Sedimentary architecture of submarine channel-lobe systems under different seafloor topography:Insights from the Rovuma Basin offshore East Africa

Seafloor topography plays an important role in the evolution of submarine lobes.However,it is still not so clear how the shape of slope affects the three-dimensional(3-D)architecture of submarine lobes.In this study,we analyze the effect of topography factors on different hierarchical lobe architectures that formed during Pliocene to Quaternary in the Rovuma Basin offshore East Africa.We characterize the shape,size and growth pattern of different hierarchical lobe architectures using 3-D seismic data.We find that the relief of the topographic slope determines the location of preferential deposition of lobe complexes and single lobes.When the topography is irregular and presents topographic lows,lobe complexes first infill these depressions.Single lobes are deposited preferentially at positions with higher longitudinal(i.e.across-slope)slope gradients.As the longitudinal slope becomes higher,the aspect ratio of the single lobes increases.Lateral(i.e.along-slope)topography does not seem to have a strong influence on the shape of single lobe,but it seems to affect the overlap of single lobes.When the lateral slope gradient is relatively high,the single lobes tend to have a larger overlap surface.Furthermore,as the average of lateral slope and longitudinal slope gets greater,the width/thickness ratio of the single lobe is smaller,i.e.sediments tend to accumulate vertically.The results demonstrate that the shape of slopes more comprehensively influences the 3-D architecture of lobes in natural deep-sea systems than previously other lobe deposits and analogue experiments,which helps us better understand the development and evolution of the distal parts of turbidite systems.

Vertical grain-size trend of mouth bar in lacustrine fan delta:Flume experiments

Field observational previously indicated a mouth bar of a fan delta could exhibit a fining-or coarseningupward trend,which bring a new challenge to the identification of mouth bar in subsurface studies due to the lack of morphological descriptions.Previous studies have indicated that effluent behavior in rivermouth system can affect the vertical grain-size trend of mouth bar,but the drivers and magnitude of this phenomenon are not understood.We conducted flume experiments to investigate the mechanism and controlling factors of vertical grain-size trend of mouth bar.Experiment with a steeper slope of the substrate layer,greater discharge,higher sediment/water ratio,and coarser sediment induced a finingupward trend of mouth bar,because the effluent was dominated by strong inertia.Mouth bar in the experiment with a gentler slope of the substrate layer,smaller discharge,lower sediment/water ratio,and finer sediment exhibited a coarsening-upward trend dominated by the friction-dominated effluent.The relationship between the vertical grain-size trend of mouth bar and the gradients of foreset bedding in small-scale flume models and the cut-off of 15°-18°are applicable in natural systems.Identifying depositional setting to infer depositional process in river-mouth system and analyzing the plane geometry of sandbodies are two steps in the interpretation of ancient fan deltaic rock record.

First-order multiples imaging aided by water bottom

Surface-related multiples frequently propagate into the subsurface and contain abundant information on small reflection angles.Compared with the conventional migration of primaries,migration of multiples offers complementary illumination and a higher vertical resolution.However,crosstalk artifacts caused by unrelated multiples during reverse time migration(RTM)using multiples severely degrade the reliability and interpretation of the final migration images.Therefore,we proposed RTM using first-order receiver-side water-bottom-related multiples for eliminating crosstalk artifacts and enhancing vertical resolution.We first backward propagate the first-order receiver-side water-bottom-related multiples using a water-layer model,followed by saving the upper boundary wavefield.Then we produce the source wavefield using a seismic wavelet and the receiver wavefield by back-extrapolating the saved boundary.Finally,the cross-correlation imaging condition is applied to generate the final image.This method transforms the receiver-side multiples into primaries,followed by the conventional migration processing procedures.Numerical examples using synthetic datasets demonstrate that our method significantly enhances the imaging quality by eliminating crosstalk artifacts and improving the resolution.

