底流在陆缘深水环境下广泛存在,可对深水沉积过程及砂体分布产生重要影响。前人对重力流与底流的交互作用机制及沉积产物开展了大量研究,但目前有关底流改造型的海底扇储层构型模式仍然研究不够深入。东非鲁伍马盆地是当前重力流—底流交互作用研究的热点地区,文中以其代表性的下始新统海底扇水道体系为例,综合岩心、测井及三维地震资料开展储层构型精细表征,建立重力流—底流交互作用下的海底扇水道体系构型模式。研究表明,目标水道体系内部发育水道、溢岸及朵叶3种构型要素,其中水道可分为水道复合体、单一水道及其内部不同级次的构型单元。底流对细粒物质的搬运可形成非对称的溢岸沉积,导致水道复合体之间呈逆底流侧向迁移叠置样式,其间泥岩隔层容易保存; 单一水道之间呈顺水道纵向迁移或逆底流侧向迁移样式,其中纵向迁移部位水道切叠连通,而侧向迁移部位容易保存泥质侧向隔挡体。受重力流沉积演化的影响,单一水道内部充填由砂泥交互型逐渐演化为富砂型,且在水道弯曲段的轴部砂体最为发育。

Bottom currents are common phenomenon in the deepwater setting of continental margins,which have important impacts on the deepwater sedimentary process and sandstone distribution. Extensive studies have been focused on the mechanisms and sedimentary products of gravity flow-bottom current interaction,but it remains much unknown regarding the submarine-fan reservoir architecture model modified by bottom currents. The Rovuma Basin offshore East Africa is a typical region to study the interaction between sediment gravity flows and bottom currents. Taking the representative submarine channel systems from the Lower Eocene as an example,this paper characterizes the reservoir architecture by integrating cores,well logs and 3D seismic data,with an aim to establish the submarine channel architecture model under interaction of sediment gravity flows with bottom currents. Results show that the target channel system developed three types of architectural elements,channel,overbank,and lobe,where channel deposits contain channel complexes, individual channels,and other smaller hierarchical units. Fine-grained materials could be transported by bottom currents forming asymmetrical distribution of overbanks deposits. This forced channel complexes to migrate laterally against bottom currents,with shale barriers easily preserved. Individual channels show downstream migration or lateral migration against bottom currents. In downstream migration,channels incised each other forming good sand connectivity,while indined shale baffles may develop in lateral migration. As influenced by sediment gravity flows,individual channel-fills evolved from mixed sand-shale to sand-prone channels,and in the mapview sandbodies are mostly distributed in the axis of channel bends.

Extensive 3D seismic datasets acquired during exploration offshore southern Tanzania have revealed the complex architecture of two contrasting styles of hybrid turbidite–contourite deposits that formed in the Cretaceous (Albian–Early Campanian) and Paleogene (Paleocene–Oligocene). Both sequences are characterized by migrating channel-levee complexes, interpreted to record, and be diagnostic of, the synchronous interaction of eastward, downslope flowing turbidity currents and northerly, along-slope flowing contour currents. Flow stripping of the fine-grained suspended part of the turbulent flow by weak contour currents led to the formation of expanded levee-drifts on the northern (downstream) side of the channels, which prograded southwards (upstream), driving southwards migration of the turbidite channel axis.

