白云凹陷陆坡峡谷沉积与迁移特征及其对天然气水合物成藏的影响

DEPOSITION AND MIGRATION OF SEDIMENTS IN SUBMARINE CANYONS OF BAIYUN SAG AND THEIR EFFECTS ON GAS HYDRATE ACCUMULATION

利用高分辨率地震资料,研究了南海北部白云凹陷中新世以来的陆坡峡谷沉积和迁移特征及其对动态似海底反射(BSR)的影响。白云凹陷陆坡区浊流和底流共同作用形成了大型单向迁移峡谷沉积体系。峡谷的沉积过程包括侵蚀为主阶段、侵蚀-沉积共同作用阶段及沉积为主阶段。峡谷沉积相主要包括峡谷侵蚀基底、谷底沉积、谷内滑塌块体搬运沉积及侧向倾斜沉积层等4个单元。峡谷的迁移造成含天然气水合物脊部两侧不同的侵蚀-沉积环境,因此,脊部两侧BSR反射特征也不同。随着峡谷迁移的进行,在峡谷侵蚀侧翼处,沉积物被侵蚀,天然气水合物稳定带底界将发生下移,BSR反射特征为多轴较连续反射;而峡谷沉积侧翼处,沉积物增厚,天然气水合物稳定带将发生上移,BSR反射特征为单轴连续反射。

A series of submarine canyons characterized by unidirectional sediment migration are investigated using a grid of high resolution seismic data in Baiyun Sag, northern South China Sea （NSCS）. The unidi- rectional migrating canyon system and the dynamic bottom simulating reflectors （BSRs） are the result of the interaction between turbidity currents and bottom currents. The South China Sea Warm Current （SCSWC）, an important bottom current in NSCS, is thought to be the main reason for the migration of canyons. The evolutionary history of a single canyon can be divided into three stages： erosion-dominated stage, erosion-deposition stage and the deposition-dominated stage. Four types of architecture elements consist of the fill of a single canyon, which are erosion surface （ES）, basal lag （BL）, mass transport deposits （MTD） and lateral inclined packages （LIP）. Two distinct BSRs developed on each flank of the canyon because of the migration： the erosional flank undergoes more erosion than deposition, while the depositional flank is just the opposite. In consequence, the base of gas hydrate stability zone （BGHSZ） tends to move down on the erosional side of the canyon where sediments are removed by turbidity currents, while BGHSZ tends to move up on the depositional side where sediments become thicker.