Basic geological features of superimposed basin and hydrocarbon accumulation in Sichuan Basin, China

Abstract

The recent breakthroughs in natural gas exploration in Sichuan Basin have been making it urgent and indispensable to reinvestigate its basic geological features and characteristics of hydrocarbon accumulation. Being a typical superimposed basin, Sichuan Basin has experienced three major phases of basin evolution during the Phanerozoic time as follows: marine carbonate platform with differential subsidence-uplift in extensional setting from Sinian to Middle Triassic, continental clastic basin with fold-and-thrust deformation under compressive background from Late Triassic to Eocene, and uplift and structural modification since Oligocene. Simultaneously, Sichuan Basin has also gone through three key periods of tectonic transformation. The first tectonic transformation in Late Triassic time is characterized by the changes of tectonic settings from extension to compression, basin property from passive continental margin to foreland basin, deposition from marine carbonate to marine clastic and to continental clastic. The second tectonic transformation in early Late Cretaceous is symbolized by the migration of foredeep, tectonic domain change from Qinling orogenic belt to Tibet Plateau, and the change from deposition to partial uplift and erosion. The last tectonic transformation in Eocene is represented by the changes from exorheic basin to endorheic basin, from sedimentary basin to topographic basin, and from deposition to overall uplift and erosion. Affected jointly by peripheral structural belts since Indosinian movement (the Qinling orogenic belt on the north side, Xuefeng intra-continental tectonic system on the east and southeast sides, and Tibet Plateau on the west and southwest sides), multi-stage and multi-direction structures were superimposed in Sichuan Basin. The present-day basin-orogen configuration in Sichuan Basin and adjacent regions exhibits clear trisection in topography, basement and surface structural features, and two types of basin-orogen boundaries, i. e. break and gradual. According to the type of basin-orogen boundary, developing history and controlling factors, the Sichuan Basin can be divided into five basin-orogen structure units: Unit I-North Sichuan break basin-orogen structure area (controlled by Qinling tectonic domain), Unit II-West Sichuan break basin-orogen structure area (controlled by Tibet tectonic domain), Unit II-East Sichuan gradual basin-orogen structure area(controlled by Xuefeng tectonic domain), Unit IV-Southwest Sichuan gradual basin-orogen structure area(controlled jointly by Tibet, Xuefeng and basement tectonic domains) and Unit V-Central Sichuan autochthonous uplift-basin area (controlled by basement tectonic domain). Sichuan Basin is one of major oil-gas-bearing basins in western China, with great natural gas exploration potential owing to abundant hydrocarbon supply and excellent preservation condition. Firstly, five sets of major source rocks developed during the multi-stage basin evolution, with a total thickness of up to 1500~2500 m, and high organic carbon content, hydrocarbon generation capacity and gas productivity. Secondly, the Lower Triassic gypsum-salt rocks played an important role in sealing marine oil and gas, while the binary texture and gentle erosion in thrust belt and adjacent foreland basin greatly enhanced the preservation condition in break basin-orogen structure areas. Further, uplift activities since the Late Cretaceous might cause not only the adjustment or destruction of ancient gas pools (gas storage center), but also the formation of present gas reservoirs (gas preservation centre), and fluid cross-formation migration and explosive gas accumulation as well. The multi-phased evolution of superimposed basin, composite-association of multi-stage and multi-direction structures, the heterogeneity of reservoirs and the mobility of natural gas decided the complexity, time-consuming and tortuosity of oil-gas exploration in Sichuan Basin. Thus, just a single exploration thinking, approach and hydrocarbon accumulation theory could not be employed to instruct oil-gas exploration; that is, exploration strategies should also be diverse.