| Deep water oil and gas is an important development trend of global oil and gas exploration with broad prospects.However,due to the scarcity of deepwater drilling,pre-drilling lithofacies prediction has always restricted the drilling success rate,especially under complex geological conditions.In the analysis of seismic lithofacies,there are generally only two rock types in the simple geological conditions: sandstone and mudstone.However,the complex geological conditions have the characteristics of various rock types(in addition to siliceous clastic rocks,there are also limestone and volcanic rocks),sedimentary facies span multiple facies belts(sedimentary facies from fluvial facies to abyssal facies),complex structure(a lot of faults and intense volcanic activity)and the inlluence of natrual gas charging,which seriously restrict the accuracy of lithofacies predition.At present,enhanced seismic lithofacies analysis is the effective method to predict the lithofacies under the complex geological conditions.This method can improve the seismic lithofacies discrimination ability by adding three indentification marks including tidiness,smoothness and special waveforms,and can recognize lithofacies directly from seismic profiles without drilling well.In this paper,the depositonal system under complex geological conditions was reconstructed for first time taking the Oligocene Series sedimentary sequence in the west region of Baiyun Sag in the north of the South China Sea as an example.The main research contents and working methods are as follows: According to velocity analysis of different rock types,lithologic profile and foward inversion of seismic records that seismic lithofacies identification marks were recognized.Using six commonly used seismic facies identification marks including position,external form,internal configuration,amplitude,frequency and continuity,as well as a total of nine seismic parameters,including tidiness,smoothness and special waveforms,to directly identify seismic lithofacies from the seismic profiles(combinations of rock types identified from the seismic profiles).The development and distribution of sandstone bodies were determined by sandstone skeleton lithofacies scanning interpretation.Optimum selection of the typical seismic profiles,and restoring the stratigraphic structure and geomorphology of five sequences in Oligocene Series(32-23.8Ma)to the depositional morphology using the restoration technique of sedimentary sequence original structural profile that the sedimentary internal structure and sedimentary environment can be analyzed.The major sedimentary environments of each sequence were classified by the coastline,the slope break line and the slope foot line,as well as the fault warping uplift landform and the volcanic cone.The main water flow channels were determined by the gully line at the top of the mound sedimentary body,and the small water flow paths were determined by the distribution of sandstone body and ancient topography.The depositional systems were reconstructed by superposition of skeleton lithofacies sandstone body,major sedimentary environment and water flow paths.The results show that:1)Eleven subdivided seismic lithofacies were identified from the seismic profiles by nine independent identifications marks.Among them,the seismic lithofacies of siliceous clastic rocks can be subdivied into five types including sandy conglomerate,sandstone intercalated with mudstone,sand and mudstone interbed,mudstone intercalated with sandstone and mudstone.The “sandstone” here consists of5 grades from conglomeratic sand to silty sand.The seismic lithofacies of carbonate rock can be subdivided into three types including calcareous sandstone,limestone-covered sandstone and limestone intercalated with mudstone.The seismic lithofacies of volcanic rock also can be subdivided into three types including volcanic agglomerates,pyroclastic sedimentary rock and volcanic lava.2)The identification path and method of complex lithofacies on continental shelf margin were established.The seismic lithofacies were divided into three categories consisting of extremely strong amplitude reflection group,weak amplitude reflection group and medium-strong amplitude reflection group:(1)Extremely strong amplitude reflection group: the gas-bearing sandstone(including the gas-bearing siltstone)presents the right downward skew-symmetrical wave,while the limestone and volcanic rocks present the left downward skew-symmetrical wave.The limestone layer is extremely smooth and continuous,while the volcanic lava is less smooth and discontinuous.