琼东南盆地深水区第四系乐东组层序地层学特征及其水合物勘探启示

琼东南盆地深水区第四系乐东组是南海北部已证实的天然气水合物主要勘探层系。近年来，其层序地层格架、构成特征以及对天然气水合物稳定域及运聚成藏的控制影响作用等科学问题引发了广泛关注。为了厘清乐东组层序地层格架、构成特征及其展布规律，基于层序地层学理论，结合研究区高分辨率地震资料，建立了乐东组精细层序地层格架及层序样式。依据典型的地震接触关系识别了乐东组复合层序界面T20、T14和T0，同时划分确定了乐东组下段6套三级层序（LDSQ1-6）和上段8套三级层序（LDSQ7-14）。在三级层序内部还进一步识别出初始海泛面、最大海泛面以及最大海退面，在三级层序中进一步划分出低位体系域（LST）、海侵体系域（TST）、高位体系域（HST）和下降体系域（FFST）。研究表明，1.8 Ma以来，红河物源体系沉积物供给充足，乐东组下段陆架边缘体系进积和加积组分厚度大，层序样式主要为H型层序；0.9～0.8 Ma至今，地层以进积叠置样式为主，且峡谷发育频次增大，低位体系域组分占比增大，主要发育L-H型层序。笔者结合国内外相似区域的沉积相模式，明确了层序格架约束下的沉积相发育类型及时空演化特征，为该区域天然气水合物储层预测提供了地质依据。

Sequence stratigraphic architecture of the Quaternary Ledong Formation in the deep-water area of the Qiongdongnan Basin and its significance to gas hydrate exploration

The Quaternary Ledong Formation in the deep-water area of the Qiongdongnan Basin is a proven gas hydrate enriched zone in the northern South China Sea. The sequence stratigraphic architecture, depositional patterns, and their influences on the gas hydrate stability, migration and accumulation have become hot topics internationally and attracted great attention from the geological society of the region. Based on the theory of sequence stratigraphy, and combined with the high-resolution seismic surveys in the working area, a detailed sequence stratigraphic framework of the Quaternary Ledong Formation is established in this paper, which includes 4 second-order sequence boundaries and 13 third-order sequence boundaries. In addition, the maximum flooding surface (MFS), first flooding surface (FTS) and maximum regression surface (MRS) are also identified. The third level of sequence is further divided into system tracts. Since 1.8 Ma, the Red River provenance system has supplied sufficient sediments to the basin, which led to the progradation of deltaic deposits of Lower Ledong Formation, and a sequence of H-type was formed. In the upper part of Ledong Formation, however, the stratigraphic sequence is dominated by an accretionary architecture, and the sequence style was mainly of L-H type. Finally, this paper established the depositional model under the constraints of the sequence framework, and discussed the deltaic-gravity flow related deposits in the study area. This study has provided an important clue for prediction and exploration of natural gas hydrate in the future.