Mechanism Of Hypogenic Karstification In The Belt Of Strike-slip Fault

The Shunnan gentle slope is a three-level structural unit within Tarim basin, located in downthrown side of I fault zone of Tazhong area, Tarim basin. And it’s the southeast section of North Slope of Tazhong. It adjoins Khattak Uplift bounded by the western 1 fault zone in the west. To the north, it is adjacent to the Shuntuoguole low Uplift. Also the gentle slope connects Guchengxu low Uplift to the south and Mangar sag to the east repectively.This paper aims to Middle to Lower Ordovician carbonate rock stratum in Shunnan area, Tarim basin. Based on seismic data, drilling data, core data, thin sections, fluid inclusion, stable isotope analysis, trace and rare earth element test, multiple discipline studies including structure geology, tectonics of petroliferous basin, petrology, petroleum geology, and geochemical have been carried out. On the macro scale, we analysed the features of fault activities and their evolutionary process and on the micro scale, the fluid properties of diagenesis of hypogenic karstification in Shunnan area have been summarized. The conclusdions are following:There are fourth periods of tectonic activities in Shunnan area of Tarim basin:the Early and Middle Caledonian phases, the Late Caledonian to the Early Hercynianstage phases, and the Late Hercynian phases. The tectonic stress strength develops moderate relatively during the Early Caledonian phases; Until the Middle- Late Caledonian to the Early Hercynianstage phases, the tectogenesis developed the most intense and then the force of the tectogenesis during Late Hercynian stage became weak. Under the multiple tectonic superposition, the faults in Shunanan area are characterized by obvious multi-phased in vertical direction, which have different structural types within different stratums.Early Caledonian, Shunnan area is mainly affected by the tension cracking movement of The ancient Tianshan ocean, The ancient Kunlun ocean and the Arkin ocean around the Tarim basin, this area develop many tectonic style, mainly deposited graben-like, semi-graben, fault-stepped breaks, positive flower structure and Vertical fault In the Cambrian strata. The fault mainly located in southeast of Shunnan area, it’s trend is NEE, NE and NW. Middle Caledonian, because of Compression shear tectonic stress action, mainly deposited positive flower structure, Negative flower, graben structure, semi-graben structure, vertical fault, positive-negative flower structure in each other; On the plane, this area developed within the two groups six strike-slip fault. SN3, SN1, SN4 fault exist as one group, their trends are NE, it is on the top of the fault which developed in Early Caledonian trending NE; Another group includes NEE1, NEE2 and NEE3, their trends are NEE; In addition to this, Region also developed a lot of small fracture trending NEE. Late Caledonian to early Hercynian, Shunnan area is affected by transtensional tectonic stress, negative flower structure, negative flower-like structure graben structure, fault-stepped breaks, positive flower structure developed in the Ordovician strata. The fault distributes SN3, SN1 and SN4 fault zone trending NE, the early strike-slip fault zone trending to NE is not inherited. Late Hercynian, the strata of Tazhong is in a week tensional environment, Shunnan area experienced a weak structure adjustment, Permian strata mainly developed a little of graben-like fault and fault-stepped breaks trending to NNW and NE, fault activity is weak.Through precise description of the three strike-slip faults zone, SN3, SN1 and SN4 in the plane, section and three-dimensional space, the author thinks that the NE trend strike-slip faults result from the tectonic movements in I episode of mid-Caledonian and the period from late Caledonian to early Hercynian. The principal stress direction changed between two tectonic movements and there is an obvious tectonic inversion. In I Episode of mid-Caledonian, the strike-slip faults are mainly transpressional faults and their principal stress direction σl is NE-SW trend which is same as the principal displacement direction of SN3 and SN1 faults zone. So, the two faults zone have a banding distribution in this period. However, the principal displacement direction of SN4 is NNE trend with a acute angle from al, and it is distributed without a singl banded structure in the plane. Their principal displacement direction is NNE trend and the corresponding principal stress direction is NE trend. The NE trend strike-slip faults are dextral in this period.In late Caledonian and early Hercynian, the strike-slip faults are mainly transpressional faults and their principal stress direction al is NNW trend, and the corresponding principal stress direction is NEE trend. The NE trend strike-slip faults have a variety of combined styles in the plane. SN3 fault zone is overall left-lateral right-step echelon arrangement in research area. The plane combined styles of SN3 fault zone is obviously segmented. In northern segment, there are left-lateral right-step echelon faults. In middle segment, there are banded strike-slip faults. In Southern segment, there are horse tail faults. The plane combined styles of SN4 fault zone is also segmented. In northern segment, there are echelon faults. In Southern segment, there are horse tail faults.Through the technology of 3D visualization comprehensive