The Oligocene Zhuhai sandstones are significant reservoirs for hydrocarbons in the Baiyun Sag, South China Sea.For effective appraisal, exploration and exploitation of such a deep-water hydrocarbon sandstone, samples ...The Oligocene Zhuhai sandstones are significant reservoirs for hydrocarbons in the Baiyun Sag, South China Sea.For effective appraisal, exploration and exploitation of such a deep-water hydrocarbon sandstone, samples of five wells from depths of 850 m to 3 000 m were studied. A series of comprehensive petrographic and geochemical analyses were performed to unravel the diagenetic features and their impact on the reservoir quality.Petrographically, the sandstones are dominated by feldspathic litharenites and lithic arenites with fine to medium grain sizes and moderate to good sorting. The reservoir quality varies greatly with a range of porosity from 0.2% to 36.1% and permeability from 0.016 ×10~(–3) μm~2 to 4 301 ×10~(–3) μm~2, which is attributed to complex diagenetic evolution related to sedimentary facies;these include compaction, cementation of calcite, dolomite, siderite and framboidal pyrite in eogenetic stage;further compaction, feldspar dissolution, precipitation of ferrocalcite and ankerite, quartz cements, formation of kaolinite and its illitization, precipitation of albite and nodular pyrite, as well as hydrocarbon charge in mesogenetic stage. The dissolution of feldspar and illitization of kaolinite provide internal sources for the precipitation of quartz cement, while carbonate cements are derived from external sources related to interbedded mudstones and deep fluid. Compaction is the predominant factor in reducing the total porosity, followed by carbonate cementation that leads to strong heterogeneity. Feldspar dissolution and concomitant quartz and clay cementation barely changes the porosity but significantly reduces the permeability.The high-quality reservoirs can be concluded as medium-grained sandstones lying in the central parts of thick underwater distributary channel sandbodies(>2 m) with a high content of detrital quartz but low cement.展开更多
Dissolution mechanism and favorable reservoir distribution prediction are the key problems restricting oil and gas exploration in deep-buried layers.In this paper,the Enping Formation and Zhuhai Formation in Baiyun Sa...Dissolution mechanism and favorable reservoir distribution prediction are the key problems restricting oil and gas exploration in deep-buried layers.In this paper,the Enping Formation and Zhuhai Formation in Baiyun Sag of South China Sea was taken as a target.Based on the thin section,scanning electron microscopy,X-ray diffraction,porosity/permeability measurement,and mercury injection,influencing factors of dissolution were examined,and a dissolution model was established.Further,high-quality reservoirs were predicted temporally and spatially.The results show that dissolved pores constituted the main space of the Paleogene sandstone reservoir.Dissolution primarily occurred in the coarse-and medium-grained sandstones in the subaerial and subaqueous distributary channels,while dissolution was limited in fine-grained sandstones and inequigranular sandstones.The main dissolved minerals were feldspar,tuffaceous matrix,and diagenetic cement.Kaolinization of feldspar and illitization of kaolinite are the main dissolution pathways,but they occur at various depths and temperatures with different geothermal gradients.Dissolution is controlled by four factors,in terms of depositional facies,source rock evolution,overpressure,and fault activities,which co-acted at the period of 23.8–13.8 Ma,and resulted into strong dissolution.Additionally,based on these factors,high-quality reservoirs of the Enping and Zhuhai formations are predicted in the northern slope,southwestern step zone,and Liuhua uplift in the Baiyun Sag.展开更多
The eastern main sub-sag(E-MSS)of the Baiyun Sag was the main zone for gas exploration in the deep-water area of the Zhujiang River(Pearl River)Mouth Basin at its early exploration stage,but the main goal of searching...The eastern main sub-sag(E-MSS)of the Baiyun Sag was the main zone for gas exploration in the deep-water area of the Zhujiang River(Pearl River)Mouth Basin at its early exploration stage,but the main goal of searching gas in this area was broken through by the successful exploration of the W3-2 and H34B volatile oil reservoirs,which provides a new insight for exploration of the Paleogene oil reservoirs in the E-MSS.Nevertheless,it is not clear on the distribution of“gas accumulated in the upper layer,oil accumulated in the lower layer”(Gas_(upper)-Oil_(lower))under the high heat flow,different source-rock beds,multi-stages