly of, coal-derived gas, and that a few gas reservoirs (Wolonghe, Naxi, Hejiang gas fields) in the Xujiahe Formation coals of Sichuan Basin are oil-associated gas.

Natural gas origins of large and medium-scale gas fields in China sedimentary basins

China sedimentary basins present abundant natural gas resource thanks to its unique geological settings. Marine highly-matured hydrocarbon source rocks, widespread coal-measure strata and low temperature Quaternary saline strata, etc., indicate the wide foreground of China natural gas resources. Up to now, most of the petroliferous basins have been discovered to have wholesale natural gas accumulation from Precambrian, Paleozoic, Mesozoic to Cenozoic in the east, the central, the west and the coast of China. These large and medium-scale gas reservoirs are mainly composed of hydrocarbon gas with big dry coefficient, tiny non-hydrocarbon, wide carbon isotope distribution and varying origin types, the hydrocarbon gas includes coal-formed gas, oil-formed gas, biogenic gas and inorganic gas, etc. Coal-formed gas is the main type of China natural gas resources, in particular several explored large-scale gas fields (>100 billion cubic meter) of Kela 2, Sulige and Daniudi, etc., they all belong to coal-formed gas fields or the gas fields consisting mostly of coal-formed gas. Oil-formed gas is also abundant in China marine basins, for example marine natural gas of Sichuan Basin generated from crude oil cracking gas. Primary and secondary biogenic gas fields were discovered respectively in the Qaidam Basin and Western Slope of Songliao Basin. In addition, inorganic gases are mainly distributed in the eastern China, in particular the Songliao Basin with abundant carbon dioxide accumulation, indicating that the eastern China present large exploration potential of inorganic gas.

Primary migration and secondary alteration of the Upper Paleozoic gas reservoir in Ordos Basin, China — Application of fluid inclusion gases

The composition of fluid inclusions (FI) often represents the initial geochemical characteristics of palaeo-fluid in reservoir rock. Influence on composition and carbon isotopic composition of gas during primary migration, reservoir-forming and subsequent secondary alterations are discussed through comparing fluid inclusion gas with coal-formed gas and natural gas in present gas reservoirs in the Ordos Basin. The results show that primary migration of gas has significant effect on the molecular but not on the carbon isotopic composition of methane. Migration and diffusion fractionation took place during the secondary migration of gas in Upper Paleozoic gas reservoir according to carbon isotopic composition of methane in Fls. Composition and carbon isotopic composition of natural gas were nearly unchanged after the gas reservoir forming through comparing the FI gases with the natural gas in present gas reservoir.

Geochemical characteristics and origin of gases from the Upper, Lower Paleozoic and the Mesozoic reservoirs in the Ordos Basin, China

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 1000×108m3 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 δ13C 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 δ13C1 values are mostly similar with those in the Upper Paleozoic, but the δ13C2 and δ13C3 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. δ13C1>δ13C2), inferring that gases would be mixed between high-mature coal-formed gases and oil-type gases. In the Mesozoic, the δ13C values for gaseous alkanes are enriched in 12C, 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 δ13C1-R0 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.

Discussion of gas enrichment mechanism and natural gas origin in marine sedimentary basin, China

There are abundant natural gas resources in Chinese marine sedimentary basin. The exploration hot shots of natural gas are the Palaeozoic marine strata here in recent years, and several large scale gas fields have been discovered. Chinese Palaeozoic high-post matured and coal measure hydrocarbon source rocks are mainly prone to gas generation in the present. This research considered that gas source rocks and TSR are the key cause of gas enrichment of marine strata. High-quality argillaceous and coal measure hydrocarbon rocks are distributed widely in the Palaeozoic marine strata, which have been in highly matured phase in the present. The argillaceous source rock generally contains various sulfates that could accelerate crude oil cracking to gas for TSR occurrence, and coal measure source rock mainly generates gas, so Chinese marine basin gives priority to accumulating gas. Marine strata have not founded oil reservoirs in the Sichuan Basin and Ordos Basin, and they consist mainly of dry gas. Marine natural gases are the mixed gases of oil cracking gas and coal-formed gas in a general way, oil cracking gases contain usually some H2S and CO2. Hydrocarbon carbon isotopes are very complicated, and methane and ethane isotopic values bear apparent reversal caused by thermal evolution and mixing among different genetic types of natural gas. Coal-formed gases are the main component of Chinese marine natural gas. The Upper Permian of the Sichuan Basin and the Carboniferous-Permian of the Ordos Basin coal measure hydrocarbon source rock present large hydrocarbon generation potential, which are the prospecting highlight of marine natural gas hereafter. Oil cracking gas exploration will be paid much attention to in the Tarim Basin because of the lack of coal measure hydrocarbon source rock.

