Characteristics and Mechanism of Thick Sandbody of Yanchang Formation at the Centre of Ordos Basin

Thick sandbody are widely and stably developed at the depositional period of Chang6 and Chang7 oil bearing formation in late Triassic in the centre of Ordos basin.The thickness surpass one hundred metres uninterruptedly in some areas.The large-scale sand belt,formed in deep lake,stretches along the facies boundary and isolates itself from delta-front sandbody in the southwest sedimentary system,while the sand belt or lobes encircle the delta-front in the northeast sedimentary system.The thick sandbody was resulted from gravity flow and deep-water tractive current and can be divided into five types,i.e.slump,sandy debrite,turbidite,delta sandbody and underflow transforming sandbody.Among them,deposition of gravity flow is the dominant sedimentary type;However,thick sanbody in Baibao area and its north area belong to delta deposition and sandy debrite,with deposition of gravity flow.The stable distribution of thick sandbody is controlled by the factors as supply rate of sediment,bottom shape of basin and tectonic activity.