Stratigraphic base-level cycles and physical properties of the reservoir sandstones in the Chang 4+5 and Chang-6 oil formations of the Upper Triassic Yanchang Formation in the Pingbei Oil Field, Shaanxi

The development and distribution of the reservoir sandstones are noticeably different in the fourth bed of the Chang 4+5 and Chang-6 oil formations of the Upper Triassic Yanchang Formation in the Pingbei Oil Field. And other differences are also manifested in the ratios of the sandstone thickness in subaqueous distributary channels, subaqueous crevasse splays and channel-mouth bars to the stratial thickness, and the proportions of the reservoir sandstone thickness in various sedimentary microfacies. The physical properties of the reservoir sandstones are almost identical in varying base levels. The base-level cycle analysis is here believed to be significant in the examination of the development and distribution of the reservoir sandstones, physical properties of the reservoir sandstones, and the exploration and development of the Pingbei Oil Field.