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Secondary migration and leakage of methane from a major tight-gas system

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ABSTRACT

Tight-gas and shale-gas systems can undergo significant depressurization during basin uplift and erosion of overburden due primarily to the natural leakage of hydrocarbon fluids. To date, geologic factors governing hydrocarbon leakage from such systems are poorly documented and understood. Here we show, in a study of produced natural gas from 1,907 petroleum wells drilled into a Triassic tight-gas system in western Canada, that hydrocarbon fluid loss is focused along distinct curvilinear pathways controlled by stratigraphic trends with superior matrix permeability and likely also structural trends with enhanced fracture permeability. Natural gas along these pathways is preferentially enriched in methane because of selective secondary migration and phase separation processes. The leakage and secondary migration of thermogenic methane to surficial strata is part of an ongoing carbon cycle in which organic carbon in the deep sedimentary basin transforms into methane, and ultimately reaches the near-surface groundwater and atmosphere.

No MeSH data available.


Cross-plot of normalized methane (C1) content versus iC4/nC4ratio of natural gas samples from the Barnett Shale in Texas.The data (blue squares) from 131 wells show a ‘normal' trend at low thermal maturity of iC4/nC4 ratio increasing with methane content up to 0.95, followed by a ‘rollover', and then a ‘reversal' at high thermal maturity (late gas generation window) of iC4/nC4 ratio decreasing with methane content. Data from Zumberge et al.21
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f2: Cross-plot of normalized methane (C1) content versus iC4/nC4ratio of natural gas samples from the Barnett Shale in Texas.The data (blue squares) from 131 wells show a ‘normal' trend at low thermal maturity of iC4/nC4 ratio increasing with methane content up to 0.95, followed by a ‘rollover', and then a ‘reversal' at high thermal maturity (late gas generation window) of iC4/nC4 ratio decreasing with methane content. Data from Zumberge et al.21

Mentions: The thermal maturity trend of indigenous hydrocarbon fluids is well illustrated by signatures of natural gas produced from the Mississippian Barnett Shale in Texas21, a major shale-gas basin with no significant modification by migrated hydrocarbons. A cross-plot of normalized methane content versus iC4/nC4 ratio shows a distribution with two main maturity segments (Fig. 2). At lower thermal maturity, in the oil to early/mid gas generation windows, the iC4/nC4 ratio increases with maturity up to a methane (C1) content of ∼0.95. At higher thermal maturity, in the late gas generation window (C1>0.95), the iC4/nC4 ratio decreases markedly with further increase in maturity and methane content. The increase in iC4/nC4 ratio at lower thermal maturity is considered to reflect higher generation rates of iC4 than nC4 from thermal cracking of kerogen, oil or bitumen20. The reversal and decrease in the iC4/nC4 ratio at higher thermal maturity is considered to reflect the thermal cracking of wet gas and, in particular, the cracking of iC4 at a higher rate than nC4 (ref. 20).


Secondary migration and leakage of methane from a major tight-gas system
Cross-plot of normalized methane (C1) content versus iC4/nC4ratio of natural gas samples from the Barnett Shale in Texas.The data (blue squares) from 131 wells show a ‘normal' trend at low thermal maturity of iC4/nC4 ratio increasing with methane content up to 0.95, followed by a ‘rollover', and then a ‘reversal' at high thermal maturity (late gas generation window) of iC4/nC4 ratio decreasing with methane content. Data from Zumberge et al.21
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5121425&req=5

f2: Cross-plot of normalized methane (C1) content versus iC4/nC4ratio of natural gas samples from the Barnett Shale in Texas.The data (blue squares) from 131 wells show a ‘normal' trend at low thermal maturity of iC4/nC4 ratio increasing with methane content up to 0.95, followed by a ‘rollover', and then a ‘reversal' at high thermal maturity (late gas generation window) of iC4/nC4 ratio decreasing with methane content. Data from Zumberge et al.21
Mentions: The thermal maturity trend of indigenous hydrocarbon fluids is well illustrated by signatures of natural gas produced from the Mississippian Barnett Shale in Texas21, a major shale-gas basin with no significant modification by migrated hydrocarbons. A cross-plot of normalized methane content versus iC4/nC4 ratio shows a distribution with two main maturity segments (Fig. 2). At lower thermal maturity, in the oil to early/mid gas generation windows, the iC4/nC4 ratio increases with maturity up to a methane (C1) content of ∼0.95. At higher thermal maturity, in the late gas generation window (C1>0.95), the iC4/nC4 ratio decreases markedly with further increase in maturity and methane content. The increase in iC4/nC4 ratio at lower thermal maturity is considered to reflect higher generation rates of iC4 than nC4 from thermal cracking of kerogen, oil or bitumen20. The reversal and decrease in the iC4/nC4 ratio at higher thermal maturity is considered to reflect the thermal cracking of wet gas and, in particular, the cracking of iC4 at a higher rate than nC4 (ref. 20).

View Article: PubMed Central - PubMed

ABSTRACT

Tight-gas and shale-gas systems can undergo significant depressurization during basin uplift and erosion of overburden due primarily to the natural leakage of hydrocarbon fluids. To date, geologic factors governing hydrocarbon leakage from such systems are poorly documented and understood. Here we show, in a study of produced natural gas from 1,907 petroleum wells drilled into a Triassic tight-gas system in western Canada, that hydrocarbon fluid loss is focused along distinct curvilinear pathways controlled by stratigraphic trends with superior matrix permeability and likely also structural trends with enhanced fracture permeability. Natural gas along these pathways is preferentially enriched in methane because of selective secondary migration and phase separation processes. The leakage and secondary migration of thermogenic methane to surficial strata is part of an ongoing carbon cycle in which organic carbon in the deep sedimentary basin transforms into methane, and ultimately reaches the near-surface groundwater and atmosphere.

No MeSH data available.