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Panel_14490
Panel_14490
8:00 AM
11:50 AM
8:00 a.m.
Introductory Remarks
Room 702/704/706
Panel_15792
Panel_15792
8:00 AM
12:00 AM
8:05 a.m.
Correlation of Rocky Mountain Produced Natural Gases to Sources by Application of Gas Isotope Kinetic Modeling
Room 702/704/706
By J. B. Curtis, S. W. Brown, H. A. Illich, J. E. Zumberge
Due to variation in maceral types and concentrations, source rocks have different gas formation kinetic characteristics which result in isotopically different gases. Additionally, cracking of crude oils to gases varies with oil composition. The identification of gases from oil cracking is possible as resulting gases have very different carbon isotopic signatures compared to gases from primary cracking of kerogen (and bitumen). Therefore, experimental calibration of oil cracking allows a differentiation of gas from primary cracking of kerogen. Gas to source correlations can not be completely successful by application of theoretical concepts, but are enhanced by gas formation simulation experiments where gas evolution is monitored and quantified during laboratory thermal maturation of source rocks and cracking of oils. Such experiments allow for the correlation of gas samples back to a more specific source unit. Gas isotope kinetic modeling as described in Tang et al., 2000 (Geochimica et Cosmochimica Acta 64, p. 2673-2687), and Tang and Schoell et al., 2005 (Abstract 96191, AAPG Annual Convention and Exhibition, Calgary, AB, Canada) was employed. Experimental data were available for three Greater Green River Basin samples: a Rock Springs coal, a Mancos Shale, and the asphaltene fraction of a Mowry-sourced oil. These three samples account for much of the types of gases generated in the Greater Green River Basin. A sample set of >400 produced gases with molecular and isotopic compositional data allowed differentiation of e.g., multiple gas sources in Pinedale and Jonah fields of Wyoming which were geologically reasonable. Using the initial laboratory Greater Green River Basin results, this approach has been expanded to include additional carbon isotopic data to evaluate produced gases from five additional Rocky Mountain basins: Denver, Piceance, Powder, San Juan and Uinta. The data commonly show carbon isotopic evolution trends that can be interpreted in terms of source type(s) and generation depths, complementing more traditional gas – source correlation techniques.
Due to variation in maceral types and concentrations, source rocks have different gas formation kinetic characteristics which result in isotopically different gases. Additionally, cracking of crude oils to gases varies with oil composition. The identification of gases from oil cracking is possible as resulting gases have very different carbon isotopic signatures compared to gases from primary cracking of kerogen (and bitumen). Therefore, experimental calibration of oil cracking allows a differentiation of gas from primary cracking of kerogen. Gas to source correlations can not be completely successful by application of theoretical concepts, but are enhanced by gas formation simulation experiments where gas evolution is monitored and quantified during laboratory thermal maturation of source rocks and cracking of oils. Such experiments allow for the correlation of gas samples back to a more specific source unit. Gas isotope kinetic modeling as described in Tang et al., 2000 (Geochimica et Cosmochimica Acta 64, p. 2673-2687), and Tang and Schoell et al., 2005 (Abstract 96191, AAPG Annual Convention and Exhibition, Calgary, AB, Canada) was employed. Experimental data were available for three Greater Green River Basin samples: a Rock Springs coal, a Mancos Shale, and the asphaltene fraction of a Mowry-sourced oil. These three samples account for much of the types of gases generated in the Greater Green River Basin. A sample set of >400 produced gases with molecular and isotopic compositional data allowed differentiation of e.g., multiple gas sources in Pinedale and Jonah fields of Wyoming which were geologically reasonable. Using the initial laboratory Greater Green River Basin results, this approach has been expanded to include additional carbon isotopic data to evaluate produced gases from five additional Rocky Mountain basins: Denver, Piceance, Powder, San Juan and Uinta. The data commonly show carbon isotopic evolution trends that can be interpreted in terms of source type(s) and generation depths, complementing more traditional gas – source correlation techniques.
Panel_15537
Panel_15537
8:05 AM
8:25 AM
8:25 a.m.
