Whole petroleum system and ordered distribution pattern of conventional and unconventional oil and gas reservoirs

The classical source-to-trap petroleum system concept only considers the migration and accumulation of conventional oil and gas in traps driven dominantly by buoyance in a basin,although revised and improved,even some new concepts as composite petroleum system,total petroleum system,total composite petroleum system,were proposed,but they do not account for the vast unconventional oil and gas reservoirs within the system,which is not formed and distributed in traps dominantly by buoyancedriven.Therefore,the petroleum system concept is no longer adequate in dealing with all the oil and gas accumulations in a basin where significant amount of the unconventional oil and gas resources are present in addition to the conventional oil and gas accumulations.This paper looked into and analyzed the distribution characteristics of conventional and unconventional oil/gas reservoirs and their differences and correlations in petroliferous basins in China and North America,and then proposed whole petroleum system(WPS)concept,the WPS is defined as a natural system that encompasses all the conventional and unconventional oil and gas,reservoirs and resources originated from organic matter in source rocks,the geological elements and processes involving the formation,evolution,and distribution of these oil and gas,reservoirs and resources.It is found in the WPS that there are three kinds of hydrocarbons dynamic fields,three kinds of original hydrocarbons,three kinds of reservoir rocks,and the coupling of these three essential elements lead to the basic ordered distribution model of shale oil/gas reservoirs contacting or interbeded with tight oil/gas reservoirs and separated conventional oil/gas reservoirs from source rocks upward,which is expressed as“S\T-C”.Abnormal conditions lead to other three special ordered distribution models:The first is that with shale oil/gas reservoirs separated from tight oil/gas reservoirs.The second is that with two direction ordered distributions from source upward and downward.The third is with lateral distribution from source outside.

Proppant transport in rough fractures of unconventional oil and gas reservoirs

A method to generate fractures with rough surfaces was proposed according to the fractal interpolation theory.Considering the particle-particle,particle-wall and particle-fluid interactions,a proppant-fracturing fluid two-phase flow model based on computational fluid dynamics(CFD)-discrete element method(DEM)coupling was established.The simulation results were verified with relevant experimental data.It was proved that the model can match transport and accumulation of proppants in rough fractures well.Several cases of numerical simulations were carried out.Compared with proppant transport in smooth flat fractures,bulge on the rough fracture wall affects transport and settlement of proppants significantly in proppant transportation in rough fractures.The higher the roughness of fracture,the faster the settlement of proppant particles near the fracture inlet,the shorter the horizontal transport distance,and the more likely to accumulate near the fracture inlet to form a sand plugging in a short time.Fracture wall roughness could control the migration path of fracturing fluid to a certain degree and change the path of proppant filling in the fracture.On the one hand,the rough wall bulge raises the proppant transport path and the proppants flow out of the fracture,reducing the proppant sweep area.On the other hand,the sand-carrying fluid is prone to change flow direction near the contact point of bulge,thus expanding the proppant sweep area.

A"One Engine with Six Gears"System Engineering Methodology for the Economic Development of Unconventional Oil and Gas in China

Unconventional oil and gas resources have become the most important and realistic field for increasing China’s domestic oil and gas reserves and production.At present,the production scale does not match the massive amount of resources and the rapid growth of proven geological reserves.The challenges of technology,cost,management,and methodology restrict large-scale and economic development.Based on successful practices,a"one engine with six gears"system engineering methodology is put forward,which includes life-cycle management,overall synergy,interdisciplinary cross-service integration,marketoriented operation,socialized support,digitalized management,and low-carbon and green development.The methodology has been proved to be effective in multiple unconventional oil and gas national demonstration areas,including the Jimusar continental shale oil demonstration area.Disruptive views are introduced-namely,that unconventional oil and gas do not necessarily yield a low return,nor do they necessarily have a low recovery factor.A determination to achieve economic benefit must be a pervasive underlying goal for managers and experts.Return and recovery factors,as primary focuses,must be adhered to during China’s development of unconventional oil and gas.The required methodology transformation includes a revolution in management systems to significantly decrease cost and increase production,resulting in technological innovation.