of oil and gas charge,and multi-fluid phases,and not yet a definite understanding of the genetic relationship and formation mechanism among volatile oil,light oil and condensate gas reservoirs,and the migration and sequential charge model of oil and gas.These puzzles directly lead to the lack of a clear direction for oil exploration and drilling zone in this area.In this work,the PVT fluid phase,the origin of crude oil and condensate,the secondary alteration of oil and gas reservoirs,the evolution sequence of oil and gas formation,the phase state of oil and gas migration,and the configuration of fault activity were analyzed,which established the migration and accumulation model of Gas_(upper)-Oil_(lower)cocontrolled by source and heat,and fractionation controlled by facies in the E-MSS.Meanwhile,the fractionation evolution model among common black reservoirs,volatile reservoirs,condensate reservoirs and gas reservoirs is discussed,which proposed that the distribution pattern of Gas_(upper)-Oil_(lower)in the E-MSS is controlled by the generation attribute of oil and gas from source rocks,the difference of thermal evolution,and the fractionation controlled by phases after mixing the oil and gas.Overall,we suggest that residual oil reservoirs should be found in the lower strata of the discovered gas reservoirs in the oil-source fault and diapir-developed areas,while volatile oil reservoirs should be found in the deeper strata near the sag with no oil-source fault area.展开更多
In this study,element geochemistry and zircon chronology are used to analyze the Oligocene sediments in the Baiyun Sag,Zhujiang River Mouth Basin.The experimental results are discussed with respect to weathering condi...In this study,element geochemistry and zircon chronology are used to analyze the Oligocene sediments in the Baiyun Sag,Zhujiang River Mouth Basin.The experimental results are discussed with respect to weathering conditions,parent rock lithologies,and provenances.The chemical index of alteration and the chemical index of weathering values of mudstone samples from the lower Oligocene Enping Formation indicate that clastic particles in the study area underwent moderate weathering.Mudstone samples exhibit relatively enriched light rare earth elements and depleted heavy rare earth elements,"V"-shaped negative Eu anomalies,and negligible Ce anomalies.The rare earth element distribution curves are obviously right-inclined,with shapes and contents similar to those of post-Archean Australian shale and upper continental crust,indicating that the samples originated from acid rocks in the upper crust.The Hf-La/Th and La/Sc-Co/Th diagrams show this same origin for the sediments in the study area.For the samples from the upper Enping deltas,the overall age spectrum shows four major age peaks ca.59–68 Ma,98–136 Ma,153–168 Ma and 239–260 Ma.For the Zhuhai Formation samples,the overall age spectrum shows three major age peaks ca.149 Ma,252 Ma and 380 Ma.The detrital zircon shapes and U-Pb ages reveal that during Oligocene sedimentation,the sediments on the northwestern margin of the Baiyun Sag were supplied jointly from two provenances:Precambrian-Paleozoic metamorphic rocks in the extrabasinal South China fold zone and Mesozoic volcanic rocks in the intrabasinal Panyu Low Uplift,and the former supply became stronger through time.Thus,the provenance of the Oligocene deltas experienced a transition from an early proximal intrabasinal source to a late distal extrabasinal source.展开更多
The relationships between crustal stretching and thinning,basin structure and petroleum geology in Baiyun deep-water area were analyzed using large area 3D seismic,gravity,magnetic,ocean bottom seismic(OBS),deep-water...The relationships between crustal stretching and thinning,basin structure and petroleum geology in Baiyun deep-water area were analyzed using large area 3D seismic,gravity,magnetic,ocean bottom seismic(OBS),deep-water exploration wells and integrated ocean drilling program(IODP).During the early syn-rifting period,deep-water area was a half-graben controlled by high angle faults influenced by the brittle extension of upper crust.In the mid syn-rifting period,this area was a broad-deep fault depression controlled by detachment faults undergone brittle-ductile deformation and differentiated extension in the crust.In the late syn-rifting period,this area experienced fault-sag transition due to saucer-shaped rheology change dominated by crustal ductile deformation.A broad-deep fault depression controlled by the large detachment faults penetrating through the crust is an important feature of deep-water basin.The study suggests that the broad-deep Baiyun sag provides great accommodation space for the development of massive deltaic-lacustrine deposition system and hydrocarbon source rocks.The differentiated lithospheric thinning also resulted in the different thermal subsidence during post-rifting period,and then controlled the development of continental shelf break and deep-water reservoir sedimentary environment.The high heat flow background caused by the strong thinning of lithosphere and the rise of mantle source resulted in particularities in the reservoir diagenesis,hydrocarbon generation process and accumulation of deep-water area in northern South China Sea.展开更多
