Analysis On Sedimentary System Of SQ5in Zhuhai Formation, Northeast Of Baiyun Depression, Pearl River Mouth Basin

The Baiyun sag, located in ZhuⅡ depression of Pearl River Mouth basin,continental slope of northern South China Sea, is the thickest, largest and the mosteconomic potential depression with water depth of200~2000m. This paperinvestigates SQ5of Zhuhai Formation in the northeast of Baiyun sag, taking fulladvantage of drilling data, based on4800km23D seismic data, by studying theskeleton sand body identification based on the waveform analysis in the framework,RMS amplitude extraction of the slice in SQ5, restoration of sedimentary basinprototype by line drawing of9skeleton survey lines which pass important structureand drilling, sedimentary system analysis, got the follow understandings:Mudstone was mainly developed in1600~3000m burial depth, thecorresponding velocity range was3300~4300m/s; The gas-bearing sandstone wasmainly developed in1500~2500m burial depth, the velocity range was2800~3600m/s. When buried depth was same, its velocity was smaller about400m/s than shale line velocity, so it was Ⅰ-type sandstone. The seismic response oftop surface of such sandstone corresponded with trough reflection and the bottomsurface corresponded with strong amplitude peak reflection in SQ5, waveformfeatures of such sandstone was declination symmetry waveforms toward right, keypoint of recognition marks of such sand is the bottom response; The oil-bearing orwater-bearing sandstone was mainly developed in1500~3000m burial depth, thevelocity range was3000m/s~4000m/s. When buried depth was same, its velocitywas smaller about70m/s than shale line velocity, so it was Ⅱ-type sandstone, Theseismic response of such sandstone was similar with mudstone, so we can only useseismic facies to identify it; The velocity range of low porosity sandstone was3800m/s~4500m/s. When burial depth was same, its velocity was greater about200m/s than shale line velocity, so it was Ⅲ-type sandstone, The seismic response of topsurface of such sandstone corresponded with strong amplitude peak reflection,waveform features was declination symmetry waveforms toward left; The velocityrange of calciferous sandstone was4500m/s~5000m/s. The seismic response of topsurface of such sandstone corresponded with strong amplitude peak reflection inSQ5; Limestone’s velocity was more than5000m/s, its top surface’s seismicresponse was very strong amplitude peak reflection and it was easy to identity. Allkinds of sandstone were widely developed in research area and calciferoussandstone and limestone were developed in local area.RMS amplitude attribute can be used to predict sedimentary facies, becauseRMS amplitude map can show the development location and sedimentary bodyshape, it is very useful to distinguish between sandstone and mudstone. But due tothe complex underground geological conditions, it is difficult to distinguish thegeological properties of amplitude distribution zone. So it can only be used topredict sedimentary facies after restoration of ancient sedimentary environment.With the restoration of sedimentary basin prototype by line drawing of9skeleton survey lines which pass important structure and drilling, we got the shelfbreak line. It passed X-5, X-11, X-16well and located in the middle of the area.The north of shelf break was shallow water shelf area and its south was continentalslope. It was calcareous clay sedimentary area in east study area.By the research of sedimentary system in the study area, we have realized thatthe main sources came from the northwest ancient Pearl River delta, thedenudation area of Northeast uplift and volcanic cone provided some volcanicdebris, in addition included small amounts of inner debris. The ancient flow systemis complex. It developed six large delta-slope fan-basin fan sedimentary bodies inthe study area, and the overall shelf break zones had great potential.