Velocity structure in the South Yellow Sea basin based on first-arrival tomography of wide-angle seismic data and its geological implications

The South Yellow Sea basin is filled with Mesozoic-Cenozoic continental sediments overlying pre-Palaeozoic and Mesozoic-Palaeozoic marine sediments.Conventional multi-channel seismic data cannot describe the velocity structure of the marine residual basin in detail,leading to the lack of a deeper understanding of the distribution and lithology owing to strong energy shielding on the top interface of marine sediments.In this study,we present seismic tomography data from ocean bottom seismographs that describe the NEE-trending velocity distributions of the basin.The results indicate that strong velocity variations occur at shallow crustal levels.Horizontal velocity bodies show good correlation with surface geological features,and multi-layer features exist in the vertical velocity framework(depth:0–10 km).The analyses of the velocity model,gravity data,magnetic data,multichannel seismic profiles,and drilling data showed that high-velocity anomalies(>6.5 km/s)of small(thickness:1–2 km)and large(thickness:>5 km)scales were caused by igneous complexes in the multi-layer structure,which were active during the Palaeogene.Possible locations of good Mesozoic and Palaeozoic marine strata are limited to the Central Uplift and the western part of the Northern Depression along the wide-angle ocean bottom seismograph array.Following the Indosinian movement,a strong compression existed in the Northern Depression during the extensional phase that caused the formation of folds in the middle of the survey line.This study is useful for reconstructing the regional tectonic evolution and delineating the distribution of the marine residual basin in the South Yellow Sea basin.

Characteristics of Meso-Cenozoic Igneous Complexes in the South Yellow Sea Basin,Lower Yangtze Craton of Eastern China and the Tectonic Setting

The South Yellow Sea Basin is partially surrounded by the East Asian continental Meso- Cenozoic widespread igneous rocks belt. Magnetic anomaly and multi-channel seismic data both reveal the prevalent occurrence of igneous rocks. We preliminarily defined the coupling relation between magnetic anomalies and igneous rock bodies. Some igneous complexes were also recognized by using multi-channel seismic and drilling data. We identified various intrusive and extrusive igneous rock bodies, such as stocks, sills, dikes, laccoliths and volcanic edifice relics through seismic facies analysis. We also forecasted the distribution characteristics of igneous complexes. More than fifty hypabyssal intrusions and volcanic relics were delineated based on the interpretation of magnetic anomaly and dense intersecting multi-channel seismic data. It is an important supplement to regional geology and basin evolution research. Spatial matching relations between igneous rock belts and fractures document that extensional N-E and N-NE-trending deep fractures may be effective pathways for magma intrusion. These fractures formed under the influence of regional extension during the Meso- Cenozoic after the Indosinian movement. Isotopic ages and crosscutting relations between igneous rock bodies and the surrounding bedded sedimentary strata both indicate that igneous activities might have initiated during the Late Jurassic, peaked in the Early Cretaceous, gradually weakened in the Late Cretaceous, and continued until the Miocene. Combined with previous studies, it is considered that the Meso-Cenozoic igneous activities, especially the intensive igneous activity of the Early Cretaceous, are closely associated with the subduction of the Paleo-Pacific Plate.

Hydrocarbon Geological Conditions and Exploration Potential of Mesozoic–Paleozoic Marine Strata in the South Yellow Sea Basin

No substantial breakthroughs have been made in hydrocarbon exploration of the South Yellow Sea Basin.It is believed that the Mesozoic-Paleozoic marine sedimentary formation in the South Yellow Sea Basin is similar to that in the Sichuan Basin.Therefore,outcrop,drilling,and seismic data were determined and compared with the research results on petroleum geological conditions in the Yangtze land area,and the hydrocarbon geological conditions were analyzed comprehensively from multiple aspects,such as basin evolution and main source rocks,reservoir characteristics,preservation conditions,and structural traps.The results show that two main stages in the South Yellow Sea Basin(the stable evolution stage of the Mesozoic-Paleozoic marine basin and the Mesozoic-Cenozoic tectonic reformation and basin formation stage)were important for the development and evolution of four sets of main source rocks.Reservoirs dominated by carbonate rocks,three sets of capping beds with good sealing capability,relatively weak magmatic activity,and multiple types of structural traps jointly constituted relatively good hydrocarbon-reservoir-forming conditions.There were four sets of main source-reservoir-cap assemblages and three possible hydrocarbon reservoir types(primary residual-type hydrocarbon reservoir,shallow reformed-type hydrocarbon reservoir,and composite-type hydrocarbon reservoir)developed in the marine strata.It is concluded that the marine strata in the South Yellow Sea Basin have relatively good hydrocarbon potential.The Laoshan Uplift is characterized by stable structure,complete preserved source reservoir cap assemblage,and large structural traps,and thus it is the preferred prospect zone for marine Paleozoic hydrocarbon exploration in this area.