Ichnofossils are well developed in clastic rock reservoirs in marine and transitional facies, which can considerably change the physical properties of the reservoir. However, this influence is not well understood, rai...Ichnofossils are well developed in clastic rock reservoirs in marine and transitional facies, which can considerably change the physical properties of the reservoir. However, this influence is not well understood, raising an important problem in the effective development of petroleum reservoirs. This paper analyzes continental shelf margin delta reservoirs through core observation, cast thin section observation and reservoir physical property test. Some important scientific insights are obtained:(1) The presence of Cruziana ichnofacies, including Asterosoma, Ophiomorpha, Planolites, Skolithos, Thalassinoides, and other ichnofossils can be used to identify in subaqueous distributary channels, subaqueous levee, frontal sheet sand, abandoned river channels, crevasse channels, main channels and channel mouth bars. Considerable differences in the types of ichnofossils and the degree of bioturbation can be observed in the different petrofacies.(2) Ichnofossils and bioturbation play a complex role in controlling reservoir properties. The reservoir physical properties have the characteristics of a decrease–increase–decrease curve with increasing bioturbation degree. This complex change is controlled by the sediment mixing and packing of bioturbation and the diagenetic environment controlled by the ichnofossils.(3) Sea-level cycle changes affect the modification of the reservoir through sediment packing. Bioturbation weakens the reservoir's physical property when sea level slowly rises and improves the reservoir's physical property when base level slowly falls.展开更多
This paper discusses deep crustal architecture of the Baiyun Sag of the Pearl River Mouth Basin, northern South China Sea based on velocity analysis, time-depth con- version and seismic interpretation of the deep seis...This paper discusses deep crustal architecture of the Baiyun Sag of the Pearl River Mouth Basin, northern South China Sea based on velocity analysis, time-depth con- version and seismic interpretation of the deep seismic reflec- tion profile DSRP-2002. The profile was acquired and proc- essed to 14 S TWT by the China National Offshore Oil Corp. (CNOOC) in 2002. It extends across the Baiyun Sag of the Pearl River Mouth Basin, from the northern continental shelf of the SCS to the deepwater province. As the first deep seismic reflection profile in the Pearl River Mouth Basin,this profile reveals seismic phases from basement down to upper most mantle. The Moho surface appears in the profile as an undulating layer of varying thickness of 1-3 km. It is not a single reflector interface, but a velocity gradient or intercon- version layer. The crust thins stepwisely from the shelf to the continental slope and the abyssal plain (from north to south), and also thins under depocenters. The crustal thickness is only 7 km in the depocenter of the main Baiyun Sag, which corresponds to a Moho upwelling mirroring the basement topography. In the lower slope and the ocean-continental transition zone of the southernmost portion of the profile, three sub-parallel, NW-dipping strong reflectors found at depths around 10-21 km are interpreted as indications of a subducted Mesozoic oceanic crust. Crustal faults exist in the northern and southern boundaries of the Baiyun Sag. The intense and persistent subsidence of the Baiyun Sag might be related to the long-term activity of the crustal faults.展开更多
