南海白云凹陷东侧巨型麻坑中自生碳酸盐岩的特征及其地质意义

CHARACTERISTICS OF AUTHIGENIC CARBONATES FROM A MEGA-POCKMARK ON THE EASTERN SIDE OF BAIYUN SAG,SOUTH CHINA SEA AND THEIR GEOLOGICAL SIGNIFICANCE

探测发现南海白云凹陷东侧存在一个宽1 500m、深达75m的巨型麻坑,在麻坑内采集了大量的碳酸盐岩结核。大部分结核中存在新鲜孔洞和流体通道。本文从中选取了5块结核,进行XRD衍射、扫描电镜、碳氧同位素分析。结果表明,5个结核中碳酸盐矿物主要是含铁白云石,其中一个还有少量方解石,碎屑矿物主要是石英和长石。结核的显微结构以纤维状为主,存在裂隙和孔洞。δ^(13)C值为-24.7‰^-10.9‰V-PDB,显示孔隙流体主要以热成因甲烷为主。δ^(18)O值为5.6‰~6.9‰V-PDB,高值主要与天然气水合物分解有关。巨型麻坑海底之下最可能存在天然气水合物埋藏。含铁白云石结核在麻坑表层沉积物之下形成,随后出露海底。碳酸盐岩结核指示巨型麻坑内曾发生的甲烷渗漏活动可能与深部油气泄露有关,浅地层剖面中疑似气体羽和结核表面管状蠕虫活体都显示巨型麻坑仍在发生甲烷渗漏。

A mega-pockmark,1 500 min diameter and 75 min depth,is discovered on the east side of the Baiyun Sag,the South China Sea.Many irregular nodules are collected from the mega-pockmark and most of them characterized by fresh holes and seep passages.The mineral composition,microstructures,stable carbon and oxygen isotopes（δ^（13）C and δ^（18）O）of the five authigenic carbonates are used to trace the fluid source and their forming environment.Mineralogical study suggests that the five carbonates are all dominated by ferrous dolomite,followed by a detrital fraction,mainly composed of quartz and feldspar.The δ^（13）C of the five carbonates falls in the range of -24.7‰^-10.9‰,indicating aprimary source of ^（13）C-depleted thermogenic methane.The δ^（18）O of five carbonates varies between 5.6‰~6.9‰ V-PDB,indicating that the ^（18）O-rich fluids are derived from the decomposition of gas hydrates and/or dehydration of clay minerals at depth.Gas hydrate may have existed in the sediment of the mega-pockmark.Ferrous dolomites in authigenic carbonates are formed in sediments and subsequently exposed to the seawater,indicating that the old methane seep was related to deep oil and gas seepage in the mega-pockmark.The live tubeworms and gas plumes in the pockmark suggest that methane seep is still active nowadays.