Research advances on the mechanisms of reservoir formation and hydrocarbon accumulation and the oil and gas development methods of deep and ultra-deep marine carbonates

Based on the new data of drilling, seismic, logging, test and experiments, the key scientific problems in reservoir formation, hydrocarbon accumulation and efficient oil and gas development methods of deep and ultra-deep marine carbonate strata in the central and western superimposed basin in China have been continuously studied.(1) The fault-controlled carbonate reservoir and the ancient dolomite reservoir are two important types of reservoirs in the deep and ultra-deep marine carbonates. According to the formation origin, the large-scale fault-controlled reservoir can be further divided into three types:fracture-cavity reservoir formed by tectonic rupture, fault and fluid-controlled reservoir, and shoal and mound reservoir modified by fault and fluid. The Sinian microbial dolomites are developed in the aragonite-dolomite sea. The predominant mound-shoal facies, early dolomitization and dissolution, acidic fluid environment, anhydrite capping and overpressure are the key factors for the formation and preservation of high-quality dolomite reservoirs.(2) The organic-rich shale of the marine carbonate strata in the superimposed basins of central and western China are mainly developed in the sedimentary environments of deep-water shelf of passive continental margin and carbonate ramp. The tectonic-thermal system is the important factor controlling the hydrocarbon phase in deep and ultra-deep reservoirs, and the reformed dynamic field controls oil and gas accumulation and distribution in deep and ultra-deep marine carbonates.(3) During the development of high-sulfur gas fields such as Puguang, sulfur precipitation blocks the wellbore. The application of sulfur solvent combined with coiled tubing has a significant effect on removing sulfur blockage. The integrated technology of dual-medium modeling and numerical simulation based on sedimentary simulation can accurately characterize the spatial distribution and changes of the water invasion front.Afterward, water control strategies for the entire life cycle of gas wells are proposed, including flow rate management, water drainage and plugging.(4) In the development of ultra-deep fault-controlled fractured-cavity reservoirs, well production declines rapidly due to the permeability reduction, which is a consequence of reservoir stress-sensitivity. The rapid phase change in condensate gas reservoir and pressure decline significantly affect the recovery of condensate oil. Innovative development methods such as gravity drive through water and natural gas injection, and natural gas drive through top injection and bottom production for ultra-deep fault-controlled condensate gas reservoirs are proposed. By adopting the hierarchical geological modeling and the fluid-solid-thermal coupled numerical simulation, the accuracy of producing performance prediction in oil and gas reservoirs has been effectively improved.

A review of reservoir damage during hydraulic fracturing of deep and ultra-deep reservoirs

Deep and ultra-deep reservoirs have gradually become the primary focus of hydrocarbon exploration as a result of a series of significant discoveries in deep hydrocarbon exploration worldwide.These reservoirs present unique challenges due to their deep burial depth(4500-8882 m),low matrix permeability,complex crustal stress conditions,high temperature and pressure(HTHP,150-200℃,105-155 MPa),coupled with high salinity of formation water.Consequently,the costs associated with their exploitation and development are exceptionally high.In deep and ultra-deep reservoirs,hydraulic fracturing is commonly used to achieve high and stable production.During hydraulic fracturing,a substantial volume of fluid is injected into the reservoir.However,statistical analysis reveals that the flowback rate is typically less than 30%,leaving the majority of the fluid trapped within the reservoir.Therefore,hydraulic fracturing in deep reservoirs not only enhances the reservoir permeability by creating artificial fractures but also damages reservoirs due to the fracturing fluids involved.The challenging“three-high”environment of a deep reservoir,characterized by high temperature,high pressure,and high salinity,exacerbates conventional forms of damage,including water sensitivity,retention of fracturing fluids,rock creep,and proppant breakage.In addition,specific damage mechanisms come into play,such as fracturing fluid decomposition at elevated temperatures and proppant diagenetic reactions at HTHP conditions.Presently,the foremost concern in deep oil and gas development lies in effectively assessing the damage inflicted on these reservoirs by hydraulic fracturing,comprehending the underlying mechanisms,and selecting appropriate solutions.It's noteworthy that the majority of existing studies on reservoir damage primarily focus on conventional reservoirs,with limited attention given to deep reservoirs and a lack of systematic summaries.In light of this,our approach entails initially summarizing the current knowledge pertaining to the types of fracturing fluids employed in deep and ultra-deep reservoirs.Subsequently,we delve into a systematic examination of the damage processes and mechanisms caused by fracturing fluids within the context of hydraulic fracturing in deep reservoirs,taking into account the unique reservoir characteristics of high temperature,high pressure,and high in-situ stress.In addition,we provide an overview of research progress related to high-temperature deep reservoir fracturing fluid and the damage of aqueous fracturing fluids to rock matrix,both artificial and natural fractures,and sand-packed fractures.We conclude by offering a summary of current research advancements and future directions,which hold significant potential for facilitating the efficient development of deep oil and gas reservoirs while effectively mitigating reservoir damage.

