Bullentin of Canadian Petroleum Geology最新文献

{"title":"A reassessment of gas resources in selected Upper Cretaceous biogenic gas accumulations in southeastern Alberta and southwestern Saskatchewan, Canada","authors":"Zhuoheng Chen, Y. Shuai, Norman Wang","doi":"10.2113/GSCPGBULL.63.1.5","DOIUrl":"https://doi.org/10.2113/GSCPGBULL.63.1.5","url":null,"abstract":"Abstract Tens of thousands of production wells drilled during the past 100 years reveal that biogenic gas accumulations in the Upper Cretaceous succession in southeast Alberta and southwest Saskatchewan are a regionally pervasive gas field with mixed reservoirs and varying resource density across the region. Historical pool boundaries have disappeared gradually as a result of in-fill drilling. This suggests that previous resource assessments that used methods based on feature counting, such as the number of pools and individual pools sizes, may have significantly underestimated the resource potential of this field because the resource potential occurring between the pool boundaries were largely ignored and the areal extent of the field is still growing geographically. This study used available historical production data and employed a well performance-based method to re-assess the natural gas potential of this giant gas field. Three major production intervals, Medicine Hat, Milk River and Second White Speckled formations in Upper Cretaceous successions, were assessed. The geographical locations of 86 561 production wells with production from one of these three intervals were used to define the play boundaries. More than ten thousand production wells with historical records were collected and analyzed. All wells with comingled production from more than one zone were excluded to eliminate the impact from mixed contributions from multiple intervals. The remaining 2783 production wells with production from a single formation were used in the Estimated Ultimate Reserve (EUR) calculation for each of the three intervals. The estimated total technically recoverable natural gas resource in the three stratigraphic intervals vary from 30.2 to 73.3 TCF (P90 to P10) with a median of 43.6 and a mean of 50.1 TCF. The total inferred resources obtained for this study are much larger than those obtained previously for this field.","PeriodicalId":56325,"journal":{"name":"Bullentin of Canadian Petroleum Geology","volume":"63 1","pages":"5-19"},"PeriodicalIF":0.0,"publicationDate":"2015-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.2113/GSCPGBULL.63.1.5","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"68206985","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Quantitative seismic interpretations to detect biogenic gas accumulations: a case study from Qaidam Basin, China","authors":"Yexin Liu, Zhuoheng Chen, Liqun Wang, Yong-shu Zhang, Zhiqiang Liu, Y. Shuai","doi":"10.2113/GSCPGBULL.63.1.108","DOIUrl":"https://doi.org/10.2113/GSCPGBULL.63.1.108","url":null,"abstract":"Abstract Quantitative seismic interpretation can be used to identify lithology and detect petroleum accumulations by integrating rock properties and attributes derived from advanced seismic inversion methods with existing petrophysical data and geological knowledge. We use quantitative seismic interpretations for detection of shallow biogenic gas accumulations in the Qaidam Basin, China, employing an integrated workflow that incorporates petrophysical data, seismic attribute analysis, Constrained Simultaneous Inversion (C-SI) and Bayesian-based Support Vector Machine (B-SVM) inference. Previous petrophysical studies have shown that it is challenging to effectively identify gas-bearing intervals using parameters such as impedance, Poisson’s ratio and porosity, because the reservoir sediments are unconsolidated and at shallow depths. The resistivity well-log response remains as an effective tool for estimating gas saturation and identifying gas-bearing intervals. In this study, we propose the use of the petroleum pore-volume, which is defined as the product of reservoir porosity and gas saturation, to detect biogenic gas accumulations seismically. Rock properties inferred from seismic inversion, such as compressional velocity (Vp), shear velocity (Vs) and density, cannot be used directly for petroleum pore-volume estimation. Therefore, we employ a Bayesian-based support vector machine approach to cross-link well-log properties, seismic AVO attributes and seismic rock properties to quantitatively predict petroleum pore-volume in 2D and 3D seismic dataset. Because seismic information is crucial to statistical inference, we propose C-SI to infer the Vp, Vs and density from seismic elastic impedance gathers, which can be generated from seismic gathers using a traditional recursive seismic inversion method. The C-SI procedures use the Interior-Point algorithm to optimize and solve elastic impedance equations. The Interior-Point method is a popular method for handling constrained non-convex, non-linear optimization problems that involve simultaneously inverting the seismic properties with thousands of seismic samples. This