Evolution of deepwater sedimentary environments and its implication for hydrocarbon exploration in Qiongdongnan Basin, northwestern South China Sea

Over the past several years, a number of hydrocarbon reservoirs have been discovered in the deepwater area of Qiongdongnan Basin, northwestern South China Sea. These oil/gas fields demonstrate that the evolution of the deepwater sedimentary environment are controlling the formation and distribution of large-scale clastic reservoirs. Integration between seismic and borehole data were necessary to best clarify the distribution and quality of these deepwater reservoirs. Geochemical and paleobiological evidence from discrete samples was also applied to document specific information regarding the sedimentary environment. Results show that the Qiongdongnan Basin has existed as a thriving marine environment since Oligocene, when several rifting depressions developed throughout the entire Qiongdongnan Basin. Triggered by the faults activities, several distinct provenances supplied the coarse sediments, transporting and depositing them in deep parts of the rifting depressions. A fan delta system then formed nearby the source in the deeper area of these rifting depressions. The sedimentary environment of Qiongdongnan gradiationally became deepwater since early Miocene. Consequently, abundances of sediments were transported from Hainan Island and Southern Uplift, and then sunk into the basin center. The submarine fans revealed by many boreholes in this area verified them as good reservoir. Because the area reached its lowest sea level at late Miocene and the Southern Uplift subsidenced under sea level, not providing any sediment, so that the carbonate mesa and biorhythms characteristic of this area also developed during this period. In the west part of Qiongdongnan Basin, sediments transported from Vietnam increased in response to the Tibetan Uplift. Consequently, a central canyon developed along the center of Qiongdongnan Basin, which has been confirmed by several boreholes as a favorable hydrocarbon reservoir. The clarification of the deepwater sedimentary environment’s evolution is potentially highly beneficial to future hydrocarbon exploration in the deepwater area of Qiongdongnan Basin.

Properties of Continental Margin and its Hydrocarbon Exploration Significance in Cambrian in the Southern Ordos Kratogen of North China

It is important to determine the properties of the tectonics in Cambrian period for the sake of prospecting deep hydrocarbon in the near future in the southern Ordos Kratogen of North China. Authors chose the marginal areas of the southern Ordos basin as the object of research, avoided the effects of both the Qinling Orogenic Belts （QOB） and Weihe River Graben （WRG） whose geological structures are too complicated. By surveying typical Cambrian outcrops and profdes in the basin edges and based on the cores of 57 wells which penetrated the Cambrian in the basin, combined with the seismic profiles, the field gammaray measuement results and the carbon isotope analysis, Authors conclude that the southern margin of the Ordos Kratogen during Cambrian was a passive continental margin which resulted from sea-floor spreading of the Ancient Qinling Ocean. Epicontinental sea carbonate sediments formed in the south Ordos continental margin during Cambrian, and were predominant as tidal flat and o61itic shoal. Both transgression-regression process and the change in palaeostructure have the obvious cyclicity. Using the junction between the late Nangao age of Qiandong epoch and the early Duyun age of Qiandong epoch as a boundary, each had a full transgression cycle at the upper and lower stages. The early cycle is characterized by high energy clastic littoral facies while the late cycle is characterized by carbonate ramp on which clear water and muddy water developed alternately changing to carbonate platform last. During the early stages, An aulacogen was formed in the middle section of the southern margin. The southern Ordos margin was uplifted and denudated by the Huaiyuan Movement which occurred from the late Furongian age to the middle Flolan age and the history of the passive continental margin ended and entering into a new tectonic cycle. The unconformity surface caused by the Huaiyuan Movement, along with its neighborhood areas where dissolved pores and cavities are developed, may be another important district for good hydrocarbon reservoirs （excluding the unconformity surface on the top of the Ordovician in the Ordos basin）.

