Strata Architectural Variability and Facies Distribution in Structural Transfer Zone: A Case Study of Fushan Sag, Northern South China Sea
Résumé
Structural transfer zones in a half‐graben rift basin play a significant role in controlling the sandy sediments and provided a target to the hydrocarbon exploration. Previous studies have classified the transfer zone in lacustrine environments into two different patterns: synthetic approaching transfer zones and synthetic overlapping transfer zones. However, the depositional pattern evolution and control factors of the above transfer zones are still unclear. In the Fushan Sag, the northern South China Sea, an overlapping transfer zone developed in the early Eocene Epoch, while a synthetic approaching transfer zone developed in the late Eocene due to tectonic uplift. This evolution process provided an opportunity to study the stack pattern of strata architectural variability and facies distribution in the structural transfer zone of the Eocene lacustrine basin. In this study, as the oriented sedimentary structures in core samples and heavy mineral assemblages of 18 wells indicating, we rebuilt the paleo‐hydrodynamic distribution evolution during the early and late Eocene. Then, we divide the sequence‐stratigraphy and calculate the sand body parameters according to the seismic data and well log interpretation. During the early Eocene, the lake level was in the low stand, and the faults broken displacement in the East block was over 50 m. We point out that the prograding delta and turbidities are oriented perpendicular to the structural transfer zone. According to the quantitative analysis of the flow rate and the depositional parameters, we speculate that its gravity transported the sediment and the sediment‐supply is dominated factors during this period. Up to the late Eocene, the rising lake level and the decreased faults displacement lead the flow diverted to a NE‐direction, resulting in it being parallel to the axis of the transfer zone. Thus, we speculated that the accommodation space is domination in this period. In comparison, the above two periods, a braid river delta with an isolated sand body and turbidities developed in the deep area is prominent in the overlapping transfer zone, while a meandering river delta characterized in the synthetic approaching transfer zone.