A new understanding of Jurassic provenance and sedimentary framework in northwestern Sikeshu Sag, Junggar Basin, China

For a basin or sag with low exploration maturity,the understanding of its provenance system and clastic distribution system is the key to the analysis of its sedimentary framework and further influence the decision making and deployment of exploration.This study analyzed the Jurassic provenance of the Sikeshu Sag based on the data from cores,well logging,and seismic survey.The analysis of seismic profile characteristics revealed that there is a remote possibility that the Chepaizi Uplift in the northern part serves as the main Jurassic provenance of the Sikeshu Sag.On the contrary,systematic indicative signs of provenance have developed in the study area,including west-to-east progradational reflection config-urations and composite mounded reflections that show bi-directional downlapping contact in both the south and north directions.These systematic indicative signs suggest that the denuded uplift area in the western part is the main Jurassic provenance in the study area.Controlled by the provenance in the western part,the study area shows an orderly distribution of paleosedimentary systems from west to east and from shallow to deep water.The paleosedimentary system mainly exhibits the following characteristics after undergoing later high-intensity transformation.(1)The western part is dominated by the subfacies of a braided river delta front and is composed of three microfacies including distributary channels,interdistributary bays,and sandbars;(2)The middle and eastern parts show a semi-deep lacustrine environment,where debris flow sedimentary systems of slumping origin or in deep can-yons,gullies and channels connected to the denudation area developed from west to east.These debris flow sedimentary systems jointly form the clastic sedimentary system under the control of western provenance.

Basin-Scale Sand Deposition in the Upper Triassic Xujiahe Formation of the Sichuan Basin, Southwest China: Sedimentary Framework and Conceptual Model

The Upper Triassic Xujiahe （须家河） Formation in the Sichuan （四川） Basin, Southwest China is distinctive for the basin-scale sand deposition. This relatively rare sedimentary phenomenon has not been well interpreted. Here we addressed this issue by discussing sedimentary framework and conceptual model. Analysis of sedimentary setting implied that the basin received transgression during the deposition. It had multiple provenance supplies and river networks, as being surrounded by old- lands in multiple directions including the north, east and south. Thus, the basin was generally charac-terized by coastal and widely open and shallow lacustrine deposition during the Late Triassic Xujiahe period. This is similar to the modern well-known Poyang （鄱阳） Lake. Therefore, we investigated the framework and conceptual model of the Sichuan Basin during the Xujiahe period with an analogue to the Poyang Lake. Results show that the conceptual model of the deposition can be divided into transgressive and regressive stages. The first, third and fifth mem- bers of the formation are in transgressive stage and the deposits are dominated by shore and shallow lacustrine mud. In contrast, the deposition is mainly of braided river channel sand deposits during the regressive stage, mainly including the second, fourth and sixth members of the formation. The sand deposited in almost the entire basin because of the lateral migration and forward moving of the cross networks of the braided rivers. The multiple alternations of short and rapid transgression and relatively long regression are beneficial to the basin-scale sand deposition. Thus, the main channel of the braided river and its extensional areas are favorable for the development of hydrocarbon reservoir. This provides practical significance to the reservoir evaluation and exploration. In addition, the results also justify the relatively distinctive sedimentary phenomenon in the study area and may also have im- plications for understanding the large-scale sand deposition elsewhere.