analysis, the spatial combined styles of two vertical sets of faults in Ordovician in the three strike-slip faults zone is obviously segmented along the strike direction. The combined styles of two vertical sets of faults can be divided into four main types. Firstly, the tensional faults develop in upper and lower part. Secondly, the tensional faults develop in upper part and the compressive faults develop in lower part. Thirdly, the compressive faults develop in upper and lower part. Fourthly, the tensional faults develop in upper part and the pure strike-slip faults develop in lower part. The lower main faults vary from north to south in the turn:tensional faultsâ†' pure strike-slip faultsâ†' compressive faultsâ†' pure strike-slip faults under the action of two stages of tectonic stresses. The combined styles of two vertical sets in SN1 faults zone can be divided into two main types. Firstly, the tensional faults develop in upper part and the compressive faults develop in lower part. Secondly, the tensional faults develop in upper and lower part. Because of the weaker downcutting function of tension-torsional stress in the later stage, the lower main faults are overall transpressional structure in SN1 faults zone. In SN4 faults zone, the tensional faults only develop in upper part of Ordovician, and tensional faults, compressive faults and compound faults all develop in lower part. In SN4 faults zone,the main faults in lower Ordovician vary from north to south in the turn: strike-slip faultsâ†'tensional faultsâ†'compressive faultsâ†'tensional faultsâ†' strike-slip faultsâ†'ompressive faultsâ†'tensional faults under the action of two stages of tectonic stresses.The key functions of hypogenic karstification are hydrothermal dissolution of the carbonate reservoir from Yingshan formation in Shunnan area and the generation of diagenetic minerals, such as:siliceous rock, calcite vein, quartz vein, iceland spar and tabular anhydrite. Through geochemical analysis, it can be concluded that the fluid’s source of hypogenic karstificationconsists of two parts, the distal hydrothermal fluid driven by magmation related to vocalism and syngenetic seawaterin deep formation, Cambrian system for example. These two kinds of fluids flow into the middle to lower Ordovician in Shunnan area almost at the same geological time, developing lots of hypogenic karstification.Using comprehensive analysis technology of 3D visualization, hypogenic karst reservoirs from SN4 fault belt and the different tectonic-evolution of strike-slip fault can be displayed clearly. The hypogenic karstification in this area develops characteristics of primary-secondary faults, dendritic conduction, vertical-lateral combination, layered distribution and flower-shaped morphology. Primary-secondary faults are of positive significance to the hypogenic karstification. The hydrothermal fluid in deep formation conducts dendritically through multiple fractures. The hypogenic karstification has good development vertically and layered distribution laterally. There are superior space matching relation between faults and hypogenic karstificationin the middle to lower Ordovician and secondary reverse faults. And hypogenic karst reservoirs develop in pairs near Well-SN4.As we know the vertical conduction through fractures is further superior to the lateral conduction through effective carbonate reservoir, fluids will choose effective and opened faults to conduct while migrating upward. So hypogenic karstification mostly develops at the end of opened faults. Although the primary faults, connecting upper fault and with disadvantages of fluids’ preservation, suffers the biggest degree of transtensional stress transformation in later period,hypogenic karstification develops relatively weaker in the primary faults.The opened lateral fracture makes hydrothermal fluids react in the regional stratigraphy, owing to the upper carbonate cover which plugs the fluids from continuing migrating upward. And the fluids’lateral migration is mainly in control of lateral-secondary faults and permeability formation existed. The hypogenic karstification in calcarenite existed and with better physical property, obviously develops stronger than in micrite.In short, the coupling relationship between hypogenic karstification and strike-slip fault in the middle to lower Ordovician in Shunnan area is, that under transtensional stress, early faults with positive flower structure occurtectonic inversion and form fault transportation system beneficial to fluids’migration in late Caledonian and early Hercynian. At the same time, acid fluids driven by the activity of igneous rocks below the basement, migrates upward through strike-slip fault cutting the basement under fluid potential, and then occurs different degree hypogenicdiagenesistowards surrounding rock in the middle to lower Ordovician. Fluids in deep formation migrates to the end of opened lateral fracture, stops migrating upward and starts accumulating owing to the plugging effect from the upper Ordovician cover. And the fluids have sufficient dissolution time towards surrounding rock and choose good permeability formation existed to migrate laterally at the middle to lower Ordovician. Thus calcarenite in the middle to lower Ordovician develops abundant hydrothermal alteration, which forms great hypogenic karst reservoirs. And they become the deep and ultra-deep exploration target of oil and gas.