Basic characteristics and accumulation mechanism of Sinian−Cambrian giant highly mature and oil-cracking gas reservoirs in the Sichuan Basin, SW China

Abstract

Old MesoproterozoicÀCambrian successions have been regarded as an important frontier field for global oil and gas exploration in the 21st century. This has been confirmed by a recent natural gas exploration breakthrough in the Sinian and Cambrian strata, central Sichuan Uplift, Sichuan Basin of SW China. However, the accumulation mechanism and enrichment rule of these gases have not been well characterized. This was addressed in this work, with aims to provide important guidance for the further exploration while enriching the general studies of the oil and gas geology in the old Mesoproterozoic-Cambrian strata. Results show that the gas field in the study area is featured by old target layers (Sinian-Lower Cambrian), large burial depth (>4500 m), multiple gas-bearing intervals (the second and fourth members of the Sinian Dengying Formation and the Lower Cambrian Longwangmiao Formation), various gas reservoir types (structural type and structural-lithologic type), large scale (giant), and superimposing and ubiquitous distribution. The giant reserves could be attributed to the extensive intercalation of pervasive high quality source rocks and large-scale karst reservoirs, which enables a three-dimensional hydrocarbon migration and accumulation pattern. The origin of natural gas is oil cracking, and the three critical stages of accumulation include the formation of oil reservoirs in Triassic, the cracking of oil in Cretaceous, and the adjustment and reaccumulations in the Paleogene. The main controlling factor of oil and gas enrichment is the inherited development of large-scale stable paleo-uplift, and the high points in the eastern paleo-uplift are the favorable area for natural gas exploration.

The formation of the Sichuan Basin, South China, during the Late Ediacaran to Early Cambrian

“…Member 2 (Z 2 dn 2 ), which varies between 20 and 1,000‐m‐thick, is characterized by the occurrence of microbial botryoidal dolomite, consisting of laminated dolomite, thrombolite, stromatolite, oncolite and bindstone. The top of Member 2, is defined by a karstified unconformity (Li et al., 2013; Luo et al., 2018; Zhou et al., 2016), but seismic and well data do not suggest that this event was an angular unconformity and most likely represents local variation in sea‐level with erosion and the development of a karstified surface. Member 3 (Z 2 dn 3 ), varies from a few centimetres to more than 100 m in thickness, and consists of a varied stratigraphy including interbedded siliciclastic, carbonate rocks and siliceous rocks with volcanic ash interpreted as a mixed shelf sedimentary environment (Deng et al., 2020).…”

“…Member 3 (Z 2 dn 3 ), varies from a few centimetres to more than 100 m in thickness, and consists of a varied stratigraphy including interbedded siliciclastic, carbonate rocks and siliceous rocks with volcanic ash interpreted as a mixed shelf sedimentary environment (Deng et al., 2020). Member 4 (Z 2 dn 4 ), which varies from 0 to 400 m thick, consists of medium‐to‐thick laminated dolomite, thrombolite, stromatolite, oncolite and bindstone (Duda et al., 2016; Li et al., 2013; Luo et al., 2018; Zhou et al., 2016). In the platform areas its top is defined by a flat karstified unconformity, although within the trough there is continuous deposition.…”

Special Issue: Advances of petroleum exploration and geology research in West China's petroliferous basins

“…Luo et al (2018) analyze petroleum accumulation in oil-cracking gas reservoirs in the Sinian Dengying and Cambrian Longwangmiao formations of the Sichuan Basin. The gas field contains old strata (Sinian to Cambrian) and is deep (generally > 4500 m), widely distributed, and contains multiple gas-bearing intervals that represent several gas accumulation types (structural, stratigraphic, and structural-stratigraphic).…”

“…Much exploration and research has been conducted on the late Neoproterozoic to Late Triassic Sichuan Basin, SW China, providing insights into marine strata, oil-cracking gas reservoirs, microbial carbonates, bioprecursor organisms, dolomite reservoirs, diagenesis, karst processes, and the temperature-pressure evolution of the basin (Du et al, 2014;Ma et al, 2010;Wei et al, 2013;Wu et al, 2017;Zou et al, 2014). Luo et al (2018) analyze petroleum accumulation in oil-cracking gas reservoirs in the Sinian Dengying and Cambrian Longwangmiao formations of the Sichuan Basin. The gas field contains old strata (Sinian to Cambrian) and is deep (generally > 4500 m), widely distributed, and contains multiple gas-bearing intervals that represent several gas accumulation types (structural, stratigraphic, and structural-stratigraphic).…”