Hydrocarbon Accumulation Characteristics And Exploration Potential Of The Upper Sinian In The Middle Yangtze Area

As a typical complex hydrocarbon accumulation area, the Middle Yangtze underwent a multi-stage tectonic evolutions, superimposed alterations and hydrocarbon generation and expulsion, and complex petroleum accumulation process. The Upper Sinian is a important sequence for hydrocarbon exploration in the Middle Yangtze area. Under the guidence of modern petroleum geology theories as tectonics, sedimentology and multi-cycle basin analysis, using the data of geological, seismic, drilling, logging and laboratory test analysis, by means of comprehensive analysis and computer simulation technology, based on the principles of combined the field macroscopic observation with office microscopic analysis, combined the surface geology and subsurface geology with seismic analysis, combined static description with dynamic research, and applying the viewpoints about natural gas accumulation and enrichment controlled by regional seal, and by palaeohigh,especially emphasizing the important actions of source and caprock, this paper has studied the basic characteristics, evolution processes, mechanisms, controlling factors, possible models of hydrocarbon accumulation of the Upper Sinian in the Middle Yangtze area, evaluated the exploration potential synthetically, and also pointed out the favorable exploration zones and directions.The analysis about basin filling and tectonic evolution showed that the Middle Yangtze basin underwent two evolution stages of the prototype basin, and strong deformation and dislocation. The first stage can further be divided into four phases as craton (Z1-O2) and foreland swell basin (O3-S) during the Caledonian period, craton basin from the Hercynian to the early Indosinian (D-T2) and foreland basin from the Late Indo-China to the early Yanshan (T3-J2), whereas the second stage contained intracontinental basin with fold and deformation strongly at the end of the early Yanshan (J3-K1), extension and alteration from the late Yanshan to the early Himalayan (K2-E), and compression deformation and dislocation strongly in basin's margin during the late Himalayan period (N-Q).Under the guidence of the theories of collision orogenesis and foreland basin, the interpretation of regional structural frame section on seismic data indicated that the structural frame of the Middle Yangtze marine strata are north-south zonation and east-west blocking. The main character before the Indo-China epoch is differential uplift, and its tectonic style associated with the basement rising; The main character of the early Yanshan is the intracontinental orogenic belt, which resulted from south-north compression, and it consists of two large overthrust orogenic belts with intracontinental compression in the south and north, and a transition belt in the middle. The tectonic units of the marine strata can be divided into three first-level units, seven second-level units and seventeen third-level units.The basic features of the Upper Sinian hydrocarbon accumulation have been studied in this paper by using the data of surface outcrops, drilling, logging, testing and laboratory analysis. The results show that the source rock mainly contain carbonaceous mudstone and marl of the Upper Sinian Doushantuo Formation, and mudstone and carbonaceous mudstone of the Lower Cambrian Niutitang Formation. The main reservoirs are granular dolomite and corrosion vugular dolomite of the Upper Sinian Dengying Formation. The main caprock are the Lower Cambrian mudstone and marl. The source rock in the Upper Sinian and Lower Cambrian evolved a hign level, and mainly producted gas. The Lower Cambrian source rock is better than that of the Upper Sinian. In the plane, the Upper Sinian source rock was distributed mainly in the the Hefeng area, the western of the Hunan and Hubei provinces, whereas the Lower Cambrian source rock was developed mostly in the areas of the Enshi-Hefeng and the north of the Jianghan plain, where have the condition of forming large-size oil and gas pools. The reservoirs in the Upper Sinian Dengying Formation distributed relatively stable and extensive in the lateral. Its reservoir spaces consisted mainly of the secondary corroded pore and hole, and the major reservoir type is the fracture-pore-hole. This reservoir height in the Lichuan and Xingshan-yichang areas is greater, and physical property is also better. The caprock in the Lower Cambrian is charatered as continuous distribution, large total thickness, and high homogeneous. Comparing with the Jianghan plain, the caprock in the western Hunan-Hubei is more effectively to develop high efficiency gas accumulation. The Shizhu synclinorium is the most favorable preservation unit, and the Mianyang interference bruchfalten, Yichang stable belt, Huaguoping synclinorium, Sangzhi-Shimen synclinorium and Lichuan synclinorium are also favorable preservation unit.Based on the viewpoints of gas distribution controlled by regional seal and paleohigh, the hydrocarbon accumulation evolution history of the Upper Sinian was analyzed dymanically in this study. It is shown that the Upper Sinian and Lower Cambrian source rocks entered the peak of hydrocarbon generation and expulsion during the Caledonian period, while the paleo-tectonic presented as great uplift and great depression (North marign uplift belt, Central slope zone, Sourth margin depression zone). Thus, The slope is the favorable direction of hydrocarbon migration, i.e., the areas as the western Hubei-eastern Chongqing, western Hubei and Hunan, and sourthern Jianghan Plain due to near the hydrocarbon generation and expulsion center of source rock, while the seal ability of the Lower Cambrian caprock in the southwest of the western Hubei and Hunan is the best, and hydrocarbon accumulation condition of the Shizhu area in the western Hubei-eastern Chongqing is the best. The generation and expulsion intensities of the Upper Sinian and Lower Cambrian source rocks decreased during the Hercynian-Indochina period, while the paleo-tectonic units consisted of the western Hubei-eastern Chongqing uplift, western Hubei and Hunan depression, Huangling-Honghu uplift, Dangyang depression zone, Zhongxiang uplift and eastern Hubei depression. The uplifts situated on the direction of hydrocarbon migration. Comparing with the Caledonian period, the area of good seal ability moved to west, so the western Hubei-eastern Chongqing area is favourable to hydrocarbon preservation. During the early Yanshanian period, the hydrocarbon generation and expulsion