| Yining Basin has well-developed Tiemulike dark shale of normal lake facies formed in late age of middle Permian and Akeshake dark shale of marine facies in late age of early carboniferous, and the two sets of shale have the conditions of shale gas reservoirs. Paleozoic unconventional shale oil and gas investigation of the basin has not been carried out. This study falls back on the research status and advances of the shale gas in China and abroad, geothermal chronology, shale nano-sized pore, the base of oil and gas geological exploration degree and geological structure in Yining Basin, being established on the outcrops profile, drilling data, seismic data interpretation, sample test and analysis results, and previous research results. Then restore paleogeothermal field of the basin in virtual of the drilling strata vitrinite reflectance Ro which changes with depth, temperature measurement of inclusions. Meanwhile, the results of apatite fission-track dating can help to finish the time-temperature forward modeling, in order to discuss the tectonic thermal evolution process of Yining Basin.According to the paleogeothermal gradient, tectonic-thermal evolution and drilling stratigraphic lithology of Yining Basin, and the simulation software "Basinmod" used to restore burial history, analyze shale gas pool forming elements, involving source rock features, the reservoir characteristics of the shale, the condition of cap rocks development, the dynamics and way of reservoir formation. At last obtain a comprehensive analysis and summary of the Yining Basin shale gas accumulation conditions in upper Palaeozoic.Yining basin, an independent unit of the Yili-Zhongtianshan micro plate, is a intermontane superimposed basin, and its tectonic framework is three depressions with one uplift:Yining sag, Nileke sag, Gongnaisi sag and Awulale uplift. Yining Basin experienced four important evolution stages from late Archeozoic:the basement evolution stage (early South China age), the breakup of the super continent and ocean continent evolution stage (South China age-middle Devonian), the Post orogenic intraplate extensional stage (late Devonian-early Permain), and the intraplate evolution to the formation of the basin stage (middle Permain -present). Judging from the change of the basin’s nature from Permain, the intraplate evolution stage of Yining Basin can be divided into Permain intracontinental rift basin evolution stage and Meso-Cenozoic intracontinental faultsubsidence basin evolution stage.Combining the Yining Basin structural feature:thermal evolution, erosion thickness restoration, paleogeothermal field restoration, subsidence history, drilling lithology and dark mudstone, shale geochemistry parameter and so on, restore the burial history and thermal evolutional history of the Ning 3 Well, Ning 4 Well.Yining Basin experienced mainly four subsidence stage and in the erosion stages. The maximal sedimentation rate existed from middle Permain to early Triassic, and basin uplift in a short time in late Permain. While basin quickly deposited from early Triassic to late Cretaceous, what is more, Yining Basin had a large scale uplift and erosion in early Cretaceous, and sedimentation rate decreased from early stage of late Cretaceous to the end of Paleogene, in which basin quickly uplift caused by the effect from Himalayan movement. The basin slowly deposited From early Neogene to late Pleistocene, and it is in the erosion condition nowadays.Tiemulike formation (P2t) was mainly made up of black-deep grey calcareous mudstone, marl, black shale, mudstone, developing at deep lake and semi-deep lake sedimentary environment. The dark mudstone thickness mostly in 150-600 meters, and terrestrial higher plants are its main source. The sedimentary environment is reducing environment for brackish water stratification and hypoxia is not strong. Organic matter abundance of hydrocarbon source rocks is high, with average content of Organic carbon (TOC) 1.86%, and average potential hydrocarbon generation amount S1+S2 (mg/g) of 1.79 mg/g, and it has a trend of organic matter abundance increase in turn from the south to the north. The kerogen type is main â…¢, and same is â…¡2-The thermal evolution degree of Yining basin presents a trend of increasing gradually from the basin edge to the basin center, with Ro is about 0.8% at edge and weighted average value of Ro is 1.29% at central place.Akeshake formation dark mudstones are bay sedimentary environment, it is much thicker at Southern of Yining-Southeast of Chabuchaer and Northeast of Nileke, with residual thickness greater than 130 meters. Average content of Organic carbon (TOC) is 1.2%, The kerogen type is main â…¢-â…¡2. Value of Ro is 1.56-2.47%, and weighted average value of Ro is 1.72%, Hydrocarbon source rock is in high mature to over mature stage.Average Brittle mineral content of Tiemulike formation mud shale is 68.8%, pore in mud shale is mainly micropore formed form volume reduction by organic matter hydrocarbon. Lots of micro cracks developed during the process of organic matter generated hydrocarbon, high angle cracks, Vertical mud shale formation, developed by tectonic stress are also visible. Porosity of Shale is 2.3-3.1%, average value is 2.7%; penetration rate is 0.002-0.008md, average value is 0.005md.There exists strong high abnormal excess pressure Mid-Permian tiemulike formation, upper Paleozoic of Yining basin was in stage of the large number of dry gas generation in Middle Jurassic, overpressure developed widely in mud shale. Because of thickened and pured dark mudstone layer with higher thermal evolution degree, some abnormal pressure suddenly increased zone are emergence in vertical. The higher excess abnormal pressure zones are With better transverse continuity, which form a plurality of high abnormal fluid pressure compartment in central depression belt of Yining sag, and provide dynamic conditions for shale hydrocarbon expulsion and shale gas reservoirs.The hydrocarbon generation, expulsion and filling of dark shale in Yining basin is with episodic characteristics, and there are two times arger hydrocarbon generation and hydrocarbon filling process. Abnormal excess pressure is the main driving force of hydrocarbon expulsion from source rocks in Yining basin, when excess pressure value of hydrocarbon containing fluid reaches fracture pressure of formation or attains the opening pressure of pre-existing cracks, a large number of micro cracks in open state is produced in mud shale. High pressure hydrocarbon containing fluid transiently bleed off, and there exist multi-episodic process of hydrocarbon generation pressurizationâ†'fformation breakdownâ†'hhydrocarbon export and reservoir discharge. Boiling fluid inclusions and secondary brine inclusion homogenization temperature display episodic reservoiring of shale gas in Yining basin, one peak temperature of homogenization temperature is from 80 to 130℃, on behalf of hydrocarbon source rocks rapidly warmed and generated oil due to high tectonic subsidence rate at early Triassic～Middle Jurassic; another peak temperature of homogenization temperature is from 160 to 180℃, standing for a large natural gas generated when Yining basin subsidence reaches the maximum value at late Jurassic-early Cretaceous. Petroliferous systems related to the "four charts and a table" are made, in order to systematicly analysis reservoir forming conditions of upper Paleozoic shale gas reservoirs in Yining basin. Upper Paleozoic Tiemulike formation has superior shale gas reservoir forming conditions in Yining basin, and shale gas accumulation conditions of Akeshake formation takes second place. |
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