| Deep-water oil and gas exploration is one of the hot points in petroleum geology.As an important reservoir for deep-water oil and gas,deep-water fans have important practical and theoretical significance for their identification and research.China has discovered the deepwater fan system of the Pearl River Mouth Basin and the Red River Deep-water Fan,and has achieved major oil and gas breakthroughs,reflecting the important practical significance of deep-water fan and hydrocarbon reservoirs exploration in deep-water areas in the northern South China Sea.Changchang Sag is the largest second-order structural unit of Qiongdongnan Basin,which is mainly located in deep-water areas and is of great potential of hydrocarbon resources,but it has not yet been fully developed.There is a lack of systematic understanding of the distribution of oil and gas resources in this region.Based on two-dimensional and three-dimensional seismic data,this paper focuses on the sedimentary facies of the Lower Miocene Sanya Formation in Changchang Sag,putting the identification of deep-water fans in the first place so that to promote the exploration of oil and gas in the middle structural layers of the depression.Based on the existing seismic data in Changchang Sag,according to limited drilling data,high accuracy horizon calibration is finished,comprehensive interpretation of the entire region's sequence interface is carried out,and a complete isochronal stratigraphic framework of the Sanya Formation is constructed;this paper clarifies sedimentary facies type and distribution characteristics of the early Miocene;the distribution characteristics of deep-water fans in the Sanya Formation of the sag can be shown through seismic facies analysis and seismic attribute analysis.Then,the paper anatomically anatomies the sedimentary period of the deepwater fans,and clarifies the “source-conduit-sink” system in the southeast of Sanya Formation in Changchang Sag.Furthermore,on the basis of these studies,it makes a summary about major factors that control the development and distribution of the deep-water fans.Finally,it analyzes systematically about petroleum geological conditions of Changchang sag,such as the hydrocarbon source,reservoir-cap assemblages and hydrocarbon migration path system,to establish hydrocarbon accumulation patterns of deep-water fans.First of all,the paper recognizes different types of seismic facies and their distribution characteristics of Sanya Formation with the help of seismic facies analysis.Then according to the correspondence between seismic facies and sedimentary facies,it achieves the transformation from seismic facies to sedimentary facies.Finally,it clarifies the types and features of sedimentary facies of Sanya Formation.Generally,Changchang Sag mainly developed neritic facies and bathyal facies in the early Miocene,and the area of coastal facies were limited,which were around Southern uplift and Denudation area with belt-like distributed.Besides,there were large deep-water fans in the middle of sag,and small scale fan delta in the southeastern slope of the sag.There existed two deep-water fans during the sedimentary period of the upper segment of Sanya Formation.This paper anatomically anatomies the sedimentary period of the deepwater fans,and clarifies the “source-conduit-sink” system in the southeast of Sanya Formation in Changchang Sag.Specifically,the area of fan ?is 770.62km2,and fan ?'s area is 340.71km2.Based on the internal filling characteristics of channel of the upper fan,the vertical superposition relationship of the sheet-like lobes and the seismic attribute characteristics,the fan ?can be divided into three sedimentary periods,and the three period fans are characterized by the progradation from SW to NE and N.However,the fan?is divided into two sedimentary periods,and shows the characteristics of lateral migration toward N.Moreover,based on fan geometry morphology analysis,palaeognomy analysis and channel identification,the source-conduit-sink coupling mode of the fan body is analyzed.Taking all the features into consideration,including their distributions,sedimentary period and source analysis,it can be concluded that the form and distribution is mainly controlled by flexure zone,broom-like fault zone,sea-level change and Palaeo geomorphology.It is determined that the deep-water fans in Changchang Sag have the conditions of hydrocarbon accumulation and on this basis,a reservoir forming model is established.There are two sets of source rocks which have a large scale and a huge hydrocarbon generation,including the lower Oligocene Yacheng Formation and the upper Oligocene Lingshui Formation.The deep-water fans formed in the sag distribute widely and deposit thickly,overlying with thick marine mudstones of the top of Sanya Formation and Meishan Formation,so have strong sealing capacity.The faults in the research region are well developed,and most of them are oil-source faults.Therefore the oil and gas mainly migrate in vertical direction with short distance.Moreover,according to the configuration relationships between faults and deep-water fans,16 traps can be identified in the area of deep-water fans.Most traps are fault block or fault nose traps controlled by antithetic normal faults,and only a few traps belong to structural-lithologic complex traps.Through the above research,it can be shown that the deep-water fan oil and gas reservoirs superimpose vertically and connecte laterally and they all have a broad prospect for exploration. |
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