Geological characteristics of unconventional tight oil reservoir (10^(9) t): A case study of Upper Cretaceous Qingshankou Formation, northern Songliao Basin, NE China

The Daqing exploration area in the northern Songliao Basin has great potential for unconventional oil and gas resources,among which the total resources of tight oil alone exceed 109 t and is regarded as an important resource base of Daqing oilfield.After years of exploration in the Qijia area,Songliao Basin,NE China,tight oil has been found in the Upper Cretaceous Qingshankou Formation.To work out tight oil’s geological characteristics,taking tight oil in Gaotaizi oil layers of the Upper Cretaceous Qingshankou Formation in northern Songliao Basin as an example,this paper systematically analyzed the geological characteristics of unconventional tight oil in Gao3 and Gao4 layers of the Qijia area,based on the data of the geological survey,well drilling journey,well logging,and test.It is that three sets of hydrocarbon source rocks(K2qn1,K2qn2+3,and K2n1)develop in the examined area,and exhibit excellent type I and II kerogens,high organic matter abundance,and moderate maturity.The reservoir is generally composed of thin-bedded mudstone,siltstone,and sandstone,and presents poor porosity(average 8.5 vol.%)and air permeability(average 4 mD).The main reservoir space primarily includes intergranular pores,secondary soluble pores,and intergranular soluble pores.Three types of orifice throats were identified,namely fine throat,extra-fine throat,and micro-fine throat.The siltstone is generally oil-bearing,the reservoirs with slime and calcium become worse oil-bearing,and the mudstone has no obvious oil-bearing characteristics.The brittleness indices of the sandstone in the tight oil reservoir range from 40%to 60%,and those of the mudstone range from 40%to 45%,indicating a better brittleness of the tight oil reservoir.Based on the study of typical core hole data,this paper gives a comprehensive evaluation of the properties of the tight oil and establishes a tight oil single well composite bar chart as well as the initial evaluation system with the core of properties in the tight oil reservoir.This study has theoretical guiding significance and practical application value for tight oil exploration and evaluation in the Qijia area.

Progress in China's Unconventional Oil & Gas Exploration and Development and Theoretical Technologies

The new century has witnessed a strategic breakthrough in unconventional oil ＆ gas.Hydrocarbon accumulated in micro-/nano-scale pore throat shale systems has become an important domain that could replace current oil ＆ gas resources.Unconventional oil ＆ gas plays an increasingly important role in our energy demand.Tight gas,CBM,heavy oil and asphaltic sand have served as a key domain of exploration ＆ development,with tight oil becoming a ＇bright spot＇ domain and shale gas becoming a ＇hotspot＇ domain.China has made great breakthroughs in unconventional oil ＆ gas resources,such as tight gas,shale gas,tight oil and CBM,and great progress in oil shale,gas hydrate,heavy oil and oil sand.China has an estimated（223-263）×10~8t of unconventional oil resources and（890-1260）×l0^（12）m^3 of gas resources.China has made a breakthrough for progress in unconventional oil ＆ gas study.New progress achieved in fine-grained sedimentary studies related to continental open lacustrine basin large-scale shallow-water delta sand bodies,lacustrine basin central sandy clastic flow sediments and marine-continental fine-grained sediments provide a theoretical basis for the formation and distribution of basin central reservoir bodies.Great breakthroughs have been made in unconventional reservoir geology in respect of research methodology ＆ technology,multi-scale data merging and physical simulation of formation conditions.Overall characterization of unconventional reservoirs via multi-method and multi-scale becomes increasingly popular and facilitates the rapid development of unconventional oil ＆ gas geological theory,method and technology.The formation of innovative,continuous hydrocarbon accumulation theory,the establishment of the framework of the unconventional oil ＆ gas geological theory system,and the determination of the implications,geological feature,formation mechanism,distribution rule and core technology of unconventional oil＆ gas geological study lays a theoretical foundation for extensive unconventional oil ＆ gas exploration and development.Theories and technologies of unconventional oil ＆ gas exploration and development developed rapidly,including some key evaluation techniques such as ＇sweet spot zone＇ integrated evaluation and a six-property evaluation technique that uses hydrocarbon source,lithology,physical property,brittleness,hydrocarbon potential and stress anisotropy,and some key development ＆engineering technologies including micro-seismic monitoring,horizontal drilling ＆ completion and ＂factory-like＂ operation pattern, ＂man-made reservoir＂ development,which have facilitated the innovative development of unconventional oil ＆ gas.These breakthroughs define a new understanding in four aspects：①theoretical innovation;② key technologies;③ complete market mechanism and national policy support;and ④ well-developed ground infrastructure,which are significant for prolonging the life cycle of petroleum industry,accelerating the upgrade and development of theories and technologies and altering the global traditional energy structure.

