Study On Sandstone Transportation System Under Ultra-deep And Wellless

Baiyun sag is a deep depression with the largest area and the most fully developed Cenozoic strata in the Pearl River Mouth Basin,and its oil-gas geological resources are estimated to be 2-8 billion tons according to the latest estimates.At present,the discovered oil-gas reserves in the Oligocene and Miocene formations of Baiyun sag account for only one-seventh of the total resources,so the deep strata of Baiyun sag have great potential for oil-gas exploration.However,the burial depth of Wenchang formation in Baiyun sag generally exceeds 4000 m,and the burial depth of formation in the center of depression reaches8000-12000 m.At present,the distribution,dominant migration path and favorable reservoir-forming area of oil-gas in Wenchang formation in Baiyun sag are not clear due to the lack of effective revelation of drilling data(greatly related to the large burial depth),which brings great risk to the subsequent oil-gas exploration.Therefore,it is very important and urgent to study the sandstone transportation system in Wenchang formation in Baiyun sag.The traditional study of sandstone transportation system relies on well data,which is not applicable to the wellless area like Wenchang formation of Baiyun sag.At present,no one has attempted to study sandstone transportation system under ultra-deep and wellless conditions,which makes it a difficult and challenging research task.Taking the sedimentary sequence of Wenchang formation in Baiyun sag as an example,this paper is the first study on sandstone transportation system under ultra-deep wellless conditions based on seismic data,and the main research contents and methods are as follows: 1)Describing the distribution of source rocks.The paleogeomorphology,paleoenvironment and sedimentary facies distribution are studied by restoring the original form structure profile of sedimentary sequence,so as to obtain the distribution of dense sections of each sequence,and then the distribution range of source rocks is described through dense sections.2)Describing the distribution of sand bodies.The seismic lithofacies is directly identified from the seismic profile by enhanced seismic phase analysis(6 traditional seismic phase markers and 3 new seismic markers such as smoothness,neatness and special reflection waveform),and then the distribution of sand body is described by seismic scan interpretation of sandstone(skeletal lithofacies).3)Study on connectivity between sand body and source rock.The sedimentary system of Wenchang formation in Baiyun sag is reconstructed by superimposing 3 elements,such as skeletal lithofacies,depositional environment and water flow path,and the planar superimposed distribution of sand body and source rocks is obtained.Then,the connectivity between sand body and source rocks is studied by the spatial relationship between them.4)Study on connectivity between sand bodies,which is carried out through scanning interpretation,tip extinction and connectivity test of sand bodies.5)Study on dominant migration path of oil-gas.By superimposing the paleomorphology of lake basin with three types of porous sand bodies,the dominant migration direction is firstly determined according to the stratigraphic tendency and paleocurrent path,and then the dominant migration path and favorable reservoir-forming area of oil-gas are determined by identifying the source rock to dominant transportation sand body to basin edge or hydrocarbon trap.6)Study on the migration traces of hydrocarbon.When hydrocarbon migrate through sandstone pores,the residual oil will be left in the pore space,which will be cracked into gas in the late high temperature and high pressure background,making the sandstone become gas-bearing sandstone and behave as a low-velocity layer.These sandstones usually show a right downdip skew-symmetrical waveform and strong amplitude reflection on the seismic profile,which can be used to identify the migration traces of oil-gas.From the above study,the following results are obtained.(1)Based on the enhanced seismic phase analysis,there are three types of seismic lithofacies in Wenchang formation in Baiyun main sag: siliceous clastic rocks,volcanic clastic rocks and calcareous clastic rock.The siliciclastic rocks are subdivided into sandstone interspersed with mudstone,sandstone interbedded with mudstone,interbedded sandstone and mudstone,mudstone interbedded with sandstone,mudstone interspersed with sandstone,and mudstone.The volcanic clastic rocks are subdivided into volcanic agglomerates,pyroclastic sedimentary rock,and pyroclastic bearing sedimentary rock.The calcareous clastic rocks are subdivided into marl and calcareous sandstone and calcareous mudstone.(2)A method is established to identify the complex lithofacies of faulted lake basin.The channel sands developed in the lithofacies of sandstone interspersed with mudstone show weak amplitude unclean reflection.The channel sands developed in the lithofacies of sandstone interbedded with mudstone show a concave base,short-axis discontinuity,right downdip skew-symmetrical waveform and unclean reflection,and the gas-bearing channel sands of high porosity show strong amplitude reflection.Underwater distributary channel sands show laminar continuity,multiple concave bases,right downdip skew-symmetrical waveform,and medium-strong amplitude reflection.The sheet sands show parallel,laminar continuous,smooth,right downdip skew-symmetrical waveform,and moderate to strong amplitude reflection.The sublacustrine fan sands show a concave base,right downdip skew-symmetrical waveform,clean,and weak-strong amplitude reflection.The volcanic agglomerates show weakly laminated,untidy,and weakly amplitude reflection.pyroclastic sedimentary rocks show extremely unsmooth,stratiform shape,and weakly amplitude reflection.Volcanic lava show left downdip skew-symmetrical waveform,unsmooth,discontinuous,strong amplitude reflection.The reflection such as concave bottom,short-axis discontinuity,right downdip skew-symmetrical waveform and strong amplitude,are the most typical identified features of porous sandstone.