Thermochemical Sulfate Reduction in Anhydrite-Sealed Carbonate Gas Reservoirs: A 3-D Reactive Mass Transport Modeling Approach
Room 702/704/706
By Y. Fu, W. vanBerk, H. Schulz
Acid gas generation by thermochemical sulfate reduction (TSR) evolves within a complex web of petroleum-water-rock-gas interactions in reservoirs under high temperature conditions of more than 100°C. These interactions lead to formation of toxic and corrosive hydrogen sulfide (H2S gas and dissolved H2S). Such interactions are caused by the instability of hydrocarbons in the presence of water and by a reactive reservoir rock matrix containing water-soluble anhydrite. The mass conversions of inorganic water-rock-gas interactions, which are triggered by the kinetically controlled sulfate reduction with aqueous hydrocarbons, establish a certain, thermodynamically defined state of chemical equilibrium. Any approach to geochemically and quantitatively model TSR-induced “H2S-risks” in petroleum systems should be based on a conceptual model that adequately reproduces the interdependent nature of all simultaneous hydrogeochemical processes contributing to TSR (more than 50 reactions). Such approaches should rely (1) on the thermodynamical calculation of chemical equilibrium species distribution, (2) on the coupling of kinetically controlled sulfate reduction with petroleum-derived reductants to the equilibrium calculations, and (3) on the calculation of diffusive mass transport of solutes through the free pore water network and the irreducible water film. TSR modeling is complex, and therefore, its failure often results from conceptual models which focus on only single reactions like the kinetically controlled sulfate reduction dependent on thermal history or other selected reactions of interest which are isolated from the web of interactions. The key to model TSR, the fate and behavior of sulfidic sulfur, and a realistic “H2S-risk” in petroleum reservoirs is a comprehensive reproduction of the hydrogeochemical reactive transport processes within the whole system. Consequently, we perform 3D hydrogeochemical, multi-component and multi-species reactive mass transport modeling for a semi-generic case study by using the PHAST computer code (provided by the U.S. Geological Survey). The aim is (1) to predict the temporal and spatial evolution of complex TSR interactions under reservoir conditions and (2) to test the effects of various parameters on the concentration of H2S in the gas, on the total amount of sulfidic sulfur present in the reservoir (H2S(g), H2S(aq), HS-(aq), S-2(aq) indicating reservoir souring; FeS2), and on the amount of newly formed elemental sulfur.
Acid gas generation by thermochemical sulfate reduction (TSR) evolves within a complex web of petroleum-water-rock-gas interactions in reservoirs under high temperature conditions of more than 100°C. These interactions lead to formation of toxic and corrosive hydrogen sulfide (H2S gas and dissolved H2S). Such interactions are caused by the instability of hydrocarbons in the presence of water and by a reactive reservoir rock matrix containing water-soluble anhydrite. The mass conversions of inorganic water-rock-gas interactions, which are triggered by the kinetically controlled sulfate reduction with aqueous hydrocarbons, establish a certain, thermodynamically defined state of chemical equilibrium. Any approach to geochemically and quantitatively model TSR-induced “H2S-risks” in petroleum systems should be based on a conceptual model that adequately reproduces the interdependent nature of all simultaneous hydrogeochemical processes contributing to TSR (more than 50 reactions). Such approaches should rely (1) on the thermodynamical calculation of chemical equilibrium species distribution, (2) on the coupling of kinetically controlled sulfate reduction with petroleum-derived reductants to the equilibrium calculations, and (3) on the calculation of diffusive mass transport of solutes through the free pore water network and the irreducible water film. TSR modeling is complex, and therefore, its failure often results from conceptual models which focus on only single reactions like the kinetically controlled sulfate reduction dependent on thermal history or other selected reactions of interest which are isolated from the web of interactions. The key to model TSR, the fate and behavior of sulfidic sulfur, and a realistic “H2S-risk” in petroleum reservoirs is a comprehensive reproduction of the hydrogeochemical reactive transport processes within the whole system. Consequently, we perform 3D hydrogeochemical, multi-component and multi-species reactive mass transport modeling for a semi-generic case study by using the PHAST computer code (provided by the U.S. Geological Survey). The aim is (1) to predict the temporal and spatial evolution of complex TSR interactions under reservoir conditions and (2) to test the effects of various parameters on the concentration of H2S in the gas, on the total amount of sulfidic sulfur present in the reservoir (H2S(g), H2S(aq), HS-(aq), S-2(aq) indicating reservoir souring; FeS2), and on the amount of newly formed elemental sulfur.
Panel_15542
Panel_15542
8:25 AM
8:45 AM
8:45 a.m.
