Hydrocarbon accumulation and reconstruction in the Ediacaran Dengying carbonate reservoirs of the southeastern and central Sichuan Basin, SW China

• Published in: Journal of Asian earth sciences 2024-12, Vol.276, p.106352, Article 106352
• Main Authors: Luo, Tao, Guo, Xiaowen, He, Zhiliang, Tao, Ze, He, Sheng, Huang, Yahao, Zhu, Yanxian
• Format: Article
• Language: English
• Subjects: Ediacaran Dengying carbonate reservoir; Fluid flow; Hydrocarbon accumulation; Pore-fluid pressure evolution; Sichuan Basin
• ISSN: 1367-9120
• DOI: 10.1016/j.jseaes.2024.106352

•Three stages of dolomite cements and two stages of quartz cements were identified in the Ediacaran Dengying reservoirs.•One-episode oil and gas charge occurred as witnessed by the development of bitumen and hydrocarbon inclusions.•Declined overpressure was associated with gas expulsion during the Yanshanian-Himalayan tectonic movements.•Current gas distribution and preservation were affected by different tectonic evolution during late Yanshanian-Himalayan. The carbonate reservoirs of the Ediacaran Dengying Formation (Z2dn) in the southeastern Sichuan Basin (SESB) and central Sichuan Basin (CSB) experienced different tectonic movements during the Caledonian, Yanshanian, and Himalayan orogenies, resulting in distinct hydrocarbon accumulation processes in these two regions. Based on a detailed analysis of diagenetic sequences, pore-fluid pressure evolution, hydrocarbon charge history, and regional tectonic structural analysis, the hydrocarbon accumulation processes in the Dingshan and Ziyang areas can be divided into three stages, namely Triassic to Jurassic, Jurassic to Cretaceous, and from Late Cretaceous to the present. The first stage was characterized by the formation of paleo-oil pools, marked by an episode of oil charge in the Z2dn reservoirs, as evidenced by solid bitumen and bitumen-bearing inclusions. During the second stage, pore fluid pressure increased from normal to overpressure due to oil cracking in the favorable preservation conditions, leading to the formation of paleo-gas pools in the SESB and CSB. The timing of gas charge in the Dingshan and Ziyang area was at 173 Ma and 165 Ma, respectively, according to the trapping temperature of gas inclusions. The third stage was marked by a pressure drop from overpressure to normal pressure, possibly due to gas dissipation during the Yanshanian-Himalayan uplift. The destruction of paleo-gas pools in the Dingshan area, accompanied by gas leakage through faults and fractures, was caused by intense tectonic movement in the SESB. In contrast, the low gas productivity in the Ziyang area of the CSB was attributed to the migration of natural gas into the Weiyuan area. The fluid evolution of carbonate reservoirs and various tectonic characteristics reveal similar hydrocarbon accumulation but different reconstruction processes in the SESB and CSB, providing greater insight into the mechanism of hydrocarbon enrichment in the Sichuan Basin and other multi-cyclonic composite basins.