case study indicates that the integrated study workflow is useful for quantitatively predicting petroleum pore-volume, especially in the depth-domain, and that it is an excellent potential indicator for biogenic gas accumulations in complicated geological settings.","PeriodicalId":56325,"journal":{"name":"Bullentin of Canadian Petroleum Geology","volume":"63 1","pages":"108-121"},"PeriodicalIF":0.0,"publicationDate":"2015-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.2113/GSCPGBULL.63.1.108","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"68207042","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Lithological and diagenetic restrictions on biogenic gas generation in Songliao Basin inferred from grain size distribution and permeability measurement","authors":"Ningxi Li, Zi-hui Feng, Haiping Huang, Xue Wang, Z. Dong","doi":"10.2113/GSCPGBULL.63.1.66","DOIUrl":"https://doi.org/10.2113/GSCPGBULL.63.1.66","url":null,"abstract":"Abstract Organically rich immature source rocks occur commonly at shallow burial depths in Songliao Basin, NE China. This has led to the general assumption that there must be a large biogenic gas resource in these organic rich, shallow strata. However, except for a few small gas fields that are generally associated with biodegraded oil, only a small amount of biogenic gas has been discovered. In order to determine the geological factors controlling biogenic gas generation and accumulation, a suite of potential source rock shale samples from burial depth less than 1600 m was analyzed for organic composition, grain size distribution and permeability. The results show that selected samples are organic rich and thermally immature. The grain size analysis shows a mixed distribution of silt and clay with D10 generally below 1.6 μm. The measured permeability values are generally below 1 μd under in situ stress conditions with pore throats that are most likely smaller than that of methanogenic bacteria. Very low permeability and very small pores resulting from compaction and diagenesis restrict bacteria movement and activity, limit nutrient transport, diminish space availability, and lead to a reduced biodiversity. Currently, no microbes can survive in these Cretaceous shales even though the shales never reached geopasturization temperatures and there is sufficient substrate. Biogenic gas generation is inferred to be restricted to near surface strata while elsewhere in this succession methanogenesis has ceased, which significantly reduces the exploration potential for biogenic gas.","PeriodicalId":56325,"journal":{"name":"Bullentin of Canadian Petroleum Geology","volume":"112 6 1","pages":"66-74"},"PeriodicalIF":0.0,"publicationDate":"2015-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.2113/GSCPGBULL.63.1.66","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"68207084","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Biogenic gas accumulations in Canada and China: geological characteristics and new insights","authors":"Zhuoheng Chen, Shuichang Zhang, S. Grasby, Y. Shuai","doi":"10.2113/GSCPGBULL.63.1.1","DOIUrl":"https://doi.org/10.2113/GSCPGBULL.63.1.1","url":null,"abstract":"Generation of biogenic gas occurs under environmental conditions different from those of thermogenic gas (Rice and Claypool, 1981; Whiticar et al. 1986; Shurr et al., 2003). Microbial activity takes place at shallow depth in a semi-open system where sediments are not fully compacted, seal integrity is not well developed, and accumulation and leakage of microbial gas are competing processes. Thus the formation of a commercial biogenic gas field needs both sufficient microbial activity to maintain gas supply and an efficient entrapment mechanism to minimize gas leakage. This depends on not only the temperature and other microbial growth conditions, but also on the characteristics and internal architecture of both source and reservoir strata. A better understanding of the essential controls and optimal conditions for critical microbial activity, and for the efficient entrapment of gas generated, is important to successful exploration for biogenic resources.\u0000\u0000This Special Issue of the Bulletin of Canadian Petroleum Geology contains nine thematic research articles on microbial gas resources and their characteristics in the Western Canada Sedimentary Basin and selected Chinese basins. The nine articles are based on the results and findings from a comparative study of shallow biogenic gas resources in Canadian and Chinese basins conducted under a collaborative research agreement between PetroChina and the Geological Survey of Canada during 2010–2013. The agreement provided a unique opportunity to study shallow biogenic gas fields with different origins and characteristics in the two countries. The primary objective of the project is to better understand the critical geological controls on biogenically derived natural gas resources in shallow strata and their resource potential. The characteristics of the biogenic gas fields and available data for describing and quantifying the biogenic gas fields in the two