The Research On Sedimentary System Of Chang6-chang8, Yanchang Formation, Fuxian Area, Ordos Basin

Fuxian area is located in the southeast of the Shanbei Slope, one of the secondarytectonic units of Ordos Basin. This paper investigates the Chang 6- Chang 8 oilbearing beds of Upper Triassic Yanchang formation . On the basis of substantive fieldand indoor practice development work, the source, the sedimentary system, the basindevelopment mode,the paleogeography characteristics and evolution were studied.The sources of Chang 6-Chang 8 oil bearing beds in Fuxian region areinvestigated based on the Dickinson triangular diagram, palaeocurrent direction, lightand heavy mineral debris analysis. These suggest that NE is the main direction ofsediment provenance, while southern part of the deposition was influenced by thesouthwestern source area. Sedimentary rocks or metamorphic sedimentary rocks areconsidered to be the source rocks.According to the comprehensive analysis of the outcrops, drilling cores, facies inwell logging, and all kinds of sedimentary facies marks. we identify three types ofsedimentary facies in Chang 6—Chang 8 bearing beds ,including lake delta facies,lake facies and turbidites facies. and make a Further research on characteristics ofthese facies . The sedimentary system of lake delta is dominated by delta frontmicrofacies,which develop in every formation.Turbidites, one of the main reservoirs,develop in deep and semi-deep lacustrine of Chang 6—Chang 8 bearing beds in thesouthwest of the study area.We study on the paleogeography characteristics and evolution of Yanchangformation,and draw the lithofacies paleogeographic maps of different periods.Thereare two depositional pattern of lake delta. One is the retrograding delta duringChang 8 to Chang7, with a large Fining-upwards sequence. This period of gradualexpansion of lake area, lake transgression process was slow.The boundary linebetween shallow lake and delta front gradually migrated to the northeast.Sediment ofdelta front gradually retreated and sediment of turbidite fan developed in southern semi-deep lake.Aonather is in Chang 6 period is a prograded delta,with a largeCoarsening-upwards sequence . Delta gradully were pushing forward to the southwestduring this period, Most parts of the study area had been converting to delta facies inchang 6 period, shallow lake deposits had retreated to the southern part of study area .On the basis of genetic type of the reservoir sandstones,there are five kinds ofsand bodies types ,such as the subaqueous distributary channel sandstones ,thesuperposition of subaqueous distributary channel sandstones,the channel-mouth barsandstones , the components of subaqueous distributary channel sandstones withchannel-mouth bar sandstones,turbidite sandstones.The major reservoir sandstonesare the components of subaqueous distributary channel sandstones and turbiditesandstones.