Depositional characteristics of complex fan delta of member 1 of Shawan formation in Chepaizi area， Junggar Basin

Key words: 
• Chepaizi area /
• Member 1 of Shawan formation /
• near-source system /
• complex fan delta /
• slope break zone /
• valley

Abstract: Near-source sedimentary system of member 1 of Shawan formation is the main reservoir in Chepaizi area of Junggar basin. Neverthless, there have been different points on depositional characteristics of it. On the basis of previous studies， using the mud logging data， core data， logging data and seismic data, combining with modern fan delta sediment, the depositional characteristics of near-source sedimentary system within the 4th sequence stratigraphic framework of member 1 of Shanwan formation were investigated, and the sedimentary model of it was built up. The results are as follows: (1) Near-source system was made of two complex fan deltas, one is shoal complex fan deltas, the other is complex fan deltas. The main microfacies were distributary and sand bar of fan delta front. (2) The lithology of the distributary was changed from shoreline to lake by conglomerate - glutenite –medium-coarse lithic sandstone bearing conglomerate - medium-coarse feldspar lithic sandstone bearing conglomerate - feldspar lithic fine sandstone bearing conglomerate. The lithic composition was dominated by igneous lithic, while the gravel composition was dominated by igneous gravel and mud-gravel. (3) Near-source system of sand group 1 of member 1 of Shawan formation mainly distributed below the third slope break zone. It reached in the the middle of the project where the far-source system reached as well, and they interacted with each other. Lacustrine range was the smallest during this stage. Near-source system of sand group 2 of member 1 of Shawan formation mainly distributed above the slope break zone and the lacustrine range was the largest this period. Near-source system of sand group 3 of member 1 of Shawan formation mainly distributed near the slope break zone. The slope of the eastern wing was steeper, the gully scale was smaller, the complex fan delta scale was smaller, the slope of the western wing was slower, the valley scale was larger, the composite fan delta scale was larger. The conclusion is that paleogeomorphology and the relative lacustrine level change controlled the sedimentary characteristics of the near-source systems.