intensities of the Upper Sinian and Lower Cambrian source rock can be neglected, and the traps forming in this period became the re-distribution targets of ancient oil reservoir in the uplift or on the slope and paleo-oil pools pyrolysis gas. The seal ability of the Lower Cambrian caprock is almost the same as it in the Hercynian-Indo-China movement, and the western Hubei-eastern Chongqing area is favourable to hydrocarbon preservation. During the middle Yanshanian-Himalayan period, the source rock can not be generated hydrocarbon again, however, it can be supplyed by some oil cracking gas from the Pre-Sinian ancient reservoir. As a result of the extensional rift and the reversal of early compression fault has the dual role of destruction and construcion for oil and gas preservation system, the seal ability of the Lower Cambrian caprock in the western Hubei-eastern Chongqing area is continuously well, and the areas of Shizhu, Lichuan, Huaguoping are favourable to hydrocarbon preservation.Hydrocarbon accumulation period of the Upper Sinian was analyzed comprehensively by using the methods of trap formation time, hydrocarbon generation and expulsion history, and fluid inclusion analysis. The large-size hydrocarbon accumulation of the Upper Sinian occurred during the Yanshan movement period at the Late Jurassic on the trap formation time analysis, while much of oil and gas were lost, and gas reservoirs were destructed and transformed. With the hydrocarbon generation and expulsion history analysis, it indicatedt that the Caledonian, Hercynian-Indosinian and early Yanshan periods are the key moments to the thermal evolution of the source rocks of the Upper Sinian and the lower Cambrian. From the fluid inclusion analysis, the fluid inclusions in the Upper Sinian Dengying Formation were formed from the Late Caledonian to the early Indo-China period, while the paleo-uplifts and paleo-structures were favorable for hydrocarbon migration due to no local structures in this area at that time.The effective hydrocarbon supplying ability of source rock and controlling factors of the Upper Sinianin hydrocarbon accumulation in the Middle Yangtze area were dicussed by using the methods of source rock devoloping, hydrocarbon generation kinetics, and hydrocarbon accumulation process kinetics comprehensively. It indicated that the muddy source rock is the main hydrocarbon-generating matter, and the tectonic-sedimentary type influenceed the whole process of hydrocarbon generation, expulsion, migration, accumulation, destruction amd preservation. The hydrocarbon generation and expulsion intensities represented the initial ability of hydrocarbon expelled from hydrocarbon kitchen, and the evolving nature of hydrocarbon kitchen offered a variety of sources for gas accumulation. The hydrocarbon loss rate affected the effective supplying ability of hydrocarbon kitchen. The effective hydrocarbon supplying efficiency was the comprehensive function of source rock properties, tectonic-sedimentary types, hydrocarbon kitchen types and their combination, hydrocarbon generation intensity evolution history and gas loss rate. And the analysis and quantitative description of influencing factors of the effective hydrocarbon supplying ability provided a new idea to reconstruct the hydrocarbon history of the oil and gas pools in the Middle Yangtze area quantitatively.With the comprehensive analysis of strata deposition, reservoir and its diagenesis evolution history, the formation mechanism and controlling factors of the high-quality reservoir in the Upper Sinian Dengying Formation were studied. It is shown that the high-quality reservoir developing of the Upper Sinian Dengying Formation was closely related with the palaeoepigenetic dissolution, burial dissolution and dolomitization. In the planar, the reservoirs of the Dengying Formation developing at the subtidal high-energy zone and platform margin had good quality, whereas at the the subtidal low-energy had a relatively poor quality. In the vertical, the highstand system tract developed algae dolomite and particle dolomite of tidal flat facies, and particle dolomite of platform margin bank facies. The dolomitization was dominated by penecontemporaneous dolomitization, seepage-reflux dolomitization and mixed water dolomitization, which occurred mainly at the Third Member, the top and middle parts, and the First and Third members of the Dengying Formation respectively. Being influence of sea level change and crustal movement at the end of the Dengying Formation deposition, ancient supergene dissolution appeared at the top of the Dengying Formation in varying degrees. However, the size, and the porosity and permeability of the Dengying Formation reservoir at present should be influenced by the selective dissolution in the stages of sediment, ancient karst and bury.With the application of the theory about natural gas accumulation and enrichment, the hydrocarbon accumulation and concentration rules of the Upper Sinian were studied by dissecting the classic reservoirs combined with the regional background analysis of reservoir forming. The hydrocarbon accumulation in the Upper Sinian Dengying Formation was charactered as mixure sources and multi-periods, and would be develop three possible patterns as accumulation at early with adjusting at the later, initial accumulation with continuous evolution, and accumulation at early with destruction and alteration at the later. The migration of hydrocarbon kitchens was the primary material foundation for hydrocarbon accumulation, and the two high-quality source rocks dominated hydrocarbon multi-period accumulation and mixure sources. The inherited paleo-uplifts and slopes controlled the petroleum enrichment, where were the best favorable areas for hydrocarbon accumulation. The tectonic movement, especially the early Yanshan Movement is the key to hydrocarbon accumulation finally, and the late superior sealing condition was the most important for petroleum preservation.Based on the study about the basic consitions and main controlling factors of hydrocarbon accumulation, the structural zones of the Upper Sinian were evaluated furtherly and the exploration pontential of hydrocarbon resources was also predicted by using the method of geologic analogy. The results showed that the exploration prospect of the Upper Sinian in the Middle Yangtze is broad with the total quantity resources of 11422.10×108m3, and the Shizhu synclinorium should be the preferred area for hydrocarbon exploration due to its most favorable conditions to hydrocarbon accumulation in the Middle Yangtze.