Sedimentary characteristics and implications for hydrocarbon exploration in a retrograding shallow-water delta:An example from the fourth member of the Cretaceous Quantou Formation in the Sanzhao depression,Songliao Basin,NE China

Based on the analysis of core,logging,and testing data,the fourth member of the Cretaceous Quantou Formation(K_(2)q^(4))in the Sanzhao depression,Songliao Basin,is investigated in order to understand the sedimentary characteristics and hydrocarbon exploration significance of a retrograding shallow-water delta.The results show that during the sedimentary period of K_(2)q^(4),the Sanzhao depression with a gentle basement experienced stable tectonic subsidence and suffered a long-term lake level rise caused by paleoclimate changes(from semiarid to semihumid),the K_(2)q^(4)in the study area were dominated by a fining-upward deltaic succession and had relatively stable thickness.From the bottom to the top,the color of mudstone gradually changes from purplish-red to gray and grayish-green,the contents of caliche nodules decrease gradually,while the presence of pyrite in sediments becomes frequent.Channel sandstones mainly composed of siltstone and fine sandstone with developed high-energy sedimentary structures constitute the main sand bodies of deltaic deposits,but the scale of channel sandstones decrease upward.Despite the long-term lake level rise and fining-upward sedimentary succession,purplish-red mudstone,caliche nodules and thin channel sandstones are still broadly distributed in the study area,and thin channel sandstones can be found at the top of K_(2)q^(4)covered by the black oil shale of Qingshankou Formation.These assertations suggest that the study area was dominated by retrograding shallow-water delta deposits during the sedimentary period of K_(2)q^(4).In comparison with modern Poyang Lake,we infer that during the sedimentary period of K_(2)q^(4),the study area experienced frequent lake level fluctuations triggered by paleoclimate changes despite the long-term lake level rise,and the lake level fluctuations control the deposition of retrograding shallow-water delta.In addition,most of the thin channel sandstones distributed at the top of K_(2)q^(4)and covered by black oil shale are generally immersed in oil,indicating that the thin channel sandstones formed at the top of a retrograding shallow-water delta sedimentary succession are favorable targets for lithological reservoir exploration.

Responsibilities of petroleum prospectors: Discussions on dual logic and development trend of hydrocarbon exploration

Some unusual events happened in petroleum industry in 2020,such as the negative WTI oil price,price soaring of melt-blown nonwoven fabric,Exxon Mobil Corp.(NYSE:XOM)removed from Dow Jones Industrial Average,and the oil demand peak theory proposed by BP Energy Outlook 2020 Edition.These events have made profound impact on petroleum exploration.Prospecting is at the forefront of petroleum industry chain,and prospectors have great influence on petroleum industry.The responsibility of petroleum prospectors is to find oil,which calls for the correct way of thinking as well as scientific and technical means,both of which are indispensable.When it comes to the cognition of petroleum exploration,we should draw lessons from predecessors’philosophy of finding oil from a development perspective.It is necessary to define the relationship between subject activity and objective structure,as there is an inherent tension between the two and a dialectical relationship that complements each other.It is also essential to illustrate the logic of initiative and decisiveness,as between the two is the dual logic of active logic that changes the world and deterministic logic based on science and technology.The strategic breakthrough in the Gulong shale oil exploration in Daqing is a typical example.Our knowledge and practice of oil exploration has overthrown the Hubbert Curve.The new curve may have more than one peak,which means hopes are always there for finding oil.Climbing to the top of a mountain must start from the foot.A journey of a thousand miles must begin with a single step.Looking forward to the future,prospectors have the wisdom,ability,and methods to find more,cleaner,and more affordable oil to drive the progress of human civilization.This is the duty of petroleum prospectors.

Hydrocarbon accumulation and orderly distribution of whole petroleum system in marine carbonate rocks of Sichuan Basin,SW China

