Study On Seismic Sedimentology And Accumulation Conditions Of Meishan Formation In Lingshui 15-2 Block,Qiongdongnan Basin

Nowadays,the deepwater exploration is becoming an important field for the oil and gas exploration around the world.Qiongdongnan Basin,an important exploration target in the northern South China Sea,has good exploration potential now.Several large gas fields,including LS17-2 and LS13-2,have been discovered in the Lingshui Sag which located in the deepwater area of Qiongdongnan Basin,which proves that Lingshui Sag is a hydrocarbon-rich sag with abundant oil and gas.Meanwhile,it means that the middle-deep layers of Lingshui Sag still has great potential for oil and gas exploration.Based on the 3D seismic data,this paper analyzes the sedimentary characteristics and reservoir forming conditions of the Meishan Formation in the northern Lingshui Sag,in order to provide a foundation for the subsequent exploration and development of middle-deep reservoir in the deepwater area of the Qiongdongnan Basin,South China Sea.The drilling data are insufficient but the seismic data are abundant in the Lingshui 15-2 block,so based on the fine interpretation of seismic data,and with the help of two seismic work area in the north and south of adjacent regions and parts of the drilling data,combining previous study results,this thesis mainly studied the sedimentary facies analysis,detailed anatomy"source to sink"system elements,and further research in oil and gas accumulation conditions of the strong reflection of lithologic body developed in the Meishan Formation,Lingshui 15-2block.The results of sedimentary facies analysis show that the submarine fan in the semi-abyssal sedimentary environment is developed in the Meishan Formation in the north of Lingshui Sag,which is distributed as a lobe shape,with a total area of about 710km~2.The east branch of submarine fan is consistent with the strong reflection lithology in Lingshui 15-2 block.The submarine fan is composed of channels and lobes,in which the channels are of strong amplitude subparallel sheet-shaped reflective seismic facies and lobes are of medium amplitude subparallel sheet-shaped reflective seismic facies.The results of"Source to Sink"system analysis show the sedimentary process of submarine fan.During the sedimentary period of Early Meishan Formation,a delta developed in the Lowstand System Tract of the northern Lingshui Sag,which formed a certain scale of gravity flow sediments due to own gravity and the slope gradient.The gravity flow sediments migrated through the incised channe,and erosional undercutting occurs in the process of migration,resulting in the evolution of the incised channel shape from"U"to"W".The incised channe,as the sediment transport channel,directly passes through the continental slope and extends to the semi-abyssal environment of Lingshui Sag,the gravity flow sediments are deposited directly into the stable environment of deepwater,forming the lobular and planar submarine fan.At the same time,combined with seismic profile tracing and root mean square amplitude attribute analysis along the layer,the submarine fan is characterized into four stages.Due to the difference of sediment supply and migration path,the sedimentation range of the four stages of submarine fan is different,but generally presents the feature of progressive deposition.Combined with previous research results,the reservoir forming conditions of Lingshui sag were analyzed,and the reservoir-forming model of submarine fan in Lingshui15-2 block was established:the Yacheng Formation and Lingshui Formation in the Lingshui Sag developed two hydrocarbon source rock.Based on the analysis resulys of organic geochemical,oil-source implication and thermal evolution of hydrocarbon source rock,this thesis ensure the hydrocarbon source rock of the Yacheng Formation is the main hydrocarbon generation layer,and entered the stage of dry gas in the sedimentary period of Yinggehai Formation(Ro>2.0%),the generated gas through low intensity activity of fault,unconformity surface and interlaminar fracture in conducting system continued migration to submarine fan of the Meishan Formation.With the end of fault activity during the sedimentary periood of the Yinggehai Formation,the fault and mudstone blocked off the submarine fan reservoir of Meishan Formation,so the internal gas does not escape any more,and forming the gas reservoir composed of the structure-lithologic compound trap.