东非海岸盆地渐新世浊积体沉积特征及油气勘探意义

Sedimentary characteristics of Oligocene turbidite and its significance for hydrocarbon exploration in coastal basins of East Africa

深水浊积砂体是东非海岸油气勘探的重要领域。利用钻井、测井及地震资料,对东非海岸的鲁伍马和坦桑尼亚盆地渐新统浊积体的类型和展布特征进行了分析,从"源—汇"系统的角度探讨了大型浊积体差异分布的控制因素。研究表明:研究区深水浊积体的沉积类型多样,可划分为两类5种不同的沉积类型。受河流—三角洲物源体系的控制,鲁伍马和坦桑尼亚盆地形成4大"源—汇"体系。物源供给和地形(陆架宽度及陆坡坡度)是浊积体差异分布的主控因素,鲁北和坦中地区深水浊积体总体规模最大,其次为坦北,坦南地区最小。鲁北为窄陆架—陡陆坡背景下的大型河流供给体系,深水区发育由水道和朵叶组成的大型浊积体。坦南为窄陆架—陡陆坡背景下的小型多源供给体系,以孤立型水道和朵叶构成的小型浊积体为主。坦中为宽陆架—缓陆坡背景下大型河流供给体系,深水区发育由水道和朵叶组成的大型浊积体,其规模与鲁北地区相当。坦北地区为宽陆架—缓陆坡背景下中型多源供给体系,但物源规模相对鲁北和坦中地区较小,浊积体规模中等。深水浊积砂体规模控制了东非海岸深水区岩性或构造-岩性圈闭的大小,进而控制了天然气藏的规模,因此,寻找大型深水浊积砂体是东非海岸天然气勘探的关键。

Deepwater turbidite sand body is an important field of oil and gas exploration in East Africa coast.Based on drilling,well logging and seismic data,the types and distribution characteristics of Oligocene turbidites in Ruvuma and Tanzania basins along East Africa coast are analyzed,and the controlling factors of differential distribution of large turbidites are discussed from the perspective of “source to sink” system.The study shows that the deep-water turbidites in the study area are diverse and can be divided into two kinds of five different sedimentation types.Controlled by river-delta provenance system,Ruvuma and Tanzania basins form four “source to sink” systems.Provenance supply and topography(shelf width and slope gradient) are the main controlling factors for the differential distribution of turbidites.The overall scale of deep-water turbidites in northern Ruvuma and central Tanzania are the largest,followed by that in northern Tanzania,while that in southern Tanzania is the smallest.Northern Ruvuma is a large river supply system under the background of narrow shelf and steep slope.Large turbidites composed of channel and lobes are developed in deepwater areas.Southern Tanzania is a small multi-source supply system under the background of narrow shelf and steep slope,which is dominated by small turbidites composed of isolated channels and lobes.Central Tanzania is a large river supply system under the background of wide shelf and gentle slope,while large turbidites composed of channels and lobes are developed in deepwater area,which has similar scale to that in northern Ruvuma.Northern Tanzania is a medium-sized multi-source supply system under the background of wide shelf and gentle slope,but its provenance scale is smaller than that of northern Ruvuma and central Tanzania,and its turbidite scale is medium.The scale of deepwater turbidite sand body controls the size of lithologic or structural-lithologic trap in the deepwater area of East Africa coast,and then controls the scale of natural gas reservoir.Therefore,finding large-scale deepwater turbidite sand body is the key to natural gas exploration in East Africa coast.