A novel method for quantitatively identifying driving forces and evaluating their contributions to oil and gas accumulation

Different driving forces govern the formation of distinct types of oil and gas accumulation and yield diverse oil and gas distributions.Complex oil and gas reservoirs in basins are commonly formed by the combination of multiple forces.It is very difficult but essential to identify driving forces and evaluate their contributions in predicting the type and distribution of oil and gas reservoirs.In this study,a novel method is proposed to identify driving forces and evaluate their contribution based on the critical conditions of porosity and permeability corresponding to buoyancy-driven hydrocarbon accumulation depth(BHAD).The application of this method to the Nanpu Sag of the Bohai Bay Basin shows that all oil and gas accumulations in the reservoirs are jointly formed by four driving forces:buoyance(Ⅰ),non-buoyance(Ⅱ),tectonic stress(Ⅲ1)and geofluid activity(Ⅲ2).Their contributions to all proven reserves are approxi-mately 63.8%,16.2%,2.9%,and 17.0%,respectively.The contribution of the driving forces is related to the depth,distance to faults and unconformity surfaces.Buoyancy dominates the formation of conven-tional reservoirs above BHAD,non-buoyant dominate the formation of unconventional reservoirs below BHAD,tectonic stress dominates the formation of fractured reservoirs within 300 m of a fault,and geoflu-ids activity dominates the formation of vuggy reservoirs within 100 m of an unconformity surface.

Novel Water-Based Drilling and Completion Fluid Technology to Improve Wellbore Quality During Drilling and Protect Unconventional Reservoirs

The efficient exploration and development of unconventional oil and gas are critical for increasing the self-sufficiency of oil and gas supplies in China.However,such operations continue to face serious problems(e.g.,borehole collapse,loss,and high friction),and associated formation damage can severely impact well completion rates,increase costs,and reduce efficiencies.Water-based drilling fluids possess certain advantages over oil-based drilling fluids(OBDFs)and may offer lasting solutions to resolve the aforementioned issues.However,a significant breakthrough with this material has not yet been made,and major technical problems continue to hinder the economic and large-scale development of unconventional oil and gas.Here,the international frontier external method,which only improves drilling fluid inhibition and lubricity,is expanded into an internal-external technique that improves the overall wellbore quality during drilling.Bionic technologies are introduced into the chemical material synthesis process to imitate the activity of life.A novel drilling and completion fluid technique was developed to improve wellbore quality during drilling and safeguard formation integrity.Macroscopic and microscopic analyses indicated that in terms of wellbore stability,lubricity,and formation protection,this approach could outperform methods that use typical OBDFs.The proposed method also achieves a classification upgrade from environmentally protective drilling fluid to an ecologically friendly drilling fluid.The developed technology was verified in more than 1000 unconventional oil and gas wells in China,and the results indicate significant alleviation of the formation damage attributed to borehole collapse,loss,and high friction.It has been recognized as an effective core technology for exploiting unconventional oil and gas resources.This study introduces a novel research direction for formation protection technology and demonstrates that observations and learning from the natural world can provide an inexhaustible source of ideas and inspire the creation of original materials,technologies,and theories for petroleum engineering.