Basin modeling in the initial stage of exploration： a case study from the North Subbasin of the South Yellow Sea Basin

Basin modeling has become an important tool for analyzing sedimentary basins. The North Subbasin of the South Yellow Sea Basin is filled with thick Meso-Cenozoic terrigenous deposits during the rift evolution stage. The accumulation of data and achievements of geological investigations in recent years have provided the preconditions for basin modeling. The necessary parameters and geological elements for simulations are collated and summarized. Modeling of tectono-thermal evolution is performed and the related trend in heat flow is reconstructed and calibrated. The heat flow value commences from an average level of 61 m W/m2during MiddleLate Jurassic, rises to about 80 m W/m2from circa 145 Ma to circa 74 Ma, and then undergoes a gradual decline to65 m W/m2until the end of Oligocene.Three evolutionary phases, namely, the initial rifting phase, syn-rifting phase, and post-rifting phase, have been identified. The modeling results show that the North Subbasin generally enters into a stage of strong rifting during Cretaceous and undergoes rapid subsidence until the Late Cretaceous,then follows by a stage of moderate rifting during the Paleogene. The input and general workflow involved in 3-D modeling are introduced. Reconstruction of the petroleum system in the North Subbasin reveals that the threshold depth of hydrocarbon generation is located near the top of the Paleogene Funing formation, and the underlying Jurassic and Cretaceous source rocks have reached or exceeded peak oil generation and have almost completed the generation and expulsion of hydrocarbons. The main generation and expulsion in the Jurassic source rocks take place during the syn-rifting and post-rifting phases, whereas the peak generation and expulsion in the Cretaceous and Paleogene source rocks take place during the post-rifting phase. Although the study area is still a relatively less explored sedimentary basin, the results of modeling can provide valuable information for exploration. A preliminary discussion of the main uncertainty factors is also presented.

Tectonic Evolution and Geological Characteristics of Hydrocarbon Reservoirs in Marine Mesozoic-Paleozoic Strata in the South Yellow Sea Basin

The South Yellow Sea Basin is the main body of the lower Yangtze area in which marine Mesozoic–Paleozoic strata are widely distributed.The latest geophysical data were used to overcome the limitation of previous poor-quality deep data.Meanwhile,the geological characteristics of hydrocarbon reservoirs in the marine Mesozoic–Paleozoic strata in the South Yellow Sea Basin were analyzed by comparing the source rocks and the reservoir and utilizing drilling and outcrop data.It is believed that the South Yellow Sea Basin roughly underwent six evolutionary stages:plate spreading,plate convergence,stable platform development,foreland basin development,faulted basin development,and depression basin development.The South Yellow Sea Basin has characteristics of a composite platform-fault depression geological structure,with a half-graben geological structure and with a ‘sandwich structure' in the vertical direction.Four sets of hydrocarbon source rocks developed – the upper Permian Longtan–Dalong formation,the lower Permian Qixia formation,the lower Silurian Gaojiabian formation,and the lower Cambrian Hetang formation/Mufushan formation,giving the South Yellow Sea Basin relatively good hydrocarbon potential.The carbonate is the main reservoir rock type in the South Yellow Sea area,and there are four carbonate reservoir types:porous dolomitic,reef-bank,weathered crust,and fractured.There are reservoir-forming horizons similar to the typical hydrocarbon reservoirs in the Yangtze land area developed in the South Yellow Sea,and there are three sets of complete source-reservoir-cap rock assemblages developed in the marine strata,with very good hydrocarbon potential.