Significance of Organic Carbon and Bulk Nitrogen Fluctuations Across the Cenomanian-Turonian Boundary, Eagle Ford Formation, Maverick Basin, Texas
Room 702/704/706
By H. Rowe, S. C. Ruppel
A stratigraphic record of organic matter deposition spanning late Cenomanian through at least early Turonian time was reconstructed from a drill core recovered from the Maverick Basin, South Texas. The strata (Eagle Ford Fm) record 1) the depositional evolution in an anoxic/euxinic shelf setting following a hiatus, represented by the contact with the underlying Buda Fm, 2) a shift to more oxygenated conditions after the initiation of the OAE-2 (as defined by a positive shift in d13CTOC), and 3) variably oxygenated (poikiloaerobic) conditions throughout much of the post-OAE-2 succession. Approximately 170 ft of TOC-rich (average 3%, maximum 6.5%) lower Eagle Ford (LEF) strata are subdivided into two sub-equal zones: a lower zone with variable d13CTOC values (ranging from -28.9 to -25.9‰), and an upper zone with a largely unidirectional d13CTOC trajectory that upwardly trends to less depleted values (~-28.1 to ~-26.8‰). Interestingly, the abrupt shift between the two zones coincides with the apex in TOC/N ratio, suggesting that a shift in organic matter source or degradation occurred at this time. The bulk organic characteristics of the upper Eagle Ford (UEF) are significantly more variable. Redox-sensitive trace elemental results show that the OAE-2 began at the LEF-UEF boundary, when the concentration of sedimentary molybdenum (a proxy for euxinia) greatly diminished. The OAE-2 interval (lowermost portion of UEF) spans a thickness of ~70 ft, and yields a d13CTOC range of ~4‰. The upper 230 ft of core (UEF and lower Austin Fm) is defined by highly variable TOC (maximum 4%), an upwardly-increasing trend in TOC/N, and variable d13CTOC (range -28 to -26‰). The majority of bulk rock ?15Ntotal values fall between -5 and -2‰, which is a typical range for strata of this age. The most interesting feature of the nitrogen isotopic record is an abrupt decrease in ?15Ntotal of several permil at the apex of the d13CTOC record in the OAE-2 interval. Following the depletion of nitrate inputs from others sources, this decrease may represent a unique set of water mass conditions under which nitrogen fixation by diazotrophic cyanobacteria was the sole process responsible for bringing nitrogen into the system. The significance of the new record will be discussed with respect to the many existing records of Cenomanian-Turonian paleoceanographic change.
A stratigraphic record of organic matter deposition spanning late Cenomanian through at least early Turonian time was reconstructed from a drill core recovered from the Maverick Basin, South Texas. The strata (Eagle Ford Fm) record 1) the depositional evolution in an anoxic/euxinic shelf setting following a hiatus, represented by the contact with the underlying Buda Fm, 2) a shift to more oxygenated conditions after the initiation of the OAE-2 (as defined by a positive shift in d13CTOC), and 3) variably oxygenated (poikiloaerobic) conditions throughout much of the post-OAE-2 succession. Approximately 170 ft of TOC-rich (average 3%, maximum 6.5%) lower Eagle Ford (LEF) strata are subdivided into two sub-equal zones: a lower zone with variable d13CTOC values (ranging from -28.9 to -25.9‰), and an upper zone with a largely unidirectional d13CTOC trajectory that upwardly trends to less depleted values (~-28.1 to ~-26.8‰). Interestingly, the abrupt shift between the two zones coincides with the apex in TOC/N ratio, suggesting that a shift in organic matter source or degradation occurred at this time. The bulk organic characteristics of the upper Eagle Ford (UEF) are significantly more variable. Redox-sensitive trace elemental results show that the OAE-2 began at the LEF-UEF boundary, when the concentration of sedimentary molybdenum (a proxy for euxinia) greatly diminished. The OAE-2 interval (lowermost portion of UEF) spans a thickness of ~70 ft, and yields a d13CTOC range of ~4‰. The upper 230 ft of core (UEF and lower Austin Fm) is defined by highly variable TOC (maximum 4%), an upwardly-increasing trend in TOC/N, and variable d13CTOC (range -28 to -26‰). The majority of bulk rock ?15Ntotal values fall between -5 and -2‰, which is a typical range for strata of this age. The most interesting feature of the nitrogen isotopic record is an abrupt decrease in ?15Ntotal of several permil at the apex of the d13CTOC record in the OAE-2 interval. Following the depletion of nitrate inputs from others sources, this decrease may represent a unique set of water mass conditions under which nitrogen fixation by diazotrophic cyanobacteria was the sole process responsible for bringing nitrogen into the system. The significance of the new record will be discussed with respect to the many existing records of Cenomanian-Turonian paleoceanographic change.
Panel_15538
Panel_15538
8:45 AM
9:05 AM
9:05 a.m.
