坦桑尼亚盆地渐新统深水重力流沉积特征及控制因素*

近年来,在东非坦桑尼亚盆地深水区相继发现大型气藏,但其沉积特征、形成机理及主控因素研究较为薄弱。综合利用钻井、测井及二维地震资料,对其沉积特征进行分析,发现研究区渐新统发育深水重力流沉积,包括水道、堤岸、朵叶等,并以水道—朵叶沉积为主。根据水道的发育位置、外部形态、内部构型、沉积方式等,将其进一步分为复合型、侧向迁移型、垂向加积型和孤立型水道。研究区渐新统自南向北重力流沉积特征存在差异: 盆地南部以小规模孤立型水道、朵叶沉积为主,呈近SW-NE向展布;盆地中部以复合型、垂向加积型、侧向迁移型水道、朵叶沉积为主,整体呈NW-SE向展布;盆地北部以侧向迁移型水道、堤岸及朵叶沉积为主,展布方向与中部基本一致。针对南北差异,以源-汇系统耦合关系研究为主旨,对盆地各部源-汇系统要素进行对比分析,结果表明,研究区深水重力流沉积体系的发育与展布主要受构造运动(构造抬升、洋中脊扩张运动、断层活动)、供源体系、陆架—陆坡地形3大因素共同控制。

Sedimentary characteristics and controlling factors of deep-water gravity flow deposits of the Oligocene in Tanzania Basin

In recent years, several large-scale gas reservoirs have been discovered in Tanzania Basin of the East Africa. However, the study of sedimentary characteristics, formation mechanism and main controlling factors are relatively weak. Based on drilling, logging and 2D seismic data, with the comprehensive analysis of the sedimentary characteristics, it is found that deep-water gravity-flow deposits were developed in Oligocene of Tanzania Basin. The deep-water deposits could be divided into 3 types: channels, levees and lobes, among which channels and lobes are dominated. Moreover, deep-water channels could be further divided into the complex channel, the lateral-migrational channel, the vertical-aggradational channel and the isolated channel, according to the occurrence location, external morphology, internal architecture and sedimentary style. There were variations in sedimentary characteristics of Oligocene gravity-flow deposits from south to north of the Tanzania Basin. In the south of the basin, the small-scale isolated channels and lobe deposits were developed, which have a SW-NE orientation. In the middle of the basin,complex channels, vertical-aggradational channels, lateral-migrational channels and lobe deposits were mainly developed, with a overall orientation of NW-SE direction. In the north of the basin, lateral-migrational channels, levees and lobe deposits were dominated, which have similar orientations to those of the middle part. Taking the variations among the south, the middle and the north parts of Tanzania Basin and the coupling relationships of Source-to-Sink into consideration, the elements of Source-to-Sink system is comparatively analyzed. Research suggests that the development and distribution of deep-water gravity-flow deposits in the study area are mainly affected by tectonic movements (the tectonic uplift, mid-ocean ridge spreading, and fault activity), source systems and the shelf-slope topography.