New insights into marine hydrocarbon geological conditions in the South Yellow Sea Basin:evidence from borehole CSDP-2

The South Yellow Sea Basin(SYSB) has multiple sets of proven source rocks and good hydrocarbon prospects,but no industrial oil and gas has been explored at present.To solve this puzzle for petroleum geologists,we systematically investigated the marine hydrocarbon geological conditions based on cores and testing data from borehole CSDP-2,the first exploration well with continuous coring in SYSB.The qualities of source rocks are evaluated in detail according to organic matter abundance,type,and maturity.The reservoir characterization mainly includes porosity,permeability,and reservoir space.Displacement pressure test and stratum thickness are the main foundations for defining the caprocks.Then,the oil-source rock correlation in the Permian and stratum model are analyzed to determine the favorable source-reservoir-caprock assemblages.The results show that three sets of effective source rocks(the Lower Triassic,Upper Permian,and Lower Permian),two sets of tight sandstone re servoirs(the Upper Permian and Lower Silurian-Upper Devonian),and two sets of caprocks(the Lower Triassic and Carboniferous) combine to constitute the hydrocarbon reservoir-forming as se mblages of "lower-ge neration and upper-accumlation" and "self-generation and self-accumlation",thus laying a solid foundation for promising petroleum prospects.The three sets of marine source rocks are characterized by successive generation and expulsion stages,which guarantees multistage hydrocarbon accumulation.Another three sets of continental source rocks distributed across the Middle Jurassic,Upper Cretaceous,and Paleogene depression areas,especially in the Northern Depression,may supplement some hydrocarbons for the Central Uplift through faults and the Indosinian unconformity.The favorable Permian exploration strata have been identified in the Central Uplift of SYSB.First,the Lower Permian and Upper Permian source rocks with high organic matter abundance and high thermal maturity supply sufficient hydrocarbons.Secondly,the interbedding relationship between the source rocks and sandstones in the Upper Permian strata ensures that hydrocarbons have been migrated into the nearby Upper Permian sandstones,reflecting near-source hydrocarbon accumulation.Finally,the good sealing property of the Lower Triassic Qinglong Formation caprocks plays an indispensable role in hydrocarbon preservation of the Permian reservoirs.This conclusion is supported by direct oil shows,gas logging anomalous layers,and hydrocarbon-bearing fluid inclusions.

Structural characteristics and evolution of the South Yellow Sea Basin since Indosinian

Based on the seismic data gathered in past years and the correlation between the sea and land areas of the Lower Yangtze Platform,the structural characteristics of the South Yellow Sea Basin since the Indosinian tectonic movement is studied in this paper.Three stages of structural deformation can be distinguished in the South Yellow Sea Basin since the Indosinian.The first stage,Late Indosinian to Early Yanshanian, was dominated by foreland deformation including both the uplifting and subsidence stages under an intensively compressional environment.The second stage,which is called the Huangqiao Event in the middle Yanshanian,was a change for stress fields from compression to extension.While in the third stage (the Sanduo Event)in the Late Himalayan,the basin developed a depression in the Neogene-Quatemary after rifting in the Late Cretaceous-Paleogene.The long-time evolution controlled 3 basin formation stages from a foreland basin,then a fault basin to a final depression basin.In conclusion,since the Indosinian,the South Yellow Sea Basin has experienced compressional fold and thrust,collisional orogen,compressional and tensional pulsation,strike-slip,extensional fault block and inversion structures,compression and convergence.The NE,NEE,nearly EW and NW trending structures developed in the basin.From west to east,the structural trend changed from NEE to near EW to NW.While from north to south,they changed from NEE to near EW with a strong-weak-strong zoning sequence.Vertically,the marine and terrestrial facies basins show a "seesaw"pattern with fold and thrust in the early stages,which is strong in the north and weak in the south and an extensional fault in later stages,which is strong in the north and weak in the south.In the marine facies'basin,thrust deformation is more prevailing in the upper structural layer than that in the lower layer.The tectonic mechanism in the South Yellow Sea Basin is mainly affected by the collision between the Yangtze and North China Block,while the stress environment of large-scale strike- slip faults was owing to subduction of the Paleo-Pacific plate.The southern part of the Laoshan uplift is a weak deformation zone as well as a stress release zone,and the Meso-Paleozoic had been weakly reformed in later stages.The southern part of the Laoshan uplift is believed,therefore,to be a promising area for oil and gas exploration.