(3)The depositional system of five sequences in Wenchang formation in Baiyun sag is constructed by superimposing the three elements of skeletal lithofacies,depositional environment and water flow path.There are four types of "source-sink" systems: 1)The northern gentle slope of main sag: the Panyu Low Uplift is the main provenance(local volcanic clastic provenance in the northeast slope).The fluvial-shallow lake-deep lake environment is mainly developed,and the large scale depositional system of braided river delta-sublustrine fan is developed;2)The southwest axial steep slope zone of main sag: takes Yunkai Uplift as the main material provenance,develops mainly semi-deep-deep lake environment and medium scale nearshore submerged fan,fan delta-sublustrine fan sedimentary system;3)The southeast steep slope zone of main sag: takes the terrigenous clastic of Yunli Uplift(main)and volcanic clastic as the mixed provenance,mainly develops the semi-deep-deep lake environment and medium-small scale nearshore submerged fan and fan delta-sublustrine fan sedimentary systems;4)The Baiyun East sag is a multi-uplift depression and multi-provenance system formed by steep slope and magmatic underplating,mainly develops nearshore submerged fan and fan delta-sublacustrine fan depositional systems,and the scale of sedimentary body is small.(4)Seven types of sandstone transmitting beds are identified in Wenchang formation in Baiyun main sag,including: 1)autochthonous transmitting beds,which is composed of deep water fan sands,and sandstones surrounded by hydrocarbon source rocks,forming lithologic trap;2)The short-distance layered sandstone transmitting beds,which is composed of delta front sand bodies or deep water fan sand bodies connected with each other;3)The long-distance layered sandstone transmitting beds consisting of deltaic sand bodies(braided channel and front sands),and they mainly present in individual branched-channel delta lobes;4)Conformable dip layered sandstone transmitting beds,which is transported by braided river delta sand layer along the dip direction,mainly in a single delta lobe;5)Conformable dip and lateral sandstone transmitting beds,which can be transported between different deltaic lobes;6)Composite transport system composed of deltaic sandstone and basal unconformity;7)Composite transport system composed of sandstone transmitting beds and faults(5)The distribution of sandstone transportation beds in different areas of Baiyun main sag during the depositional period of Wenchang formation is described: 1)the northwest gentle slope zone of Baiyun main sag: on the whole,the sandstones of braided river delta have good porosity and high connectivity,and the delta sand body(channel sand,mouth bar sand and sheet sand)in multiple periods can form a long-distance sandstone transport conductor.In addition,the sand bodies within sequence are connected with the source rock of deep depression area.Therefore,the sandstone transport layers are the most developed in this area;2)The southwest steep slope zone of Baiyun main sag: A large number of fan delta sandbodies are developed in WCSQ1 and WCSQ2 in the fault depression area.The front sandbodies have good porosity and are well connected with deep lacustrine mudstones,so they have the potential to form large rolling anticlinal traps;3)The west of Baiyun main sag:WCSQ1 and WCSQ2 develop a large number of fan delta front sand bodies,which can form an effective sandstone transport layer and have the potential to form rolling anticline trap oil-gas reservoirs.WCSQ3 and WCSQ4 develop effective sandstone transmitting beds in the slope setting,and have the potential to form updip thinning-out sandstone reservoirs;4)The northeast slope zone of Baiyun main sag:due to the influence of volcanic debris,WCSQ1-WCSQ4 mainly develops short-distance sandstone transmitting beds,while WCSQ5 develops long-distance sandstone transmitting beds;5)The southeast of Baiyun main sag: affected by the volcanic debris of lake basin margin,the short-distance sandstone transmitting beds constructed by delta front sand bodies are mainly developed.(6)The favorable oil-gas accumulation areas and intervals of Wenchang formation in Baiyun main sag are pointed out.The best play is the northwest slope zone,and the sequences of WCSQ2-WCSQ4 have a great development potential of paleoreservoir;The second play is the southern fault step zone,and the sequences of WCSQ1-WCSQ2 have the potential to develop ancient oil-gas reservoirs;The third paly is the western edge of Baiyun main sag,the sequences of WCSQ3-WCSQ4 have good reservoir-forming potential.The northeastern,southeastern of main sag and Baiyun East sag mainly develop volcanic clastic sedimentary rocks,which lead to a poor transport capacity of sandstones and small scale of ancient oil-gas reservoirs.the research on sandstone transportation system under ultra-deep and wellless is carried out by using seismic data for the first time in this paper.Innovative achievements include: 1)The seismic identification technology of complex lithofacies in faulted lake basin is established;2)A method to study sandstone transportation system under ultra-deep and wellless by seismic data has been explored.These innovative achievements have important value and significance for the study of lacustrine sedimentoloyg and ultra-deep oil-gas migration in wellless area,and can guide the oil-gas exploration of deep strata in Baiyun sag.