Possible Sources of Dissolved Inorganic Carbon in the Formation of Middle and Upper Devonian Carbonate Concretions, Appalachian Basin
Room 702/704/706
By G. Lash
Calcium carbonate concretions are common to the Middle and Upper Devonian shale succession of the Appalachian Basin. Geologic and microtextural evidence suggests that the authigenic carbonate formed at shallow burial depth, perhaps no more than a few tens of meters below the sediment-water interface, in a diagenetic environment resulting from the anaerobic oxidation of methane (AOM). Burial histories of the Middle Devonian Marcellus Shale and Upper Ordovician Utica Shale suggest that only the latter had generated a small amount of thermogenic methane by the time the Devonian shale succession started to accumulate. Thus, oxidized biogenic methane appears to have been the principal dissolved inorganic carbon source of the authigenic carbonate. However, modestly depleted d13C values of concretions, from the Marcellus Shale upward through the Upper Devonian Dunkirk Shale, are well in excess of d13C values of authgenic carbonate formed within a diagenetic environment induced by the anaerobic oxidation of biogenic methane. One explanation of this seemingly incongruent relationship entails a combination of the prolonged oxidation of shallow biogenic methane mixed with methanogenic CO2, both of which were sourced at the bottom of the Marcellus Shale. Alternatively, some volume of the methane inventory consumed by AOM within the Devonian shale succession may have originated within the Upper Ordovician Utica Shale. It is plausible that “ancient” biogenic methane was released from the Utica shale, either over an extended period of time or as a geologically rapid event, to the overlying sedimentary column. Residual biogenic methane that finally reached the accumulating Middle and Upper Devonian deposits would have been only modestly depleted in 13C due to a protracted oxidation history. The obvious shortcoming of a scenario involving the expulsion of methane from the Utica Shale into the Middle and Upper Devonian shale succession is the presence of such intervening units as the Silurian Lockport Dolomite and overlying Salina Formation salt deposits. However, transport of methane from the Utica could have been enhanced by Acadian foreland basin dynamics, including salt removal, reactivated basement faults and the formation of Acadian faults and related fractures.
Calcium carbonate concretions are common to the Middle and Upper Devonian shale succession of the Appalachian Basin. Geologic and microtextural evidence suggests that the authigenic carbonate formed at shallow burial depth, perhaps no more than a few tens of meters below the sediment-water interface, in a diagenetic environment resulting from the anaerobic oxidation of methane (AOM). Burial histories of the Middle Devonian Marcellus Shale and Upper Ordovician Utica Shale suggest that only the latter had generated a small amount of thermogenic methane by the time the Devonian shale succession started to accumulate. Thus, oxidized biogenic methane appears to have been the principal dissolved inorganic carbon source of the authigenic carbonate. However, modestly depleted d13C values of concretions, from the Marcellus Shale upward through the Upper Devonian Dunkirk Shale, are well in excess of d13C values of authgenic carbonate formed within a diagenetic environment induced by the anaerobic oxidation of biogenic methane. One explanation of this seemingly incongruent relationship entails a combination of the prolonged oxidation of shallow biogenic methane mixed with methanogenic CO2, both of which were sourced at the bottom of the Marcellus Shale. Alternatively, some volume of the methane inventory consumed by AOM within the Devonian shale succession may have originated within the Upper Ordovician Utica Shale. It is plausible that “ancient” biogenic methane was released from the Utica shale, either over an extended period of time or as a geologically rapid event, to the overlying sedimentary column. Residual biogenic methane that finally reached the accumulating Middle and Upper Devonian deposits would have been only modestly depleted in 13C due to a protracted oxidation history. The obvious shortcoming of a scenario involving the expulsion of methane from the Utica Shale into the Middle and Upper Devonian shale succession is the presence of such intervening units as the Silurian Lockport Dolomite and overlying Salina Formation salt deposits. However, transport of methane from the Utica could have been enhanced by Acadian foreland basin dynamics, including salt removal, reactivated basement faults and the formation of Acadian faults and related fractures.
Panel_15541
Panel_15541
9:05 AM
9:25 AM
Panel_15793
Panel_15793
9:25 AM
12:00 AM
10:10 a.m.
Carbon and Noble Gas Isotope Banks in Two-Phase Flow: Changes in Gas Composition During Migration
Room 702/704/706
By K. J. Sathaye, T. E. Larson, M. Hesse
A dramatic expansion of natural gas exploration and extraction in unconventional reserves is underway. However, there is public concern that hydraulic fracturing will also cause natural gas, reservoir brines and associated fracturing fluids to contaminate shallower groundwater reservoirs. Considerable scientific research is currently focused on attributing methane found in shallow groundwater sources to either thermogenic or low temperature bacterial sources. Attribution techniques use concentration ratios of methane, ethane and propane and their stable carbon and hydrogen isotope ratios, as well the abundance of atmospheric and crustal derived noble gases. These distinct properties can be used to differentiate bacterial and thermogenic methane assuming negligible change in composition and stable isotope ratios during transport. We use experimental results, theoretical models, and existing field data to determine whether hydrocarbon gas will show any appreciable change while migrating a distance greater than 1km. Theoretical two-phase gas displacement models predict that methane will become enriched at the front of a migrating gas plume due to mixing with dissolved biogenic methane in shallow groundwater.. Propane will dissolve more readily into the subsurface brines as the plume rises, leaving the final gas plume in the shallow groundwater heavily enriched in methane. We show that a mixture of a thermogenic gas plume with a small, dissolved biogenic methane supply in the groundwater will cause significant isotopic changes in the gas plume. Furthermore, atmospheric derived noble gases will be swept ahead of the methane pulse, leaving the main gas plume depleted of atmospheric gas components. We present results of experiments investigating these processes. All experiments used a 1m long, sand-packed steel column saturated with water containing dissolved noble gases. We then displaced the water by injecting methane, and measured the composition and carbon isotope ratio of the effluent gas. In this series of ongoing experiments, we are able to test both theory and field observations. Preliminary experimental results agree with theory and field observations, and show that dissolved gases and high volatility gases present in the injection gas are enriched in banks at the front of the displacement. These enrichment processes can be used to aid source identification of both fugitive gas plumes and migration of natural gas from source to reservoirs.
