A CASE STUDY OF THE BRAIDED-RIVER DELTA IN THE PALAEOGENE SHAHEJIE FORMATION IN THE GUDONG AREA, SHENGLI OIL FIELD

Braided-river deltas that braided river systems prograde into an inland lake are first found in the Palaeogene Shahejie Formation in the Gudong area, Shengli Oil Field. Their characters are distinct and significantly different from the other types of deltas. Three sedimentary subfacies, that is, braided-river delta plain subfacies, braided-river delta front subfacies and prodelta subfacies, and eight microfacies are recognised. The sediments of braided-river deltas are mainly composed of pebbled sandstone and medium-to coarse-grained sandstones. The bars of the distributary channels and subaqueous distributary channels are the main bodies of deltas. A single channel bar has a thickness of 0.15~2.2 m. Many finger-upward lenticular channel sand bodies pile vertically up each other. Cross-stratification is abundant in the braided-river deltas, but the lateral accretion cross-bedding resulting from lateral accretion of channel sand bodies is the most principal sedimentary structure and one of the most distinct characters of braided-river deltas.