Based on the situation and progress of marine oil/gas exploration in the Sichuan Basin,SW China,the whole petroleum system is divided for marine carbonate rocks of the basin according to the combinations of hydrocarbon accumulation elements,especially the source rock.The hydrocarbon accumulation characteristics of each whole petroleum system are analyzed,the patterns of integrated conventional and unconventional hydrocarbon accumulation are summarized,and the favorable exploration targets are proposed.Under the control of multiple extensional-convergent tectonic cycles,the marine carbonate rocks of the Sichuan Basin contain three sets of regional source rocks and three sets of regional cap rocks,and can be divided into the Cambrian,Silurian and Permian whole petroleum systems.These whole petroleum systems present mainly independent hydrocarbon accumulation,containing natural gas of affinity individually.Locally,large fault zones run through multiple whole petroleum systems,forming a fault-controlled complex whole petroleum system.The hydrocarbon accumulation sequence of continental shelf facies shale gas accumulation,marginal platform facies-controlled gas reservoirs,and intra-platform fault-and facies-controlled gas reservoirs is common in the whole petroleum system,with a stereoscopic accumulation and orderly distribution pattern.High-quality source rock is fundamental to the formation of large gas fields,and natural gas in a whole petroleum system is generally enriched near and within the source rocks.The development and maintenance of large-scale reservoirs are essential for natural gas enrichment,multiple sources,oil and gas transformation,and dynamic adjustment are the characteristics of marine petroleum accumulation,and good preservation conditions are critical to natural gas accumulation.Large-scale marginal-platform reef-bank facies zones,deep shale gas,and large-scale lithological complexes related to source-connected faults are future marine hydrocarbon exploration targets in the Sichuan Basin.

Palaeogeographical development of two merging delta systems(Eocene Shahejie Formation)in the Bohai Bay Basin,E China and implications for hydrocarbon exploration

Two deltas developed simultaneously during the Eocene on the eastern side of a large lake that existed in the Dongying Sub-basin,which forms part of the Bohai Bay Basin in eastern China.The rivers that built the deltas had different catchment areas,which resulted in sediments with different permeability and porosity,due to differences in sorting and mud content.Both deltas prograded,and mass flows that originated frequently on their fronts formed lobes that expanded laterally.This eventually led to merging of both deltas,a feature that has rarely been described from ancient deposits thus far.Core analysis and seismic reflection data show that the merging of the two deltas took place in nine phases,determined by phases of slower progradation or even temporary retrogradation in between.The alternation of sediments from both deltas and their eventual mixing makes the architecture of the merged deltas much more complex than that of'classical'single deltas.This affects the predictability of the spatial distribution of possible reservoir characteristics significantly,but detailed core analysis shows that the best hydrocarbon reservoirs consist of sand bodies formed in distributary channels on the delta plains,and of sandy turbidites formed in the deep-lacustrine environment in front of the merged deltas.

Hydrocarbon Exploration in the South Yellow Sea Based on Airborne Gravity,China

Differences between the Cenozoic and pre-Cenozoic strata and structures in the middlesouth South Yellow Sea are analyzed using high-resolution airborne gravity data combined with data from offshore wells, seismic exploration and the regional geological background, using forward and inverse methods. We discuss why hydrocarbon exploration has so far failed in the South Yellow Sea, and put forward alternative future exploration plans. The results show that there are thick Cenozoic strata over Jurassic-Cretaceous continental strata in the southern basin of the South Yellow Sea, contrasted with thick Mid-Paleozoic marine strata preserved in the middle uplift area. In the mid-southern South Yellow Sea, airborne Bouguer gravity anomalies are fragmentized with scattered local anomalies. Many tensile normal faults and minor fault blocks occur in the Cenozoic strata. In contrast, reverse faults and nappe structures are found in pre-Cenozoic strata. The essential reasons for the lack of a breakthrough in hydrocarbon exploration are the complexity of geological structures, hydrocarbon accumulation environments, seismic-geologic conditions, and the difficulty of interpreting and understanding these features, rather than an imbalance of exploration framework. Hydrocarbon exploration should be targeted at Mid-Cenozoic continental strata, especially of Paleogene age, in the middle to southern parts of the South Yellow Sea. Special attention should be paid to the favorable structural belts SYI1, SYI2, SYI3 and SYI4 in the southern basin of the South Yellow Sea, where detailed seismic exploration or drilling should be carried out. Attention should also be given to Mid-Paleozoic marine strata in the middle uplift area of the South Yellow Sea. Key strategies that could lead to a hydrocarbon exploration breakthrough are improving exploration resolution, conducting detailed studies of fine structures, and accurately locating minor structures.