A dramatic expansion of natural gas exploration and extraction in unconventional reserves is underway. However, there is public concern that hydraulic fracturing will also cause natural gas, reservoir brines and associated fracturing fluids to contaminate shallower groundwater reservoirs. Considerable scientific research is currently focused on attributing methane found in shallow groundwater sources to either thermogenic or low temperature bacterial sources. Attribution techniques use concentration ratios of methane, ethane and propane and their stable carbon and hydrogen isotope ratios, as well the abundance of atmospheric and crustal derived noble gases. These distinct properties can be used to differentiate bacterial and thermogenic methane assuming negligible change in composition and stable isotope ratios during transport. We use experimental results, theoretical models, and existing field data to determine whether hydrocarbon gas will show any appreciable change while migrating a distance greater than 1km. Theoretical two-phase gas displacement models predict that methane will become enriched at the front of a migrating gas plume due to mixing with dissolved biogenic methane in shallow groundwater.. Propane will dissolve more readily into the subsurface brines as the plume rises, leaving the final gas plume in the shallow groundwater heavily enriched in methane. We show that a mixture of a thermogenic gas plume with a small, dissolved biogenic methane supply in the groundwater will cause significant isotopic changes in the gas plume. Furthermore, atmospheric derived noble gases will be swept ahead of the methane pulse, leaving the main gas plume depleted of atmospheric gas components. We present results of experiments investigating these processes. All experiments used a 1m long, sand-packed steel column saturated with water containing dissolved noble gases. We then displaced the water by injecting methane, and measured the composition and carbon isotope ratio of the effluent gas. In this series of ongoing experiments, we are able to test both theory and field observations. Preliminary experimental results agree with theory and field observations, and show that dissolved gases and high volatility gases present in the injection gas are enriched in banks at the front of the displacement. These enrichment processes can be used to aid source identification of both fugitive gas plumes and migration of natural gas from source to reservoirs.
Panel_15543
Panel_15543
10:10 AM
10:30 AM
10:30 a.m.
Application of Noble Gas Isotopic Signatures at McElmo Dome-DOE Canyon to Investigate CO2 Source and System Characterization
Room 702/704/706
By J. G. Adams, D. Gonzales, T. Darrah
The McElmo Dome-DOE Canyon field in the Four Corners region is one of the largest sources of CO2 in the Rocky Mountain region. In prior studies, hypotheses in favor of CO2 generation by thermal in situ decomposition of carbonate-sulfate assemblages in the Leadville Limestone or magmatic-gas release were proposed. The fundamental source of the gases, however, remained poorly understood. In this investigation, noble gas isotope signatures were used in an attempt to fingerprint the source of the CO2 gas and test competing hypotheses on its origin, migration, and evolution. Analyses of noble gas isotopes, stable isotopes, and major gas compositions across the McElmo-DOE field reveal variable and mixed mantle-crust signatures which are dominated by the addition of radiogenic crustal signatures (4He, 21Ne, 40Ar). A comparison of CO2/3He against CO2 concentrations are consistent with a magmatic 3He source that mixed with crustal contributions. The crustal contributions are indicated by helium isotope ratios 3He/4He (where the ratio of RAIR=1) from 0.057 to 0.215 (R/Ra), nucleogenic (following U and Th decay) 20Ne/22Ne (<8.5), 21Ne/22Ne (>0.10), and highly elevated radiogenic Ar with 40Ar/36Ar* >15,000. Our preliminary data suggests that CO2 gas was likely sourced from Cenozoic magmatic activity in the region that filled Leadville Formation traps at the time of magmatism. Magmatic events spanned the period from 75-5 Ma and involved melting of Proterozoic lithospheric mantle which was a key source of carbonated mantle melts in the Oligocene. Mafic rocks generated from these melts have elevated K, U, Th and F, and these magmas could have been a major source of the exceptionally high nucleogenic (21Ne, 22Ne) and radiogenic (4He,40Ar) signatures of noble gases in the McElmo Dome and Doe Canyon CO2 fields.
