广东三水盆地古近系(土布)心组地球化学特征及其所指示的氧化还原条件

古近系心组红岗段是广东三水盆地的主要生油层,以一套黑—灰黑色页岩沉积为主,其富含有机质、纹层理发育良好的岩层与相对贫有机质、含微体生物化石的微扰动层交替出现。根据纹层状页岩与生物扰动页岩的C-S-Fe关系以及对氧化还原条件敏感的痕量元素(Mo、U、V、Co、Ni、Pb和Cu)地球化学记录,对这两种不同岩相的地球化学特征及其所反映的水体氧化还原条件进行了分析。纹层状页岩的TOC、S含量和DOPT值均较高,TOC-S、TOC-DOPT显示较弱的相关性;而生物扰动页岩的TOC、S含量和DOPT值相对较低,TOC-S、TOC-DOPT相关性显著。痕量元素在两种岩相中的分布也有差别,各元素在纹层状页岩中的丰度均高于生物扰动页岩,尤其是Mo和U具有较高的丰度水平、且在不同岩相中显示明显差别。黑色页岩中的C-S-Fe关系和痕量元素(特别是Mo和U)分布反映了心组红岗段沉积时期湖盆水体的氧化还原条件变化。生物扰动页岩沉积期间,底部水体主要为氧化环境,间歇性出现贫氧条件,硫酸盐还原作用发生于沉积物/水界面及其以下沉积物中。纹层状页岩形成于缺氧的底部水体条件下,水体和沉积物中均可含H2S,为有机质保存以及对氧化还原条件敏感的痕量元素的富集提供了有利条件。

GEOCHEMICAL PALEOREDOX INDICATORS IN BLACK SHALES OF THE PALEOGENE BUXIN FORMATION, SANSHUI BASIN ( SOUTH CHINA )

The Honggang Member of the Buxin Formation (Early Paleogene) is the main source rock in the Sanshui Basin.It is composed mainly of alternating laminated, organic-rich, and microbioturbated, organic-lean shales. In this study, C-S-Fe relationships and redox-sensitive trace elements (Mo, U, V, Co, Ni, Pb and Cu) of the laminated and bioturbated shales are analyzed in order to examine the differences of the two shale facies in geochemical records and depositional conditions. TOC, S and DOPT values are higher and TOC-S and TOC-DOPT show a weaker correlation in the laminated intervals than in the bioturbated intervals. The abundance of trace elements in the two shale types is different. The laminated shales have higher concentration of trace elements (especially Mo and U). The geochemical contrasts between the two shale facies of the Honggang Member indicate that their sedimentary environments were different. The difference was related to variations in bottomwater redox conditions. The bioturbated shales were formed under oxygenated to dysoxic bottomwater conditions with microfaunal activity, in which sulfate reduction took place at/below the sediment-water interface. During the accumutlation of laminated shales, bottom water environments were anoxic, in which H2S could have been present above the sediment-water interface as well as within the sediments. This was favorable for organic matter preservation and enrichment of redox-sensitive trace elements.