countries are complimentary. The long production history and large amount of public data make the …","PeriodicalId":56325,"journal":{"name":"Bullentin of Canadian Petroleum Geology","volume":"63 1","pages":"1-3"},"PeriodicalIF":0.0,"publicationDate":"2015-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.2113/GSCPGBULL.63.1.1","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"68206800","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Geochemistry and distribution of biogenic gas in China","authors":"Shuichang Zhang, Y. Shuai","doi":"10.2113/GSCPGBULL.63.1.53","DOIUrl":"https://doi.org/10.2113/GSCPGBULL.63.1.53","url":null,"abstract":"Of twenty-nine known biogenic gas fields in China, three with reserves of nearly 100 × 109 m3 are located in Qaidam Basin, western China, and two with reserves of 50 × 109 m3 are located in Yinggehai Basin, southern China. The other fields, concentrated in eastern and south-eastern China, have smaller reserves. The gas geochemical characteristics, including data from more than 300 gas samples, over 200 isotopic analyses and 12 inert gas analyses, reveals the following. Gases are predominated by methane (CH4) (95%), followed by nitrogen (N2; 0–15%) and carbon dioxide (CO2; <3% but commonly <1%). Methane δ13C1 values are very light, usually <−55‰. The ethane δ13C2 values range widely from −60‰ to −20‰ and the δ13CCO2 values are between −39‰ and 5‰. Hydrogen isotope values of CH4 range from −260‰ to −190‰, which indicate that the gases are formed by CO2 reduction. 3He/4He ratios are between n × 10−8 and n × 10−7. 40Ar/36Ar ratios are between 231 and 439 and R/Ra ratios are 0.03. These biogenic gases exhibit geochemical characteristics and occur in geological settings that indicate two major biogenic gas types in China: early, or primary, biogenic gas and secondary biogenic gas that formed through crude oil biodegradation. Primary biogenic gas reserves are large while secondary biogenic gas reserves are smaller but widely distributed. The geological settings of the two types differ significantly. The primary biogenic gases are concentrated in Cenozoic successions characterized by rapid sedimentation, high organic matter content and syndepositional entrapment. Most secondary biogenic gases are associated with biodegraded heavy oil occurrences and these are not confined to any specific sedimentary strata or epoch.","PeriodicalId":56325,"journal":{"name":"Bullentin of Canadian Petroleum Geology","volume":"63 1","pages":"53-65"},"PeriodicalIF":0.0,"publicationDate":"2015-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.2113/GSCPGBULL.63.1.53","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"68207065","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Geochemistry and origin of shallow gas in the Baise Basin, southern China","authors":"Y. Shuai, P. Peng, Y. Zou, Shuichang Zhang, Ling Huang","doi":"10.2113/GSCPGBULL.63.1.84","DOIUrl":"https://doi.org/10.2113/GSCPGBULL.63.1.84","url":null,"abstract":"Abstract Many small and medium-sized Paleogene pull-apart basins in southern China contain an abundance of biogenic gas shows. Such shallow gas with biogenic characteristics has been thoroughly investigated only in Baise Basin. Hence, the research results from Baise Basin could serve as a model for the origin and characteristics of shallow gases in similar basins and areas in Southern China. There are ten gas fields with a total of proven reserves of 20 billion cubic meters discovered in Baise Basin. Three gas fields are located on the western and southern flank of the basin, and they are gas caps to heavy oil pools from depths between 600 m and 850 m. These accumulations contain dry biogenic gases, with C1/C1-5 exceeding 0.99, light δ13C1 (–76 to −54‰), and heavy δD1 (−218‰). Trace heavy gaseous hydrocarbons are strongly biodegraded (iC4/nC4>3). The other seven gas fields are located in the northern fault zone and the central Nakun uplift of the basin at depths between 300 m and 700 m. Gases are mainly unassociated and condensed gases. The condensed gases are depleted in 13C (δ13C1: −67 to −53.7‰, δ13C2: −52.3 to −36.1‰, δ13C3: −43.3 to −33.4‰), but wet with C1/C2+3 mostly less than 20, suggesting a mainly thermal origin at low maturity. The unassociated gases are dominated by methane, with C1/C2+3 ratio above 100, variable N2 (0 to 5.4%), and traces of CO2. The δ13C value of methane in the unassociated gases is between −76 to −60‰, and δD1 values from −248 to −213.7‰. These gases also contain isotopically light ethane with δ13C2 values of −64.5 to −42‰, which we infer to have originated from deeper horizons as a result of migration or diffusion from low-maturity thermal gases with light stable carbon isotopic compositions (C1/C2+3 <20, δ13C2 from −60‰ to −58‰). A similar thermal origin is inferred for the other heavy gaseous hydrocarbons. Formation water with the shallow gases of this basin is mainly NaHCO3-type with low TDS ranging from 1000 to 4500 ppm. The (HCO3+CO3)/Cl ratios range from 1.5 to 100, indicating a relative open hydrodynamic condition and the possible intrusion of meteoric