The Research On Formation Condition Of Gas Pool And Target Evaluation Of Feixianguan Formation, Lower Triassic, Northeast Sichuan

Based on the theory of hydrocarbon (petroleum) system and pool-formation dynamics, and guided by the theory and method of carbonate sedimentology, reservoir geology and petroleum geochemistry, formation and evolution of source rock, reservoir, cap rock and trap have been studied in considerable detail. Thermal evolution history of organic matter of source rock, time and period of hydrocarbon generation, and characteristics of oil and gas migration, accumulation and dispersion have been documented. On the basis of quantitative evaluation of petroleum geological static elements, and by taking the evolution and arrangement of "four histories" in the time and space as a main clue, formation mechanism and distribution rule of gas pool have been illustrated, and the favorable zones of gas pool-formation and exploration targets have been predicted and evaluated. The main conclusions are stated as below:1. In Feixianguan period, Northeast Sichuan was largely located in a environment of carbonate isolated platform-trough (basin). The platform is surrounded by deep-water Kaijiang-Liangping trough, south Qinling ocean and Exi trough (basin). Oolitic shoal on the platform margin is the favorable area of reservoir distribution, whereas fine sediments of trough facies are beneficial for the formation of source rock.2. The abundance of carbonate organic matter is comparatively the highest in trough areas, Upper Pemian and Member 1 of Feixianguan Formation, Northeast Sichuan. The organic matter is post-mature at present. The capacity of their hydrocarbon generation is strong. These carbonates belong to good source rock.3. Results from the resumption of burial history and thermalevolution history show that Upper Permian source rock of trough area entered into hydrocarbon-generating threshold when Lower Triassic sediments began to deposit. The source rock achieved oil-generating peak during middle-Jurassic sedimentation and reached gas-generating peak in Cretaceous sedimentation. The largest burial depth was realized at the end of the Cretaceous period.4. Nature gas of Feixianguan oolitic shoal belongs to high sulphur dry gas in northeast Sichuan, whose average content of hydrogen sulfide is above 10% and the content of carbon dioxide is of middle level.5. By integrating carbon isotope composition of natural gas, source rock, and geochemical characteristics of reservoir bitumen, and combining carbon isotope characteristics of natural gas during different geological periods of Sichuan basin, it is considered that natural gas of study area mainly comes from marine carbonate source rock. Moreover, it is principally oil cracking gas, and secondly kerogen cracking gas. The natural gas is mainly related to carbonate organic matter in Upper Permian(P2) and lowest Triassic(T₁f¹) in trough facies.6. It confirmed that H₂S in natural gas is formed by thermal chemical sulphate reduction (TSR) according to exhaustive research of lithology characteristics, sulphur isotope of anhydrite, hydrogen sulfide and pyrite, and oxygen and carbon isotope composition of carbonate and calcite which replaces anhydrite nodule.7. There are four periods of solution in carbonate reservoir, one in contemporaneous period and three in burial period. Contemporaneous solution was caused by the meteoric water, and its intensity was comparatively low. The first burial solution was linked to organic acidic water produced by trough source rock, and its intensity was strong. The second burial solution was connected with H₂S generated by TSR (Thermal chemical Sulfate Reduction) in lagoon facies, and its intensity was also strong. The third burial solution was linked to reallocation and adjustment of underground water caused by tectonic activity in Himalayan period, and its intensity was low.8. There are three types of dolomitization in Feixianguan Formation. Mixing-water dolomitization exists in Oolitic shoal on the platform margin. Seepage-reflux dolomitization occurs in lagoons and point bank sediments. The burial dolomitization is weak. Dolomites distribute mainly along fracture and stylolite or replace oolith selectively. Dolomite characteristics of all genetic types are quite different from each other in the aspects of sedimentology, lithology and geochemistry.9. Pore evolution of the reservoir is studied in detail, and evolutionary model of reservoir "four histories" is put forward, by which we can combine occurrence, development and disappearance of pores in reservoir with tectonic burial conditions, hydrocarbon generation and trap evolution.10. The dolomite reservoir in oolitic shoal rise from the internal part of the platform to the edge in the horizon. Furthermore, the nearer the location to the platform edge is, the better the physical property of the reservoir is and the thicker the thickness of the reservoir is. The analysis of factors which control the formation of reservoir indicates that oolitic shoal sediments of platform margin is the basis for high-quality reservoir, while mixing-water dolomitization is essential condition and burial dissolution is the key.11. P-T hydrocarbon system of northeast Sichuan can be named as Upper Permian - Changxing organic reef and Feixianguan oolitic shoal hydrocarbon system(!) of Kaijiang-Liangping trough area. Key times for oil and gas formation in this hydrocarbon system are in the middle of Middle-Jurassic and middle Cretaceous respectively.12. Oolitic shoal gas pool in Feixianguan Formation is an independent pool-formation dynamic system, namely P2 — T^ pool-formation dynamic system, or oolitic shoal gas pool-formation dynamic system of northeast Sichuan. It can be named as normal pressure, mixed source, and semi-enclosed pool-formation dynamic system. Paleofluid potential of oil and gas formation periods of the dynamic system has been resumed. There are totally four modes of oil and gas migration and accumulation in the P2—Tif pool-formationdynamic system of northeast Sichuan. They are not isolated from each other, but are connected and constitute a unified fluid migration and accumulation system. They are a historical, systematic and dynamic process.13. Oolitic shoal gas pool in Feixianguan Formation is formed through three processes, namely the formation of palaeo oil pool in late Indosinian - early Yanshan priod, the formation of primary gas pool in the middle and late Yanshan period, and the reformation, regulation and reallocation of primary gas pool in Himalayan period. Formation model of oolitic shoal gas pool in Feixianguan Formation of northeast Sichuan has been established according to migration rule, and generating time of oil and gas, trap formation history, reservoir evolution history and pool formation history in the pool-formation dynamic system.14. Traps are evaluated by adopting expert system. Among 31 traps for evaluation, there are 15 traps of categories I and II, with trap area of 181.9 Km2 and resource quantity 1549.82 x 108m3. Two favorable oil and gas exploration zones are pointed out: One is in the northwest direction of Dukouhe until the south section of Tieshanpo structure, and the other is Wenqianjing tectonic zone. Finally, the thesis makes comprehensive geological evaluation to the most favorable category I traps, providing scientific basis for the selection of exploration targets.