The Baiyun (白云) sag in the southern Pearl River Mouth basin is the largest and deepest sag in deepwater northern South China Sea. Researches and exploration in this sag have revealed many distinct features of the sa...The Baiyun (白云) sag in the southern Pearl River Mouth basin is the largest and deepest sag in deepwater northern South China Sea. Researches and exploration in this sag have revealed many distinct features of the sag. This article reports its filling history through backstripping of depth data of interpreted sequence boundaries. Maps of sediment rates of 10 sequences from 49 Ma to the present were constructed, showing the spatio-temporal variation of the sediment rate. Three stages of sediment infilling, 49-17.5 Ma, 17.5-10.5 Ma, and 10.5-0 Ma, were divided by abrupt changes of sedimentary patterns. If the breakup of the South China Sea took place at ～30 Ma, significant post-breakup accel-eration of sedimentation was observed at 17.5-15.5 Ma and 13.8-12.5 Ma, indicating acceleration of subsidence at these times. We propose that the onset of strong post-breakup subsidence at ～17.5 Ma was an important tectonic event that changed the pattern of sedimentation from discrete and medium-rate deposition centers in both main and south subsags to restricted but high-rate deposition in the main subsag. The cause and implications of this newly recognized event need to be investigated.展开更多
The Baiyun Sag is the deepest sag in the Pearl River Mouth Basin in northern continental margin of South China Sea,with the maximum sediment thickness over 12.5 km above the basement including >6.5 km sediments abo...The Baiyun Sag is the deepest sag in the Pearl River Mouth Basin in northern continental margin of South China Sea,with the maximum sediment thickness over 12.5 km above the basement including >6.5 km sediments above the 30 Ma breakup unconformity.According to the theoretical models for the rifted basins,the post-rift subsidence is driven solely by the thermal contraction and can be calculated as the function of the lithospheric stretching factor.A method combining the forward modeling and reverse backstripping was designed to estimate lithospheric stretching factor.Using the 2D forward modeling based on the flexural cantilever model,we simulated the multi-rifting process of the Baiyun Sag with constrain of the backstripped profiles.By doing this the lithospheric stretching factor was obtained,and then the theoretical post-rift subsidence was calculated.The calculated theoretical subsidence was much smaller than the observed subsidence given by backstripping.Along the 1530 line in the Baiyun Sag,the anomalous post-rift subsidence is over 2 km in the sag center,and varies slightly to the north and south edges of the sag.This suggests that the anomalous post-rift subsidence continues beyond the sag both in the continental shelf to the north and in the continental slope to the south.The sensitivity tests in the forward modeling process indicate that only the use of low-angle faults (≤13°) can we simulate the shape of the backstripped profile.展开更多
基金The National Natural Science Foundation of China under contract No. 42262020the Research Program of Science and Technology at Universities of Inner Mongolia Autonomous Region under contract No. NJZY22445。
文摘The Oligocene Zhuhai sandstones are significant reservoirs for hydrocarbons in the Baiyun Sag, South China Sea.For effective appraisal, exploration and exploitation of such a deep-water hydrocarbon sandstone, samples of five wells from depths of 850 m to 3 000 m were studied. A series of comprehensive petrographic and geochemical analyses were performed to unravel the diagenetic features and their impact on the reservoir quality.Petrographically, the sandstones are dominated by feldspathic litharenites and lithic arenites with fine to medium grain sizes and moderate to good sorting. The reservoir quality varies greatly with a range of porosity from 0.2% to 36.1% and permeability from 0.016 ×10~(–3) μm~2 to 4 301 ×10~(–3) μm~2, which is attributed to complex diagenetic evolution related to sedimentary facies;these include compaction, cementation of calcite, dolomite, siderite and framboidal pyrite in eogenetic stage;further compaction, feldspar dissolution, precipitation of ferrocalcite and ankerite, quartz cements, formation of kaolinite and its illitization, precipitation of albite and nodular pyrite, as well as hydrocarbon charge in mesogenetic stage. The dissolution of feldspar and illitization of kaolinite provide internal sources for the precipitation of quartz cement, while carbonate cements are derived from external sources related to interbedded mudstones and deep fluid. Compaction is the predominant factor in reducing the total porosity, followed by carbonate cementation that leads to strong heterogeneity. Feldspar dissolution and concomitant quartz and clay cementation barely changes the porosity but significantly reduces the permeability.The high-quality reservoirs can be concluded as medium-grained sandstones lying in the central parts of thick underwater distributary channel sandbodies(>2 m) with a high content of detrital quartz but low cement.
