Muglad盆地Sufyan凹陷下白垩统AG组2段沉积特征与成因模式

下白垩统AG组2段(后简称AG2段)是苏丹Muglad盆地Sufyan凹陷的主力烃源岩和最重要的含油层系。本文基于岩石学、测井相、地震相和地震属性研究,对Sufyan凹陷下白垩统AG2段沉积体系进行精细研究,结合其沉积期构造特征、古地貌特征、古气候环境及内物源和外物源发育特征,建立了其断陷期沉积模式。Sufyan凹陷AG组断陷初期为双断模式,南北两侧高陡断层发育规模相当,随着盆地的演化和湖平面的变化,到AG2段进入高水位期,凹陷北部逐渐演变为缓坡沉积环境,南部仍然为陡坡沉积环境。凹陷北部为Babanusa凸起和中非走滑带,为Muglad盆地的盆外物源,凹陷南部以Tomat凸起与Nugara凹陷分割,发育内物源。从钻井和岩心资料来看,凹陷北部以分选较好、细粒沉积为主,在断层下降盘发现中粗粒重力流沉积,反映了北部物源长距离搬运和长期供应的特点,据此建立了断陷后期AG2段北部曲流河三角洲沉积模式,北部物源为主力物源区;南部临近凹陷断裂带钻井揭示其发育较高砂地比的细粒沉积,显示物源供给的周期性和近源性,研究认为发育辨状河三角洲,沉积模式类似于扇三角洲,总体物源供给能力较弱,具有近源水下搬运特征。南部物源和西部物源一起,构成凹陷的次物源。本文综合编制了Sufyan凹陷AG2段沉积期的两期沉积相图,建立了立体沉积模式,认为盆地优质储层应以凹陷中部近烃源岩的区带为目标进行寻找。

The depositional characteristics and formation mechanism of Lower Cretaceous AG Formation in Sufyan sag, Muglad Basin, Sudan

The Lower Cretaceous AG2 Formation is the main source rock and dominant oil-bearing layer of Sufyan sag, Muglad Baisn, Sudan. This paper based on the study of lithology, log facies, seismic facies and seismic attribute, studied the AG2 depositional system of Sufyan sag. And analyzed its tectonic features, paleogeomorphology, paleoclimate and provenances, built the sedimentary model of rifting period. The Sufyan is a graben, faulted in the north and south part at the initial rift period, as the basin evolution and water level fluctuation, the north part of sag became gentle slope environment and the south was steep slope yet in AG2 Period. There are Babanusa uplift and Central Africa locate at the northern part of Sufyan sag, which are the extra-provenance of Muglad Basin, and divided with Nugara sag by Tomat small uplift in south direction, and developed the inner-provenance. From the drill data and core data, most of the sandstone is fine-grained and well sorted in northern part of the sag, we find turbidite of medium grained sandstone in the downthrow side, which means the gravity current and the long-distance sediment transportation. We built the AG2 meandering river delta depositional model of northrn part, and judge the north provenance is the dominant one. The drill data shows high sand gross ratio and fine-grained sediment of southern part, and found its near sediment supply and periodicity, we built braided-river delta model of the south sediment supply system. The depositional model of the south is similar with the fan-delta, but has weak sediment supply and subsea sediment transportation characteristics. The south and west are the secondary provenance. This paper compiled two sediment facies maps of AG2 in two periods, built three-dimensional depositional model, and suggest the middle structure area of the sag are favorable prospecting area, which have good reservoir and near the source rock.