Sedimentary characteristics of shallow-water deltas in Chang 8 oil-bearing interval in Eastern Gansu, Ordos Basin

The Chang 8 oil-bearing interval of the Upper Triassic Yanchang Formation in the north-western Ordos Basin was deposited in a stable tectonic setting and it was favorable in terms of geological condition for the shallow-water deltas with the humid climate and the slope gradient at only 0.1°. It is found that shallow water delta was developed during Chang 8 deposition by a great number of core observations, seismic reflection characteristics analysis and modern shallow water lake sediments. It is chcharacterized with low component maturity and middle textural maturity, traction current deposits indicated by grain-size probability curves. In the study area, the mouth bar and the gravity flow deposits are not well developed, and the river has been branched off or diverted frequently due to the lake water obstruction and are widely distributed to a long distance with larger scales. The delta front facies belt is characterized by its wide distribution, the gently connecting of delta plain and delta front with no obvious transition. And three-member structure of the Gilbert Delta is not developed. Therefore, the sandbodies of distributary channels are widely distributed, overlapping vertically, and extending laterally, which is favorable for the formation of lithologic hydrocarbon reservoir.