Provenance and Tectonic Implications of Paleozoic Strata in the South Yellow Sea Basin, China-Revealed from the Borehole CSDP-2

Well CSDP-2 is a fully coring deep drilling hole, penetrating the thick Paleozoic marine strata in the South Yellow Sea Basin(SYSB) in the lower Yangtze Block(LYB). Based on the petrological and geochemical analysis of 40 sandstone samples from the core CSDP-2, the provenance and tectonic features of Paleozoic detrital rocks from SYSB are analyzed and systematically delineated in this article. The results show that the Silurian – Carboniferous sandstones are mature sublitharenite, while the Permian sandstones are unstable feldspathic litharenite. The average CIA(chemical index of alteration) is 74.61, which reflects these sediments were derived from source rocks with moderate chemical weathering. The REE(rare earth element) patterns are characterized by LREE enrichment, flat HREE and obviously negative Eu anomaly, which are similar to that of the upper continental crust. Dickinson QFL discrimination results indicate the recycled orogeny provenance. Various diagrams for the discrimination of sedimentary provenance based on major and trace element data show all the sediments were derived predominantly from quartz sedimentary rocks, of which the Permian strata contain more felsic sedimentary rocks. Geochemical data for these detrital rocks suggest they occur at the passive continental margin and island arc settings, and the Permian sandstone presents active continental margin setting.

Tectonic Evolution of the North Depression of the South Yellow Sea Basin Since Late Cretaceous

On the basis of subsidence history analysis and balanced cross-section analysis, the vertical uplift/subsidence history and horizontal extension/compression history of the north depression of the south Yellow Sea basin are quantitatively studied. The results show that the tectonic evolution of the north depression of the south Yellow Sea basin since late Cretaceous can be divided into a rifting phase(late Cretaceous to Paleogene) and a post-rifting phase(Neogene to Quaternary). The rifting phase can be further subdivided into an initial rifting stage(late Cretaceous), an intensive rifting stage(Paleocene), a rifting termination stage(Eocene), and an inversion-uplifting stage(Oligocene). Together, this division shows the characteristics of an episodic-evolved intracontinental rift-depression basin. The deformation of the north depression of the south Yellow Sea basin since late Cretaceous was mainly fault-related. The horizontal extension and tectonic subsidence were controlled by the activity of faults. The differential evolution of faults also caused variations in local uplift/subsidence movements and the regional heterogeneity in extension. The late Cretaceous initial rifting of the north depression of the south Yellow Sea basin is related to the Pacific-Eurasia convergence. From the Paleocene intensive rifting stage to present, the Pacific-Eurasia convergence and India-Eurasia convergence have played important roles in the evolution of this region.

Types,characteristics and implication for hydrocarbon exploration of the Middle Miocene deep-water sediments in Beikang Basin,southern South China Sea

The internal seismic architectures of the Middle Miocene in Beikang Basin, southern South China Sea, were investigated and described using regional 2D seismic data from Guangzhou Marine Geology Survey. In particular, five typical seismic facies were identified based on an integrated analysis of the amplitude, continuity, contact relationship, and morphologies of seismic reflections. Bathyal-abyssal fine-grained sediments, deltaic front sandy bodies, turbidites, and small-scale turbidite channels were developed in the Middle Miocene according to the tectonic-sedimentary evolution of the sedimentary basins in the southern South China Sea. The findings of this study suggest that deltaic front sandy bodies and turbidites can be considered as the two major types of deep-water clastic reservoirs for the depression stage of Beikang Basin. A well-developed source-reservoir-cap assemblage was composed by deep rift-stage source rocks, deep-water clastic reservoirs of the Middle Miocene, and bathyal-abyssal deep-water fine-grained sediments after the Middle Miocene, implying a good potential for hydrocarbon exploration.

Composition, Source and Environmental Indication of Clay Minerals in Sediments from Mud Deposits in he Southern Weihai Offshore, Northwestern Shelf of the South Yellow Sea, China