Geological characteristics and ‘‘sweet area'' evaluation for tight oil

Tight oil has become the focus in exploration and development of unconventional oil in the world, especially in North America and China. In North America, there has been intensive exploration for tight oil in marine. In China, commercial exploration for tight oil in conti- nental sediments is now steadily underway. With the dis- covery of China＇s first tight oil field--Xin＇anbian Oilfield in the Ordos Basin, tight oil has been integrated officially into the category for reserves evaluation. Geologically, tight oil is characterized by distribution in depressions and slopes of basins, extensive, mature, and high-quality source rocks, large-scale reservoir space with micro- and nanopore throat systems, source rocks and reservoirs in close contact and with continuous distribution, and local ＂sweet area.＂ The evaluation of the distribution of tight oil ＂sweet area＂ should focus on relationships between ＂six features.＂ These are source properties, lithology, physical properties, brittleness, hydrocarbon potential, and stress anisotropy. In North America, tight oil prospects are distributed in lamellar shale or marl, where natural fractures are fre- quently present, with TOC 〉 4 %, porosity 〉 7 %, brittle mineral content 〉 50 %, oil saturation of 50 %-80 %, API 〉 35~, and pressure coefficient 〉 1.30. In China, tight oil prospects are distributed in lamellar shale, tight sand- stone, or tight carbonate rocks, with TOC 〉 2 %, poros- ity 〉 8 %, brittle mineral content 〉 40 %, oil saturation of 60 %-90 %, low crude oil viscosity, or high formation pressure. Continental tight oil is pervasive in China and its preliminary estimated technically recoverable resources are about （20-25） × lO8^ t.

Geological and Engineering‘Sweet Spots'in the Permian Lucaogou Formation,Jimusar Sag,Junggar Basin

Unconventional oil and gas resources require petrophysical logs to answer the question of how best to optimize geological and engineering‘sweet spots'.Therefore,the establishment of a key well with comprehensive descriptions of lithology,reservoir properties,hydrocarbon-bearing properties,electronic well log responses,source rock properties,brittleness,and in situ stress magnitude and direction is important for the effective exploration and production of unconventional hydrocarbon resources.Cores,thin sections,scanning electron microscopy(SEM)and comprehensive well log suites are used to build a key well for the Permian Lucaogou Formation,Jimusar Sag of the Junggar Basin.The results show that there are three main types of lithologies,including siltstone,mudstone and dolostone.Lithologies can be predicted using the combination of conventional well and image logs.The pore spaces consist of interparticle pores,intragranular dissolution pores and micropores.Nuclear Magnetic Resonance(NMR)T_(2)components longer than 1.7 ms are superposed as effective porosity.Permeability is calculated using the Coates model from NMR T_(2)spectra.The ratio of T_(2)components>7.0 ms to T_(2)components>0.3 ms is used to calculate oil saturation.TOC is calculated using theΔlog R method.Brittleness index is calculated using Poisson-Young's method,ranging from 13.42%-70.53%.In situ stress direction is determined,and in situ stress magnitudes(maximum horizontal stress SH_(max),minimum horizontal stress Sh_(min),vertical stress S_(v))are calculated using density and sonic logs.The strike-slip stress type(SH_(max)>S_(v)>Sh_(min))is encountered.The key well which comprehensively includes the above seven properties is established.Geological and engineering(geomechanical)‘sweet spots'are then optimized from the key well by fully analyzing lithology,reservoir property,oilbearing potential,in situ stress magnitude and brittleness.It is hoped that the results support engineers'and geologists'decisions for the future exploitation of unconventional hydrocarbon resources.

Petroleum geology of marl in Triassic Leikoupo Formation and discovery significance of Well Chongtan1 in central Sichuan Basin,SW China

