Coalbed methane genesis, occurrence and accumulation in China

New progress and future prospects of CBM exploration and development in China

Structural controls on coalbed methane accumulation and high production models in the eastern margin of Ordos Basin, China

Chapter 11 - CBM

Challenges and development direction of deep fragmented soft coalbed methane in China

Alberta's coalbed methane (CBM) resource has been estimated to be comparable in size to that of the province's conventional natural gas resource base. Although only limited drilling for CBM targets has occurred in the province to date, considerable interest in how development of this resource might proceed has been shown by the gas and coal industries, the public, and regulatory agencies. Because CBM is a form of natural gas, it is regulated in the same manner as other natural gas. An overview of CBM in Alberta is given, including the potential resource base, elaboration on the application of natural gas regulations to CBM, activity to date, and some of the main regulatory issues and initiatives that have arisen. It is shown that there is a need to collect Alberta specific CBM data before decisions can be made on the necessity of regulatory change specifically for CBM development. Introduction Numerous papers and articles have been published over the past few years on various aspects of CBM. The growing number of CBM wells and increasing amount of CBM production in the United States (USA) has caused somewhat of a surge in published case histories and field experiences for the major CBM basins ill that country. Much less CBM field experience currently is available for Alberta, but the potential CBM resources of the province have not been overlooked. Early investigations into the production of CBM in Alberta date back to the late 1970's, but interest waned in the early to mid 1980's with falling gas prices and a levelling in gas demand. The more recent interest in CBM development ill Alberta was spurred on by the surge in CBM activity that occurred in parts of the USA from 1989 to 1992. A summary of the current CBM situation in Alberta is given in this paper. The province's CBM resource potential and regulatory framework are overviewed along with activity to date and some of the issues that have been raised regarding the development of this resource. Why the Interest in Coalbed Methane in Alberta? The recent interest in CBM in Alberta can be attributed to three main factors:the large CBM resource potential of the province.the large amount of recent CBM development activity in the USA, andenvironmental concerns regarding the development of CBM in Alberta. Alberta's Coalbed Methane Resource Potential CBM is a naturally occurring hydrocarbon gas, consisting largely of methane, that is generated by and stored in coal seams. Because Alberta has extensive coal deposits, it follows that the CBM resource associated with these deposits has the potential to be significant. Coals of varying thermal maturity, or rank, underlie about 45 per cent of the province(1), with essentially all of the coals being located south of Township 75 as shown ill Figure I. These coals are associated with Upper Jurassic, Cretaceous, and Lower Tertiary strata as shown in Figure 2, and much of it may be of suitable rank to hold potential for CBM development.

Geological conditions of deep coalbed methane in the eastern margin of the Ordos Basin, China: Implications for coalbed methane development

For finding out favorable coalbed methane development districts, the evaluation of geological controls of CBM distribution and accumulation is essential during CBM exploration and development. In this paper, the No. 3 coal seam in the Zhengzhuang coal zone located at southern Qinshui Basin, China, was selected as a focus for comprehensively evaluating geological controls of CBM. The findings of this study are: (1) Coalbed gas contents increase with increasing mudstone roof thickness (ranging from 0-9 m), and will always be as high as 20 m3/t in the coal reservoirs with the mudstone roof thickness greater than about 9 m. (2) The weak transmission and stagnant coalbed water are favorable for CBM accumulation in coal reservoirs resulting from the typical hydraulic preservation of CBM. (3) High coalbed gas contents will commonly occur in vitrinite-rich and low-ash yield coals with high CH4 adsorptive capacity.

Implications of the pore pressure and in situ stress for the coalbed methane exploration in the southern Junggar Basin, China