(2)Weak amplitude reflection group: mudstone has a clean reflection with weak amplitude,sandstone has an unclean reflection with weak amplitude,volcanic agglomerates are distributed in the core of high steep mound sedimentary body and have an unclean reflection with weak amplitude,and pyroclastic sedimentary rocks have an extremely unsmooth laminar(dirty)reflection with weak amplitude.(3)Medium-strong amplitude reflection group: it is seismic lithofacies of three types of sandstone and mudstone with different sand-bearing rates,in which the reflection events of sand and mudstone interbed is the most and the continuity is the best.The amplitudes of sandstone intercalated with mudstone and mudstone intercalated with sandstone are medium,but the background of the former(sandstone)is an unclean reflection,and the background of the latter(mudstone)is a clean reflection.3)The plane distribution of sandstone bodies can be determined by the scanning interpretation of sandstone layers.The plane distribution of subdivision seismic lithofacies sand bodies can be described by subdivision seismic lithofacies calibration in the seismic profiles.4)Based on the coastline(the boundary line between fluvial and marine facies),the slope break line(the boundary line of shelf and slope)and the slope foot line(the boundary line of slope and basin),as well as the fault warping uplift landform and the volcanic cone,the major sedimentary environments can be clearly and accurately expressed.On the west region of Baiyun Sag,there are six major types of sedimentary environments: coastal alluvial plain,shelf,slope,basin,distal uplift and carbonate platform.5)The flow paths can be constructed based on paleotopography,gully line and sand body distribution.There are three water flow systems on the west side of Baiyun Main Sub-Sag: the west side of the fault warping uplift of the well B7-B13,the southwest sub-sag area of Baiyun Main Sub-Sag,and the northwest slope of Baiyun Main Sub-Sag.6)The depositional systems of the HST-FRST system tracts at the top of SQ30(the numerical number represents the age of the top of the sequence),SQ28.4,SQ26.5,SQ25.5 and SQ23.8 were constructed by combining the three elements of skeleton lithofacies,sedimentary environment and water flow paths.The depositional systems are mainly composed of the continental shelf margin delta and deep-water fan,distal flood mudstone on the top of the fault warping uplift,the mixed shelf limestone in the uplift area,the volcanic cone and the carbonate platform sedimentary bodies on the top of the volcanic cone.Delta consists of three subfacies: delta plain,delta front edge and front delta,and eight main microfacies such as channel sand,channel edge,flood plain,mouth bar,distributary channel,distal bar,front delta mudstone and slope fan sand body.Slope fan and basin floor fan are divided into two parts: main channel sand body and branch channel superimposed fan sand body.7)Under the cycle control of the second order sea-level change,the continental shelf margin delta-deep-water fan depositional systems generally migrated to the direction of land from SQ30 to SQ26.5 and the depositional systems migrated to the direction of sea basin from SQ26.5 to SQ23.8.8)The provenance of the rock type(sandstone or siltstone)of deep-water“seismic sandstone” can be determined by the water flow path between continental shelf margin delta and deep-water fan.If the provenance is from the delta branch channel and delta front,the high probability of deepwater fan sandstone is high porosity and permeability sandstone.If the provenance is from the no sand shelf,the high probability of deepwater fan sandstone is siltstone.Based on the water flow paths in the newly reconstructed depositional systems of SQ23.8,it indicates that the sandy conglomerate in the SQ23.8 of Liwan Sag comes from the delta lobe in the northwest slope of Baiyun Main Sub-Sag(well P35),instead of the delta lobe sand body in the southwest sub-sag of Baiyun Main Sub-Sag.According to the the water flow paths in the newly reconstructed depositional systems,it indicates that sand body in the Heshan Sag which on the west side of Baiyun Main Sub-Sag is mainly transported by the water flow of the west side.There is a potential for the development of fine sandstone reservoirs in Heshan Sag.High-precision depositional systems under the complex geological conditions were reconstructed for the first time in this paper.Innovative achievements include:(1)The seismic identification technology of complex lithofacies on continental shelf margin was established.(2)Enhanced seismic lithofacies analysis was first used to reconstruct the depositional systems of continental shelf margin under the complex geological conditions.(3)The provenance channel and sediment type in Heshan Sag was proved.These innovative achievements are of great value and significance to deep-water sedimentology research and oil and gas exploration. |