Studying the sedimentary characteristics of the muddy area along the Yellow Sea shelf is of great significance for deep-ening the understanding of the climate and environment evolution since the last glacial period.Recently,a small muddy area developed in the southern Weihai offshore has attracted a lot of attention.Based on high-resolution grain size,clay mineral,AMS14C,and OSL data of sediments from the core WHZK01 in the muddy area,we acquired a new understanding of sedimentary characteristics,sources and paleoclimatic environment during the last 13 kyr.The results show that the core WHZK01 sediments were mainly from the Yellow River and local rivers along the coast.However,the sources varied for different deposition units.The riverbed(before 13 kyr BP)and fluvial plain deposits(13-10 kyr BP)were mainly from local river inputs.The very thin littoral deposits from 10 kyr BP to 8 kyr BP also mainly originate from coastal river inputs,while the littoral-neritic deposits since 8 kyr BP were dominated by the Yellow River materials and partly supplied by the coastal rivers and the island erosion.In addition,five events related with the strong East Asian winter monsoon(EAWM)during the intervals of 13.0-11.0 kyr BP,10.0-8.2 kyr BP,7.0-5.0 kyr BP,3.5-2.5 kyr BP,and 1.2-0.5 kyr BP were iden-tified.Three events related with the enhancement of the EAWM since the middle Holocene have strengthened the transport capacity of the coastal currents of the Shandong Peninsula.Meanwhile,more Yellow River-derived distal sediments were deposited to the southern Weihai offshore and formed wedge-shaped muddy bodies.In short,the ratio of smectite/illite could be used as an effective EAWM indicator since 13 kyr BP on the northwestern shelf of the South Yellow Sea.

Geochemistry of the upper Permian sandstones in the Dalong Formation in the South Yellow Sea Basin,East China:implications to provenance,weathering,and tectonic setting

To reconstruct the paleoenvironment,the petrography and geochemical compositions of the upper Permian Dalong Formation(Fm)(P_(3)d)sandstones in the Central Uplift,South Yellow Sea Basin,was investigated,after which the provenance,tectonic setting,weathering conditions,and diagenetic history were determined.The Dalong Fm is mainly composed of sandstones with a few mudstones and limestones.The petrographic analysis showed that the P_(3)d sandstones are in low maturity and could be classified into feldspar sandstone and lithic sandstone on average framework composition of Q_(63)F_(16)L_(21).The major and trace element results show that Dalong Fm sandstones are enriched in Rb and depleted in Cr and Ni compared with the upper continental crust(UCC).The chemical weathering intensity is weak to low moderate in the source area as indicated by the chemical index of alteration(CIA)and other results from a variety of methods.The diagenetic process included three main stages:eodiagenesis,mesodiagenesis,and telodiagenesis.Tectonic discrimination plots show that the source areas were an active continental margin(ACM)and a passive continental margin(PCM),which may have been related to the northward movement of the Lower Yangtze Plate(LYP)and the collision between the LYP and the North China Plate(NCP).

Characteristics and classification of paleozoic tight reservoirs in the central uplift of the South Yellow Sea Basin

Several sets of Paleozoic tight reservoirs are developed in the Central Uplift of the South Yellow Sea Basin.A qualitative analysis of the microscopic pore structure of the tight reservoir rocks was carried out through cast thin slice and scanning electron microscopic image observation.Based on reservoir pet-rophysical properties,thirty core samples in the Central Uplift of the South Yellow Sea Basin were selected for high-pressure mercury intrusion(HPMI)analysis,which was then combined with fractal calculation to classify and evaluate the tight reservoirs.The analysis of the HPMI curves and related parameters shows that the Paleozoic tight reservoirs can be divided into three types:Type-A,Type-B and Type-C.Type-A sandstone reservoirs contain pores with size mostly ranging between 0.01 and 0.1 mm,followed by pores with size range of 0.001-0.01 m m,and relatively fewer pores larger than 0.1 m m.The Type-B reservoirs are carbonate rocks with extremely heterogeneous pore size distribution,which is closely related to the development of dissolution pores and microfractures.Type-C sandstone reservoirs are dominated by nanopores and submicron pores that distribute more heterogeneously than pores in Type-A reservoirs.The pore distribution in sandstone reservoirs shows significant fractal characteristics and is closely related to the pore size.The heterogeneity of nanopore distribution has a negative cor-relation with porosity and median pressure and a relatively weak correlation with permeability.Our study has important implications for petroleum exploration in the South Yellow Sea Basin.

