Geochemistry, origin and accumulation of natural gases in the deepwater area of the Qiongdongnan Basin, South China Sea

A comprehensive analysis on high‐resolution seismic data and the geological characteristics of the Qiongdongnan Basin (QDNB) in the northern South China Sea reveals multiple seismic evidence indicative of hydrocarbon migration and gas hydrate accumulation, including mud diapirs, gas chimneys, bottom simulating reflectors (BSRs), acoustic blanking, enhanced reflections, V‐AMP structures, elevated stacking velocity of sediments, and seafloor expressions. Mud diapirs and gas chimneys, which are characterized by acoustic turbidity, fuzziness, and blanking reflections on seismic profiles, with a diversity of structures and scales, are widespread and concentrated in the centres of sags and the transitional zones between sags and uplifts. Five BSR regions, with a total area of approximately 11,000 km2, have been delineated in the QDNB. They have high or medium‐high continuity amplitudes with overlying acoustic blanking on seismic profiles, indicating the occurrence of gas hydrate. Distinct fluid seepage phenomena are common below the BSRs, demonstrating that abundant free gas has migrated and accumulated beneath the gas hydrate. The distribution of BSRs and the developed zones of mud diapirs and gas chimneys presents a spatial superposition feature, and the BSRs are usually located on top of or within the structural highs of the mud diapirs and gas chimneys, indicating a close relationship between them. Mud diapirs, gas chimneys, and associated faults are efficient pathways for migrating hydrocarbons, through which the mature to highly mature thermogenic gas of the Palaeogene strata and the biogenic gas of the Neogene sediments migrate to the gas hydrate stability zones and form gas hydrate. Gas hydrate samples were recovered from the gas chimney area during the remote operated vehicle (ROV) investigation in deepwater QDNB in 2015. Based on the seismic interpretation and geochemical results, we propose two models of gas hydrate accumulation in the deepwater area of the QDNB: (a) near source biogenic gas hydrate with self‐generation and self‐accumulation, and (b) distal source thermogenic gas hydrate with lower generation and upper accumulation. We believe that the areas with a fine spatial coupling configuration between the mud diapirs, gas chimneys, and BSRs are favourable exploration targets for gas hydrate in the QDNB.

Section: Geologic Settingmentioning

confidence: 99%

Section: Geologic Settingmentioning

confidence: 99%

Section: Figure 13mentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Distribution and characteristics of mud diapirs, gas chimneys, and bottom simulating reflectors associated with hydrocarbon migration and gas hydrate accumulation in the Qiongdongnan Basin, northern slope of the South China Sea

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Gong

et al. 2018

Formation of an anomalously high‐porosity zone of a very fine‐grained deep clastic reservoir in the Qiongdongnan Basin, South China Sea

Duan

Luo³

et al. 2020

Development of anomalously high porosities in very fine‐grained deep clastic reservoirs is still largely unknown. The reservoirs in the S13 block of the lower Meishan Formation in the Qiongdongnan Basin, South China Sea, developed a zone of anomalously high porosity up to 28.5%, and thus offers an ideal opportunity to quantitatively study the formation mechanism of the abnormally high porosity but fine‐grained deep clastic reservoir from the diagenetic perspective. Our study demonstrates that the first phase charging of oil and organic acid promoted the early‐stage dissolution. This process, however, restrained the early‐stage compaction, cementation, and transformation of clay minerals in sandstone reservoirs. CO2‐rich thermal fluid at the fourth phase enhanced the dissolution of late‐stage feldspar and early‐stage cements and authigenic clay minerals. The massive Ca2+, Mg2+, Fe2+, and other ions generated by the dissolution facilitated the formation of late‐stage siderite and ankerite, which filled the pore throats and lowered the reservoir permeability. Furthermore, the rapid deposition produced high primary porosities, and the early‐stage overpressure also inhibited the compaction, cementation, and clay transformation in the S13 block. All these processes promoted the preservation of primary pores, and the S13 block developed reservoirs with anomalously high porosity and low permeability. By contrast, the C24 block was only charged by oil and hydrocarbon‐rich gas at the second and third phases, and experienced strong compaction, weak dissolution, and strong cementation. Consequently, fewer primary and secondary pores, but higher contents of cements and authigenic clays were generated in the C24 block. The reservoirs in the C24 block exhibit medium porosity but low permeability.

Gas hydrate accumulation in shelf break setting: Example from the Qiongdongnan Basin in the northern slope of the South China Sea

Zhang

Deng

et al. 2021

Based on interpretation and inversion of high‐resolution seismic data, a previously undocumented occurrence of pore‐filled gas hydrates that are associated with a submarine canyon system was discovered in the shelf break of the north‐east Qiongdongnan Basin (QDNB) in the northern South China Sea (SCS). Continuous bottom simulating reflections (BSRs) were recognized in the submarine striking NW‐SE ridges along the shelf break. The two‐way travel times (TWTs) of the BSRs are about 200–250 ms below the sea floor and the burial depths are mapped at ~180–225 mbsf after time‐depth conversion. The distribution of the enhanced reflections (ERs) and acoustic blanking zone overlying the BSRs indicate possible pore‐filled gas hydrate enrichment. ERs have been commonly observed immediately below the BSRs, indicating the accumulation of free gas, which exhibits distinct amplitude versus offset (AVO) anomalies. The acoustic impedance inversion results show that the gas hydrate saturation is 20–50%, and the thickness of the gas hydrate‐bearing layers is usually ~10–40 m, with a maximum thickness of up to ~90 m in local regions. Large‐scale listric faults originating from the deep sag connect the hydrocarbon source region and the shallow gas hydrate stability zone (GHSZ) and act as migration pathways for biogenic gas and potentially thermogenic gas. In addition, hydrocarbons may also be transported into the GHSZ along the sidewalls of the canyon. Canyon incision is believed to cause the increase in the burial depth of the BSRs on the flank of the ridge toward the base of the channel and thereby to trap the hydrocarbons migrating along the sidewall in permeable beds due to the decrease in permeability caused by the gas hydrate accumulation, resulting in the occurrence of ERs immediately below the BSRs. It is speculated that in the channel‐levee system, the coarse‐grained sediments should be deposited within the GHSZ, and the reservoir's physical properties are conducive to the wide distribution and enrichment of pore‐filled gas hydrates in the shelf break of the northern QDNB.