Deep-water Gravity Flow Sediments Reservoir Description And Modeling Of The Ch7 Member In The Jinghe Oilfield,Southern Ordos Basin,China

The studied Ch7 Member in the Jinghe oilfield is a part of the late Triassic period of the Yanchang Formation in the southern Ordos Basin.The depositional system belongs to the deep-water gravity flow which has been investigated comprehensively and,characterized to be sandy debris flow reservoir,modified from the earlier classifications of a braided river delta fronts,deep lacustrine turbidity fans,and seismite deposits following the underlying Ch8 Member depositional background.However,the structure of the Basin was deformed asymmetrically by the Indosinian movement and strongly affected the Western Qinling zone,next to the Jinghe oilfield in the south-western margin boundary of the basin.Further,during this period of the basin,the tectonic evolution creates some fault systems and the paleogeomorphology the basin changes to become steep in the south and gentle in the north.Therefore,the deposition system of the Ch7 Member formed during the basin slope break that has determined from the contour plan and the oil shale thickness.The overall studied oil interval forms a deep-water lake environment with a very complex sedimentary sequence.However,it becomes necessary to inspect the exact elements and the origin of sedimentary symptoms of gravity flow accumulates in the Jinghe oilfield of the Ordos Basin.As a result,the studied formation comprises lithology mainly fine-grained sediment,which comprises fine sandstone,siltstone,mudstone,and oil shale.For this context,a large number of core data samples were analyzed and identifying 15different lithofacies.Thus,fine sandstone?S_f?,including,S_fm,S_fmfc,S_figb,S_fpl,S_ftmc,S_fp,S_fgb,S_fls,S_fliq,S_fd,and siltstone facies?S_S?,comprises,S_sw,S_spl,and S_sd.Therefore,argillaceous lithofacies include mudstone facies?M?and oil shale(M_osh).Also,various sedimentary structures have observed through the core description,including massive bedding,dish structure,ejected structure,liquefied convolute beddings,liquefied breccia structure,and parallel laminae beddings.Consequently,this study identified three main sedimentary microfacies,which includes sandy debris flow,turbidity current flow,and gravity deformation this producing a sliding of the massive volumes of delta front deposits on the margin slope break zone.Together,surrounding lake water influenced the seismite-slump microfacies.These major sedimentary facies types were proved by a vertical distributions of the reservoir sandbody.The overall sedimentary remodeling has been affected by the earthquake and sediment,creating seismite-slump micro-facies with load,liquefied,and the slump deformation structures.Subsequently,with the continuous sediment liquefaction,the sandy debris flow microfacies?massive bedding sandstones?were formed during gravity flow transformation to the turbidity current flows which remolded by a weak bottom currents in discontinuous period of sandy debris flow.However,the reservoir properties and oil-bearing analysis prove that sandstones of sandy debris flow have the best properties and oil shows,followed by turbidity sandstones,and the seismite-slump are being the poorest.Beyond,the sandy debris flow,a part of gravity flow sandstones have shown a good characteristics of oil potential.Furthermore,the statistical analysis of thin sections,core samples,and the rock classification result,shows that the reservoir sandstone composed of fine-grained sediments mainly feldspar lithic sandstone and lithic arkose sandstone which are dominant in the tight sandstone.Hence,the sandstone composition consists of quartz content of 38%to 62%,average being 48.02%.Thus,the feldspar content varies from 9%to 47%,with an average of28.05%.Whereas the debris content is between 12%and 56%,and the average is being23.43%.Overall,the sandstone compositional maturity is low.Similarly,the reservoir sandstone contains the carbonate debris with an average content being 0.5%.Reservoir physical properties?porosity-permeability?evaluation the Ch7 sandstone shows a porosity of 0.2%and 17.68%,with an average of being 9.36%.On the other hand,permeability of 0.04 and 61mD with an average of 0.22mD.In contrast,the low porosity and permeability of the sandstone suggest the study interval forms a typical tight sandstone reservoir.In this context,the most favorable reservoir properties show with massive bedding and the parallel bedding sandstones that are deposits of the sandy debris flow.These sandstones show average porosity of 10.3%and the average permeability is0.64mD.Accordingly,the turbidity microfacies show as poorly sorted sandstones they often show deposits of thin interbedded sandstone and mudstone.Thus,they have an average porosity and permeability of 6.7%and 0.21mD,respectively.Besides,physical properties of the seismite-slump microfacies are lowest with an average porosity of 5.6%and average permeability of 0.16mD,this expected to the mix of the sandstone and mudstone with deformed and convoluted beddings.Besides,logging interpretation of the gamma-ray response shows box-shaped curve,bell-shaped curve,bell-shaped curve,symmetrical-shaped curve,and finger-like shape.Moreover,through the vertical sand-body correlation,thick beds of sandstone accumulations were determined in the upper part of the Ch7 Member.These thick beds of sandstones were interpreted to be deposits of debris flow based on their presence of sedimentary structures.However,in the middle part medium-thick beds of sandstones were deposited,these deposits show typical characteristics seismite-slump debris flow sediments.At the base of the Ch7 Member thick layers of argielaciouse oil shale,were developed during the the Ch7^-3 period;these deposits are productions of a typical deep-water basin plain turbidities.