In 2022,the risk exploration well Chongtan1(CT1)in the Sichuan Basin revealed commercial oil and gas flow during test in a new zone–the marl of the second submember of the third member of Leikoupo Formation(Lei-32)of Middle Triassic,recording a significant discovery.However,the hydrocarbon accumulation in marl remains unclear,which restricts the selection and deployment of exploration area.Focusing on Well CT1,the hydrocarbon accumulation characteristics of Lei-32 marl are analyzed to clarify the potential zones for exploration.The following findings are obtained.First,according to the geochemical analysis of petroleum and source rocks,oil and gas in the Lei-32 marl of Well CT1 are originated from the same marl.The marl acts as both source rock and reservoir rock.Second,the Lei-32 marl in central Sichuan Basin is of lagoonal facies,with a thickness of 40–130 m,an area of about 40000 km^(2),a hydrocarbon generation intensity of(4–12)×10^(8) m^(3)/km^(2),and an estimated quantity of generated hydrocarbons of 25×10^(12) m^(3).Third,the lagoonal marl reservoirs are widely distributed in central Sichuan Basin.Typically,in Xichong–Yilong,Ziyang–Jianyang and Moxi South,the reservoirs are 20–60 m thick and cover an area of 7500 km^(2).Fourth,hydrocarbons in the lagoonal marl are generated and stored in the Lei-32 marl,which means that marl serves as both source rock and reservoir rock.They represent a new type of unconventional resource,which is worthy of exploring.Fifth,based on the interpretation of 2D and 3D seismic data from central Sichuan Basin,Xichong and Suining are defined as favorable prospects with estimated resources of(2000–3000)×10^(8) m^(3).

Gas hydrate saturation model and equation for logging interpretation

Since gas hydrate exists in three different forms at the same time such as pore filling,particle support and separate stratification,the calculation method of hydrate saturation using traditional shaly sand formation interpretation models is equivalent to considering only the simple case that hydrate exists as pore filling,and does not consider other complex states.Based on the analysis of hydrate resistivity experimental data and the general form of the resistivity-oil(gas)saturation relationship,the best simplified formula of hydrate saturation calculation is derived,then the physical meaning of the three items are clarified:they respectively represent the resistivity index-saturation relationship when hydrate particles are completely distributed in the pores of formation rocks,supported in the form of particles,and exist in layers,corresponding quantitative evaluation method of hydrate saturation is built.The field application shows that the hydrate saturation calculated by this method is closer to that obtained by sampling analysis.At the same time,it also provides a logging analysis basis for the effective development after hydrate exploration.

On the connotation, challenge and significance of China’s “energy independence” strategy

The world’s energy is in the "third major transformation period" from fossil energy to new energy, and all countries in the world have formulated energy development strategies. Through advanced deployment, the United States is about to achieve "energy independence" relying on "unconventional oil and gas revolution". China’s energy development is faced with four challenges:(1) The population base and economic development scale determine the "totally huge amount" of energy consumption;(2) the "coal rich but oil and gas insufficient" resource structure determines the "unclean" energy consuming structure;(3) the increasing dependence on imported oil and gas determines the "unsafe" energy supply;and(4) the unconventional oil and gas endowment makes it impossible to achieve energy independence by copying the American model. From the perspective of the world energy trend and the unique situation of China’s energy, we put forward a "three-step" strategy for China to achieve "energy independence": From 2020 to 2035, "energy supply security" will be addressed by "cleaning coal, stabilizing oil and gas production and vigorously developing new alternative energy";from 2035 to 2050, the vision of "production independence" will be realized by relying on "domestic production and overseas oil and gas mining rights";from 2050 to 2100, "intelligent energy and new energy" will help China realize "energy independence". The two important signs of China’s "energy independence" are that domestic production accounts for more than 90% of the domestic consumption and clean energy production accounts for more than 70%, and energy security realizes "independence and self-control" and "long-term security". The strategic significance of "energy independence" is to ensure national energy security, drive the development of relevant major industries, achieve energy management reform, and implement the environmental protection goal of zero carbon emissions. The "energy independence" of China is a strategic mission, it might be fulfilled in the future with the growth of the state’s power, even when the domestic energy production does not catch up with the domestic consumption. Perhaps the world’s new technological revolution will exceed expectations, and China’s "energy independence" dream will eventually come true.