Sedimentary-tectonic interaction on the growth sequence architecture within the intraslope basins of deep-water Niger Delta Basin

This paper presents a 3D seismic-based case study from the deep-water Niger Delta Basin to investigate sedimentary-tectonic interaction on growth sequence architecture within the thrust-related intraslope or piggyback basins. Gravitational contraction in the lower continental slope had yielded a series of thrust faults and associated folds in the study area, which formed several piggyback basins. These basins were filled by a suite of growth sequences with varying stratigraphic architecture. Analysis of the 3D seismic data recognized three primary seismic facies types respectively as: convergent, draping and chaotic, which contain seven subtypes. These facies types are combined to form different filling successions for convergent or chaotic growth sequences. The convergent growth sequences mainly occur in the deep section of basin fills during strong gravitational deformation, and always began with convergent-baselapping strata succeeded by convergent-thinning strata, representing pond-to-bypass transition in the ponded-basin accommodation space. The chaotic growth sequences mainly occur in the shallow section of basin fills in response to weak gravitational deformation, and usually began with debris-flow deposits succeeded by channel-levee complexes, reflecting dominant erosion-bypass processes in the slope accommodation space. A dynamic fill-andspill model considering relationship between episodic sedimentation rate and structural growth rate is proposed to explain the formative mechanisms of growth strata units and associated successions. Interaction between glaciation or deglaciation and sea-level change and gravitational deformation history are suggested to be the factor which resulted in the complex stratal stacking patterns, including progradational or retrogradational stacking patterns within convergent growth sequences, and progradational stacking patterns within chaotic growth sequences.

Research on the architecture of submarine-fan lobes in the Niger Delta Basin,offshore West Africa

As one of the most important reservoirs in submarine fan, submarine lobes are hot fields for the deep-water oil-gas exploration in the world. Although a large amount of researches have been carried out on the architecture model of submarine lobes throughout the world, some controversies still exist on aspects such as the differential distribution of composite patterns and the quantitative relationships among different sedimentary settings. This paper, which took an oilfietd in the Niger Delta Basin offshore West Afnca as an example and utilized abundant core, welt-logging and seismic data comprehensively, established the semi- quantitative to quantitative architecture model for individual lobe and lobe complex. Results show that （1） A lobe complex was composed of multiple individual lobes as the compensational stacking pattern. According to the stacked relationship among individual lobes, four types of compensational stacking pattern were classified as the inordered type, the lateral migration type, the retrograding type and the prograding type. Influenced by the laterally confined degree of palaeotopography, the basin floor fan lobe complex was easily superimposed as inordered type, showing relatively large lateral width but relatively small thickness; the slope fan lobe complex was easily superimposed as lateral migration, retrograding and pro- grading types, showing relatively small lateral width but relatively large thickness. （2） Influenced by the laterally confined degree of palaeotopography, the basin floor fan individual lobes showed relatively large planar distribution area but relatively small thickness, while the slope fan individual lobes showed relatively small planar distribution area but relatively large thickness. An individual lobe was composed of multiple lobe stories by the way of aggradation-progradation pattern. From proximal to middle and distal part of individual lobes, the bottom mass-transport-deposits and the top branch channels disappeared gradually, and the amalgamated degree of the middle tabular sandbodies weakened as well with gradually developing of muddy interlayers. The formation of an individual lobe generally includes three stages as ＂rapid accumulation-progradation-aggradation＂.

Quantitative lithofacies palaeogeography

Quantitative lithofacies palaeogeography is an important discipline of palaeogeography.It is developed on the foundation of traditional lithofacies palaeogeography and palaeogeography,the core of which is the quantitative lithofacies palaeogeographic map.Quantity means that in the palaeogeographic map,the division and identification of each palaeogeographic unit are supported by quantitative data and quantitative fundamental maps.Our lithofacies palaeogeographic maps are quantitative or mainly quantitative.A great number of quantitative lithofacies palaeogeographic maps have been published,and articles and monographs of quantitative lithofacies palaeogeography have been published successively,thus the quantitative lithofacies palaeogeography was formed and established.It is an important development in lithofacies palaeogeography.In composing quantitative lithofacies palaeogeographic maps,the key measure is the single factor analysis and multifactor comprehensive mapping method—methodology of quantitative lithofacies palaeogeography.In this paper,the authors utilize two case studies,one from the Early Ordovician of South China and the other from the Early Ordovician of Ordos,North China,to explain how to use this methodology to compose the quantitative lithofacies palaeogeographic maps,and to discuss the palaeogeographic units in these maps.Finally,three characteristics,i.e.,quantification,multiple orders and multiple types,of quantitative lithofacies palaeogeographic maps are conclusively discussed.