Abstract: Based on the analysis of lithologic sections, log data, sedimentology characteristics, reservoir properties, depositional structures and sedimentary rhythm in Arnan sag of Erlian basin, it is considered that the third member of Arshan formation(Lower Cretaceous) in Arnan sag was product due to one event of sedimentation with multiple provenances and compositions conglomerate of near provenance, coarser clast, low maturation of the composition and texture. This paper firstly presented that A-3 member conglomerate deposited in large size of braided river delta, possessed three beds of structures(i. e. delta plain, delta front and prodelta) of long provenance delta, had sedimentary structure dominated by traction current and positive rhythm section characteristics. Hence ten microfacies such as subwater channel etc. are subdivided, reconstructing the lithofacies and Paleogeography environment of A-3 member and establishing model of these facies. It is pointed out that reservoir space of conglomerate is composed of primary intergranular pore, hole and fracture, secondary denudation pore and structural fracture. Reservoir quality and oil-bearing property of conglomerate and reservoir distribution are all controlled strictly by sedimentary microfacies, gravel propping pattems and structure development degree.

Key words: Erian basin, Early Cretaceous, Braided river, Delta, Conglomerate, Depositional model, Reservoir characteristic