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Coalbed Methane

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Using diverse geologic and geophysical data from recent exploration and development, and experimental results of analysis of gas content, gas capacity, and gas composition, this article discusses how geologic, structural, and hydrological factors determine the heterogeneous distribution of gas in the Weibei coalbed methane (CBM) field.

The coal rank of the Pennsylvanian no. 5 coal seam is mainly low-volatile bituminous and semianthracite. The total gas content is 2.69 to 16.15 m3/t (95.00–570.33 scf/t), and gas saturation is 26.0% to 93.2%. Burial coalification followed by tectonically driven hydrothermal activity controls not only thermal maturity, but also the quality and quantity of thermogenic gas generated from the coal.

Gas composition indicates that the CBM is dry and of dominantly thermogenic origin. The thermogenic gases have been altered by fractionation that may be related to subsurface water movement in the southern part of the study area.

Three gas accumulation models are identified: (1) gas diffusion and long-distance migration of thermogenic gases to no-flow boundaries for sorption and minor conventional trapping, (2) hydrodynamic trapping of gas in structural lows, and (3) gas loss by hydrodynamic flushing. The first two models are applicable for the formation of two CBM enrichment areas in blocks B3 and B4, whereas the last model explains extremely low gas content and gas saturation in block B5. The variable gas content, saturation, and accumulation characteristics are mainly controlled by these gas accumulation models.

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Sequence stratigraphy and coal cycles based on accommodation trends were investigated in the coal-bearing Lower Cretaceous Mannville Group in the Lloydminster heavy oil field, eastern Alberta. The study area is in a low accommodation setting on the cratonic margin of the Western Canada sedimentary basin. Geophysical log correlation of coal seams, shoreface facies, and the identification of incised valleys has produced a sequence-stratigraphic framework for petrographic data from 3 cored and 115 geophysical-logged wells. Maceral analysis, telovitrinite reflectance, and fluorescence measurements were taken from a total of 206 samples. Three terrestrial depositional environments were interpreted from the petrographic data: ombrotrophic mire coal, limnotelmatic mire coal, and carbonaceous shale horizons. Accommodation-based coal (wetting- and drying-upward) cycles represent trends in depositional environment shifts, and these cycles were used to investigate the development and preservation of the coal seams across the study area.

The low-accommodation strata are characterized by a high-frequency occurrence of significant surfaces, coal seam splitting, paleosol, and incised-valley development. Three sequence boundary unconformities are identified in only 20 m (66 ft) of strata. Coal cycle correlations illustrate that each coal seam in this study area was not produced by a single peat-accumulation episode but as an amalgamation of a series of depositional events. Complex relations between the Cummings and Lloydminster coal seams are caused by the lateral fragmentation of strata resulting from the removal of sediment by subaerial erosion or periods of nondeposition. Syndepositional faulting of the underlying basement rock changed local accommodation space and increased the complexity of the coal cycle development.

This study represents a low-accommodation example from a spectrum of stratigraphic studies that have been used to establish a terrestrial sequence-stratigraphic model. The frequency of changes in coal seam quality is an important control on methane distribution within coalbed methane reservoirs and resource calculations in coal mining. A depositional model based on the coal cycle correlations, as shown by this study, can provide coal quality prediction for coalbed methane exploration, reservoir completions, and coal mining.

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Regional variations in thickness and facies of clastic sediments are controlled by geographic location within a foreland basin. Preservation of facies is dependent on the original accommodation space available during deposition and ultimately by tectonic modification of the foreland in its postthrusting stages. The preservation of facies within the foreland basin and during the modification stage affects the kinds of hydrocarbon reservoirs that are present.

This is the case for the Cretaceous Mowry Shale and Frontier Formation and equivalent strata in the Rocky Mountain region of Colorado, Utah, and Wyoming. Biostratigraphically constrained isopach maps of three intervals within these formations provide a control on eustatic variations in sea level, which allow depositional patterns across dip and along strike to be interpreted in terms of relationship to thrust progression and depositional topography.