Organic matter occurrence and pore-forming mechanisms in lacustrine shales in China

The evolution of pore structure in shales is affected by both the thermal evolution of organic matter(OM)and by inorganic diagenesis,resulting in a wide variety of pore structures.This paper examines the OM distribution in lacustrine shales and its influence on pore structure,and describes the process of porosity development.The principal findings are:(i)Three distribution patterns of OM in lacustrine shales are distinguished;laminated continuous distribution,clumped distribution,and stellate scattered distribution.The differences in total organic carbon(TOC)content,free hydrocarbon content(S_(1)),and OM porosity among these distribution patterns are discussed.(ii)Porosity is negatively correlated with TOC and plagioclase content and positively correlated with quartz,dolomite,and clay mineral content.(iii)Pore evolution in lacustrine shales is characterized by a sequence of decreasing-increasing-decreasing porosity,followed by continuously increasing porosity until a relatively stable condition is reached.(iv)A new model for evaluating porosity in lacustrine shales is proposed.Using this model,the organic and inorganic porosity of shales in the Permian Lucaogou Formation are calculated to be 2.5%-5%and 1%-6.3%,respectively,which correlate closely with measured data.These findings may provide a scientific basis and technical support for the sweet spotting in lacustrine shales in China.

Application of geophysical well logs in solving geologic issues:Past,present and future prospect

Geophysical well logs are widely used in geological fields,however,there are considerable incompatibilities existing in solving geological issues using well log data.This review critically fills the gaps between geology and geophysical well logs,as assessed from peer reviewed papers and from the authors’personal experiences,in the particular goal of solving geological issues using geophysical well logs.The origin and history of geophysical logging are summarized.Next follows a review of the state of knowledge for geophysical well logs in terms of type of specifications,vertical resolution,depth of investigations and demonstrated applications.Then the current status and advances in applications of geophysical well logs in fields of structural geology,sedimentary geology and petroleum geology are discussed.Well logs are used in structural and sedimentary geology in terms of structure detection,in situ stress evaluation,sedimentary characterization,sequence stratigraphy division and fracture prediction.Well logs can also be applied in petroleum geology fields of optimizing sweet spots for hydraulic fracturing in unconventional oil and gas resource.Geophysical well logs are extending their application in other fields of geosciences,and geological issues will be efficiently solved via well logs with the improvements of advanced well log suits.Further work is required in order to improve accuracy and diminish uncertainties by introducing artificial intelligence.This review provides a systematic and clear descriptions of the applications of geophysical well log data along with examples of how the data is displayed and processed for solving geologic problems.

New advances in the assessment of tight oil resource in China

The research on tight oil becomes another hotspot in the field of unconventional oil and gas after the boom of shale gas.The global recoverable resources of tight oil are estimated to be around 47.3×10^(9)t.In recent years,significant progress has been made in the technologies of tight oil exploration and development in North America,thus stimulating the dramatic increase of tight oil production.Meanwhile,China has also acquired remarkable achievements in tight oil exploration.Mature assessment methods have been established for tight oil resources by the US,generally dominated by analogy and statistical methods with their own advantages and disadvantages as well as applicable conditions.In China,improvement of resource evaluation techniques becomes an urgent issue in increasing tight oil reserve and production.This study mainly discusses the resource evaluation methods and resource enrichment characteristics of tight oil.Seven kinds of assessment methods in three categories(i.e.,analogy,statistical and genetic method)and evaluation parameters have been preliminary established,and are specifically applied in the Sichuan,Ordos,Songliao,Junggar,Bohai Bay and other tight oil basins through the newly hierarchical resource abundance analogy method,the estimated ultimate recovery(EUR)analogy method and the small-cell volumetric method.The preliminary evaluation results reveal that China has great potential in tight oil resource,and the geological resources amount to 20×10^(9)t,providing a resource base for large-scale development.

A review of the microseismic focal mechanism research

Reservoir reconstructions implemented in unconventional oil and gas exploration usually adopt hydraulic fracturing techniques to inject high-pressure fluid into the reservoir and change its pore-fracture connection structure to enhance production.Hydraulic fracturing changes the reservoir stress and causes the rocks to crack,thus generating microseismic events.One important component of microseismic research is the source mechanism inversion.Through the research on the microseismic focal mechanism,information on the source mechanisms and in-situ stress status variations can be quantitatively revealed to effectively optimize the reservoir reconstruction design for increasing production.This paper reviews the recent progress in hydraulic fracturing induced microseismic focal mechanism research.We summarize their main principles and provide a detailed introduction of the research advances in source modeling,microseismic data synthesis,and focal mechanism inversion.We also discuss the challenges and limitations in the current microseismic focal mechanism research and propose prospects for future research ideas and directions.