The distribution regularity of sand bodies in Chang-6 Member of Upper Triassic Yanchang Formation, Fuxian area, Ordos Basin

Abstract：Based on a study of core, logging and seismic data, this paper studied characteristics of high-resolution sequence stratigraphy of Chang-6 Subsection of Yanchang Formation. The authors found that from northeast to southwest, the distribution model of base-level cycle varies in order of typeA1→typeA2→typeB1→typeB2→typeC or typeA2, and recognized one long-term and four medium-term cyclic sequences (MSC1, MSC2, MSC3 and MSC4) . A high-resolution sequence stratigraphic framework of medium-term cyclic sequences was established. With the medium-term cyclic sequences as mapping units, four sequence-based lithofacies and paleogeography were drawn which show that the delta was mainly developed during the MSC1 stage and MSC3 stage. The sand bodies were mainly developed at the same time. It is found that sand bodies were mainly developed in the subaquatic distributary channels of prograding deltas whose base-level cycle was of type A1 and type A2, and the sand bodies were also developed in the alluvial fan whose base-level cycle type was type A2. Consequently, the distribution regularity of sand bodies revealed by sequence-based lithofacies and paleogeography has an important guiding significance for the petroleum exploration in the prospecting area of Fuxian located in the southeast hydrocarbon-enriched belt of Shanbei slope, Ordos Basin.