Fluid inclusion and stable isotopic studies of thermochemical sulfate reduction: Upper permian and lower triassic gasfields, northeast Sichuan Basin, China

Kaikai Li, Simon C. George, Chunfang Cai, Se Gong, Stephen Sestak, Stephane Armand, Xuefeng Zhang

Research output: Contribution to journalArticleResearchpeer-review

Abstract

Fluid inclusions hosted in different stages of TSR-derived diagenetic minerals are expected to record compositions and isotopes of paleo-fluids at the time of trapping during different TSR extents. Here we report the first set of data on carbon isotopes of CH4 and CO2 and hydrogen isotopes of H2O trapped in fluid inclusions in TSR calcites. We find that the NE Sichuan sour dolostones have initially experienced oil- and wet gas-dominated TSR, as recorded in H2S-bearing oil inclusions with lower homogenization temperatures (Th) values (e.g., ≤137 °C) and the coexistence of C2+ hydrocarbon gas and H2S in fluid inclusions. The subsequent dry gas-dominated TSR occurred in higher reservoir temperatures (> about 161.5 °C) when most C2+ hydrocarbons were exhausted. The three-stage TSR resulted in CH4 δ13C values becoming progressively heavier from −46.7‰ to −29.6‰, H2O δ2H values shifting negatively from −36.4‰ to −67.8‰ and salinities decreasing to as low as 0.9 wt% NaCl. The dry gas-dominated TSR reaction seems to be the most efficient at water production, which, however, was limited by available reactive sulfate, and shows significant differences within the reef and shoal reservoirs along the platform margin, and the anhydrite-bearing reservoirs in the paleo-lagoon area. The TSR reaction within the porous shelf-margin reservoirs is capable of causing carbonate dissolution owing to high porosity and good connectivity of the micropore network and the resulting mass transport away from TSR sites. This resulted in CO2 δ13C positive shift from −9.3‰ to +6.3‰, and a positive correlation of this parameter with Th. In contrast, in the tight anhydrite-bearing reservoirs, slow mass transport and quick saturation of calcium and dissolved CO2 in the pore waters is expected to precipitate TSR calcite near the anhydrite crystals, resulting in calcite crystals having more depleted δ13C values (−1.4‰ to −18.9‰). This study shows that there are essential differences in the process and effects of TSR reaction due to geological differences in the settings of TSR sites.
LanguageEnglish
Pages86-108
Number of pages23
JournalGeochimica et Cosmochimica Acta
Volume246
DOIs
Publication statusPublished - 1 Feb 2019

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Bearings (structural)
fluid inclusion
Sulfates
Calcium Carbonate
Permian
Triassic
Gases
sulfate
anhydrite
Fluids
Hydrocarbons
basin
Isotopes
calcite
mass transport
Oils
Mass transfer
gas
Carbon Isotopes
Crystals

Cite this

@article{779582bda3eb4e4db7f71c288eb4eff5,
title = "Fluid inclusion and stable isotopic studies of thermochemical sulfate reduction: Upper permian and lower triassic gasfields, northeast Sichuan Basin, China",
abstract = "Fluid inclusions hosted in different stages of TSR-derived diagenetic minerals are expected to record compositions and isotopes of paleo-fluids at the time of trapping during different TSR extents. Here we report the first set of data on carbon isotopes of CH4 and CO2 and hydrogen isotopes of H2O trapped in fluid inclusions in TSR calcites. We find that the NE Sichuan sour dolostones have initially experienced oil- and wet gas-dominated TSR, as recorded in H2S-bearing oil inclusions with lower homogenization temperatures (Th) values (e.g., ≤137 °C) and the coexistence of C2+ hydrocarbon gas and H2S in fluid inclusions. The subsequent dry gas-dominated TSR occurred in higher reservoir temperatures (> about 161.5 °C) when most C2+ hydrocarbons were exhausted. The three-stage TSR resulted in CH4 δ13C values becoming progressively heavier from −46.7‰ to −29.6‰, H2O δ2H values shifting negatively from −36.4‰ to −67.8‰ and salinities decreasing to as low as 0.9 wt{\%} NaCl. The dry gas-dominated TSR reaction seems to be the most efficient at water production, which, however, was limited by available reactive sulfate, and shows significant differences within the reef and shoal reservoirs along the platform margin, and the anhydrite-bearing reservoirs in the paleo-lagoon area. The TSR reaction within the porous shelf-margin reservoirs is capable of causing carbonate dissolution owing to high porosity and good connectivity of the micropore network and the resulting mass transport away from TSR sites. This resulted in CO2 δ13C positive shift from −9.3‰ to +6.3‰, and a positive correlation of this parameter with Th. In contrast, in the tight anhydrite-bearing reservoirs, slow mass transport and quick saturation of calcium and dissolved CO2 in the pore waters is expected to precipitate TSR calcite near the anhydrite crystals, resulting in calcite crystals having more depleted δ13C values (−1.4‰ to −18.9‰). This study shows that there are essential differences in the process and effects of TSR reaction due to geological differences in the settings of TSR sites.",
author = "Kaikai Li and George, {Simon C.} and Chunfang Cai and Se Gong and Stephen Sestak and Stephane Armand and Xuefeng Zhang",
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Fluid inclusion and stable isotopic studies of thermochemical sulfate reduction : Upper permian and lower triassic gasfields, northeast Sichuan Basin, China. / Li, Kaikai; George, Simon C.; Cai, Chunfang; Gong, Se; Sestak, Stephen; Armand, Stephane; Zhang, Xuefeng.

