The Characteristics Of Reservoir And Prospecting Advantageous Regions For The Sixth Unit Of Triassic Yanchang Formation In DaLuGou Area, The Ordos Basin

Ordos basin is the second largest sedimentary basin on land in China which possesses abundant oil and gas resources and has bright prospects in oil and gas exploration. It has been becoming one of the important tactical area in oil and gas exploration in China. The amout of oil and gas resources and the degree which it is proven will directly influence the development of China petroleum Industry.Dalugou oilfield is one important generative area of oil in the Ordos basin.As one primary oil bed in Dalugou area, exploration and exploitation of the sixth reservoir unit of Triassic Yanchang formation are significant.However, the reservoir is characterized with low porosity,low permeability and low pressure that restricts to its output.It is exigent to understand the formation conditions and control factors for petroleum reservoir in order to fine out advantageous regions in the area.On the basis of the geological investigations in the field and analysis indoors, combined with the theories on the sedimentary facies and petroleum geology, by means of various analytical techniques including conventional analysis of the core, analysis of capillary pressure,examination for resin-impregnated thin sections under a microscope, scanning electron microscope, analysis by x-way diffractometer,et al.the paper divide and correlate the Yangchang formation stratigraphic framework in Dalugou area. And stress to study the sixth reservoir unit of Yanchang formation in the sequence framework,discuss the characteristics of petrological,sedimentary facies and lithofacies palaeogeography,diagenesis and reservoir physical property in detail.These studys indicate that sedimentary time of the sixth sandstones of Yanchang formation is the peak period of formation of Zhidan delta in Dalugou area ,the main sedimentary microfacies are submerged distrbutary channel,interdistributay bay,et al; the reservoir rocks are characterized with low compositionally maturity and relatively high textural maturity of fine grained arkose and the least lithic arkose;and its diagenetic mineral includes chlorite,laumonite,calcite,authigenic quartz,et al.The diagenesis mainly includes compaction,pressure solution,cementation,dissolution,et al.Based on the pattern of diagenetic and porosity evolution established,the reserveoir diagenetic stages can be classified into A-substage of later diagenesis.The main reserveoir space of the sixth reservoir unit of Yanchang formation in this area is intergranular pore and intergranular dissolution pore.The sixth reservoir unit of Yanchang formation have small pores and fine throat.The average porosity is 11.85%,and the average permeability is 1.27×10~3μm~2.Reservoir properties are mainly controlled by primary continental detrital composition,sedimentary microfacies and diagenesis;the effect of the tectonic movement on the reservoir is relatively limited.The research shows that the physical property of submerged distributary channel sandbodies is the best.The cementation of chlorite,laumontite,calcite,silicicastic,authigenic clay minerals,as well as thedissolution of laumontite,feldspar are of main factors which affect reservoir properties.The more favorable reservoir sandbodies are submerged distributary channel sandbodies on delta front.At last, the reservoirs are classified and predicted prospecting advantageous regions.