In recent years, several huge natural gas reservoirs have been discovered consecutively in Paleogene deep-water sedimentary sand bodies of deep-water regions in Rovuma Basin of the East Africa, and this basin has been a hot spot for global natural-gas exploration. However, due to the low exploration degree of Rovuma Basin and the relatively lagging research on deep-water sedimentary sand bodies, the further hydrocarbon exploration is seriously restricted. Based on cores, logging and 3D seismic data, the Oligocene sequence stratigraphic framework of the continental-slope deep-water regions in Rovuma Basin of the East Africa is established. Meanwhile, through the analysis on water channel-lobe characteristics in combination with regional geological data, the controlling factors of the rich-sand deep-water sediments such as water channel and lobe are discussed to establish the corresponding sedimentary model. The Oligocene Rovuma Basin is divided into three third -order sequences from bottom to top, i.e., SQ1, SQ2 and SQ3, and the channel-lobe sediments are developed in the low-stand system tract of each sequence. In SQ1 and SQ2 of the study area, the channel-lobe sediments are developed, while SQ3 is dominated by channel sediment. Affected by the East Africa continent uplift and global "icehouse" climate at the end of Eocene, the basin provenance supply was increased and the deltas prograded seawards, leading to the gravity flow sediments such as continental slope channel-lobe. The channel-lobe sediment is reworked by Antarctic bottom water flowing northwards, and the sand-stratum ratio of channel-lobe is increased by washing gravity flow sediment to form lateral accretion bodies and drift bodies on the north side of channel, while the late gravity flow sediment is restricted, resulting in the southward migration of channel and the southward extension of lobe. The research results can provide a certain theoretical basis for the prediction of deep-water reserving sand bodies and hydrocarbon exploration in Rovuma Basin and even the whole East African continental margin basin.

According to the 2D and 3D seismic data from the deepwater area, it is interpreted that several stages of tectonic-sedimentary evolution have undergone in Rovuma Basin. Fluvial and lacustrine sedimentary environments developed during Permian to Early Jurassic Gondwana intracontinental-intercontinental rifting. Marine-continental transitional sedimentary environment developed during Middle Jurassic to Early Cretaceous Madagascar drifting. Deep-water gravity flow deposits widely developed in the slope, and extended to Davie Ridge, during Late Cretaceous to Oligocene passive continental margin stage. Deep-water gravity flow and abyssal argillaceous deposits developed in both the slope and Kerimbas Graben during Miocene to Quaternary faulting of the East African Rift System Sea Branch. A“fault-depression-fault” structure pattern that formed in vertical in the basin just brought from the important Permian-Early Jurassic and Miocene-Quaternary riftings. The marine mudstone deposited during Madagascar drifting is the chief source rock and the Paleogene deepwater turbidite sand body in the slope acts as the main reservoirs. The faulting of East African Rift System connects the underlying source rocks with Paleogene reservoirs in the western slope, thus oil and gas accumulate generally in the western slope where faults have hardly developed late.

Based on the latest seismic and drilling data, the formation conditions of gas reservoir and the controlling factors of gas accumulation of the Rovuma Basin in East Africa have been analyzed. The results show that the Rovuma Basin has excellent conditions for hydrocarbon accumulation. In the study area, the source rock is mud shale of Permian-Triassic, Jurassic and Cretaceous which developed in the rift period, and the reservoir is the Paleogene turbidite sand, and the regional caprock has several sets of the marine mudstone widely distributed in Paleogene. It is concluded that the tectonic activities controlled the oil and gas accumulation in this area, and good caprock conditions are the key factor for hydrocarbon accumulation and preservation. It is pointed out that the relatively stable area, especially the continental slope area with weak tectonic deformation during the latest extensional deformation is a priority exploration region, and the optimal selection of exploration block should avoid the area with large scale faults that extend to sea floor, and the regional caprock conditions should be evaluated emphatically.

东非被动大陆边缘盆地近年来发现了一系列大气田，主要分布于鲁武马盆地，其可采储量达3.8×1012 m3。盆地经历了卡鲁裂谷期、马达加斯加裂谷期和马达加斯加漂移期等3期构造演化。其中，马达加斯加裂谷期形成下侏罗统—中侏罗统湖相滨浅海相烃源岩，于早白垩世进入生油门限，晚白垩世达到生油高峰，渐新世进入生气窗。受马达加斯加漂移期东非陆上断裂系统活动影响，大量富砂沉积物以块体搬运和深海滑塌方式向深水区堆积，形成了盆地内面积广、厚度大、岩性相对均一且物性良好的超大型深水重力流沉积砂体；漂移期海相泥页岩则为良好的区域性盖层。东非陆上早期隆升和三角洲进积作用使鲁武马盆地形成重力滑动和盐底辟构造，渐新统—上新统内形成东非被动陆缘正断层带(EANFZ)和逆冲断层带(EATFZ)，下伏中—下侏罗统烃源岩生成油气沿正断层和深水区逆冲断层向上运移，聚集于构造岩性地层、构造圈闭中，形成大气田。

