Western Canada Sedimentary Basin petroleum systems: A working and evolving paradigm

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.

Recognising biodegradation in gas/oil accumulations through the δ13C compositions of gas components

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.

Geochemistry and sources of hydrate-bound gas in the Shenhu area, northern south China sea: Insights from drilling and gas hydrate production tests

Coalbed methane genesis, occurrence and accumulation in China

Geochemical heterogeneity, origin and secondary alteration of natural gas inside and outside buried hills of Xinglongtai area, West Sag, Liaohe Depression, Bohai Bay Basin

Gas-source rock correlation in Thrace basin, Turkey

This study researched the geochemical origin of coalbed methane (CBM) in three mining areas of the Huaibei coalfield and the distribution of different origin gases. The Rock-Eval data indicate that coal and coaly mudstone in this coalfield are both good gas-source rocks and are within the gas generation window. The large ranges of isotope data (δ13CCH4 = −42.8‰ to −75.5‰; δDCH4 = −153.2‰ to −228.1‰; δ13CCO2 = +8.2‰ to −24.3‰) reflect the complicated formation mechanism and evolutionary process of CBM. Both thermogenic and biogenic gas are present within the mining areas to varying degrees, with the biogenic gas being predominately generated via the CO2 reduction pathway. Gas migration was most prevalent in the Suixiao mining area, low in the Linhuan mining area, and not identified in the Suzhou mining area. An appropriate distribution model for the gases of different origin is, from bottom to top, a thermogenic gas zone, a mixture zone, a secondary biogenic gas zone, and a gas-weathered zone. Spatially, s...

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.

Summary We collected and investigated published data on >1,500 samples of gases recovered from shales in >20 basins in Argentina, Canada, China, France, Poland, Saudi Arabia, Sweden and the USA. Most shale gases are relatively dry (average dryness C1(C1–C1 is 0.9) and contain nitrogen (average about 6.6%) and CO2 (average about 2.5%). Methane has carbon isotopic composition δ¹³C ranging from −82‰ to −27‰ and averaging around −43‰. Most shale gases have thermogenic origin, although gases from New Albany shale in the Illinois basin, Antrim shale in the Michigan basin, and Colorado shale in the Western Canada Sedimentary Basin have predominantly primary or secondary microbial origin. Isotopic reversal of the normal trend in carbon isotopic composition such that δ³C of methane is larger than δ¹³C of ethane is a common phenomenon observed in many studied samples. The vast majority of samples showing isotopic reversal have thermogenic origin and are very dry (ratio C1/(C1–C5) exceeding 0.97 and averaging 0.99) suggesting that isotopic reversal is common among thermogenic gases with very high maturity. However, isotopic reversal of the normal trend in hydrogen isotopic composition such that δ2H of methane is larger than δ2H of ethane is observed among both thermogenic and microbial shale gases.

Due to the current concern regarding the remaining reserve of oil and gas, there is a strong incentive for the innovative exploitation of fossil fuels occurring in unconventional situations. The latter include a wide variety of resources, some of them known for a long time, but set aside in deference to the profit of the more conventional oil and gas; others rely on more recent identification or on new production technologies; and some are already in the operational stage, with others in the pilot stage or even only prospective.In this respect this chapter first briefly reviews the concept of the petroleum system and then tentatively extends this concept to the definition of the different hydrocarbon occurrences associated with “nonconventional” settings. These settings include: coal, oil shale, heavy oils and bitumens, primary and secondary biogenic gas, methane hydrate, shale gas, coal bed methane, tight oil/oil producing shale, tight gas, aquifer gas, and so on.It does not pretend to address exhaustively all aspects of nonconventional hydrocarbons, but to provide a synthetic overview of the geological meaning of these new players in the energy domain.

Biogenic gas is generated at low temperatures by decomposition of organic matter by anaerobic microorganisms. More than 20% of the world’s discovered gas reserves are of biogenic origin. A higher percentage of gases of predominantly biogenic origin will be discovered in the future. Biogenic gas is an important target for exploration because it occurs in geologically predictable circumstances and in areally widespread, large quantities at shallow depths. In rapidly accumulating marine sediments, a succession of microbial ecosystems leads to the generation of biogenic gas. After oxygen is consumed by aerobic respiration, sulfate reduction becomes the dominant form of respiration. Methane generation and accumulation become dominant only after sulfate in sediment pore water is depleted. The most important mechanism of methane generation in marine sediments is the reduction of CO2 by hydrogen (electrons) produced by the anaerobic oxidation of organic matter. CO2 is the product of either metabolic decarboxylation or chemical decarboxylation at slightly higher temperatures. The factors that control the level of methane production after sediment burial are anoxic environment, sulfate-deficient environment, low temperature, availability of organic matter, and sufficient space. The timing of these factors is such that most biogenic gas is generated prior to burial depths of 1,000 m. In marine sediments, most of the biogenic gas formed can be retained in solution in the interstitial (pore) waters because of higher methane solubility at the higher hydrostatic pressures due to the weight of the overlying water column. Under certain conditions of high pressures and (or) low temperatures, biogenic methane combines with water to form gas hydrates. Biogenic gas usually can be distinguished from thermogenic gas by chemical and isotopic analyses. The hydrocarbon fraction of biogenic gas consists predominantly of methane. The presence of as much as 2% of heavier hydrocarbons can be attributed to admixture of minor thermogenic gas due to low-temperature degradation of organic matter. The amounts of hydrocarbon components other than methane generally are proportional to temperature, age, and organic-matter content of the sediments. Biogenic methane is enriched in the light isotope 12C (δ13C1 lighter than –55 ppt) owing to kinetic isotope fractionation by methanogens. The variations in isotopic composition of biogenic methane are controlled primarily by δ13C of the original CO2 substrate, which reflects the net isotopic effect of both addition and removal of CO2. The methane isotopic composition also can be affected by mixing of isotopically heavier thermogenic gas. The possible complicating factors require that geologic, chemical, and isotopic evidence be considered in attempts to interpret the origin of gas accumulations. Accumulations of biogenic gas have been discovered in Canada, Germany, Italy, Japan, Trinidad, the United States, and USSR in Cretaceous and younger rocks, at less than 3,350 m of burial, and in marine and nonmarine rocks. Other gas accumulations of biogenic origin have undoubtedly been discovered; however, data that permit their recognition are not available.

Secondary biogenic coal seam gas reservoirs in New Zealand: A preliminary assessment of gas contents

Modification of exogenous CO2 on the pathway of microbial degradation coal to CH4: From the view of stable isotope