Geochemical features and genesis of the natural gas and bitumen in paleo-oil reservoirs of Nanpanjiang Basin, China

Bitumen from the Nanpanjiang Basin occurs mainly in the Middle Devonian and Upper Permian reef limestone paleo-oil reservoirs and reserves primarily in holes and fractures and secondarily in minor matrix pores and bio-cavities. N2 is the main component of the natural gas and is often associated with pyrobitumen in paleo-oil reservoirs. The present study shows that the bitumen in paleo-oil reservoirs was sourced from the Middle Devonian argillaceous source rock and belongs to pyrobitumen by crude oil cracking under high temperature and pressure. But the natural gas with high content of N2 is neither an oil-cracked gas nor a coal-formed gas generated from the Upper Permian Longtan Formation source rock, instead it is a kerogen-cracked gas generated at the late stage from the Middle Devonian argillaceous source rock. The crude oil in paleo-oil reservoirs completely cracked into pyrobitumen and methane gas by the agency of hugely thick Triassic deposits. After that, the abnormal high pressure of methane gas reservoirs was completely destroyed due to the erosion of 2000–4500-m-thick Triassic strata. But the kerogen-cracked gas with normal pressure was preserved under the relatively sealed condition and became the main body of the gas shows.

Distribution and fractionation mechanism of stable carbon isotope of coalbed methane

The stable carbon isotope values of coalbed methane range widely, and also are generally lighter than that of gases in normal coal-formed gas fields with similar coal rank. There exists strong carbon isotope fractionation in coalbed methane and it makes the carbon isotope value lighter. The correlation between the carbon isotope value and R o in coalbed methane is less obvious. The coaly source rock maturity cannot be judged by coalbed methane carbon isotope value. The carbon isotopes of coalbed methane become lighter in much different degree due to the hydrodynamics. The stronger the hydrodynamics is, the lighter the CBM carbon isotopic value becomes. Many previous investigations indicated that the desorption-diffusion effects make the carbon isotope value of coalbed methane lighter. However, the explanation has encountered many problems. The authors of this article suggest that the flowing groundwater dissolution to free methane in coal seams and the free methane exchange with absorbed one is the carbon isotope fractionation mechanism in coalbed methane. The flowing groundwater in coal can easily take more 13CH4 away from free gas and comparatively leave more 12CH4. This will make 12CH4 density in free gas comparatively higher than that in absorbed gas. The remaining 12CH4 in free gas then exchanges with the adsorbed methane in coal matrix. Some absorbed 13CH4 can be replaced and become free gas. Some free 12CH4 can be absorbed again into coal matrix and become absorbed gas. Part of the newly replaced 13CH4 in free gas will also be taken away by water, leaving preferentially more 12CH4. The remaining 12CH4 in free gas will exchange again with adsorbed methane in the coal matrix. These processes occur all the time. Through accumulative effect, the 12CH4 will be greatly concentrated in coal. Thus, the stable carbon isotope of coalbed methane becomes dramatically lighter. Through simulation experiment on water-dissolved methane, it had been proved that the flowing water could fractionate the carbon isotope of methane, and easily take heavy carbon isotope away through dissolution.

Kinetic modeling the effects of primary migration, diffusion and waterwashing on Upper Paleozoic coal-derived gas at the center of Ordos Basin

Gold reactor pyrolysis system is used to model the gas formation from the Upper Paleozoic coal measures and the results show coal-derived gas characterized with δ13C1 of −33.46‰, δ13C2 of −23.1‰, dryness (C1/C1–4) 85.6%. And then, effects of the post-genetic processes on coalderived gas are analyzed in turn: (i) About 27% coal-derived gases constituted with more methane are calculated to be lost during diffusion. The residual in reservoir are characterized with almost the same compositions as the original, which suggests faint influence by diffusion (the residual, δ13C1 of −32.78‰, δ13C2 of −23.1‰, C1/C1-4 83%); (ii) Water washing made about 8% coal-formed gases lost and their components and stable carbon isotopes are stable; (iii) In the final, it is speculated that primary migration makes much more wet gas (C2–4) leave in coal measures. The variance of gas dryness induced by this factor is estimated to be about 10%.

04/01799 Study of accumulation coefficient of coal-formed gas below the salt sequence in Wenliu area of Dongpu depression: Xiong, L. et al. Shiyou Shiyan Dizhi, 2003, 25, (1), 64–68, 75. (In Chinese)

The gas in Es3(4) below the salt sequence has single source in Wenliu area of Dongpu depression, and the boundary condition is clear for the source rocks in the CarboniferousPermian coalbearing sequence. Exploration degree is high in this area. Final proven reserves can be calculated accordingly by profound understand of natural gas reservoir formation condition, reservoir formation history and gas reservoir features. The quantity of gas generated from coal measure in the reference section can be calculated by the method of basin modeling, and natural gas accumulation coefficient is calculated as 7.3%  7.6%.

