Sedimentary characteristics and petroleum geological implications of the first lacustrine transgression period of Yanchang Formation in Zhijing-Ansai area of Ordos Basin

Using comprehensive methods such as the features of sediment color, particle size distribution, sedimentary structure, transitional fossils and single well analysis, this paper discusses the development and distribution characteristics of sedimentary facies with the purpose of guiding the oil and gas exploration and exploitation of Chang 9 reservoir which is newly developed. The results show that the delta-lacustrine sedimentary system was developed in Zhijing-Ansai area during Chang 9 stage, the first lacustrine transgression period of Ordos Basin, mainly including delta front subfacies and shallow lake subfacies. During the depositional period of the lower part of Chang 9, delta front was mainly developed in the northern part of the study area, and was less formed in the southwestern part. During the depositional period of the upper part of Chang 9, the shallow lake area was small and extended southeastward in the trumpet form. Deposition pattern during the period of the upper part of Chang 9 basically inherited the characteristics of the lower part of Chang 9, except that the shallow lake distribution range was enlarged. Underwater distributary channel of delta front subfacies stretched from northeast to southwest. The mudstone which had hydrocarbon-generating capability was developed during the first lacustrine transgression period, and was distributed around Wuqi-Zhidan-Ansai area, and the greatest thickness lay in the southeast area of Zhidan which was up to 20m. The thickness tended to increase southeastward. The existence of reservoir space and the development of source rock have created favorable conditions for the prospecting work in Chang 9 reservoir.