Characterization Of Braided River Reservoir Architecture In Northeastern Ordos Basin

The major gas pay zone lies in the 8th member of the Upper Paleozoic Lower Shihezi Formation(He 8 member),eastern part of Sulige region,Ordos basin.Its reservoir architecture acts as a predominant geological factor for oil gas distribution.Targeting at Tianshengqiao outcrop in Fugu county in eastern Ordos Basin,similar to eastern Sulige region in provenance and sedimentary setting,we finely studied the architecture of the outcropped sand body,which can of fer critical information for the establishment of geological model of eastern Sulige region.Based on the outcrop survey,petrographic investigation,architecture analysis and fine measurement of sand body volume,combined with the sedimentary and petrologic analyses,Main contents and results are as follows:(1)According to the classification scheme of base-level cycles in continental basins,2 long-term cycles,4 medium-term cycles and 12 short-term cycles can be divided in section 8 of the study area.It is identified that braided river deposits in block 8 of the study area developed two sub-facies of channel and dike flooding,the channel flooding includes channel filling and the core beach(bar)microfacies,and the dike flooding includes the transshore sedimentary microfacies.(2)we recognized twelve types of lithofacies in the braided river deposits of He 8 member.Six levels of architecture interfaces were classified within the reservoirs,namely composite channel-belt,single channel-belt,singlesand body,silty layer accompanied with accretion,bed set,single bed and laminae.Six reservoir architecture units are indentified,that is,channel filling(CH),longitu dinal bar(LB),diagonal bar(DB),transverse bar(TB),abandoned channel(ACH)and overflow deposits(OF).Among these,sand bar is a dominant reservoir architecture unit.The thickness of longitudinal bar ranges between 2.5m and 5.8m with the width/ thickness ratio varying from 27.6 to 37.7;the thickness of diagonal bar ranges between 2.2m and 6m with a width/ thickness ratio varying from 38.4 to 42.1;while the thickness of transverse bar ranges between 1.5m and 3.6m with a width/ thickness ratio varying from 47.0 to 61.8.(3)according to the average analysis of pore and seepage,the reservoir performance of braided river configuration unit in section 8 of the research area is from good to bad,including transverse sand dam,longitudinal sand dam,oblique row sand dam,abandoned river channel and river filling.Among them,the longitudinal sand bar and the inclined row sand bar have weak inseam heterogeneity,while the channel filling,the transverse sand bar and the abandoned channel have strong heterogeneity.(4)Based on the detailed anatomical analysis of different configuration units of tianshengqiao section,we summed up the spatial superimposition patterns of the reservoir,established the 3D architecture model of braided river composite sand body in the He 8 member of Tianshengqiao outcrop in Fugu county,both of which will provide geological evidences in the fine description of He 8 member in eastern Sulige region.