Research On Characteristics Of The Palaeozoic Gas Accumulation In Southwest Of Ordos Basin

The Ordos Basin，plentiful in resouces, has long been the focus and hotspot offor geologist. In recent years, in the Upper Palaeozoic the He8section of the ShiheziFormation and the Shan1setion is the target for the exploration and development.Based on the research to regional tectonic background and the new drilling wells datain recent years, combined with field investigation, regional geological characteristicsof southwestern Ordos Basin was studied. The diagenesis and reservoir characteristicstogether with mechanism of gas accumulation of the Upper Palaeozoic were discussed,by using the modern test and analysis technology, especially the technology of fluidinclusions.The southwestern Ordos Basin generally has an uncomplete stratigraphicsequence which is lack of the deposition of the upper Ordovician, the Silurian, theDevonian and the lower Carbonaceous. It experienced a transformation from marinefacies to continental facies in the Upper Palaeozoic. The Shanxi Formation and theShihezi Formation in the southwest of the Ordos Basin were deposited in lacustrineand delta environment whose provenance are mainly from the Qilian-Qinling orogenand the LiuPanShan orogen. The He8section and the Shan1section all developedsandstone reservoirs.An research of the He8section and the Shan1section reservoir characteristicswas undertaken in the southwestern Ordos Basin.The result shows that rock aremostly defined quartz sandstone, lithic quartz sandstone and feldspar quartz sandstonewith high compositional maturity and low structure maturity, the matrix of interstitialmaterial are mainly argillaceous material, then silica, calcium and argillaceous cementare dominating cements. The pore consists of residual intergranular pore,intergranular dissolved pore, intragranular dissolved pore and a small amount ofmicro cracks. The physical property of the sandstone reservoir is poor indicating lowporosity and low permeability reservoirs. The diagenesis types of sandstone arecomplex and diverse, including compaction, pressolution, cementation which leads toreservoir compacting and forming low porosity and low permeability reservoir. The secondary pores and fractures generated by later period dissolution have veryimportant significance in improving reservoir physical property and forming effectivereservoir.On the basis of petrography and inclusion analysis technology, southwesternbasin accumulation characteristics are analyzed. The results show that in the reservoirsandstone of the Upper Palaeozoic, two periods of organic inclusions areidentified-the early inclusions and the late inclusions these, which respectivelyrepresent twice of gas migration and accumulation. The early organic fluid inclusionsrepresent natural gas generation and primary migration into the reservoir, while thelater inclusions represent the peak of gas generation and mass gas migration andgathering into gas accumulation in the large scale. The Organic inclusions aredistributed in the dissolution pores and fractures which generated before the formationof late cement. The early and late inclusion petrography reflect the natural gas has thecharacteristics of accumulation first and then densification. The He8section and Shan1section in the study area have the same palaeogeothermal system and brine systemcharacteristics. Using the Tt method (inclusions homogenization temperature method),the research shows that in this area, the gas migrated and began to enter the reservoir200and220million years ago(the Late Triassic), the time of the natural gas massmigration and accumulation of the upper palaeozoic in the Weibei uplift is80millionyears (the early Cretaceous), and in the Tianhuan Depression and the center paleouplift, the time of the natural gas mass migration and accumulation is150millionyears (the late Jurassic).