The Research Of Sedimentary System Of The Triassic Yangchang Formation In The South Of The Slope Of Shanbei

The yanchang formation in the south of the slope of shanbei is important oil-bearing series in Ordos basin. Based on outcrop observation , core description , single log facies analysis and connecting-well section , combined with the rock types , depositional texture , structure , paleontology and sedimentary cycle , etc , the Upper Triassic Yanchang formation is composed of the fluvial system , the lacustrine system and the delta system in the south of the slope of shanbei ,which are farther divided carefully them into 11 types sediment subfacies and 22 types sediment micfacies .and After researching carefully the delta system in research aera , There are two types delta sedimentary . It is braided river delta sedimentary and meandering river delta sedimentary , and two different types of stratigraphic modes was identified in lacustrine - delta system , that is , the deep water delta and the water platform delta .The study showed , the yanchang formation has the southewest and norheast provenances , the northeast provenance come from the Archeozic Wulashan group and lower proterozic Erdaoao group of high-grade metamorphic crystalline schist and gneiss in Yinshan mountain and Daqing mountain in the north part of Ordos basin , the southwest provenance come from the Simian and Paleozoic neritopelagic detrital rock and carbonatite . In addition , there are subprovenancs in the northwest, but their size are small.The study showed , the yanchang formation in the south of the slope of shanbei have two main deposition systems , that is , the northeast and southwest deposition system , the deposition system in the northeast of the research area is the main of meandering river delta deposition system , the deposition system in the southwest of the research area is the main of braided river delta deposition system , while , the turibidite fan appears in Qingyang—Heshui in the southwest. There is a general trend of progradation to the south in the cross section from changlO to chang2 period , however , because the basement was uplifted , the part of region appeared differential subsidence ,the continental shallow small lake was formed in chang 1 period .Because the large region appeared paludification , the lake disappeared at last, so the history of yanchang formation deposition ended . The characteristics of distribution of sedimentary system in plane controls the distribution of sand body , the northeast sedimentary system's sand body has a good continuity and presents silllike of northeast-southwest trend in the plane , the southwest sedimentary system's sand body presents a band shape of east -west or northeast-southwest trend . The procession of Yanchang formation sediment come through three stages, that is , plain-river environment in the early lacustrine basin development era , lacustrine environment inthe metaphase era ,and the flood plain-river plain environment ,and then lacustrine disappear in the late era.The source rock and the distribution of the centre of oil generation all have well coincidence relation with the distribution of deep lake in research area . The property and distribution of reservoir is controlled by sedimentary system , the stream channel sandbody , deltaic plain distributary channel sand body , deltaic front subaqueous distributary channel sand body and turbidite fan sand body are good reservoirs . The favorable source-reservoir-caprock group can be formed for the frequency of lacustrine transgression and regression . Through analysis of sedimentary facies , the distribution of sandbody , the character of structure and the property of reservoir, etc , the favorable exploration area can be found out in the main oil-bearing beds in Yanchang formation .