Organic geochemical characteristics and significance of Permian Gufeng formation siliceous shale in well CSDP-2 of South Yellow Sea central uplift

In order to understand the hydrocarbon generation potential of the siliceous shale in the Permian Gufeng Formation of the South Yellow Sea,the organic geochemistry and organic petrology of the siliceous shale in the Gufeng Formation of the CSDP-2 well were studied and analyzed.It is found that the TOC of the samples is mainly between 9%and 12%,and Tmaxis mainly distributed between 510 and 540℃.The value of the organic microscopic type index(TI)is between 25.71 and 31.00,which is a set of II_(2)high-quality source rocks with high organic matter abundance in the over mature stage of largescale gas generation.According to the distribution characteristics of Molecular fossils in siliceous shale of Gufeng Formation of well CSDP-2,bacterial organic matter is the main hydrocarbon generating parent material,and aquatic animal organic matter is an important hydrocarbon generating parent material.The siliceous shale in Gufeng Formation of CSDP-2 well is in the stage of large-scale gas generation;Therefore,the Gufeng Formation in the central uplift of the South Yellow Sea has the material basis for the formation of large-scale shale gas reservoirs.At the same time,the siliceous metasomatism of the siliceous shale reservoir of Gufeng Formation is not conducive to the primary migration of oil,and there is much oil and gas remaining in the reservoir.

Prospective prediction and exploration situation of marine Mesozoic-Paleozoic oil and gas in the South Yellow Sea

The South Yellow Sea Basin is a large sedimentary basin superimposed by the Mesozoic-Paleozoic marine sedimentary basin and the Mesozoic-Cenozoic terrestrial sedimentary basin, where no oil and gas fields have been discovered after exploration for 58 years. After the failure of oil and gas exploration in terrestrial basins, the exploration target of the South Yellow Sea Basin turned to the marine Mesozoic- Paleozoic strata. After more than ten years' investigation and research, a lot of achievements have been obtained. The latest exploration obtained effective seismic reflection data of deep marine facies by the application of seismic exploration technology characterized by high coverage, abundant low-frequency components and strong energy source for the deep South Yellow Sea Basin. In addition, some wells drilled the Middle-Upper Paleozoic strata, with obvious oil and gas shows discovered in some horizons. The recent petroleum geological research on the South Yellow Sea Basin shows that the structure zoning of the marine residual basin has been redetermined, the basin structure has been defined, and 3 seismic reflection marker layers are traceable and correlatable in the residual thick Middle-Paleozoic strata below the continental Meso-Cenozoic strata in the South Yellow Sea Basin. Based on these, the seismic sequence of the marine sedimentary strata was established. According to the avaliable oil and gas exploration and research, the marine Mesozoic-Paleozoic oil and gas prospects of the South Yellow Sea were predicted as follows.(1) The South Yellow Sea Basin has the same sedimentary formation and evolution history during the sedimentary period of the Middle-Paleozoic marine basin with the Sichuan Basin.(2) There are 3 regional high-quality source rocks.(3) The carbonate and clastic reservoirs are developed in the Mesozoic- Paleozoic strata.(4) The three source-reservoir-cap assemblages are relatively intact.(5) The Laoshan Uplift is a prospect area for the Lower Paleozoic oil and gas, and the Wunansha Uplift is one for the marine Upper Paleozoic oil and gas.(6) The Gaoshi stable zone in the Laoshan Uplift is a favorable zone.(7) The marine Mesozoic-Paleozoic strata in the South Yellow Sea Basin has the geological conditions required to form large oil and gas fields, with remarkable oil and gas resources prospect. An urgent problem to be addressed now within the South Yellow Sea Basin is to drill parametric wells for the Lower Paleozoic strata as the target, to establish the complete stratigraphic sequence since the Paleozoic period, to obtain resource evaluation parameters, and to realize the strategic discovery and achieve breakthrough in oil and gas exploration understanding.

Geothermal field and thermo-tectonic evolution in Southern South Yellow Sea Basin

Based on the available borehole temperature data and measurements of thermal conductivities on 10 core samples in the Southern South Yellow Sea Basin, 8 heat flow values are obtained. The results show that the mean values of temperature gradient and heat flow are 28.6℃/km and 69 mW/m2, respectively. The thermal history reconstruction from the inversion of vitrinite reflectance data, using the temperature-gradient method, indicates that the highest paleo-heat flow occurred at the end of the Mesozoic, and then the basin began to cool to present day. Tectonic subsidence analysis shows that the basin experienced at least four episodes of quick subsidence since the late Paleozoic and that the tectonic evolution was quite strong and complex.