The most highly subsiding parts of the Rocky Mountain foreland basin, near the fold and thrust belt to the west, typically contain a low number of coarse-grained sandstone channels but limited sandstone reservoirs. However, where subsidence is greater than sediment supply, the foredeep contains stacked deltaic sandstones, coal, and preserved transgressive marine shales in mainly conformable successions. The main exploration play in this area is currently coalbed gas, but the enhanced coal thickness combined with a Mowry marine shale source rock indicates that a low-permeability, basin-centered play may exist somewhere along strike in a deep part of the basin.

In the slower subsiding parts of the foreland basin, marginal marine and fluvial sandstones are amalgamated and compartmentalized by unconformities, providing conditions for the development of stratigraphic and combination traps, especially in areas of repeated reactivation. Areas of medium accommodation in the most distal parts of the foreland contain isolated marginal marine shoreface and deltaic sandstones that were deposited at or near sea level lowstand and were reworked landward by ravinement and longshore currents by storms creating stratigraphic or combination traps enclosed with marine shale seals.

Paleogeographic reconstructions are used to show exploration fairways of the different play types present in the Laramide-modified, Cretaceous foreland basin. Existing oil and gas fields from these plays show a relatively consistent volume of hydrocarbons, which results from the partitioning of facies within the different parts of the foreland basin.

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It’s all about staying on target: Technological advances are helping to make geosteering an increasingly valuable tool for geologists involved in horizontal wells.

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Recount: The U.S. Geological Survey offers a new estimate of the world’s undiscovered conventional oil and gas resources.

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A world-class technical program is in place for the AAPG International Conference and Exhibition in Singapore.

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My co-author for this month’s column is Jim McGhay, AAPG Treasurer. Later this month, you will receive your dues statement, where there are two items that might attract your attention. We address them here.   

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If you have a pulse and have been to an AAPG Annual Convention and Exhibition (ACE) over the past couple of years or so, or a recent AAPG International Convention and Exhibition (ICE) such as those held in Calgary or Milan, you would have noticed.

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Anne Oudinot has won the A.I. Levorsen Award for presenting the best paper at the recent Eastern Section annual meeting.

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In-Person Training
Muscat Oman 16 May, 2016 18 May, 2016 26806 Desktop /Portals/0/PackFlashItemImages/WebReady/advances-in-subsurface-imaging-hero.jpg?width=100&height=100&mode=crop&anchor=middlecenter&quality=75amp;encoder=freeimage&progressive=true Geophysics, Direct Hydrocarbon Indicators, Gravity, Magnetic, Seismic, Engineering, Reservoir Characterization, Tight Gas Sands, Subsalt Traps, Structural Traps, Stratigraphic Traps, Shale Gas, Oil Shale, Oil Sands, Fractured Carbonate Reservoirs, Diagenetic Traps, Deep Basin Gas, Coalbed Methane
Muscat, Oman
16-18 May 2016

This three-day workshop aims at sharing knowledge and ideas on the advancements in subsurface mapping. This includes recent technologies in acquiring seismic and non seismic data, improvements in imaging the subsurface and advances in data interpretation.

Online Training
29 October, 2009 29 October, 2009 1445 Desktop /Portals/0/PackFlashItemImages/WebReady/oc-es-application-of-thermal-maturation.jpg?width=100&height=100&mode=crop&anchor=middlecenter&quality=75amp;encoder=freeimage&progressive=true
29 October 2009

Expanded package for CEU credit is $100 for AAPG members, and $145 for non-members. Special Student Pricing: $25 for Webinar only; $35 for Expanded package.

01 January, 2013 01 January, 9999 1473 Desktop /Portals/0/PackFlashItemImages/WebReady/oc-cc-unconventional-resources.jpg?width=100&height=100&mode=crop&anchor=middlecenter&quality=75amp;encoder=freeimage&progressive=true
1 January 2013 - 1 January 9999

Unconventional Resources is an online course that enables participants to learn about shale gas, shale oil and coalbed methane.

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