Deepwater Hydrocarbon Exploration in China Seas:Yesterday,Today and Tomorrow

Deepwater area has been one of the hottest areas in the global hydrocarbon exploration.However,deepwater hydrocarbon exploration in China started late and mainly concentrated in the South China Sea(SCS) with low level of researches.Although there are some achievements,there are even more challenges.For this reason,this paper reviewed the history of deepwater exploration in China Seas and summarized the current exploration situation.The future directions of deepwater hydrocarbon exploration mainly consist of five aspects:establishing key technology system of complex structure and reservoir seismic acquisition and processing in deepwater areas of SCS,clarifying the development mechanism of high quality source rock and establishing matching assessment technology,studying the formation conditions of favorable reservoir and creating reservoir identification technology,improving accumulation theory of large and medium-sized oil and gas field of deepwater,deepening the researches of petroleum geology conditions of middle and south part of SCS.Simultaneously,the concept of portfolio should be utilized in the exploration process,with economic benefits considered.These understandings will help guiding the future deepwater hydrocarbon exploration in China Seas.

Reservoir characteristics,formation mechanisms and petroleum exploration potential of volcanic rocks in China

Characterized by complex lithology and strong heterogeneity, volcanic reservoirs in China developed three reservoir space types： primary pores, secondary pores and fractures. The formation of reservoir space went through the cooling and solidification stage （including blast fragmentation, crystallization differentiation and solidification） and the epidiagenesis stage （including metasomatism, filling, weathering and leaching, formation fluid dissolution and tectonism）. Primary pores were formed at the solidification stage, which laid the foundation for the development and transformation of effective reservoirs. Secondary pores were formed at the epidiagenesis stage, with key factors as weathering and leaching, formation fluid dissolution and tectonism. In China, Mesozoic-Cenozoic volcanic rocks developed in the Songliao Basin and Bohai Bay Basin in the east and Late Paleozoic volcanic rocks developed in the Junggar Basin, Santanghu Basin and Ta- rim Basin in the west. There are primary volcanic reser- voirs and secondary volcanic reservoirs in these volcanic rocks, which have good accumulation conditions and great exploration potential.

The mechanism of unconventional hydrocarbon formation: Hydrocarbon self-sealing and intermolecular forces

The successful development of unconventional hydrocarbons has significantly increased global hydrocarbon resources, promoted the growth of global hydrocarbon production and made a great breakthrough in classical oil and gas geology. The core mechanism of conventional hydrocarbon accumulation is the preservation of hydrocarbons by trap enrichment and buoyancy, while unconventional hydrocarbons are characterized by continuous accumulation and non-buoyancy accumulation. It is revealed that the key of formation mechanism of the unconventional reservoirs is the self-sealing of hydrocarbons driven by intermolecular forces. Based on the behavior of intermolecular forces and the corresponding self-sealing, the formation mechanisms of unconventional oil and gas can be classified into three categories:(1) thick oil and bitumen, which are dominated by large molecular viscous force and condensation force;(2) tight oil and gas, shale oil and gas and coal-bed methane, which are dominated by capillary forces and molecular adsorption;and(3) gas hydrate, which is dominated by intermolecular clathration. This study discusses in detail the characteristics, boundary conditions and geological examples of self-sealing of the five types of unconventional resources, and the basic principles and mathematical characterization of intermolecular forces. This research will deepen the understanding of formation mechanisms of unconventional hydrocarbons, improve the ability to predict and evaluate unconventional oil and gas resources, and promote the development and production techniques and potential production capacity of unconventional oil and gas.

