The relationship between fan delta and hydrocarbon accumulation in Benxi-Taiyuan Formation, Ordos Basin

Abstract：Based on abundant well datas and core and outcrop observations, combined with geological setting as well as particle sizes and rock and mineral logging datas, the authors analyzed the sedimentary characteristics of widely distributed Late Carboniferous-Early Permian pebbly sandstones. It is suggested that they belong to the fan delta sedimentary system and were deposited in the epicontinental marine basin in the form of channel alluviation characterized by clear zonation in plane and obvious retrogradation structure in vertical section. The depositional structure was formed by the channel traction flow, with no obvious gravity flow deposition. The sandy conglomerate of high-energy braided channels and underwater distributary channels are their main skeletal bodies, whereas the river mouth bars are not well developed. Nevertheless, because of the effects of provenance, paleogeomorphic features and tide, the high-energy channel sand bodies of fan deltas with an important fluviation role have usually been reformed into barrier bars by tides, and hence they can also be regard as favorable reservoirs. Due to the difference in depositional characteristics, the reservoir space, reservoir property and oil-bearing properties of the fan delta are also evidently different. It is therefore concluded that the thick quartz sand belts formed by high energy channels seem to be the favorable facies belts.