Characteristics and sand body distribution patterns of retrogradational fan-delta sediments of first member of Shawan Formation in Chepaizi area on western margin of Junggar Basin
摘要
Massive fan-delta sediments are developed in the first member of the Neogene Shawan Formation （N1s1） in the Chepaizi area of the Junggar Basin. However， the sedimentary evolution models and sand body distribution remain poorly understood， which has limited oil and gas exploration and development in the Chepaizi area. According to the core， well logging， and seismic data， this study investigated the characteristics and sand body distribution patterns of the retrogradational fan-delta sediments of N1s1 during the lake basin expansion period. The results show that the sedimentary environment of N1s1 is dominated by the fan-delta front subfacies， which develop the main channel， distributary channel， interchannel bay， channel edge， and other microfacies. From landward to lakeward， the thickness of the sand body gradually decreases， and the grain size of the sediments becomes finer， leading to the formation of estuary bar and sheet sand microfacies in the fan-delta outer-front subfacies. The fan-delta inner-front sand body is mainly composed of conglomerate and sandstone and generally shows the retrogradational sedimentary sequence with rising lake water levels. With the rise in lake water level， the boundaries of the fan-delta sediments shift towards the direction of provenance， and the sedimentary sand bodies shift landward. Horizontally， the scale of sand body development gradually decreases. Longitudinally， it is formed by the superimposed three-phase sand body retrogradation and is characterized by the positive rhythmic characteristic. In the first phase， the main channel develops sand bodies with large amounts of widely distributed gravel and poor physical properties and oiliness. In the second phase， the distributary channel develops sand bodies with large thickness， good connectivity， small gravel diameter， and good oiliness. In the third phase， sediments in the interchannel bay and channel edge increase； the thickness of the sand body becomes thinner， and the con