Characteristics of the turbidite fan in the Wenchang Formation of the Enping Sag, Pearl River Mouth Basin, China and its hydrocarbon significance

A turbidite fan in the Eocene upper Wenchang Formation in the Enping Sag, Pearl River Mouth Basin （PRMB） has been studied using seismic, logging and borehole data. The fan is characterized by parallel progradation on the dip seismic profile and is mound-shaped or lenticular-shaped on the strike seismic profile. The study of the core and logging data from well EP17-3-1, which is located in the front side of the turbidite fan, shows that this fan is a set of normal grading sand beds, interbedded within thick dark grey mudstones of semi-deep to deep lake deposits in the Wenchang Formation. The fan is interpreted as a sand/mud-rich turbidite fan that has an area of over 140 km2 and a maximum thickness of over 340 m. Combined with a study of the regional geological background and previous provenance analysis of the Eocene Wenchang Formation, the main potential provenances for the turbidite fan are considered to be the Panyu low-uplift and northern fault terrace zone. The Enping Sag is considered to be a half graben-like basin whose north side is faulted and whose south side is overlapped. Basement subsidence in the Eocene was mainly controlled by boundary faults which dip relatively steeply on the north side, causing the subsidence center of the Enping Sag in this stage to be close to the north boundary faults. Sustained faults developed in the Enping Sag during the Eocene caused an increase of the relative height difference between the north and the south uplift zone in the Enping Sag. Affected by the second episode of the Zhuqiong movement （39-36 Ma） in late Eocene, sediments which had accumulated on the Panyu low-uplift zone were triggered and moved toward the subsidence center of the Enping Sag and formed the turbidite fan. The second episode of the Zhuqiong movement is the most important triggering factor for the formation of the turbidite fan in the Wenchang Formation. Seismic attribute characterization shows that the low frequency energy is enhanced and high frequency energy is weakened when seismic waves propagate through the oil-bearing zone in this fan. Amplitude versus offset （AVO） anomalies are observed in the seismic data and abnormally high pressure is encountered. The turbidite fan in the Wenchang Formation has provided important information for sedimentary evolution in deep layers of the Enping Sag and pointed to a new direction for the hydrocarbon exploration in the study area.

Geochemical,mineralogical,and petrological analyses for the interpretation of the sedimentary environment of the Middle-Late Ordovician Majiagou Formation(northern China)as a tool for more effective gas exploration

Core samples from the deeply buried Ordovician Majiagou Formation below the Huainan Coalfield(E China) have been investigated for their carbonate types,major and trace elements(including rare earth elements) and C and O isotopes,The objective was to get a better insight into the possible occurrences of gas(and possibly oil) derived from Carboniferous coals.It was found that the carbonates are dolomites with strongly varying amounts of CaO and MgO.The low concentrations of SiO_(2),Al_(2)O_(3),and Fe_(2)O_(3)indicate deposition in a normal marine environment with little terrigenous input,The Na_(2)O/K_(2)O,Fe/Mn and Sr/Ba ratios,as well as the Ga values indicate mainly a marine salinity and a hot and humid climate.The slight depletion of Ce and Eu,the depletion of heavy rare earth elements(HREE) and the enrichment of light rare earth elements(LREE) indicate deposition in a reducing environment.It thus appears that the Majiagou Formation below the Huainan Coalfield closely resembles that in the eastern part of the Ordos Basin,where several gas reservoirs are present,so that the Majiagou Formation under the Huainan Coalfield represents a promising target for hydrocarbon exploration.

Early Cenozoic paleontological assemblages and provenance evolution of the Lishui Sag,East China Sea

The East China Sea Shelf Basin generated a series of back-arc basins with thick successions of marine-and terrestrial-facies sediments during Cenozoic.It is enriched with abundant oil and gas resources and is of great significance to the petroleum exploration undertakings.Therein,the Lishui Sag formed fan delta,fluvial delta and littoral-to-neritic facies sediments during Paleocene–Eocene,and the research on its sedimentary environment and sediment source was controversial.This study analyzed the paleontological combination characteristics,and conducted a source-to-sink comparative analysis to restore the sedimentary environment and provenance evolution of the Lishui Sag during Paleocene–Eocene based on the integration of detrital zircon U-Pb age spectra patterns with paleontological assemblages.The results indicated that Lishui Sag was dominated by littoral and neritic-facies environment during time corroborated by large abundance of foraminifera,calcareous nannofossils and dinoflagellates.Chronological analysis of detrital zircon U-Pb revealed that there were significant differences in sediment sources between the east and west area of the Lishui Sag.The western area was featured by deeper water depths in the Paleocene–Eocene,and the sediment was characterized by a single Yanshanian peak of zircon U-Pb age spectra,and mainly influenced from Yanshanian magmatic rocks of South China Coast and the surrounding paleo-uplifts.However,its eastern area partly showed Indosinian populations.In particular,the upper Eocene Wenzhou sediments were featured by increasingly plentiful Precambrian zircons in addition to the large Indosinian-Yanshanian peaks,indicating a possible impact from the Yushan Low Uplift to the east.Therefore,it is likely that the eastern Lishui Sag generated large river systems as well as deltas during time.Due to the Yuquan Movement,the Lishui Sag experienced uplifting and exhumation in the late stage of the late Eocene and was not deposited with sediments until Miocene.Featured by transitional-facies depositions of Paleocene–Eocene,the Lishui Sag thus beared significant potential for source rock and oil-gas reservoir accumulation.

