Control of slope-pattern on the deposition of fan-delta systems: a case study of the Upper Karamay Formation, Junggar Basin

The Mesozoic fan deltas in the north-west margin of the Junggar Basin, as important petroleum reservoirs, exhibited complex facies change and internal structures with strong heterogeneity which were controlled by the transformation of slope-patterns, bringing great challenges to the study of sedimentary characteristics. The Upper Karamay Formation at north-west margin of the Junggar Basin was the objective in this paper which attempts to clarify the mechanism of sedimentary response and sand-body distribution of fan delta systems under the control of slope-pattern change. Based on a data set of cores, well logs and seismic, two types of slope-pattern were identified in the study area, which include steep-to-gentle in the south and gentle-to-steep in the north. The control of difference slope-patterns on the sand-body distribution was clarified based on the analysis of the sedimentary dynamics, facies characteristics, and depositional evolution of the fan deltas. The study shows that the transport mechanism of sediments on the steep-slope was dominated by debris flows, developing coarse-grained, thick-bedded lobes with poor structural maturity of clasts. On the gentle-slope, the deposition was dominated by hyperconcentrated-traction currents, forming relatively fine-grained, thin-bedded lobes with increased sandy matrix. The sand-bodies show frequent bar-channel transformation and channel down-cutting under the steep slope setting, which exhibit migration of isolated river channels on the gentle slopes. Under the steep-to-gentle pattern, the coase-grained sediments were mainly accumulated at slope toe, generally developed equiaxial lobes. However, the coarse-grained clasts were preserved both at proximal and distal lobes on the gentle-to-steep slopes, showing obvious lateral extension of the fan delta. The slope patterns controlled sedimentary respond rates of the fan deltas during lake level change. By comparing the modern cases of fan systems worldwide, the control of slope patterns on deposition of coarse-grained fans was clarified, providing insight into hydrocarbon exploration on basin margins.