In: Geochimica et Cosmochimica Acta, Vol. 246, 01.02.2019, p. 86-108.

Research output: Contribution to journalArticleResearchpeer-review

TY - JOUR

T1 - Fluid inclusion and stable isotopic studies of thermochemical sulfate reduction

T2 - Geochimica et Cosmochimica Acta

AU - Li, Kaikai

AU - George, Simon C.

AU - Cai, Chunfang

AU - Gong, Se

AU - Sestak, Stephen

AU - Armand, Stephane

AU - Zhang, Xuefeng

PY - 2019/2/1

Y1 - 2019/2/1

N2 - Fluid inclusions hosted in different stages of TSR-derived diagenetic minerals are expected to record compositions and isotopes of paleo-fluids at the time of trapping during different TSR extents. Here we report the first set of data on carbon isotopes of CH4 and CO2 and hydrogen isotopes of H2O trapped in fluid inclusions in TSR calcites. We find that the NE Sichuan sour dolostones have initially experienced oil- and wet gas-dominated TSR, as recorded in H2S-bearing oil inclusions with lower homogenization temperatures (Th) values (e.g., ≤137 °C) and the coexistence of C2+ hydrocarbon gas and H2S in fluid inclusions. The subsequent dry gas-dominated TSR occurred in higher reservoir temperatures (> about 161.5 °C) when most C2+ hydrocarbons were exhausted. The three-stage TSR resulted in CH4 δ13C values becoming progressively heavier from −46.7‰ to −29.6‰, H2O δ2H values shifting negatively from −36.4‰ to −67.8‰ and salinities decreasing to as low as 0.9 wt% NaCl. The dry gas-dominated TSR reaction seems to be the most efficient at water production, which, however, was limited by available reactive sulfate, and shows significant differences within the reef and shoal reservoirs along the platform margin, and the anhydrite-bearing reservoirs in the paleo-lagoon area. The TSR reaction within the porous shelf-margin reservoirs is capable of causing carbonate dissolution owing to high porosity and good connectivity of the micropore network and the resulting mass transport away from TSR sites. This resulted in CO2 δ13C positive shift from −9.3‰ to +6.3‰, and a positive correlation of this parameter with Th. In contrast, in the tight anhydrite-bearing reservoirs, slow mass transport and quick saturation of calcium and dissolved CO2 in the pore waters is expected to precipitate TSR calcite near the anhydrite crystals, resulting in calcite crystals having more depleted δ13C values (−1.4‰ to −18.9‰). This study shows that there are essential differences in the process and effects of TSR reaction due to geological differences in the settings of TSR sites.

AB - Fluid inclusions hosted in different stages of TSR-derived diagenetic minerals are expected to record compositions and isotopes of paleo-fluids at the time of trapping during different TSR extents. Here we report the first set of data on carbon isotopes of CH4 and CO2 and hydrogen isotopes of H2O trapped in fluid inclusions in TSR calcites. We find that the NE Sichuan sour dolostones have initially experienced oil- and wet gas-dominated TSR, as recorded in H2S-bearing oil inclusions with lower homogenization temperatures (Th) values (e.g., ≤137 °C) and the coexistence of C2+ hydrocarbon gas and H2S in fluid inclusions. The subsequent dry gas-dominated TSR occurred in higher reservoir temperatures (> about 161.5 °C) when most C2+ hydrocarbons were exhausted. The three-stage TSR resulted in CH4 δ13C values becoming progressively heavier from −46.7‰ to −29.6‰, H2O δ2H values shifting negatively from −36.4‰ to −67.8‰ and salinities decreasing to as low as 0.9 wt% NaCl. The dry gas-dominated TSR reaction seems to be the most efficient at water production, which, however, was limited by available reactive sulfate, and shows significant differences within the reef and shoal reservoirs along the platform margin, and the anhydrite-bearing reservoirs in the paleo-lagoon area. The TSR reaction within the porous shelf-margin reservoirs is capable of causing carbonate dissolution owing to high porosity and good connectivity of the micropore network and the resulting mass transport away from TSR sites. This resulted in CO2 δ13C positive shift from −9.3‰ to +6.3‰, and a positive correlation of this parameter with Th. In contrast, in the tight anhydrite-bearing reservoirs, slow mass transport and quick saturation of calcium and dissolved CO2 in the pore waters is expected to precipitate TSR calcite near the anhydrite crystals, resulting in calcite crystals having more depleted δ13C values (−1.4‰ to −18.9‰). This study shows that there are essential differences in the process and effects of TSR reaction due to geological differences in the settings of TSR sites.

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