The McElmo Dome-DOE Canyon field in the Four Corners region is one of the largest sources of CO2 in the Rocky Mountain region. In prior studies, hypotheses in favor of CO2 generation by thermal in situ decomposition of carbonate-sulfate assemblages in the Leadville Limestone or magmatic-gas release were proposed. The fundamental source of the gases, however, remained poorly understood. In this investigation, noble gas isotope signatures were used in an attempt to fingerprint the source of the CO2 gas and test competing hypotheses on its origin, migration, and evolution. Analyses of noble gas isotopes, stable isotopes, and major gas compositions across the McElmo-DOE field reveal variable and mixed mantle-crust signatures which are dominated by the addition of radiogenic crustal signatures (4He, 21Ne, 40Ar). A comparison of CO2/3He against CO2 concentrations are consistent with a magmatic 3He source that mixed with crustal contributions. The crustal contributions are indicated by helium isotope ratios 3He/4He (where the ratio of RAIR=1) from 0.057 to 0.215 (R/Ra), nucleogenic (following U and Th decay) 20Ne/22Ne (<8.5), 21Ne/22Ne (>0.10), and highly elevated radiogenic Ar with 40Ar/36Ar* >15,000. Our preliminary data suggests that CO2 gas was likely sourced from Cenozoic magmatic activity in the region that filled Leadville Formation traps at the time of magmatism. Magmatic events spanned the period from 75-5 Ma and involved melting of Proterozoic lithospheric mantle which was a key source of carbonated mantle melts in the Oligocene. Mafic rocks generated from these melts have elevated K, U, Th and F, and these magmas could have been a major source of the exceptionally high nucleogenic (21Ne, 22Ne) and radiogenic (4He,40Ar) signatures of noble gases in the McElmo Dome and Doe Canyon CO2 fields.
Panel_15540
Panel_15540
10:30 AM
10:50 AM
10:50 a.m.
Noble Gases Help Trace the Behavior of Hydrocarbons in Unconventional Oil and Gas Shales
Room 702/704/706
By T. Darrah, R. J. Poreda, R. Perkins
The occurrence, distribution, and composition of hydrocarbons in the Earth's crust, result from the complex interplay between the tectonic and hydrologic cycles. For example, there is complex association between the tectonics of fold-thrust belts, the deformation of foreland basins, and the generation and migration of hydrocarbons and other geologic fluids in the subsurface. Accurately characterizing the relationship between these factors is critical to predicting the economic success of conventional and unconventional energy plays. One technique that is traditionally used in these studies is the analysis of gas geochemistry, specifically stable isotopic compositions (e.g., d13C, d18O, and ?2H) of hydrocarbon gases or CO2. The inert noble gases provide a complementary geochemical technique that can be used in concert with hydrocarbon molecular and stable isotope composition to evaluate the source and migrational history of hydrocarbons in conventional and unconventional plays. Additionally, in some cases, noble gases can be used as an external variable to evaluate the timing of closure for hydrocarbon reservoirs, open vs. closed system behavior and to determine and monitor the residual fluids in place during exploration and production. Herein, we will present noble gas and hydrocarbon molecular and stable isotope data from hydrocarbon plays in the Appalachian Basin (Utica, Trenton-Black River, and Marcellus) and Dallas-Fort Worth (Barnett) basins. Our presentation will focus on insights gained about hydrocarbon stable isotopic roll overs and reversals based on noble gas isotope data. Our preliminary data suggests that producing natural gas wells that exhibit isotopic reversals display distinct noble gas evidence consistent with relatively closed system behavior. Additionally, samples with isotopic reversals retain more than 3x the concentrations of atmospheric (air-saturated water) noble gases suggesting that significantly higher levels of formational waters remain in black shale source rocks that exhibit isotopic reversals.