基金The National Natural Science Foundation of China under contract No.42202157the China National Offshore Oil Corporation Co.,Ltd.Major Production and Scientific Research Program under contract No.2019KT-SC-22。
文摘Dissolution mechanism and favorable reservoir distribution prediction are the key problems restricting oil and gas exploration in deep-buried layers.In this paper,the Enping Formation and Zhuhai Formation in Baiyun Sag of South China Sea was taken as a target.Based on the thin section,scanning electron microscopy,X-ray diffraction,porosity/permeability measurement,and mercury injection,influencing factors of dissolution were examined,and a dissolution model was established.Further,high-quality reservoirs were predicted temporally and spatially.The results show that dissolved pores constituted the main space of the Paleogene sandstone reservoir.Dissolution primarily occurred in the coarse-and medium-grained sandstones in the subaerial and subaqueous distributary channels,while dissolution was limited in fine-grained sandstones and inequigranular sandstones.The main dissolved minerals were feldspar,tuffaceous matrix,and diagenetic cement.Kaolinization of feldspar and illitization of kaolinite are the main dissolution pathways,but they occur at various depths and temperatures with different geothermal gradients.Dissolution is controlled by four factors,in terms of depositional facies,source rock evolution,overpressure,and fault activities,which co-acted at the period of 23.8–13.8 Ma,and resulted into strong dissolution.Additionally,based on these factors,high-quality reservoirs of the Enping and Zhuhai formations are predicted in the northern slope,southwestern step zone,and Liuhua uplift in the Baiyun Sag.
基金The Major Science and Technology Project of China National Offshore Oil Corporation during the“14th Five-Year Plan”under contact No.KJGG2022-0103-03。
文摘The eastern main sub-sag(E-MSS)of the Baiyun Sag was the main zone for gas exploration in the deep-water area of the Zhujiang River(Pearl River)Mouth Basin at its early exploration stage,but the main goal of searching gas in this area was broken through by the successful exploration of the W3-2 and H34B volatile oil reservoirs,which provides a new insight for exploration of the Paleogene oil reservoirs in the E-MSS.Nevertheless,it is not clear on the distribution of“gas accumulated in the upper layer,oil accumulated in the lower layer”(Gas_(upper)-Oil_(lower))under the high heat flow,different source-rock beds,multi-stages of oil and gas charge,and multi-fluid phases,and not yet a definite understanding of the genetic relationship and formation mechanism among volatile oil,light oil and condensate gas reservoirs,and the migration and sequential charge model of oil and gas.These puzzles directly lead to the lack of a clear direction for oil exploration and drilling zone in this area.In this work,the PVT fluid phase,the origin of crude oil and condensate,the secondary alteration of oil and gas reservoirs,the evolution sequence of oil and gas formation,the phase state of oil and gas migration,and the configuration of fault activity were analyzed,which established the migration and accumulation model of Gas_(upper)-Oil_(lower)cocontrolled by source and heat,and fractionation controlled by facies in the E-MSS.Meanwhile,the fractionation evolution model among common black reservoirs,volatile reservoirs,condensate reservoirs and gas reservoirs is discussed,which proposed that the distribution pattern of Gas_(upper)-Oil_(lower)in the E-MSS is controlled by the generation attribute of oil and gas from source rocks,the difference of thermal evolution,and the fractionation controlled by phases after mixing the oil and gas.Overall,we suggest that residual oil reservoirs should be found in the lower strata of the discovered gas reservoirs in the oil-source fault and diapir-developed areas,while volatile oil reservoirs should be found in the deeper strata near the sag with no oil-source fault area.
基金The National Natural Science Foundation of China under contract No.91528303the National Science and Technology Major Project under contract Nos 2016ZX05026,2011ZX05025 and 2008ZX05025+1 种基金the National Basic Research Program(973 Program)of China under contract No.2009CB219400the Foundation for Excellent Youth Scholars of NIEER,CAS
文摘In this study,element geochemistry and zircon chronology are used to analyze the Oligocene sediments in the Baiyun Sag,Zhujiang River Mouth Basin.The experimental results are discussed with respect to weathering conditions,parent rock lithologies,and provenances.The chemical index of alteration and the chemical index of weathering values of mudstone samples from the lower Oligocene Enping Formation indicate that clastic particles in the study area underwent moderate weathering.Mudstone samples exhibit relatively enriched light rare earth elements and depleted heavy rare earth elements,"V"-shaped negative Eu anomalies,and negligible Ce anomalies.The rare earth element distribution curves are obviously right-inclined,with shapes and contents similar to those of post-Archean Australian shale and upper continental crust,indicating that the samples originated from acid rocks in the upper crust.The Hf-La/Th and La/Sc-Co/Th diagrams show this same origin for the sediments in the study area.For the samples from the upper Enping deltas,the overall age spectrum shows four major age peaks ca.59–68 Ma,98–136 Ma,153–168 Ma and 239–260 Ma.For the Zhuhai Formation samples,the overall age spectrum shows three major age peaks ca.149 Ma,252 Ma and 380 Ma.The detrital zircon shapes and U-Pb ages reveal that during Oligocene sedimentation,the sediments on the northwestern margin of the Baiyun Sag were supplied jointly from two provenances:Precambrian-Paleozoic metamorphic rocks in the extrabasinal South China fold zone and Mesozoic volcanic rocks in the intrabasinal Panyu Low Uplift,and the former supply became stronger through time.Thus,the provenance of the Oligocene deltas experienced a transition from an early proximal intrabasinal source to a late distal extrabasinal source.
