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

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.


Map showing location of the study area in western Canada.Green shaded area shows extent of Montney Formation and age-equivalent strata. Blue box shows study area. Scale bar, 100 km.
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f1: Map showing location of the study area in western Canada.Green shaded area shows extent of Montney Formation and age-equivalent strata. Blue box shows study area. Scale bar, 100 km.

Mentions: The Lower Triassic Montney Formation is up to 320 m thick and forms part of an overpressured, tight-gas fairway in the Western Canadian Sedimentary Basin678. The formation is composed mainly of siltstone and was deposited dominantly in lower shoreface to offshore marine environments8910. Turbidite facies were also deposited locally in the succession1112. Basin modelling suggests that the Montney tight-gas fairway passed into the thermogenic hydrocarbon generation window ∼80–90 Ma (million years ago)13. Present-day total organic carbon, typically in the range from 0.25 to 4.0 wt%, is virtually all in the form of solid bitumen/pyrobitumen14151617. The solid bitumen represents a previous oil phase that partially filled the pore network of Montney siltstones during hydrocarbon charging1617. With further burial to maximum depths of 4–7 km (ref. 13) and the accompanying increase in temperature, oil cracked in situ to solid bitumen and light hydrocarbon fluids, and significant overpressuring developed. From maximum burial at ∼60 Ma to the present day, the Montney tight-gas fairway underwent considerable depressurization and cooling in response to relaxation of tectonic compression and erosion of 1.4–3.0 km of overburden13. In the geographic area of study, which straddles the border between Alberta and British Columbia (Fig. 1), present-day pore pressures in the Montney Formation conform broadly to structural elevation with normally pressured strata in the northeast and overpressured strata located down-dip to the southwest (cf. refs 6, 8, 13). The present-day depth of the Montney Formation ranges from 850 m in the northeast to 3,900 m in the southwest, and the transition from normal pressure to overpressure is at a depth of 1,800–2,600 m.


Secondary migration and leakage of methane from a major tight-gas system
Map showing location of the study area in western Canada.Green shaded area shows extent of Montney Formation and age-equivalent strata. Blue box shows study area. Scale bar, 100 km.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f1: Map showing location of the study area in western Canada.Green shaded area shows extent of Montney Formation and age-equivalent strata. Blue box shows study area. Scale bar, 100 km.
Mentions: The Lower Triassic Montney Formation is up to 320 m thick and forms part of an overpressured, tight-gas fairway in the Western Canadian Sedimentary Basin678. The formation is composed mainly of siltstone and was deposited dominantly in lower shoreface to offshore marine environments8910. Turbidite facies were also deposited locally in the succession1112. Basin modelling suggests that the Montney tight-gas fairway passed into the thermogenic hydrocarbon generation window ∼80–90 Ma (million years ago)13. Present-day total organic carbon, typically in the range from 0.25 to 4.0 wt%, is virtually all in the form of solid bitumen/pyrobitumen14151617. The solid bitumen represents a previous oil phase that partially filled the pore network of Montney siltstones during hydrocarbon charging1617. With further burial to maximum depths of 4–7 km (ref. 13) and the accompanying increase in temperature, oil cracked in situ to solid bitumen and light hydrocarbon fluids, and significant overpressuring developed. From maximum burial at ∼60 Ma to the present day, the Montney tight-gas fairway underwent considerable depressurization and cooling in response to relaxation of tectonic compression and erosion of 1.4–3.0 km of overburden13. In the geographic area of study, which straddles the border between Alberta and British Columbia (Fig. 1), present-day pore pressures in the Montney Formation conform broadly to structural elevation with normally pressured strata in the northeast and overpressured strata located down-dip to the southwest (cf. refs 6, 8, 13). The present-day depth of the Montney Formation ranges from 850 m in the northeast to 3,900 m in the southwest, and the transition from normal pressure to overpressure is at a depth of 1,800–2,600 m.

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.