琼东南盆地乐东北坡梅山组海底扇沉积特征及沉积模式

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

杨宏宇 胡明毅 蔡全升 陈志宏 刘为

Hongyu YANG Mingyi HU Quansheng CAI Zhihong CHEN Wei LIU

深入研究琼东南盆地乐东北坡梅山组海底扇的沉积特征及其沉积模式，对于指导该区域梅山组天然气勘探具有重要意义。通过岩心精细观察，结合地震、测井以及分析测试资料，并运用层序地层学与沉积学理论，研究了琼东南盆地乐东北坡梅山组海底扇的沉积特征和沉积模式。研究表明：① 乐东北坡梅山组海底扇的物源分别来自西部和北部。西部物源海底扇主要分布于坡折带以下的盆地沉积区。受构造限制西部物源海底扇在低洼区沉积，剖面上水道密集分布，扇体侧边缘具明显上超特征。北部物源海底扇在乐东凹陷东北陆坡发育，剖面上扇端砂体不断前积，表现为下超特征。西部物源海底扇与北部物源海底扇扇体明显不同。西部物源海底扇没有典型的扇体外形和水道地震响应，厚层砂岩直接超覆沉积于两侧斜坡区上，岩性以灰色细砂岩为主，砂岩厚度较大，砂岩组分以石英为主，石英含量平均为76 %，长石含量平均为12 %；北部物源海底扇岩性以灰色粉砂岩为主，砂岩厚度较小，砂岩组分以石英和岩屑为主，石英含量平均为55 %，岩屑含量平均为30 %。② 乐东北坡梅山组海底扇的发育主要受控于海平面变化、物源供给及古地貌特征。坡折带的形成以及较低海平面是海底扇形成的有利条件，而物源供给量与盆地古地貌则约束了海底扇的类型，高物源供给易于形成限制型海底扇，而低物源供给则有利于非限制型海底扇的形成。西部物源海底扇属于限制型海底扇，局限的盆地边界导致西部扇体砂质碎屑流沉积极为发育，水道频繁迁移改道，浊流沉积发育较弱。据此，总结了乐东北坡梅山组限制型海底扇发育特征及主控因素，建立了限制型海底扇沉积模式，以期为类似沉积体油气勘探工作提供参考。

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.

10.11743/ogg20250214