Research On The Source-reservoir-cap Conditions And Tight Gas Accumulation Rule In Shahezi Group Of Xujiaweizi Fault Depression

Xujiaweizi fault depression is one of the most important oil and gas bearing unit in the north part of Songliao Basin, and Shahezi group develops the most important source rocks in the depression. Shahezi group was studied as gas source rock unit in the past, in this paper, it is studied as the tight gas enrichment horizon, the purpose of this study is to make clear the gas accumulation characteristics and main controlling factors of gas reservoir,through analying the basic reservoir conditions.Based on the cores, thin sections, logging, well logs, seismic, testing datas and gas datas,study the quality of source rocks and ptedicting the organic carbion content form longing curvies; predicting the distribution and thickness of source rocks by using drilling and seismic dada;calculating the gas generation period and content using chemical kinetics method. By means of core observation, scanning electron microscopy, constant-speed mercury injection,CT scan, and nuclear magnetic resonance test, the petrology, diagenesis types and microscopic characteristics of pore structure were studied, and lower limit of physical property for effective reservoir, key factors for gas accumulation were studied too.Study the reservoir-cap assemblages features in Shahezi group, dividing the reservoir-cap assemblages into three types, the gas storage and gas sealing ability of each type was analized; Study the gas accumulation period by using gas inclusion datas, then,combined with the regional tectonic features, the gas accumulation characteristic and the controlling factors were analized.Through the analysis of reservoir forming conditions,including source rocks,reservoir,reservoir-cap assemblages, fracture, structure,and the gas accumulation rules, the following conclusions were got.1.Source rocks in Shaehezi group can provide sufficient gas for tight gas accumulation.Source rocks distributed widely in almost all the depression, the thickness is more than 400 m in most palces,and gas generation content is high and lasted a long period.2.The pore size of reservoir in Shahezi group is nano-micron grade, reservoir property.is controlled by pore structure, and affected by deposition and diagenesis both. The pore throat radius is mostly2nm-100μm, the greater the pore throat radius, the better of the pore pore-throat –connectivity.The higher of plastic filler content,especially the caly content,the worse the reservoir physical properties. The compaction damages the prosity and permeability through reducing the bigger pore volume, while the dissolution and fracture improves theprosity and permeability through increasing the pore radius.3.Takeing porosity 4.0% as the lower limit of favorable reservoir. Because, when the porosity is bigger than 4%,“gas storage capacity” based on the constant speed mercury injection and “gas production capacity”based on gas production both raised significantly, on the contrast, when the porosity is less than 4%,they keep in a low level.4.Dividing the reservoir-cap assemblages of Shahezi group into three types, including mudstone type., conglomerate type.and volcano type. Reservoir thickness and prosity are relatively better in conglomerate and volcano type,the favorable reservoir which generate more gas after fracturing mainly developed in these two types.5.Gas accumulation in Shahezi group paralleled with that in Yingcheng formatin,and accumulated throuth two models,in first mode, gas is reservoired and caped by Shahezi group alone, in the other mode, gas is reservoired in shahezi gourop,but caped mainly by upper caprocks or layers.