Many giant gas fields, concentrated in the Rovuma Basin of the East Africa passive margin basins, were discovered in recent years with a total recoverable reserves up to 3.8 trillion cubic meters. The Rovuma Basin has gone through three stages of tectonic evolution, i.e. the Karoo rift, Madagascar rift and Madagascar drift. The LowerMiddle Jurassic lacustrine and shallow sea source rocks formed in the Madagascar rift period, and came to the oil window, the peak of oil generation and the gas window, in the Early Cretaceous, the Late Cretaceous and the Oligocene, respectively. Affected by the East Africa onshore faults during the Madagascar drift period, massive sandrich sediments accumulated towards the deep water region by way of block transport and deep water sliding. It formed a super giant deepwatergravityflow sandbody, wide and thick, with relatively uniform lithology and favorable physical property within the basin. The marine mudstone during the Madagascar drift period provided a good regional overlying strata. The early uplift onshore East Africa and delta progradation led to the formation of gravity sliding and salt diapir within the Rovuma Basin, and subsequently the East Africa normal fault zone (EANFZ) and the East Africa thrust fault zone (EATFZ), between the Oligocene and Pliocene. As a result, hydrocarbons from the underlying LowerMiddle Jurassic source rocks migrated upward along the normal and deep water thrust faults, accumulating in the structurallithologicalstratigraphy and structural traps to form the giant gas fields.<br>

以层序地层学经典模式为指导,利用岩心、测井、录井及地震资料,总结了东非鲁伍马盆地深水区中始新统三级和四级层序界面特征,将深水沉积中发育的水道-朵体复合体与沉积相对应,划分了沉积亚相和微相,并在此基础上探讨了深水沉积的演化规律及其对储层的影响。研究结果表明:①鲁伍马盆地中始新统三级层序顶界位于凝缩段和偶发的碳酸盐碎屑流顶部,底界为逐期南向迁移的重力流底界;四级层序由半深海泥岩顶界和地震剖面上连续性好的沉积界面确定,但仅可以在水道-朵体复合体分布范围内开展解释。②研究区深水沉积可识别出水道复合体和朵体复合体2种沉积相,复合水道、朵体、决口扇和溢岸/漂积沉积4种亚相,水道轴部/边部充填、内天然堤、块体搬运沉积(MTD)、水道底部滞留沉积、朵体单元主体/边缘、决口扇和溢岸/漂积沉积等9种沉积微相;决口扇和溢岸/漂积沉积均分布于复合水道的北侧,受底流影响的决口扇在平面上呈向北发散的脉状。③研究区中始新统深水沉积的演化分为SQ1—SQ4共4个阶段,整体表现为先进积、后退积的过程,水道-朵体复合体受重力流与底流交互作用影响,逐期向南迁移。④研究区储层的发育主要受控于沉积微相,朵体单元主体和水道轴部充填微相发育的储层品质好,其中朵体单元主体微相中储层最发育、物性最好,孔隙度为13.00%~21.00%,渗透率为5.0~118.0 mD,水道轴部充填微相次之,储层孔隙度为13.00%~19.00%,渗透率为0.8~23.0 mD;溢岸/漂积沉积微相中发育的储层物性差,决口扇不发育储层。

