Research Of Sedimentary System And Reservoir Characteristics Of The Shanxi Formation And The Lower Shihezi Formation In The Southeast Of Ordos Basin

Ordos Basin is one of China's large inland oil-bearing basins with multi-layered system.The clastic rock series of the Shanxi Formation and the Lower Shihezi Formation in the basin have acquired a good economic benefit.The southeastern basin has large prospecting potential but with great research difficulty because of low exploration degree.Based on the current research status,the sequence stratigraphic framework of the Upper Paleozoic was described in this paper,the sedimentary system and the characteristics of lithofacies and palaeogeography of the Shanxi Formation and the Lower Shihezi Formation in the southeastern Ordos Basin were analyzed in details.A corresponding depositional model was established for the study area to describe its sedimentary evolution process.The purpose is to clarify the differences in the characteristics of sandstone reservoirs and discuss the controlling factors that affect the difference through comprehensive comparative analysis.Based on the established of sequence stratigraphic framework,braided river delta and meandering river delta deposition systems were identified in the study area.In the Shanxi period,shallow water meandering river delta deposits were mainly supported on the background of the offshore lacustrine basin,and two major sub-phases of delta plain and delta front were developed.Yijun-Huanglong-Jixian is the boundary between the two sub-phases.The south side is the delta plain and the north is the delta front.In the lower Shihezi period,the delta features of accumulative-accretive shallow braided river deltas are mainly developed,and two major sub-phases of delta plain and delta front were developed.Yijun-Baishui-Chengcheng-Linyi is the boundary between the two sub-phases.The south side is the delta plain and the north is the delta front.In the Shanxi period,there is a source-transportation-convergence system in meandering river delta system from the southeast direction,which develops the sedimentary environment of alluvial plain-delta plain-delta front-pre-delta.It is affected by tectonic activities of the basin,from the early Shanxi period to the late Shanxi period,delta sedimentary systems and facies belts contracted and moved southwards.In the lower Shihezi period,there is a source-transport-accumulation system of the Jurassic delta system from the southeast direction.The depositional environments of alluvial plain-delta plain-delta front-pre-delta are developed successively.Due to the strengthening of tectonic activities and the supply of sources,the delta system developed during the deposition period gradually moved northwards.There are obviously differences in reservoir characteristics.They are controlled by various factors such as source,sedimentary environment,and diagenesis.The Lower Shihezi Formation reservoir,which is dominated by rapid deposition near the braided river delta,exhibits lower compositional maturity and structural maturity.The Shanxi Formation under the meandering river sedimentary environment has better physical properties due to higher quality pore content and better pore connectivity.During the diagenetic stage,intergranular pores and dissolution pores are developed better with higher quartz content and more acidic fluids in Shanxi period,the intercrystalline pores in kaolinite and hydromica are mainly appeared in the Lower Shihezi Formation,The Shanxi Formation in the southeastern Ordos Basin has better reservoir performance than the Lower Shihezi Formation.