Abstract:

The distal braided-river delta differs from the typical braided river delta in terms of sediment supply, sedimentary environment, sedimentary process, etc., which leads to distinct sedimentary characteristics, depositional sequences, and sand body distributions, and affects its identification and prediction in hydrocarbon exploration. The 4th member of Xujiahe Formation in the western Sichuan depression is currently facing issues such as unclear sedimentary microfacies and ambiguous depositional processes, which constrain the accurate characterization of favorable sand bodies. Based on clarifying the depositional background of gentle slope, shallow water, and remote provenance, along with the warm and humid paleoclimate conditions in the study area, this article integrates field outcrops, drill cores, and a series of test data to determine the typical sedimentary facies markers, lithofacies types, sedimentary sequences and microfacies distribution patterns of the distal braided-river delta. The research results show that: The 4th member sandstones of Xujiahe Formation in Jianyang area exhibit typical characteristics such as remote provenance, fine grain size, considerable cumulative thickness, sand-rich and mud-poor composition, and sand-on-sand stacking. The sandstones are also characterized by low matrix content, good sorting, and high maturity, with abundant traction current sedimentary structures, occasionally interrupted by flood events and gravity collapse occurrences. Based on the identification of 11 types of lithofacies and five types of vertical sedimentary sequences, it is inferred that the distal braided delta-front subfacies are mainly dominated by the development of subaqueous distributary channels, while sedimentary microfacies such as distributary mouth bars, distal bars, and interdistributary bays are relatively scarce.  Under the comprehensive influence of continuous sediment supply, seasonal climatic alternation, and lake-level fluctuations, the main river channels evolve into distributary and terminal channels in an orderly manner, exhibiting patterns of inheritance and lateral migration, ultimately presenting a sand body distribution pattern of vertically continuous stacking and extensive lateral spread.

Key words: braided river delta, sedimentary characteristics, microfacies types, 4th , Member of Xujiahe Formation, western Sichuan depression