Bohai Bay basin, in eastern China, is rich in oil resources, but recently large natural gas accumulations have been found, mainly in Miocene reservoirs, at depths shallower than 1500 m. Molecular and stable carbon isotopic gas compositions show that the gases have several distinctive biogenic origins. The first type of biogenic gas is generated in anaerobic environments from thermally immature organic matter and this type of gas accumulates typically in reservoirs interbedded within the immature source rock succession. The second type occurs up dip or above heavy-oil reservoirs and the gas is inferred derived from crude oils in shallow reservoirs as a biodegradation product. The third type originates from source rocks that were buried deeply previously (>2800 m in the basin), where they were “geopasteurized”, but which are now uplifted and recolonized by microbes at their current shallow depths. The first type is generally referred to as primary biogenic gas. The second type is commonly referred to as secondary biogenic gases elsewhere, but herein is called biogenic gas from biodegraded oil (BG-BO). This distinguishes them from the third type which is described herein as secondary biogenic gas and which is distinguished from BG-BO. Bohai Bay Basin gas pools often contain one of the three biogenic gas types and are often mixed, more or less, with thermogenic natural gas from petroleum pools in Eocene reservoirs. It is therefore important to distinguish the genetic affinities of the biogenic natural gases as an aid to the development and exploration for additional biogenic gas resources in Bohai Bay Basin.

Junlian coalfield is one of the main targets for coalbed methane (CBM) exploration and development in the southwest China. Based on field geological survey, core observation, gas content statistics, coal maceral composition, vitrinite reflectance ( R O ), proximate analysis and trace element test, lithological types, lithofacies, sedimentary environment, and structural analysis, this research established the sequence stratigraphy frame, revealed the plane distribution characteristics of sedimentary facies, and defined CBM accumulation mode. The results show that six rock types were identified and further subdivided into twenty lithofacies types. Four types of sedimentary systems such as alluvial plains, delta, lagoon-tidal flat, and carbonate platform were summarized according to their combination characteristics. Additionally, 12-14 fourth-order sequences and three third-order sequences CSI, CSII, and CSIII were divided, and a sequence stratigraphic framework of the Lopingian coal-bearing series was established. Among them, the features of third-order sequence CSIII paleogeography from west to east are alluvial plains, deltas, lagoon-tidal flats, and limited carbonate platforms. Thick coal seams are mainly developed in the sedimentary environment of tidal flats, delta plains, and floodplains behind banks. Closely related to coal seam thickness, gas contents of Lopingian coal seams are generally higher than 8 m3/t, except the low level in northwest and partial denudation areas. CBM accumulation is significantly controlled by the fold structure, and the hydraulic plugging effect makes the syncline core favorable for CBM accumulation. Furthermore, favorable geostress conditions enable the secondary anticline to become a favorable area for CBM accumulation when the sealing conditions are better. This research will provide a theoretical guide for the exploration and development of CBM in the study area.

Hydrodynamic and stratigraphic controls for a large coalbed methane accumulation in Ferron coals of east-central Utah

Upper Cretaceous coals within the Ferron Sandstone Member of the Mancos Shale contain large volumes of coalbed methane. Coals at the northern end of the 80-mi (129 km)-long Ferron trend have produced 469 bcf of coalbed methane, as of the end of July 2003. At that time, 735 wells were producing 275 mmcfd. Analysis of cores from exploratory wells indicate that the gas content of the coals decreases dramatically from north to south, even though the rank and maturity of the coals decreases only slightly. The area with lowest gas content correlates with the outcrop of the Ferron coals in the southern portion of the trend. Hydrodynamic studies have shown that the Ferron coals are aquifers that are recharged from the Wasatch Plateau to the west. Regional mapping indicates that the productive fields are large stratigraphic traps in the central and northern part of the trend where the coals pinch-out updip into tight marine shales to the east. Much of the produced gas is secondary biogenic gas and migrated thermogenic gas that has moved from the Wasatch Plateau and south margin of the Uinta Basin, respectively. In the southern part of the trend, near the town of Emery, coals are present at the surface, but most of the gas has been flushed out of the coals due to reduction of reservoir pressure and active water flow from the west. Therefore, the entire Ferron trend probably contained tremendous volumes of stratigraphically trapped coalbed methane before uplift and erosion exposed the southern coals to the atmosphere.

