Study On Depositional System Of Sequence SB23.8in Baiyun Sag Of Northern South China Sea

Baiyun sag locates in the northern continental slope deepwater area of the SouthChina Sea. Baiyun sag which attaches to Zhu II depression in Peal River Mouth Basin.The area of study region is4000km2which sits in the east of Baiyun sag. Target zoneSB23.8which locates in the upper part of Zhuhai Formation. The sequence is namedSQ6which is between ZHSB6(about SB25.2) and SB23.8.By counting the velocity characteristics of sandstone and mudstone of W1-W17wells in SQ6sequence, we summarizes the change regulations of sandstone andmudstone velocity with the increasing depth. Sandstones are divided into type I, typeII and type III. The velocity of type I is more concentrated on the1200-2200m/s, thethreshold depth of bright spot is about2000m. Gas horizon can be detected by brightspot method above2000meters. Buried depth of type II is more concentrated on the1700-2200m, a few are in the range of3000-3500m. Type II sandstones’ velocity aresimilar to mudstone in the same depth. Velocities of type III sandstones are obviouslyhigher than surrounding rocks, because of higher gray content. Velocity values of typeIII are more concentrated in the4500-5500m/s. The increasing velocity of type IIIwith depth variation is not obvious.We selected W1-W11wells and made one-dimensional convolution model instudy area. By waveshape analysis, we summaried wave reflection characteristics ofcorresponding sedimentary subfacies and sedimentary microfacies. We dividedsedimentary facies in single well. Sedimentary facies analysis results of W1-W11wells in the SQ6sequence are as follows, W1well mainly developed delta frontsubfacies. The lower part of W2well mainly developed mixed continental shelf facies,in the upper part W2well developed delta front subfacies. W3well was determined asdelta plain facies. W4well developed delta and coastal deposition. W5wells developed delta front. W6wells developed mixed continental shelf facies. W7welldeveloped delta front and plain facies. The lower part of W8well was deltaic facies, middle part was shelf and underwater fan environment, the upper part was delta frontsubfacies. The W9well was mainly delta front. The W10wells mainly developedslope fan facies. W11well mainly developed restricted platform. Through the analysisof sedimentary facies in W1-W11well, sedimentary environment beside wells wouldbe concluded by similar seismic records.Four lines which were in east of Baiyun sag liked crossline985, crossline2605,crossline3625and ilnline2475were applied in the seismic drawing line technologyand they were maked to restore the sedimentary basin prototype of SQ6. We identifiedthe location of the shelf break points and projected them onto the plane. Finally wecan determine roughly the planar distribution of shelf-break zone. By it we canidentify the plane distribution of delta plain, delta front, slope fan, floor fan andlimestone uplift.Based on interpretation and repeated revising of ZHSB6and SB23.8sequenceinterface, We encrypted interfaces to1×1grid and extracted RMS amplitude attributebased on stratal slicing. Combined with recovery results of sedimentary basinprototype, waveshape analysis and sedimentary facies analysis of single well, we willturn seismic facies into sedimentary facies effectively and analyse paleocurrentdirection.4main water system were found in SQ6sequence, in which3water systemwere in the northwest direction, another is the north or NNE direction. Sedimentaryenvironment of SQ6sequence was delta-slope fan-basin floor fan.Based on the artifical scan interpretation of lowstand sand named S-1within theSQ6sequence, S-1developed in the perpendicular to the flow direction, liked abanded distribution in plane, and the high quality reservoir located more in thewestern region.