Digital twin in hydrocarbon industry

The hydrocarbon industry is considering a range of digital technologies to improve productivity,efficiency,and safety of their operations while minimizing capital and operating costs,health and environmental risks,and variability in oil and gas project life cycles.Due to the emergence of industry 4.0 the improvement in performance,efficiency,and cost reduction,the hydrocarbon industry is gradually shifting towards solutions that are data-oriented.Understanding such complex systems involves the analysis of data from various sources at the same time.Digital Twin(DT)modelling is the foundation for the next generation of real-time production monitoring and optimization systems.It is a solution that boosts productivity by combining information,simulation,and visualization throughout the entire value chain of an operational firm,from subsurface equipment to central production plants.Oil and gas companies can majorly benefit from Hydrocarbon Exploration with the right use of such advanced technologies.This study focuses on the advancements in technology in the context of DT and how it has been used by the hydrocarbon industry.The study discusses about the emergence of the DT concept,various types,5D representation,and tools for DT.Further,the study tries to implement fields of DT in hydrocarbon industry especially in the domains of exploration,drilling,and production.Challenges associated with DT strategy like accessibility,confidentiality integration,and maintenance are also discussed.

Effects of water discharge on river-dominated delta growth

River-dominated deltas are commonly developed at modern bays and lakes and ancient petroliferous basins.Water discharge is an important variable at pay zone scales in river-dominated delta reservoirs,which affects deltaic sand distributions and evolutions.However,it's unclear how it influences riverdominated delta growth.This paper integrates Delft3 D simulations and modern analogs to analyze the effects of water discharge,considering growth time,sediment supply,and coupled effects of sediment properties.High water discharges lead to the formation of lobate deltas,and the water discharge of 1,000 m~3/s is a referenced threshold value.Fine-grained,highly-cohesive sediments increase the threshold values of water discharge at which the deltas become lobate from digitate,and vice versa.For the same simulation time,high water discharges favor more rugose shorelines,more distributary channels(especially secondary distributaries),and longer and wider deltas with more land areas.However,for the same sediment supply,high water discharges have few effects on shoreline roughness and the number of distributary channels.

Tectonic Evolution and Hydrocarbon Potential in Northern Area of the South Yellow Sea

The northern area of the South Yellow Sea, located in the offshore region of China, resulted from the continental-continental collision orogeny during the Mesozoic and can be divided into four stages in terms of tectonic evolution： （1） pre-orogenic passive continental margin stage （Z-T2）; （2） foreland basin stage corresponding with the late phase of the Sulu （苏鲁） orogeny （J3-K）; （3） post-orogenic intracontinental rifted basin stage （K2t-E）; and （4） regional subsidence and coverage stage （N-Q）. Based on detailed investigation and study of the intracontinental rifted basin, hydrocar- bon source rocks of Late Cretaceous Taizhou （泰州） Formation distributed well in the basin, and four reservoir-cap combinations as well as numerous trap structures were found. As a result, the geological conditions would be excellent for reservoir formation in the basin, and the oil resource amount is estimated at about 20×10^8 t, which makes the basin a major target for hydrocarbon exploration in the South Yellow Sea.