基金Supported by the Science and Technology Project of CNOOC Ltd.(YXKY-2012-SHENHAI-01)China National Science and Technology Major Project(2011ZX05025-003+1 种基金 2016ZX05026-003)the National Natural Science Foundation of China(91128207)
文摘The relationships between crustal stretching and thinning,basin structure and petroleum geology in Baiyun deep-water area were analyzed using large area 3D seismic,gravity,magnetic,ocean bottom seismic(OBS),deep-water exploration wells and integrated ocean drilling program(IODP).During the early syn-rifting period,deep-water area was a half-graben controlled by high angle faults influenced by the brittle extension of upper crust.In the mid syn-rifting period,this area was a broad-deep fault depression controlled by detachment faults undergone brittle-ductile deformation and differentiated extension in the crust.In the late syn-rifting period,this area experienced fault-sag transition due to saucer-shaped rheology change dominated by crustal ductile deformation.A broad-deep fault depression controlled by the large detachment faults penetrating through the crust is an important feature of deep-water basin.The study suggests that the broad-deep Baiyun sag provides great accommodation space for the development of massive deltaic-lacustrine deposition system and hydrocarbon source rocks.The differentiated lithospheric thinning also resulted in the different thermal subsidence during post-rifting period,and then controlled the development of continental shelf break and deep-water reservoir sedimentary environment.The high heat flow background caused by the strong thinning of lithosphere and the rise of mantle source resulted in particularities in the reservoir diagenesis,hydrocarbon generation process and accumulation of deep-water area in northern South China Sea.
基金The National Natural Science Foundation of China under contract Nos 41872112 and 42077410the Key Research Project of Higher Education Institutions in Henan University under contract No. 20A170010+1 种基金the Program for Innovative Research Team (in Science and Technology) of Henan Polytechnic University under contract No. T2022-05the Foundation of Shandong Provincial Key Laboratory of Depositional Mineralization and Sedimentary Mineral under contract No. DMSM2018027。
文摘Ichnofossils are well developed in clastic rock reservoirs in marine and transitional facies, which can considerably change the physical properties of the reservoir. However, this influence is not well understood, raising an important problem in the effective development of petroleum reservoirs. This paper analyzes continental shelf margin delta reservoirs through core observation, cast thin section observation and reservoir physical property test. Some important scientific insights are obtained:(1) The presence of Cruziana ichnofacies, including Asterosoma, Ophiomorpha, Planolites, Skolithos, Thalassinoides, and other ichnofossils can be used to identify in subaqueous distributary channels, subaqueous levee, frontal sheet sand, abandoned river channels, crevasse channels, main channels and channel mouth bars. Considerable differences in the types of ichnofossils and the degree of bioturbation can be observed in the different petrofacies.(2) Ichnofossils and bioturbation play a complex role in controlling reservoir properties. The reservoir physical properties have the characteristics of a decrease–increase–decrease curve with increasing bioturbation degree. This complex change is controlled by the sediment mixing and packing of bioturbation and the diagenetic environment controlled by the ichnofossils.(3) Sea-level cycle changes affect the modification of the reservoir through sediment packing. Bioturbation weakens the reservoir's physical property when sea level slowly rises and improves the reservoir's physical property when base level slowly falls.
基金the NSFC Key Project(Grant No.40238060)the National 97T3 Project(Grant No.G2000046702),SCSIO Project(Grant No.SJ200402)+1 种基金the CAS Key Project(Grant No.KZCX2-117-5)the Field Frontier Project of the SCSIO.and the Key Laboratory Project(Grant No.BYH03A05).
