Sedimentary characteristics and models of submarine fans in the Meishan Formation on the north slope of the Ledong Sag, Qiongdongnan Basin

Gaining a deep understanding of the sedimentary characteristics and models of submarine fans in the Meishan Formation on the north slope of the Ledong Sag in the Qiongdongnan Basin holds great significance for guiding natural gas exploration in this formation. Using detailed core observations and descriptions, as well as data from seismic surveys, well logging, analysis, and tests, we investigate the sedimentary characteristics and models of submarine fans in the study area based on sequence stratigraphy and sedimentology. The results indicate that the submarine fans developed in the Meishan Formation on the north slope of the Ledong Sag are supplied by the western and northern provenance areas. The western provenance area-sourced submarine fan is primarily distributed in the sedimentary basin region below the slope break zone. Since the deposition occurs in the low-lying area as restricted by structures, the western provenance area-sourced submarine fan exhibits intensively distributed channels on the seismic section, with their literal edges displaying a pronounced onlap pattern. While the northern provenance area-sourced submarine fan occurs on the northeastern continental slope of the Ledong Sag. On the seismic section, its sand bodies close to the distal end of the fan show continuous progradation, leading to a downlap pattern. The two submarine fans differ significantly. Specifically, the western provenance area-sourced submarine fan lacks typical fan body shape and seismic responses of channels, with thickly laminated sandstones directly overlapping the slope areas on both sides, and it mainly involves gray fine-grained sandstones at great thickness and with a composition of quartz (average content of 76 %) and feldspar (average content of 12 %). The northern provenance area-sourced submarine fan, on the other hand, is composed predominantly of gray siltstones at small thickness, with sandstones mainly composed of quartz (average content of 55 %) and lithic fragments (average content of 30 %). The formation and evolution of submarine fans in the study area are principally governed by sea level, provenance, and paleogeography. The formation of slope break zones and low sea level create favorable conditions for the formation of submarine fans, while sediment supply and basin landforms determine submarine fan types. Sufficient sediment supply greatly facilitates the formation of confined submarine fans; otherwise unconfined submarine fans are likely to develop. The western provenance area-sourced submarine fan is classified as the confined type, where restricted basin boundaries result in extremely developed sandy debris-flow deposits, frequent migration and diversion of channels, and weakly developed turbidite-flow deposits. Based on these findings, we summarize the developmental characteristics and major factors controlling the confined submarine fan of the Meishan Formation on the north slope of the Ledong Sag and establish its sedimentary model, aiming to provide a reference for hydrocarbon exploration in similar sedimentary bodies.