Sedimentary facies and lithologic characters as main factors controlling hydrocarbon accumulation and their critical conditions

Taking more than 1000 clastic hydrocarbon reservoirs of Bohai Bay Basin, Tarim Basin and Junggar Basin, China as examples, the paper has studied the main controlling factors of hydrocarbon reservoirs and their critical conditions to reveal the hydrocarbon distribution and to optimize the search for favorable targets. The results indicated that the various sedimentary facies and lithologic characters control the critical conditions of hydrocarbon accumulation, which shows that hydrocarbon is distributed mainly in sedimentary facies formed under conditions of a long lasting and relatively strong hydrodynamic environment; 95% of the hydrocarbon reservoirs and reserves in the three basins is distributed in siltstones, fine sandstones, conglomerates and pebble-bearing sandstones; moreover, the probability of discovering conventional hydrocarbon reservoirs decreases with the grain size of the clastic rocks. The main reason is that the low relative porosity and permeability of fine-grained reservoirs, lead to small differences in capillary force compared with surrounding rocks and insufficiency of dynamic force for hydrocarbon accumulation; the critical condition for hydrocarbon entering reservoir is that the interfacial potential in the surrounding rock（ Un） must be more than twice of that in the reservoir（ Us）; the probability of hydrocarbon reservoirs distribution decreases in cases where the hydrodynamic force is too high or too low and when the rocks have too coarse or too fine grains.

A Fast Evaluation and Research Method for Exploration Potential of Oversea Basins

Oversea hydrocarbon exploration always faces some problems,such as multiple basin types,different exploration prospects,various exploration degrees,diverse data quantities and geological recognitions.This paper focuses on overseas basins with different exploration degrees and offers a new evaluation system and research method for fast evaluation and risk analysis on hydrocarbon potential.Based on the theory of probability,this new evaluation system evaluates each parameter and core accumulation factor with comprehensive and quantitative assignment,to accelerate geological evaluation.In addition,this paper suggests that source rock evaluation should be the core criterion in screening evaluation of poorexplored basins,and also proposes that both the exploration potential and geological recognition should drive the evaluation for basins with high exploration degrees.This new system and method,which is an effective system for fast basin evaluation,is suggested to be used in oversea evaluation and decision-making objectively,scientifically and efficiently.

Structure of weathered clastic crust and its petroleum potential

Weathered clastic crust can be subdivided into weathered clay and leached zone in terms of variable weathering of different minerals and mobility of weathered products.On the basis of clastic outcrops and well cores in the Junggar Basin,the dark red Fe-rich weathered clay is formed in an arid environment,whereas the light blue Al-rich weathered clay under humid conditions.According to the geochemical analysis,a new weathering index for weathered clastic crust is built mainly on Fe and Al contents,accurately indicating the weathered clay,sandy leached zone,and muddy leached zone in the Junggar Basin.The breaking pressure of weathered clay is rather large,the same as that of normal muddy cap,effectively to seal oil or gas.The porosity of underlying leached zone is greatly enhanced by weathering and leaching,but its permeability is a function of clay mineral content,i.e.,the higher the clay content,the worse the permeability.Weathered crust provides effective sealing conditions for both top and bottom layers of a petroleum reservoir,and is important in the clastic hydrocarbon exploration.

Sedimentary evolution of Neoproterozoic rift basin in northern Tarim

Based on the outcrop,drilling and seismic data,the sedimentary successions,evolution and hydrocarbon exploration potential of the Neoproterozoic rift basin in the northern Tarim were firstly analyzed.Due to assembly and breakup of the Rodinia supercontinent,the Tabei paleocontinent and the Tarim paleocontinent were developed in the Tarim craton with an EW-trending back-arc rift basin between them during Neoproterozoic time;very thick marine clastic rocks,carbonate rocks and volcanic rocks(including tillite)were deposited in the Kuruktag and Aksu area of the northern Tarim,which experienced deep sea,bathyal sea and littoral sea environments with transitional delta and ice-sea.During the Early Cryogenian and the Late Ediacaran,the northern rift basin evolved from the deep sea to the littoralneritic sea,while the lithology changed from clastic rocks to carbonate rocks.According to the field and production data,the formation and preservation of the source rocks and reservoirs indicate a good exploration potential of the Neoproterozoic rift basin.