Figure 1. 1 Schematic diagram of main structural system and distribution of Paleozoic large oil and gas fields in China (revised according to Sun Dianqing 1990).
(1) Multi-stage and multi-type prototype basins are superimposed to form oil.
The evolution of Chinese mainland experienced continental rift in early Neoproterozoic, ocean basin expansion from late Sinian to early Middle Ordovician, subduction and subduction from late Ordovician to Silurian, collision orogeny from late Silurian to Devonian, and the formation of ancient China United continent, as well as stretching and squeezing again in Carboniferous and Permian, thus completing the evolution history of Paleozoic in Chinese mainland.
The above evolution made Tarim, North China, Yangtze, Junggar and other blocks form many types of prototype basins, such as rift valley, continental margin, intra-craton depression, compressional craton and other basins, in which huge and extensive basin facies, shelf facies and platform facies carbonate rocks and shale were deposited. The superposition of prototype basins in different periods laid the foundation for the formation of Paleozoic large oil and gas fields in China.
(2) Multi-stage source rocks and multi-stage reservoir formation.
1) Both Sinian and Permian can generate oil, mainly Cambrian-Ordovician and Carboniferous-Permian.
2) The main source rocks are dark mudstone, shale, limestone and marl.
3) Hydrocarbon generation characteristics: the organic matter abundance of carbonate rocks is low, and the organic carbon content is above 0.3%, but the hydrocarbon generation conversion rate is high; It can generate hydrocarbons in secondary and multistage; High maturity of organic matter.
4) Multi-stage reservoir formation: generally, there are 2-4 stages of reservoir formation. For example, there are four reservoir-forming periods in Tarim Basin, namely, late Caledonian-early Variscan, late Variscan, Indosinian-Yanshan and Himalayan, but Himalayan is the main reservoir-forming period.
(3) Multi-stage oil-forming combination
1) Multiple reservoir rocks: there are clastic rocks, carbonate rocks and magmatic rocks, but mainly carbonate rocks; The reservoir space of carbonate rocks is mainly fracture-cave type or pore-fracture type, which has strong heterogeneity.
2) The caprock is a multi-stage caprock, and the caprock rocks are mudstone, shale, gypsum salt, dense limestone, etc.
3) There are mainly two sets of vertical oil-forming assemblages: one is Paleozoic self-generation and self-storage assemblage; The second is the combination of Paleozoic oil generation and Mesozoic and Cenozoic reservoir formation. Paleozoic assemblages are mainly CAMBRIAN-Ordovician, CAMBRIAN-Ordovician, SILURIAN-Devonian and Carboniferous-Permian.
(4) The migration distance of oil and gas is far
Oil and gas have the characteristics of long-distance migration. Oil and gas can cross several geological ages vertically and tens of kilometers to hundreds of kilometers horizontally.
(5) Multiple accumulation mode
Paleozoic is the main source rock, and the generated oil and gas are accumulated in multi-stage reservoirs, forming various models:
1) Paleozoic ancient oil reservoir: oil and gas generated in Paleozoic are stored in Paleozoic strata (Tarim basin and Sichuan basin);
2) Supergene paleo-reservoir: the oil and gas generated in Himalayan period are stored in Paleozoic strata (Tarim basin and Ordos basin);
3) Supergene storage: oil and gas generated in Himalayan period are stored in Mesozoic strata (Tarim Basin);
4) Supergene new reservoir: oil and gas generated in Himalayan period are stored in Cenozoic strata (Tarim Basin).
(6) coexistence of various oil and gas phases
Oil and gas coexist in source rocks of different ages, oil and gas of different maturity and oil and gas of different phases.
(7) Distribution characteristics of oil and gas
Oil and gas are distributed in the national structure, with oil in the west and natural gas in the east. Oil and gas fields are mainly distributed in Tarim, Junggar, Ordos, Sichuan, Bohai Bay and other basins. The characteristics of oil and gas distribution are:
1) The paleo-uplift and paleo-slope in the craton basin control the regional accumulation of oil and gas;
2) Regional unconformity is an important factor of hydrocarbon migration and accumulation;
3) Faults control the distribution of oil and gas.
(8) Paleozoic marine oil generation theory effectively guided oil and gas exploration.
Since the breakthrough of Paleozoic marine oil and gas in Tarim Basin 1984 Well Shashan 2, we have adhered to the oil exploration ideas such as ancient uplift, ancient slope, fault zone and regional unconformity. China Petrochemical Company and China Petroleum Company have successively discovered more than 30 oil and gas fields in northern and southwestern Tarim, with proven reserves reaching 7.5× 108t oil equivalent. In particular, Tahe Oilfield and Hotan River Gas Field were discovered.
Well Tabei 1984 Shashan 2 achieved the first major breakthrough of Paleozoic marine oil and gas in China, and the understanding of geological theory has important enlightenment, reference and guiding significance for Paleozoic oil and gas exploration in China. In recent years, great progress has been made in oil and gas exploration in northwest and south China, and many Paleozoic large and medium-sized oil and gas fields have been discovered.