The occurrence, distribution, and composition of hydrocarbons in the Earth's crust, result from the complex interplay between the tectonic and hydrologic cycles. For example, there is complex association between the tectonics of fold-thrust belts, the deformation of foreland basins, and the generation and migration of hydrocarbons and other geologic fluids in the subsurface. Accurately characterizing the relationship between these factors is critical to predicting the economic success of conventional and unconventional energy plays. One technique that is traditionally used in these studies is the analysis of gas geochemistry, specifically stable isotopic compositions (e.g., d13C, d18O, and ?2H) of hydrocarbon gases or CO2. The inert noble gases provide a complementary geochemical technique that can be used in concert with hydrocarbon molecular and stable isotope composition to evaluate the source and migrational history of hydrocarbons in conventional and unconventional plays. Additionally, in some cases, noble gases can be used as an external variable to evaluate the timing of closure for hydrocarbon reservoirs, open vs. closed system behavior and to determine and monitor the residual fluids in place during exploration and production. Herein, we will present noble gas and hydrocarbon molecular and stable isotope data from hydrocarbon plays in the Appalachian Basin (Utica, Trenton-Black River, and Marcellus) and Dallas-Fort Worth (Barnett) basins. Our presentation will focus on insights gained about hydrocarbon stable isotopic roll overs and reversals based on noble gas isotope data. Our preliminary data suggests that producing natural gas wells that exhibit isotopic reversals display distinct noble gas evidence consistent with relatively closed system behavior. Additionally, samples with isotopic reversals retain more than 3x the concentrations of atmospheric (air-saturated water) noble gases suggesting that significantly higher levels of formational waters remain in black shale source rocks that exhibit isotopic reversals.
Panel_15535
Panel_15535
10:50 AM
11:10 AM
11:10 a.m.
Using Noble Gas Geochemistry to Characterize Sources and Migration of Fluids in the Eagle Ford Shale
Room 702/704/706
By J. Harrington, J. C. Williams, T. Darrah
The Eagle Ford Shale in south Texas has become one of the most prolific shale plays in the United States in recent years. While production data suggests that oil and natural gas can be produced across a vast area of the field, the source of H2S and hydrocarbons, and the extent to which fluids have migrated into and out of the Eagle Ford, have yet to be determined. This study uses noble gas isotopes, gas composition, and stable isotopes to evaluate the source gases, to characterize the fluids-in-place, and to characterize the extent of fluid migration from the Eagle Ford Shale. The inert nature and distinct isotopic compositions make noble gases ideal tracers of crustal fluid processes. In most shales, the noble gas isotopic composition reflects a binary mixture of: 1) air-saturated water (ASW), containing 20Ne, 36Ar, and 84Kr derived from solubility equilibrium with the atmosphere during groundwater recharge and 2) radiogenic noble gases such as 4He*, 21Ne*, and 40Ar* sourced from the decay of U, Th, and K. Once noble gases incorporate into crustal fluids, they fractionate only by well-constrained physical mechanisms (e.g., diffusion, phase-partitioning). For example, although the decay of U and Th, produces a fixed ratio of 4He/21Ne (2.2x107) and the initial 4He/21Ne of minerals in shale are fixed, 4He will be preferentially released with respect to 21Ne at hydrocarbon-producing temperatures. Over time, the isotopic ratios vary as fluids equilibrate with the shale matrix. Variations occur as a function of temperature, porosity, and the volume of fluid flow. Thus, the isotopic values can be used to reconstruct the history of fluid flow in specific formations. Our data from the Eagle Ford show that mantle-derived gases (elevated 3He/4He= 0.15-0.25Ra and 20Ne/22Ne=10.2-11.1) and radiogenic gases (4He, 21Ne, 40Ar) dominate the overall gas composition. We anticipate that volcanism during Cretaceous/Cenozoic rifting activity caused the observed mantle-gas contributions. Interestingly, higher mantle contributions appear to correlate with elevated H2S in the production wells from this study suggesting thermal sulfate reduction induced by magmatic activity. Additionally, ASW and radiogenic noble gases can be used to model the relative volume of residual fluids-in-place for this Eagle Ford play. Initial data suggests that there has been minimal fractionation of noble gases implying minimal loss of the initial hydrocarbon fluids.
The Eagle Ford Shale in south Texas has become one of the most prolific shale plays in the United States in recent years. While production data suggests that oil and natural gas can be produced across a vast area of the field, the source of H2S and hydrocarbons, and the extent to which fluids have migrated into and out of the Eagle Ford, have yet to be determined. This study uses noble gas isotopes, gas composition, and stable isotopes to evaluate the source gases, to characterize the fluids-in-place, and to characterize the extent of fluid migration from the Eagle Ford Shale. The inert nature and distinct isotopic compositions make noble gases ideal tracers of crustal fluid processes. In most shales, the noble gas isotopic composition reflects a binary mixture of: 1) air-saturated water (ASW), containing 20Ne, 36Ar, and 84Kr derived from solubility equilibrium with the atmosphere during groundwater recharge and 2) radiogenic noble gases such as 4He*, 21Ne*, and 40Ar* sourced from the decay of U, Th, and K. Once noble gases incorporate into crustal fluids, they fractionate only by well-constrained physical mechanisms (e.g., diffusion, phase-partitioning). For example, although the decay of U and Th, produces a fixed ratio of 4He/21Ne (2.2x107) and the initial 4He/21Ne of minerals in shale are fixed, 4He will be preferentially released with respect to 21Ne at hydrocarbon-producing temperatures. Over time, the isotopic ratios vary as fluids equilibrate with the shale matrix. Variations occur as a function of temperature, porosity, and the volume of fluid flow. Thus, the isotopic values can be used to reconstruct the history of fluid flow in specific formations. Our data from the Eagle Ford show that mantle-derived gases (elevated 3He/4He= 0.15-0.25Ra and 20Ne/22Ne=10.2-11.1) and radiogenic gases (4He, 21Ne, 40Ar) dominate the overall gas composition. We anticipate that volcanism during Cretaceous/Cenozoic rifting activity caused the observed mantle-gas contributions. Interestingly, higher mantle contributions appear to correlate with elevated H2S in the production wells from this study suggesting thermal sulfate reduction induced by magmatic activity. Additionally, ASW and radiogenic noble gases can be used to model the relative volume of residual fluids-in-place for this Eagle Ford play. Initial data suggests that there has been minimal fractionation of noble gases implying minimal loss of the initial hydrocarbon fluids.
