A new genetic mechanism of natural gas accumulation

Abstract

Natural gas of organic origin is primarily biogenic or thermogenic; however, the formation of natural gas is occasionally attributed to hydrothermal activity. The Precambrian dolomite reservoir of the Anyue gas field is divided into three stages. Dolomite-quartz veins were precipitated after two earlier stages of dolomite deposition. Fluid inclusions in the dolomite and quartz are divided into pure methane (P-type), methane-bearing (M-type), aqueous (W-type), and solid bitumen-bearing (S-type) inclusions. The W-type inclusions within the quartz and buried dolomite homogenized between 107 °C and 223 °C. Furthermore, the trapping temperatures and pressures of the fluid (249 °C to 319 °C and 1619 bar to 2300 bar, respectively) are obtained from the intersections of the isochores of the P-type and the coeval W-type inclusions in the quartz. However, the burial history of the reservoir indicates that the maximum burial temperature did not exceed 230 °C. Thus, the generation of the natural gas was not caused solely by the burial of the dolomite reservoir. The results are also supported by the presence of paragenetic pyrobitumen and MVT lead-zinc ore. A coupled system of occasional invasion by hydrothermal fluids and burial of the reservoir may represent a new genetic model for natural gas accumulation in this gas field.

High temperature methane emissions from Large Igneous Provinces as contributors to late Permian mass extinctions

“…2a, b ). In contrast, clumped methane isotope temperatures are closer to the highest trapping temperatures (249–319 °C) of the quartz inclusions that have been reported in the same gas reservoirs in the Anyue gas field 30 , representing the invasion of deep-to-epizonogenic hydrothermal fluids corresponding to the ELIP, EMP 18 , 25 (Fig. 2b ).…”

“…The reservoirs with dolomite diagenesis have allowed storage of the pre-existing oils and subsequent oil-cracked gases 30 . The modeled temperatures correspond to the geological evolution history in the successive order of early cementation, surface dolomitization, syngenetic dissolution, meteoric karstification, burial dissolution, burial dolomite precipitation, hydrothermal invasion, and deepest burial dissolution 30 , 66 . b Diagram of Δ 13 CH 3 D-based temperatures (Δ 18 -based T) vs. C 1 /ΣC 2-5 ratios.…”

“…Large basaltic eruptions, crustal melting, hydrothermal activity, and abnormal heating events occurred in the basin and may have initially covered >500,000 km 2 from inner to outer zones 1 . A thick succession of marine carbonates overlies the ~800 Ma old basement, where a large-scale paleo-uplift (Leshan-Longnvsi) formed in the center of the basin and became a large petroleum system over the early Paleozoic 29 , 30 (Fig. 1b ).…”

Abiotic methane generation through reduction of serpentinite-hosted dolomite: Implications for carbon mobility in subduction zones

“…In Type I fluid inclusions, the gaseous and liquid species are rich in CH 4 and H 2 O, respectively, whereas the strong fluorescence interference of the host carbonates sometimes impedes direct observation of the H 2 O peaks (Fig. 5A and B; see also Yang et al, 2018). In contrast, dolomite hosted Type II fluid inclusions certainly contain CH 4 , while H 2 was occasionally detected (Fig.…”

Tracing interaction between hydrocarbon and groundwater systems with isotope signatures preserved in the Anyue gas field, central Sichuan Basin, China

“…The basement rock of the Sichuan Basin was formed around 800 Ma ago. During early Caledonian movement, a thick succession of marine carbonates was deposited overlying the basement (Yang et al, 2018) and Sinian and Cambrian source rock generated oil for the first time. The Leshan-Longnvsi paleohigh was formed in the middle of the basin at the end of the Silurian (Zhai, 1989).…”