Study On Sequence Filling Patterns And Reservoir Distribution Of The Xujiahe Formation In Sichuan Foreland Basin

Foreland basin between orogenic belt and craton is a sedimentary basin, which contains many of gas or oil field, and it is also the first exploration area and discovered the gas and oil place. The Foreland basin in Midwest of China is rich in hydrocarbon resources also, and it has become the most important strategy replace area in Chinese exploration area of gas and oil. In particular, after few decades efforts, Sichuan Foreland Basin has found many gas and oil field, but according to preliminary statistics of predecessors, explored gas and oil in foreland basin was only 17% and 6.7%, so it is of great potential and importance area for exploration to our country.Because the provenance, sequence and diagenesis of Upper Triassic Xujiahe Formation in Sichuan Basin are complicated, reservoirs generally densify, the main reservoir type is with low porosity and permeability. only a few with middle porosity and low permeability in partial, the structure in Sichuan Foreland Basin is complicated, and the sequence- stratigraphy, lithology, lithofacies, sedimentary burial history and diagenetic fluid are different, so it decided the diagenesis, diagenetic facies and reservoir types of secondary structural unit are distinct, but some of these are lack of study.The paper is basis on the technology of sedimentology, structural geology, palaeontology, logging geology and reservoir sedimentology, according to analysis of the sediment-tectonic setting when the formation of Sichuan Foreland Basin, also, including the study of provenance and high-resolution sequence stratigraphy between Norian to Rhactian period of Upper Triassic, the sequence stratigraphy frameworks from east to west and from north to south have been built, and the palaeogeographic frameworks of different sequence have been further analyze. On this basis, according to the comparison of diagenesis, diagenetic facies and pore types in thrust belt, foredeep depression zone, foreland slope and foreland uplift, the purpose is to conclude the distribution regularity and control factors of reservoir types in plane and vertical, main contents are as follows:(1)The region sedimentology comparison, mineralogy and sedimentary geochemistry have been used in provenance analysis of Xujiahe Formation sandstone, the conclusion is that the deposit of Late Triassic Xujiahe stage is of several provenances, the idea of several provenances provides basic for the palaeogeography study, combining with the surface section, drilling cores and the well logging data, the Upper Triassic Xujiahe Formation is divided into four sedimentary facies types, it is alluvial fan, fan delta, braid river delta and lake, which is ordinal from basin margin closer to provenance to basin subside-deposit center.(2) Based on the recognition of surface section, seismic section, sequence boundary features of well logging section, and features of maximum flooding surface of Xujiahe Formation of Upper Triassic in Sichuan analogous foreland basin, the Xujiahe Formation is divided into 2 super long-term sequence cycles, 5 long-term sequence cycles ( LSC1～LSC5 ) and 17 middle-term sequence cycles(MSC1-MSC17), which correspond to lower and upper the Xujiahe Formation of structure sequence, Xujiahe 2 member to Xujiahe 6 member respectively. On the basis of the sequence features of secondary structural unit and regional correlation, the sequence stratigraphy frameworks from east to west and from north to south have been built. The frameworks of the two directions are of the feature that is a wedge uplapping from depression to uplift, and the regional correlation. This phenomenon is the result of differently directional and nonsynchronous thrusting-napping process of Longmenshan, Micangshan-Dabashan, which correspond to lower and upper the Xujiahe Formation of structure sequence. On the basis of the rising and falling base-level phases of each long-term sequence cycles, the sequence-based lithofacies and paleogeography maps of Xujiahe Formation have been mapped. The result indicates that the sedimentary facies systems are controlled strictly by a so-called"three depressions and one uplift"tectonic pattern, which is made up of western Sichuan depression, northeast Sichuan depression, east Chongqing-southwest Sichuan depression and Sichuan central palaeo uplift.(3) Because the structure is complicated in Sichuan Foreland Basin, diagenesis and diagenetic facies are of significant differences in different tectonic zone, it has decided the distribution rule of reservoir. Based on the analysis of sedimentary facies, cast slice and rock and mineral logging datas, diagenesis of the Xujiahe Formation sandstone in different tectonic zone have been researched systematically, it is concluded that compaction and cataclasis are leading in thrusting nappe tectonic zone, compaction and pressolution are mainly in western depression, while in ramp region and uplift zone, compaction is relatively weak, rim agglutination of chlorite and denudation developed, It showed that the destructive diageneses mainly include compaction-pressure solution,cementation, and that the constructive diageneses mainly include rim agglutination of chlorite, denudation and cataclasis. Basis on the research of diagenesis, the diagenetic facies was studied, diagenetic facies association of Xujiahe Formation sandstone is divided into three type, and the diagenetic facies of most beneficial to the reservoir development is compaction- clastation- cementation and cementation- denudation types.(4) Reservoirs of the Upper Triassic Xujiahe Formation is of the characteristics that low compositional maturity, low cement content and medium maturity in Sichuan Analogous Foreland Basin, Lithology in the plane with some regularity, in the Northwest region dominated by lithic sandstone, there are lithic sandstone and feldspathic sandstone mixed zone in Central west Sichuan, In Southwest dominated by lithic feldspathic sandstones, while In central-South Sichuan region, there are feldspathic lithic and lithic quartz sandstone mixed zone. Reservoirs are generally bad, the main reservoir type is with low porosity and permeability. Only a few with middle porosity and low permeability in partial. Because of the sedimentation, diagenesis and tectonic difference in thrust belt, foredeep depression zone, foreland slope and foreland uplift, It determines the type of porosity and zonation, fractured reservoirs are mainly in Longmen Mountain thrust belt, there are a small amount of emposieu and original intergranular pore, the pore type in foredeep depression zone is fracture-secondary dissolved pore, the primary pore is disappearance, in foreland slope, the pore type is secondary dissolved pore and primary pore, while, in foreland uplift, mixed hole consist of corrosion hole and primary hole is mainly, the development of reservoirs is controlled by deposition, diagenesis and cataclasis.