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.