water. These data indicate that early biogenic gas generated syndepositionally was probably not preserved, and that the current biogenic gas accumulation formed mainly as a result post-depositional of crude oil biodegradation in the western basin and coal biodegradation in the northern fault zone. Shallow gases in Baise Basin are mainly secondary biogenic gases, with an admixture of low maturity thermogenic gases from deeper horizons.","PeriodicalId":56325,"journal":{"name":"Bullentin of Canadian Petroleum Geology","volume":"63 1","pages":"84-95"},"PeriodicalIF":0.0,"publicationDate":"2015-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.2113/GSCPGBULL.63.1.84","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"68207426","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"The origin of low molecular weight hydrocarbons associated with biogenic gas from the Eastern Depression in Qaidam Basin, China","authors":"G. Hu, Jin Li, Song-Mei Hu","doi":"10.2113/GSCPGBULL.63.1.96","DOIUrl":"https://doi.org/10.2113/GSCPGBULL.63.1.96","url":null,"abstract":"Abstract The Eastern Depression of Qaidam Basin in China is the largest Quaternary gas producing area in the world. The chemical compositions, C1–C2 carbon and hydrogen isotopic compositions, low molecular weight hydrocarbons (C6–C7LMWHs) in 10 natural gas samples from three large gas fields (Tainan, Sebei 1 and Sebei 2) in Eastern Depression of Qaidam Basin were analyzed. The carbon and hydrogen isotopic composition of methane in the three gas fields is relatively enriched in 12C and 1H with δ13C1 values ranging from −69.6 ‰ to −65.5 ‰ (average −67.4 ‰) and δ1H from −251 ‰ to −231 ‰ (average −244 ‰). The data shows that the gases are defined as microbial in origin and generated primarily by bacterial CO2 reduction. The composition distribution of C6–C7LMWHs differs between the fields. LMWHs associated with the gas in the Sebei 1 gas field are different from those in the Tainan and Sebei 2 gas fields. The relative content of cycloalkane among C6–C7LMWHs in the Tainan and Sebei 2 gas fields is very high, ranging from 37.02% to 43.36% and 33.99% to 43.71% respectively. However, in the Sebei 1 gas field, of the total C6–C7LMWHs, the relative content of isoalkane is the highest, ranging from 37.17% to 45.92% with an average of 42.48. These traits indicate that the origins of C6–C7LMWHs are probably different from the bacterial action that is predominant in the Sebei 1 gas field. We infer that catalytic reaction affects the C6–C7LMWHs compositions in the Tainan and Sebei 2 gas fields.","PeriodicalId":56325,"journal":{"name":"Bullentin of Canadian Petroleum Geology","volume":"63 1","pages":"96-107"},"PeriodicalIF":0.0,"publicationDate":"2015-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.2113/GSCPGBULL.63.1.96","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"68207590","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Comparison of biogenic gas fields in the Western Canada Sedimentary Basin and Qaidam Basin: implications for essential geological controls on large microbial gas accumulations","authors":"Zhuoheng Chen, Y. Shuai, K. Osadetz, T. Hamblin, S. Grasby","doi":"10.2113/GSCPGBULL.63.1.33","DOIUrl":"https://doi.org/10.2113/GSCPGBULL.63.1.33","url":null,"abstract":"Abstract Compared to conventional thermogenic petroleum, the exploration for large biogenic gas accumulations is more challenging because the gas occurs commonly at shallow depths in a petroleum system with weak top and lateral seals. In addition to fundamental environmental requirements, such as temperature and formation water composition, microbial gas generation and retention require adequate petroleum system elements in order to form economic accumulations. The geological characteristics of Southeast Alberta Gas Field (SAGF), a giant biogenic gas accumulation located in southeastern Alberta and southwestern Saskatchewan within the Western Canada Sedimentary Basin (WCSB), are compared to those of Sanhu Sag, Qaidam Basin in northwestern China, to provide insight into the critical geological controls for economic biogenic gas accumulations. It was found that multilayered reservoirs, consisting of multiple layers of thin porous units and interbedded with source rocks, provide a most efficient storage mechanism. Low relief structural and stratigraphic traps with lateral permeability seals, commonly of stratigraphic or diagenetic origin, are the most effective trapping mechanisms. The recognition of these common constraints provides insight for future exploration of other biogenic gas fields and prospects in the basins studied elsewhere.","PeriodicalId":56325,"journal":{"name":"Bullentin of Canadian Petroleum Geology","volume":"63 1","pages":"33-52"},"PeriodicalIF":0.0,"publicationDate":"2015-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.2113/GSCPGBULL.63.1.33","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"68206786","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Quaternary biogenic gases in the Qaidam Basin, Western China","authors":"Y. Shuai, Shuichang Zhang, Dade Ma, Liquan