Research On Forming Mechanism Of The Primary Gas Reserviors Of The C-P Coal-Bearing Sequence In Jiyang Depression

Having a case study of the primary gas reserviors of the Permo-Carboniferous coal-bearing sequence from Jiyang Depression, this paper studied the forming mechanism of CBM gas reservoirs, coal-type gas reservoirs, coalbed methane-coal-type gas superposed gas reservoirs and other typical primary gas reservoirs. Applying the methods of sedimentology and dynamics of hydrocarbon accumulation, this paper explored the sedimentary geology background, characteristic of accumulation factors, reservoir-forming processing and reservoir formation model of coal-type gas reservoirs and superposed gas reservoirs systematically, and carried out a detailed analysis and potential forecast for the favorable accumulation areas in Gubei area from Zhanhua concave. The main achievements are shown below.The tidal-flat facies in Taiyuan formation and the interdistributary bay microfacies in Shanxi formation are the main types of depositional environment of source rocks in study area. The vitrinite is main maceral in source rock, and humic kerogen(Ⅲ types) is main producing gas matrix in the hydrocarbon precursor types. The hydrocarbon potential of Carboniferous coal is better than Carboniferous mudstone, Permian coal and mudstone. The source rocks in Zhanhua concave have better quality than other concaves. Upper and lower Shihezi formation which develops channel subfacies of meandering river are main reservoirs of coal-type gas reservoirs. The secondary pore is main void type in sandstone. Zhuahua-Chezhen area develops reservoir in Type Ⅰ, while reserviors in Huimin and Dongying area is mainly reservoirs in type Ⅱ and type Ⅲ. The sandstone of distributary channel in Shanxi Formation is reservoir of superposed gas reservoirs, having a banding distribution from Zhanhua-Chezhen concaves to Huimin-Dongying cancave. The physical property of Shanxi Formation is worse than upper and lower Shihezi formation, developing reservoirs in type Ⅲ and type Ⅳ. Saturated adsorption quantity of coal reservoir has a rule of firstly increasing and then decreasing with the increasing of buried depth, and the critical depth appears in 800~1500m.Impacting by Indosinian, Yanshan and Himalayan structural movement, upper Paleozoic has a series of NW faults which control the distribution of coal measure strata and migration-accumulation of coal-type gas reservoirs. In Indosinian period, upper Paleozoic has the first subsidence stage, and the max burial depth can reach 3000 m. However, due to strong uplift in later Indosinian period, the organic matter hydrocarbon generation function is ended, and quantity of hydrocarbon generation is not obvious. In Yanshan period, buried depth of source rocks occurre different settlement, but buried depth is generally less than 2500 m. Due to the modulation of the mantle heat flow mechanism, the second hydrocarbon-generation has occurred in part area. It is not easy to save hydrocarbon produced by coal seam because of uplift and denudation in Later Yanshan period. Himalayan period is divided into two phases, early and late phase. The early Himalayan period is main evolution phase of source rock, and the buried depth is generally deeper than 3000 m. It is the most important hydrocarbon generation stage and accumulation formation stage.From the study of the reservoir-forming processing, primary gas reservoirs of the Permo-Carboniferous coal-bearing sequence from Jiyang depression belongs to interrupted multi-stages accumulation model. From the study of the main factors, coal-type gas reservoirs belong to fracture controlling model of ‘from far to close’. Superposed gas reservoirs belong to sedimentation controlling model of ‘self-generating and self-preserving reservoir’. The Gubei area has superior accumulation conditions of primary gas reservoirs of the coal-bearing sequence. The gas reservoirs in lower Shihezi Formation from Yi 155 are coal-type gas reserviors which belongs to syncline accumulation controlled by fracture. The gas reservoirs in Kuishan Member of upper Shihezi formation from Gubeigu 1 are coal-type gas reserviors which belongs to structural of buried hill and faulted nose structure gas reserviors controlled by fracture, and lower Shihezi Formation and Shanxi Formation also have indication of oil and gas. It is possible to find coal-type gas reservoirs and superposed gas reservoirs in above wells.