IDENTIFICATION, FORMATION CONDITIONS AND DISTRIBUTION REGULARITY OF BEACH BAR SAND BODIES IN MAJOR DEPRESSION LAKE BASINS: A CASE STUDY OF THE CHANG8 MEMBER IN THE ORDOS BASIN, CHINA

The origin analysis of sand bodies in the Upper Triassic Yanchang Formation of the Ordos Basin was carried out, on the basis of the field geological investigation, elaborate core observation and characterization, and analyses of seismic reflection characteristics and systematic sampling and testing. A new sedimentary facies, namely the beach bar sand body, was identified in the Yanchang Formation. In the outcrop, the geometric shape with a flat base and a convex top is seen. Linear conformity is found between the base of the sand body and the underlying lacustrine shale, and markers of exposure are observed at the top. Various sedimentary structures attributed to wave action are developed within the sand body, with lack of erosion and filling structures. The single sand body presents inverse grading from bottom to top, while the compositional maturity of the beach bar sand formed in one stage gradually increases upwards and reaches its maximum at the top of the bar. The grain size probability curve is mainly triple-segmented with "double saltation load and single suspended load". In terms of seismic reflection, the beach bar sand shows growing events, discontinuous amplitudes, and an external form of seismic unit with a flat base and convex top. The main controlling factors of formation and preservation of beach bar sands were discussed, which are preliminarily concluded to be the basal topography of the lake basin, the transition of the stable lake coastline, etc. In the Chang82 sub-stage, a vast favorable development belt of beach bar sands, parallel to the coastline, exists in the western and southern parts of the Ordos Basin. The area, which meets the requirement of formation of large-scale reservoir bodies, is a promising major oil and gas reserve supplement.