Wang, G. Jiang, Ziyuan Xu, Ling Huang, Yirui Xu","doi":"10.2113/GSCPGBULL.63.1.75","DOIUrl":"https://doi.org/10.2113/GSCPGBULL.63.1.75","url":null,"abstract":"Abstract The Quaternary Qigequan Formation, a continental clastic sedimentation system in Sanhu Depression of eastern Qaidam Basin, west-central China, contains abundant biogenic gas resources. Favorable characteristics of the Sanhu Depression’s geological history and framework responsible for biogenic gas production include rapid sedimentation, shallow burial depth, organic-rich sediments, high porosity/permeability, and a saline depositional environment. Gases are dominated by methane (>99%), with trace ethane/propane (C2+3<0.5%) and minor non-hydrocarbon gases (CO2 <0.5%; N2 <3%). Methane has δ13C1 values of −70 to −62‰ and δD1 values of −240 to −220‰, suggesting generation following a CO2 reduction pathway. Ethane is very light with δ13C2 values of −50 to −44‰; propane δ13C3 values range from −34 to −32‰. Ethane and propane are inferred to have the same thermocatalytic origin under low organic maturity levels and are unrelated to the biogenic origin of methane. Biogenic methane is equilibrated with the saline formation waters. The formation water geochemistry data, including stable isotope values (δD and δ18O) and 36Cl age, suggest recent dilution by meteoric waters. Methanogenesis is currently active, as indicated by the presence of both abundant hydrogen and microbes. The recent and ongoing biogenic gas generation model proposed explains why biogenic gas accumulations are so abundant in a shallow geological setting where the conditions for accumulation and preservation are otherwise considered relatively poor.","PeriodicalId":56325,"journal":{"name":"Bullentin of Canadian Petroleum Geology","volume":"63 1","pages":"75-83"},"PeriodicalIF":0.0,"publicationDate":"2015-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.2113/GSCPGBULL.63.1.75","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"68207200","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Recognition and sources of secondary biogenic gases in the oil sand areas, Western Canada Sedimentary Basin","authors":"Haiping Huang","doi":"10.2113/GSCPGBULL.63.1.20","DOIUrl":"https://doi.org/10.2113/GSCPGBULL.63.1.20","url":null,"abstract":"Abstract Western Canada Sedimentary Basin oil sand deposits are derived from light oils generated in southwestern Alberta, which migrated to the north and east for more than 100 km. Biodegradation is the primary process that transformed the original light crude oil into heavy oil and bitumen, although other alteration mechanisms were present. Biodegradation levels increase from the southwest (non-biodegraded) to the northeast (extremely biodegraded) and are associated with decreasing reservoir paleo-temperature that plays the primary role in controlling the biodegradation regime. Compositional gradients and variable biodegradation within a single reservoir column indicate that water-leg size is a critical local control on vertical variations of biodegradation degree and oil physical properties. Secondary biogenic gas is a by-product of the formation of heavy oils and bitumens by anaerobic biodegradation, and “gas over bitumen” is a common feature of the bitumen accumulations. Observed 13C-depleted methane and 13C-enriched CO2 provides direct evidence for methanogenic biodegradation and biogenic methane generation. Supplemental evidence for anaerobic biodegradation is found in the geochemistry of associated formation water. The gases associated with anaerobic biodegradation are easily differentiated from primary biogenic gases using the isotopic signature of C2+ alkane components. Based on the observed crude oil biodegradation levels, the stoichiometry of methanogenic alkane biodegradation, and assuming a conversion rate of carbon dioxide to methane, approximately 141.3 × 1012 m3 (4991 Tcf) of secondary biogenic methane was generated accompanying the biodegradation of these petroleum accumulations. However, assessing how much secondary biogenic gas is preserved currently in the subsurface is difficult. Gas resource assessments performed prior to and independent of biodegradation studies suggests that McMurray Formation is expected to contain approximately 58.7 × 109 m3 natural gas in place and that all Mannville Group reservoirs associated with the oil sand regions (Athabasca and Lloydminster) are expected to contain approximately 608.5 × 109 m3 of raw gas in place. Even if the estimates of retained gas are conservative, it is apparent that the vast majority of the secondary biogenic methane generated during biodegradation is leaked into the overburden, dissolved in formation water, or escaped into the atmosphere.","PeriodicalId":56325,"journal":{"name":"Bullentin of Canadian Petroleum Geology","volume":"63 1","pages":"20-32"},"PeriodicalIF":0.0,"publicationDate":"2015-03-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://sci-hub-pdf.com/10.2113/GSCPGBULL.63.1.20","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"68207210","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}