文摘This paper discusses deep crustal architecture of the Baiyun Sag of the Pearl River Mouth Basin, northern South China Sea based on velocity analysis, time-depth con- version and seismic interpretation of the deep seismic reflec- tion profile DSRP-2002. The profile was acquired and proc- essed to 14 S TWT by the China National Offshore Oil Corp. (CNOOC) in 2002. It extends across the Baiyun Sag of the Pearl River Mouth Basin, from the northern continental shelf of the SCS to the deepwater province. As the first deep seismic reflection profile in the Pearl River Mouth Basin,this profile reveals seismic phases from basement down to upper most mantle. The Moho surface appears in the profile as an undulating layer of varying thickness of 1-3 km. It is not a single reflector interface, but a velocity gradient or intercon- version layer. The crust thins stepwisely from the shelf to the continental slope and the abyssal plain (from north to south), and also thins under depocenters. The crustal thickness is only 7 km in the depocenter of the main Baiyun Sag, which corresponds to a Moho upwelling mirroring the basement topography. In the lower slope and the ocean-continental transition zone of the southernmost portion of the profile, three sub-parallel, NW-dipping strong reflectors found at depths around 10-21 km are interpreted as indications of a subducted Mesozoic oceanic crust. Crustal faults exist in the northern and southern boundaries of the Baiyun Sag. The intense and persistent subsidence of the Baiyun Sag might be related to the long-term activity of the crustal faults.
基金supported by the National Natural Science Foundation of China (Nos. 40576027, 40238060)the High-Tech R&D Program of China (No. 2008AA09Z306)
文摘The Baiyun (白云) sag in the southern Pearl River Mouth basin is the largest and deepest sag in deepwater northern South China Sea. Researches and exploration in this sag have revealed many distinct features of the sag. This article reports its filling history through backstripping of depth data of interpreted sequence boundaries. Maps of sediment rates of 10 sequences from 49 Ma to the present were constructed, showing the spatio-temporal variation of the sediment rate. Three stages of sediment infilling, 49-17.5 Ma, 17.5-10.5 Ma, and 10.5-0 Ma, were divided by abrupt changes of sedimentary patterns. If the breakup of the South China Sea took place at ～30 Ma, significant post-breakup accel-eration of sedimentation was observed at 17.5-15.5 Ma and 13.8-12.5 Ma, indicating acceleration of subsidence at these times. We propose that the onset of strong post-breakup subsidence at ～17.5 Ma was an important tectonic event that changed the pattern of sedimentation from discrete and medium-rate deposition centers in both main and south subsags to restricted but high-rate deposition in the main subsag. The cause and implications of this newly recognized event need to be investigated.
文摘The Baiyun Sag is the deepest sag in the Pearl River Mouth Basin in northern continental margin of South China Sea,with the maximum sediment thickness over 12.5 km above the basement including >6.5 km sediments above the 30 Ma breakup unconformity.According to the theoretical models for the rifted basins,the post-rift subsidence is driven solely by the thermal contraction and can be calculated as the function of the lithospheric stretching factor.A method combining the forward modeling and reverse backstripping was designed to estimate lithospheric stretching factor.Using the 2D forward modeling based on the flexural cantilever model,we simulated the multi-rifting process of the Baiyun Sag with constrain of the backstripped profiles.By doing this the lithospheric stretching factor was obtained,and then the theoretical post-rift subsidence was calculated.The calculated theoretical subsidence was much smaller than the observed subsidence given by backstripping.Along the 1530 line in the Baiyun Sag,the anomalous post-rift subsidence is over 2 km in the sag center,and varies slightly to the north and south edges of the sag.This suggests that the anomalous post-rift subsidence continues beyond the sag both in the continental shelf to the north and in the continental slope to the south.The sensitivity tests in the forward modeling process indicate that only the use of low-angle faults (≤13°) can we simulate the shape of the backstripped profile.
基金Research in this paper was supported by the National Natural Science Foundation of China （Grant No. 41602251）, the Chinese Postdoc Fund, No.58 General Fund, 2015 （No. 2015M582636） and the Research Fund for New Teachers of Xi＇an Jiaotong University. We are grateful to CNOOC Research Institute for providing the seismic profiles. We also thank anonymous reviewers for their careful and insightful suggestions on the paper.