As the oil and gas industry weathers a transitional phase brought on by a drop in oil prices, burgeoning markets in Southeast Asia stand poised to establish themselves as legitimate players in the future. Attendees of the recent Offshore Technology Conference Asia in Kuala Lumpur saw the myriad ways in which the region’s potential could make it into an industry stronghold. In a series of sessions devoted to individual countries in the Asia Pacific region, representatives from national oil companies, multinational operators, and service companies presented their thoughts on the roles they will play in the overall development of the region and the global oil market. China Panel speakers at the China country session described a country flush with natural resources but unable to exploit them to the fullest extent. Led by its national oil company, the China National Petroleum Corporation (CNPC), the country is eager to acquire the tools needed to increase production from its oil and gas reserves. While the Chinese market is still in a nascent developmental stage, Zhang Xiaofeng said the CNPC is optimistic about its progress in developing specific types of reservoirs. Zhang, a project manager at CNPC, cited volcanic reservoirs as a particular source of optimism. China has one of the largest collections of proven volcanic reserves in the world; 900 billion m3 of the country’s 3 trillion m3 (Tcm) total proven reserves is in volcanic reservoirs. However, Zhang said there are some technical difficulties CNPC faces in actualizing these resources. He said the company must improve its reservoir prediction techniques, as well as its crack detection. Zhang was equally optimistic about shale gas prospects in China, but operational costs must continue to decline in order for the country to increase development of the resource. CNPC estimated that in 2015, the cost of drilling a horizontal well in shale formations in the Sichuan Basin ranged between USD 11.3 million and USD 12.9 million per well, representing a 23% cost reduction from 2013. “Of course, shale gas is an important resource for China,” Zhang said. “It is a rich gas area, and hopefully some of our successful projects will push shale.” China’s focus on shale gas has been driven in large part by its inability to develop its coalbed methane (CBM) resources. The country’s total resource of CBM is approximately 36.8 Tcm, but the coal depositional structure in China is too complex for the existing technological infrastructure to exploit. Overcoming this complexity will be critical for the country in the near future. Zhang said there is, at present, no large-scale commercial production of CBM in China. Xiangyu Wang, a professor of construction management at Curtin University and the moderator of the session, said renewables should play a larger role in China’s energy policy, even as the country continues to develop its oil and gas economy. Coal made up 65.7% of China’s total energy consumption in 2013, compared with 18.9% for oil and 5.5% for gas.

Hartshorne coal rank applied to Arkoma Basin coalbed methane activity, Oklahoma, USA

From the new perspective of the dynamic isotopic fractionation of microbes and the mixture of secondary biogenic gas and thermogenic gas in coalbeds, this study evaluates the geochemical characteristics of mixed coalbed gas in Enhong. Additionally, by using isotopes, the study traces the microbial activities and the results of those activities, such as the methane production of bacteria, and reveals the formation mechanism and genetic types of coalbed gas. The measured δ13C1 values of the coalbed gas samples are from −54.5‰ to −32.0‰. Based on the values of Ro and δ13C2, by calculation, the δ13C1 values of thermogenic methane range from −30.6‰ to −28.1‰ and from −30.7‰ to −28.3‰, respectively. The results of the two calculations are highly consistent, and both are approximately 20‰ higher than the measured values. The measured δDCH4 values are from −217‰ to −196‰, being between thermogenic methane and microbiogenic methane; the δ13CCO2 values are from −30.5‰ to −23.9‰, growing heavier with respect to the original thermogenic gas; the measured δ13C2 values are from −25.7‰ to −22.6‰, and the estimated δ13C2 values are from −21.8‰ to −21.2‰, also indicating growing heavier. The δ13C1 and δ13C2 values are negatively correlated; both the Δδ13CC2–C1 and Δδ13CCO2–C1 values are increasing. All of the above characteristics indicate that CO2 is reduced into microbial genetic methane by methane-producing bacteria and it mixes with thermogenic gas, which is new evidence showing the existence of secondary biogenic gas. Via calculations using a variety of data, such as the values of δ13C1, δ13C2 and Ro, it is found that thermogenic methane accounts for approximately 38% to 58% of the total amount, and microbial genetic methane accounts for approximately 42% to 62%. The proportion of micro-biogenic methane reduced from the top down, which occupied more than 50% of that in the coalbed buried within 1000 m deep. It increased the content of coalbed gas by more than 1 times. Within 1000 m deep, coalbed temperatures are generally lower than 40 °C, which is the most appropriate section for methanogenic bacteria activity and secondary biogenic gas generation. Coalbed uplift to the shallow parts in the late stage is the basic geological condition for the formation of secondary biogenic gas, which has significant resource value.