The Development Feature And Development Mechanism Of Deep Water Gravity Sediment

Deep water sediment clastic reservoirs plays a more and more important role in petroleum exploration nowadays.More and more discoveries in this exploration region,attract researchers interests in it.On the other hand,the progress of the theory and technology on deep water sediment during decades,brought out more and more oil and gas discoveries.Most of the international oil and gas companies aimed to his exploration region,and deployed plenty of exploration activities all around the world.The giant and huge reserves guaranteed considerable revenue for these companies,even most of the reserves distributed in deep and ultra-deep water area.It is predicted that the deep water reserves and productions will be continue increasing in future for a long time,and occupy a very important status in petroleum exploration.Thus,more and more investment from these companies will pour into deep water region.During this period,Rovuma Basin in East African offshore revealed great exploration potential and reserves.As a "hot pot" of petroleum exploration,plenty of seismic data,well data and other geological and geophysical data were acquired in this area,provided advances for the deep water sediment research in this area.Based on the deep water sediment theory and the modeling from outcrop in field trip,combining with the dataset in study area,especially high resolution 3D seismic data,we identified and analyzed deep water sediment system in East African offshore.We focus on the deep water sediment system which developed in Late Eocene,including its origin,source supply,evolution feature and et al.The study area developed multiple deep water sediment systems in Tertiary,composed of several canyon-channel systems,which acted sediment path for the current and sediment materials.The source supply could be from paloe-Rovuma deltas.Because of narrow shelf and steep slope in East African continent margin,the deep water sediment systems were quit straight,while transported and deposited coarse grained sediment.The deep water sediment system belongs to proximal sand rich sediment system developed in steep slope,which could be defined to "linear proximal sand-rich deep water system in steep slope environment".Upper Eocene sediment system,which is located in the center of study area,could be divided into 6 stages.From 1st to 6th,the sediment system flowed from west to east,with the migration direction from north to south.These 6 channel systems developed similar feature,parallel to each other.There was obvious margin and boundary between two adjacent channels,where the younger channel incised the elder channel.Among the 6 channels,4 of which develop with a lobe,correspondingly marked as lobe-3,lobe-4,lobe-5 and lobe-6,while lobe-1,love-2 are missing.Part of channel-1,channel-2,and channel-3 are also missing.Different elements in deep water system have different geophysical response.On seismic profiles,deepwater channel complex deposits expressed U-or V-shaped feature.The lower part of deepwater channel complex deposits corresponds to low-amplitude,poor-continuity and chaotic seismic reflection signatures,probably indicating slump-debris flow deposits,while the upper part is characterized by strong-amplitude,poor to moderate continuity seismic reflection signatures.The well logging curves of channel sand were characterized by low box-shaped Gammar(GR)value and high resistivity value.The shale content is quite low,which could achieve a Net Gross Ratio(NGR)of 0.97.Lobes expressed flat,continuous and high amplitude features in seismic sections,while serrated shape of ascending amplitude and high frequency in GR curve and descending amplitude and also high frequency in resistivity curve in well logging data.This feature indicated normal grading character,which reflected deceasing energy of the deep water current.This feature represented the open environment in the mouth splays and lobes which located in the basin floor.The NGR generally dropped down to 0.58 in lobes.Several factors controlled or effected the deep water sediment system in study area,however,in which the main factor should be seismicity in East Africa.The development of channel-lobe system related to the tectonic uplifting,which implied tectonic movements controlled the source supply and deep water current activities.The paleo-Rovuma deltas provided abundant source for giant Eocene sediment system.The uplifting episodes response to the development of channel-lobe complexes.The carbonate debris low and calcite cement sandstone from west indicate the strong uplifting during Eocene.The carbonate and calcite cement sandstone could collapse in this tectonic background,transported to the basin floor by high energy turbidity currents and debris flows.Comparing with the straight channels,the lobes expressed different strike from Northeast to Southeast,where showed U shape on the plain view from channel to lobe.The feature implied the high geometry of Northwest of complex constrained the accommodation space for channels and lobes.The transitional point between channel and lobe also played a important role for the blend feature developed here.The straight feature also related to the steep slope and coarse clasts in turbidity currents.The steep slope usually caused steep deep water systems because of high energy.Meanwhile,the coarse clasts were apt to incise the seafloor and deepen the channel,also caused the straight feature of this system.The paleo-Rovuma Deltas provided source for the deep water sediment system,however,the clasts of Rovuma Delta supplied from Mozambique Fold in the west.Based on lithologic analysis,the granite and gneiss is the parent rock for the sandstones.These crystallized rock experienced uplifting movements and weathering activities during Eocene.Thus,the huge amount of clasts were transported to the offshore in the east area,then the Rovuma Delta formed firstly.During special period,the gravity current occurred in this area under the tectonic events,which caused the unstable environment.The gravity current easily transported the plenty of clasts from shelf and deltaic area to the basin floor along the canyons and channels.The clasts will deposit in channels and lobes when the current energy decreased finally.Thus,the Mozambique Fold provided the source for deep water sediment system basically.