Genesis and distribution of natural gas in the foreland basins of China

This article discusses the geochemical characteristics of various types of natural gas and features of gas reservoirs in the foreland basins in China. We conclude that there are four types of natural gas in origin: coal-formed cracking gas, coal-formed thermal gas, oil-type thermal gas and mixed gas. Coal-formed gases are generated from three sets of coal measures: Permo-Carboniferous Formations in the Ordos Basin, Upper Triassic Formations in the Qaidam Basin, and Triassic Formations in northwest China. Source rocks for the oil-type gas include Permian Formations in the Junggar Basin, Triassic Formations in the Qaidam Basin, and lacustrine source rocks in basins along the Hexi Corridor. Abnormally high pressures commonly exist in the gas reservoirs in foreland basins where multiple sets of source–reservoir–seal combinations are present, and the major trap type is the anticlinal trap type and faults are the major hydrocarbon migration pathways.

An initial discussion on major controlling factors on formation of coal-formed large-medium gas fields

The so-called “large-medium gas field” means a gas field with proven reserves equal to or larger than 100 × 108 m3. Up to 1997, 23 coal-formed great-medium fields are discovered in China, excluding two in Taiwan Province. Their proven reserves take 50.9% in total natural gas reserves. Therefore, the coal-formed great-medium gas field plays a decisive role in natural gas reserves and the study of their controlling factors is very important both in theory and in practice. There are six major controlling factors on the formation of coal-formed large-medium gas fields: (i) The generating center of coal-formed gas and its surrounding area; (ii) low potential area; (iii) late pool-forming period; (iv) traps related to the paleo-uplifits in coal-formed gas area; (v) above or between abnormally pressured compartments; (vi) faulted traps related to coal-bed.

98/02020 Coal-formed gas potential of Yingjisu sag in Tarim basin : Li, X. et al. Shiyou Kantan Yu Kaifa, 1996, 23, (3), 6–10. (In Chinese)

Carboniferous and Jurassic coal-measure rocks with their relative coal-formed gas are developed in Junggar Basin. Carboniferous source rocks have high organic material abundance, Type III organic material, and high maturity. Jurassic source rocks are widely distributed with big thickness and great maturity variety. The Middle and Lower Jurassic are the predominant sources of natural gas, and the potential is large. Carboniferous coal-formed gas is dry gas, its carbon isotope is heavy, the methane carbon isotope composition is about −30‰, the ethane carbon isotope composition is above −28‰, the carbon isotope series is positive (δ13C1δ13C2δ13C3δ13C4). Gas reservoir is self-generating and self-preserving. Gas from the Jurassic source rocks has wide distribution and is wet. Methane carbon isotope composition varies greatly, and the carbon isotope reversal of butane and propane is common, reflecting that natural gas has different maturity and the mixing of different-maturity natural gas originated from the same source results in the light isotopic values of heavy hydrocarbons. The potential of the Carboniferous rocks is huge, and is the major exploration area. Gas from the Jurassic source rocks is distributed widely and the southern margin of the basin is a favorable area.

Irish Sea innovator

Carboniferous and Jurassic coal-measure rocks with their relative coal-formed gas are developed in Junggar Basin. Carboniferous source rocks have high organic material abundance, Type III organic material, and high maturity. Jurassic source rocks are widely distributed with big thickness and great maturity variety. The Middle and Lower Jurassic are the predominant sources of natural gas, and the potential is large. Carboniferous coal-formed gas is dry gas, its carbon isotope is heavy, the methane carbon isotope composition is about −30‰, the ethane carbon isotope composition is above −28‰, the carbon isotope series is positive (δ13C1δ13C2δ13C3δ13C4). Gas reservoir is self-generating and self-preserving. Gas from the Jurassic source rocks has wide distribution and is wet. Methane carbon isotope composition varies greatly, and the carbon isotope reversal of butane and propane is common, reflecting that natural gas has different maturity and the mixing of different-maturity natural gas originated from the same source results in the light isotopic values of heavy hydrocarbons. The potential of the Carboniferous rocks is huge, and is the major exploration area. Gas from the Jurassic source rocks is distributed widely and the southern margin of the basin is a favorable area.

Generation, Migration, and Accumulation of Deep Natural Gas in Songliao Basin

The Songliao basin, the main oil-bearing basin of China is a continental sedimentary one. Giant and middle oil fields had been discovered in the past 30 years, but only small gas fields have been discovered in shallow and middle depth. No giant or middle gas fields have been found. The exploration and research work undertaken in recent years has proved that a lot of gas fields occur in the lower and deep oil- and gas-bearing formations. The gas fields are found in two zones: a lower gas-bearing zone (Lower Cretaceous Quantou Group) and deep gas-bearing zone (Lower Cretaceous Denlouku Group and Upper Jurassic). This paper describes the geochemistry, sedimentary structure, and the various types of gas reservoirs of Lower Cretaceous and Upper Jurassic formations; analyzes the generation, migration, accumulation of natural gas; and points out that the source rock is the black mudstone and coal bed of Upper Jurassic. The gas was coal-formed gas that migrated upward along the faults and accumulated in the sandstone of Lower Cretaceous (Quantou Group and Denlouku Group). Finally, the paper points out the prospect of deep gas in Songliao basin.