Guided by the classic model of sequence stratigraphy,using core,welllog,mudlog and seismic data,the characteristics of the third-order and fourth-order sequence boundaries of Middle Eocene in the deep-water area of Rovuma Basin in East Africa were summarized. Corresponding to channel-lobe complexes in deep-water depo-sits,sedimentary subfacies and microfacies were divided. Based on this,the evolution laws of deep-water deposits and the influence of sedimentary microfacies on reservoirs were explored. The results show that:（1）The top boundary of the third-order sequence of Middle Eocene in Rovuma Basin is located at the top of the condensed section and occasional carbonate debris flow,and the bottom boundary is located at the bottom of the gravity flow that migrates southward in stages. The fourth-order sequence is determined by the top boundary of the bathyal deposit and the sedimentary interface with good continuity on the seismic section,but can only be interpreted within the distribution range of the channel-lobe complexes.（2）Two sedimentary facies,including channel complex and lobe complex,have been identified in deep-water deposits in the study area. The two sedimentary facies can be subdivided into four subfacies:composite channel,lobe,crevasse splay,and overbank/drift deposits. There are nine microfacies,including channel axis/edge filling,internal natural levee,mass transport deposits（MTD）,channel bottom lag deposit,lobe element main body/edge,crevasse splay,and overbank/drift deposits. Both crevasse splay and overbank/drift deposits are distributed in the northern part of the composite channel,and the crevasse splay affected by bottom flow is in a northward divergent vein shape.（3）The evolution of deep-water depo-sits of Middle Eocene in the study area can be divided into four stages,namely SQ1-SQ4 in sequence,showing a process of progradational deposition and retrograde deposition as a whole. Affected by the interaction between gravity flow and bottom current,the channel-lobe complexes gradually migrated southward.（4）The development of reservoirs in the research area is mainly controlled by sedimentary microfacies. The main body of the lobe element and the axis of the channel developed high-quality reservoirs. The reservoirs are the most developed and the physical properties are the best in the main body of the lobe element,with a porosity of 13.00%-21.00% and a permeability of 5.0-118.0 mD. The reservoir properties at the axis of the channel are secondary,with a poro-sity of 13.00%-19.00% and a permeability of 0.8-23.0 m D. The reservoir properties of overbank/drift deposits are poor,and the reservoirs in the crevasse splay are not developed.

The interaction of deep-marine bottom currents with episodic, unsteady sediment gravity flows affects global sediment transport, forms climate archives, and controls the evolution of continental slopes. Despite their importance, contradictory hypotheses for reconstructing past flow regimes have arisen from a paucity of studies and the lack of direct monitoring of such hybrid systems. Here, we address this controversy by analyzing deposits, high-resolution seafloor data, and near-bed current measurements from two sites where eastward-flowing gravity flows interact(ed) with northward-flowing bottom currents. Extensive seismic and core data from offshore Tanzania reveal a 1650-m-thick asymmetric hybrid channel levee-drift system, deposited over a period of ∼20 m.y. (Upper Cretaceous to Paleocene). High-resolution modern seafloor data from offshore Mozambique reveal similar asymmetric channel geometries, which are related to northward-flowing near-bed currents with measured velocities of up to 1.4 m/s. Higher sediment accumulation occurs on the downstream flank of channel margins (with respect to bottom currents), with inhibited deposition or scouring on the upstream flank (where velocities are highest). Toes of the drift deposits, consisting of thick laminated muddy siltstone, which progressively step back into the channel axis over time, result in an interfingering relationship with the sandstone-dominated channel fill. Bottom-current flow directions contrast with those of previous models, which lacked direct current measurements or paleoflow indicators. We finally show how large-scale depositional architecture is built through the temporally variable coupling of these two globally important sediment transport processes. Our findings enable more-robust reconstructions of past oceanic circulation and diagnosis of ancient hybrid turbidite-drift systems.

Turbidity currents and contour currents are common sedimentary and oceanographic processes in deep-marine settings that affect continental margins worldwide. Their simultaneous interaction can form asymmetric and unidirectionally migrating channels, which can lead to opposite interpretations of paleocontour current direction: channels migrating against the contour current or in the direction of the contour current. In this study, we performed three-dimensional flume-tank experiments of the synchronous interaction between contour currents and turbidity currents to understand the effect of these combined currents on channel architecture and evolution. Our results show that contour currents with a velocity of 10–19 cm s−1 can substantially deflect the direction of turbidity currents with a maximum velocity of 76–96 cm s−1, and modify the channel-levee system architecture. A lateral and nearly stationary front formed on the levee located upstream of the contour current, reduced overspill and thus restrained the development of a levee on this side of the channel. Sediment was preferentially carried out of the channel at the flank located downstream of the contour current. An increase in contour-current velocity resulted in an increase in channel-levee asymmetry, with the development of a wider levee and more abundant bedforms downstream of the contour current. This asymmetric deposition along the channel suggests that the direction of long-term migration of the channel form should go against the direction of the contour current due to levee growth downstream of the contour current, in agreement with one of the previously proposed conceptual models.