Interrelationship Between Tectonism And Hydrocarbon Accumulation In Marine Strata At Critical Tectonic Moment In West Hubei-East Chongqing, South China

Based on the principles and methods of Tectonic dynamics, Fluid mechanics, Petroleum geochemistry and Reservoir dynamics, this article focuses on the characteristics of tectonic stress field, temporal-spacial distribution regularities, mechanism of tectonic deformation of Yanshan-himalayan, fault features and distribution, fluid dynamic field and energy field of critical tectonic stage, the relationships between fault active and fluid activity, distribution of traps along the strike and crossing the direction, tectonic dynamic evolution, fluid dynamic of gas accumulation, fluid geochemistry of gas preservation, and accumulation model.With the help of field investigation and seismic profile interpretation, we redefine the structure boundaries of study area, divide the main fault system, classify and describe the structural layers, and provide a new scheme for the large structural layer and the associated sub-structural layers. While dividing and discussing the structural styles in this study area, this paper introduces the space distribution and typological of main local structures in Exi- yudong on the foundation of plane and section analysis. On the foundation of the study of contacted relationships between Cretaceous and the under layer, structural chronology, this paper suggests the main tectonic movement happened between Late Jurassic and Early Cretaceous, which involved the Jurassic and pre-Jurassic into folds and the Upper Cretaceous covered the Jurassic as an unconformity developed between them. This paper also studies the tectonic framework of early Yanshan, and with the analysis of geology, the direction and size of the stress field during this stage have been researched qualitatively and semi-quantitatively, and a new 3D stress field model with 6319 units and 8888 points has been established, which allows us to propose that the intracontinental orogenic layered extrusion slip model could explain the tectonic pattern in early Yanshan. The stress field environment changed from compression to extension during Late Jurassic Yanshan Movement in Exi-Yudong. This study further identifies the developing scope of tectonic period, tectonic framework, and normal fault during the same time. The main extension stress field was controlled by extensional regime along SE direction, and the tensional stress in southeast was much larger than that in Northwest; however, the changing of extension and space position and fault occurrence have a close relationship considering the influence of fault reactivation, which proves that orogenic crust-mantle adjustment after stretching mode could be used to explain the tectonic activity during late Yanshan.This paper centers on the fluid dynamic field and energy field since early Yanshan. The spatial distribution of fluid field and the spatial changing of fluid potential field are summarized. We also summarize the gas migration trend in different periods, by which gas accumulation can be justified. The relationships between paleo-fluid activity and faulting have also been researched. The data of fluid inclusion of faults indicates that there are at least three times of fluid charge, namely, the Early, Middle and Late Jurassic when folds and faults formed with the Early Yanshanian compression gradually strengthened. And hydrocarbons participated in the fluid filling. Normal fault and tension fissure formed in the stage of Late Yanshanian tension, with a new cycle of fluid filling and redistribution. And there were also at least three times of fluid charge, that is, medium-latest Early Cretaceous, early Late Cretaceous and latest Late Cretaceous and accompany with hydrocarbon fluid. In the western Hubei-eastern Chongqing area, faults were always the channels of fluid migration and charge during the complex process of hydrocarbon generation, expulsion and migration, both reverse and normal fault. Multiple hydrocarbon migration indicates the process was multi-period and oil and gas reservoir experienced multiple destruction. From the Jurassic, the Early Yanshanian compression made the Jurassic stratum fold. And this destroyed the paleo-oil reservoirs in which the hydrocarbon migrated to new traps through faults and reaccumulated. With fault activity increasing, the hydrocarbon that had just reaccumulated could dissipate again.Late Yanshanian tension resulted greatly in redistribution and dissipation of the hydrocarbon that had accumulated. And this had a bad effect on oil-gas accumulation zone in the research area. Fortunately the intense tension was mainly in Qiyue Mountain and eastwards. But tension joints were widespread in Shizhu synclinorium, which had an important potential impact on hydrocarbon accumulation.The tectonic dynamics background and the evolution of trap in the western Hubei-eastern Chongqing area can be concluded as follows:four stages of tectonic movement, three tectonic transformations, two trap formations, one trap finality, and three trap reformations. The four stages of tectonic movement are the Indo-China period, Early Yanshanian period, Late Yanshanian period-the Early Himalayan period and Late Himalayan period, respectively. The first time of tectonic transformation is that tectonism transformed from main vertical lifting movement before Yanshanian period to main horizontal motion during Early Yanshanian period. The second time of tectonic transformation happened during Late Yanshanian period, cretaceous-early Paleogene, to be exact. The tectonism in the western Hubei-eastern Chongqing area transformed from main horizontal motion during Early Yanshanian period to main extension during Late Yanshanian period-the Early Himalayan period. Then it transformed to main vertical lifting movement with intermittent compression during Late Himalayan period. Two trap formations are the Indo-china period and the Early Yanshanian period. However, the traps formed during the Indo-china period were mainly related to the palaeohigh, while the traps formed during the Early Yanshanian period were mainly related to the faulting and folding. One trap finality is that the shape and distribution of traps were finalized during the Early Yanshanian period. The three trap reformations happened during the Early Yanshanian period, Late Yanshanian period-the Early Himalayan period and Late Himalayan period. The faulting and folding during the Early Yanshanian period reformed the traps intensively; and the tension during Late Yanshanian period-the Early Himalayan period reformed the traps to various degrees. The weak compression and differential uplifting had some influence s on the traps.The genetic type of the natural gas in the western Hubei-eastern Chongqing area belongs to the typical high - over mature oil type gas. The hydrocarbon source rocks at the bottom of the Silurian was mature and began to generate oil during Late Hercynian (Permian) and reached a oil generation peak and wet gas level during the Triassic period. The source rock on the top of the Permain reached over - mature level before the Early Yanshanian period (Late Jurassic). Nature gas accumulation after the Early Yanshanian period was mainly though? reformment and adjustment. Therefore we gain the gas accumulation zone of different periods based on the fluid dynamics of gas accumulation. This research also shows the fluid geochemical evidence for gas preservation and has a discussion of zone in the aspects of formation water and paleofluid.On the basis of the research above, the accumulation models in the western Hubei-eastern Chongqing area can be divided into four genetic types--- primary-weak reforming reservoirs, primary-strong reforming reservoirs, secondary-weak reforming reservoirs and secondary-strong reforming reservoirs. The four models have a big difference in oil and gas enrichment and distribution. Whether it’s primary or secondary reservoir, the strong reforming types are all distributed in Qiyu mountain tectonic zone, because the main reform factor is the tension during Late Yanshanian period. So it is difficult to form medium and large reservoirs of this type. That is to say, Lichuan synclinorium and Qiyu mountain tectonic zone have a relatively low exploration potential. The secondary-weak reforming reservoirs mainly distribute in the south of the Shizhu synclinorium and Fangdou mountain tectonic zone. They may capture the gas from the indosinian traps which were destroyed later as the favourable position. And they possess excellent reservoir forming condition, such as Chayuanping tectonic zone. The primary-weak reforming reservoirs mainly distribute in the north of the Shizhu synclinorium and Fangdou mountain tectonic zone where the traps formed and finalized by the Early Yanshanian compression and the weak tension and compression during Late Yanshanian period and Himalayan period having very little effect on the traps. So primary-weak and second-weak reforming traps are in favour of enrichment and accumulation and could form medium and large reservoirs.