Characteristics of seismostratigraphy and analysis of structurallithofacies in the southern Yellow Sea Cenozoic basin

In this paper,the upper structural layer of the southern Yellow Sea Basin (SYSB) is further divided into three layers based on the latest obtained high-resolution single-channel seismic profiles.Combined with the borehole data,the characteristics and evolvement of structure and environment since middle-late Tertiary are discussed.The study shows that the acoustic-base indicated by shallow seismic profiles represents the basic shape of the SYSB in late Oligocene-early Miocene.Based on the comprehensive analysis of some typical profiles crossing the basin about seismostratigraphy,gravity and magnetics,the basin about seismostratigraphy,gravity and magnetics,the basin edge fault and some basin inner structure are identified.Furthermore,the acoustic-base is classified into four types of basal lithofacies:Eogene basin basement,Paleozoic basement,Mesozoic basement and magma intrusion basement.

Study of Residual Basin and Tectonolayering Based on Airborne Gravity and Magnetic Data

This paper takes the South Yellow Sea as an example to show a new method for comprehensive geological-geophysical research such as residual basin and tectonolayering using airborne gravity and magnetic data in China. Based on airborne gravity and magnetic data, by measuring and analyzing stratigraphic density and susceptibility, the depths to the pre-Sinian magnetic basement top, the pre-Jurassic top and the Cenozoic bottom, are obtained by forward and inverse methods constrained by seismic and drilling data; and furthermore, the residual thicknesses of the Sinian-Triassic, the Jurassic-Cretaceous and the Cenozoic are calculated. Based on airborne gravity and magnetic anomalies, the faults in the pre-Sinian magnetic basement, the Jurassic-Cretaceous and the Cenozoic are respectively interpreted by the qualitative and quantitative methods. On the basis of the above study, and combining regional important tectonic events, four tectonolayers are divided in the vertical succession in South Yellow Sea, namely the pre-Sinian magnetic basement, the Sinian-Triassic, the Jurassic-Cretaceous and the Cenozoic. The result shows that there are thick Cenozoic and Jurassic-Cretaceous strata and thin residual Sinian-Triassic strata in the Suzhong-Huangnan depression area, but there are thin or only sporadic Mesozoic-Cenozoic terrestrial strata and thick Sinian-Triassic marine strata reserved in Subei-Huangzhong uplift area and Sunan-Wunansha uplift area. The four tectonolayers are very different in structures as well as distributions in plane.

Preliminary Prediction and Evaluation on Source Rock in Low Exploration Basin—A Case Study from the Northeast Depression,South Yellow Sea Basin,East China

Low exploration activity fields（e.g.,deep formation,deep water,and new exploration areas） are of great importance for petroleum exploration.The prediction of source rocks is critical to the preliminary evaluation of low exploration activity fields,which will determine the early strategic electoral district and decision of exploration activity.The northeast depression is a new exploration area at the South Yellow Sea basin at present.Based on lots of seismic data in the study area and a few of well data in adjacent homologous depression,the early evaluation of source rock in the northeast depression was finished by comprehensively using the techniques of geological analogy,geophysical inversion and basin modeling,and an integrated methodology about preliminary evaluation of source rocks to low exploration activity fields is also presented.The methodology consists of three aspects：（1） prediction of the main formation of major source rocks,based on analogy analysis of seismic reflection characteristics and quality of source rocks in adjacent homologous depression;（2） method of seismic attribution to determine the thickness of source rocks;and（3） prediction of vitrinite reflectance,based on basin modeling technique to determine the maturation of source rocks.The results suggest that the source rock of the Taizhou（泰州） Formation is the most important interval for hydrocarbon generationin the northeast depression,which is characterized as high TOC,fine OM type,huge thickness,and high maturity.The western subdepression is the most important hydrocarbon kitchen in the northeast depression.

YZ1斜井定向井小井眼侧钻技术

YZ1斜井是位于南黄海盆地南五凹陷北部斜坡带南中11号构造的第一口风险探井。YZ1斜井215.9 mm井眼段在钻至3588 m后无进尺,起钻至井口发现钻头落井,多次尝试打捞失败后,决定在裸眼井段注200 m水泥塞后侧钻开窗。侧钻点较深、地层强度高、地层岩性多变、井况复杂,侧钻作业难度大。通过减少扶正器数量,选择5刀翼、强攻击性的胜利天工S323 PDC型钻头搭配1.5°螺杆马达的钻具组合,进行控压控时钻进、划槽、造台阶,克服了该段侧钻位置深、井况复杂、地层强度高等困难,一次性成功侧钻。侧钻过程顺利、新井眼轨迹平滑、井斜降低。