Panel_15536
Panel_15536
11:10 AM
11:30 AM
11:30 a.m.
Assessing Compositional Variability and Migration of Natural Gas in Antrim Shale in the Michigan Basin Using Noble Gas Geochemistry
Room 702/704/706
By T. Wen, M. Castro, B. Ellis, C. Hall, K. Lohmann, L. Bouvier
Recent studies in the Michigan Basin looked at the atmospheric and terrigenic noble gas signatures of deep brines to place constraints on the past thermal history of the basin and to assess the extent of vertical transport processes within this sedimentary system. In this contribution, we present noble gas data of shale gas samples from the Antrim shale formation in the Michigan Basin. The Antrim shale was one of the first economic shale-gas plays in the U.S. and has been actively developed since the 1980’s. This study pioneers the use of noble gases in subsurface shale gas in the Michigan Basin to clarify the nature of vertical transport processes within the sedimentary sequence and to assess potential variability of noble gas signatures in shales. Antrim Shale gas samples were analyzed for all stable noble gases (He, Ne, Ar, Kr, Xe) from samples collected at depths between 300 and 500m. Preliminary results show R/Ra values (where R and Ra are the measured and atmospheric 3He/4He ratios, respectively) varying from 0.022 to 0.21. Although most samples fall within typical crustal R/Ra range values (~0.02-0.05), a few samples point to the presence of a mantle He component with higher R/Ra ratios. Samples with higher R/Ra values also display higher 20Ne/22Ne ratios, up to 10.4, and further point to the presence of mantle 20Ne. The presence of crustally produced nucleogenic 21Ne and radiogenic 40Ar is also apparent with 21Ne/22Ne ratios up to 0.033 and 40Ar/36Ar ratios up to 312. The presence of crustally produced 4He, 21Ne and 40Ar is not spatially homogeneous within the Antrim shale. Areas of higher crustal 4He production appear distinct to those of crustally produced 21Ne and 40Ar and are possibly related the presence of different production levels within the shale with varying concentrations of parent elements.
Recent studies in the Michigan Basin looked at the atmospheric and terrigenic noble gas signatures of deep brines to place constraints on the past thermal history of the basin and to assess the extent of vertical transport processes within this sedimentary system. In this contribution, we present noble gas data of shale gas samples from the Antrim shale formation in the Michigan Basin. The Antrim shale was one of the first economic shale-gas plays in the U.S. and has been actively developed since the 1980’s. This study pioneers the use of noble gases in subsurface shale gas in the Michigan Basin to clarify the nature of vertical transport processes within the sedimentary sequence and to assess potential variability of noble gas signatures in shales. Antrim Shale gas samples were analyzed for all stable noble gases (He, Ne, Ar, Kr, Xe) from samples collected at depths between 300 and 500m. Preliminary results show R/Ra values (where R and Ra are the measured and atmospheric 3He/4He ratios, respectively) varying from 0.022 to 0.21. Although most samples fall within typical crustal R/Ra range values (~0.02-0.05), a few samples point to the presence of a mantle He component with higher R/Ra ratios. Samples with higher R/Ra values also display higher 20Ne/22Ne ratios, up to 10.4, and further point to the presence of mantle 20Ne. The presence of crustally produced nucleogenic 21Ne and radiogenic 40Ar is also apparent with 21Ne/22Ne ratios up to 0.033 and 40Ar/36Ar ratios up to 312. The presence of crustally produced 4He, 21Ne and 40Ar is not spatially homogeneous within the Antrim shale. Areas of higher crustal 4He production appear distinct to those of crustally produced 21Ne and 40Ar and are possibly related the presence of different production levels within the shale with varying concentrations of parent elements.
Panel_15539
Panel_15539
11:30 AM
11:50 AM