Geochemical characteristics and exploration significance of ultra-deep Sinian oil and gas from Well Tashen 5,Tarim Basin,NW China

The Well Tashen 5(TS5),drilled and completed at a vertical depth of 9017 m in the Tabei Uplift of the Tarim Basin,NW China,is the deepest well in Asia.It has been producing both oil and gas from the Sinian at a depth of 8780e8840 m,also the deepest in Asia in terms of oil discovery.In this paper,the geochemical characteristics of Sinian oil and gas from the well were investigated and compared with those of Cambrian oil and gas discovered in the same basin.The oil samples,with Pr/Ph ratio of 0.78 and a whole oil carbon isotopic value of31.6‰,have geochemical characteristics similar to those of Ordovician oils from the No.1 fault in the North Shuntuoguole area(also named Shunbei area)and the Middle Cambrian oil from wells Zhongshen 1(ZS1)and Zhongshen 5(ZS5)of Tazhong Uplift.The maturity of light hydrocarbons,diamondoids and aromatic fractions all suggest an approximate maturity of 1.5%e1.7%Ro for the samples.The(4-+3-)methyldiamantane concentration of the samples is 113.5 mg/g,indicating intense cracking with a cracking degree of about 80%,which is consistent with the high bottom hole temperature(179℃).The Sinian gas samples are dry with a dryness coefficient of 0.97.The gas is a mixture of kerogen-cracking gas and oil-cracking gas and has Ro values ranging between 1.5%and 1.7%,and methane carbon isotopic values of41.6‰.Based on the equivalent vitrinite reflectance(R_(eqv)=1.51%e1.61%)and the thermal evolution of source rocks from the Cambrian Yu'ertusi Formation of the same well,it is proposed that the Sinian oil and gas be mainly sourced from the Cambrian Yu'ertusi Formation during the Himalayan period but probably also be joined by hydrocarbon of higher maturity that migrated from other source rocks in deeper formations.The discovery of Sinian oil and gas from Well TS5 suggests that the ancient ultra-deep strata in the northern Tarim Basin have the potential for finding volatile oil or condensate reservoirs.

Supramolecular polymer-based gel fracturing fluid with a double network applied in ultra-deep hydraulic fracturing

A gel based on polyacrylamide,exhibiting delayed crosslinking characteristics,emerges as the preferred solution for mitigating degradation under conditions of high temperature and extended shear in ultralong wellbores.High viscosity/viscoelasticity of the fracturing fluid was required to maintain excellent proppant suspension properties before gelling.Taking into account both the cost and the potential damage to reservoirs,polymers with lower concentrations and molecular weights are generally preferred.In this work,the supramolecular action was integrated into the polymer,resulting in significant increases in the viscosity and viscoelasticity of the synthesized supramolecular polymer system.The double network gel,which is formed by the combination of the supramolecular polymer system and a small quantity of Zr-crosslinker,effectively resists temperature while minimizing permeability damage to the reservoir.The results indicate that the supramolecular polymer system with a molecular weight of(268—380)×10^(4)g/mol can achieve the same viscosity and viscoelasticity at 0.4 wt%due to the supramolecular interaction between polymers,compared to the 0.6 wt%traditional polymer(hydrolyzed polyacrylamide,molecular weight of 1078×10^(4)g/mol).The supramolecular polymer system possessed excellent proppant suspension properties with a 0.55 cm/min sedimentation rate at 0.4 wt%,whereas the0.6 wt%traditional polymer had a rate of 0.57 cm/min.In comparison to the traditional gel with a Zrcrosslinker concentration of 0.6 wt%and an elastic modulus of 7.77 Pa,the double network gel with a higher elastic modulus(9.00 Pa)could be formed only at 0.1 wt%Zr-crosslinker,which greatly reduced the amount of residue of the fluid after gel-breaking.The viscosity of the double network gel was66 m Pa s after 2 h shearing,whereas the traditional gel only reached 27 m Pa s.

Control of Earth system evolution on the formation and enrichment of marine ultra-deep petroleum in China

Taking the Paleozoic of the Sichuan and Tarim basins in China as example,the controlling effects of the Earth system evolution and multi-spherical interactions on the formation and enrichment of marine ultra-deep petroleum in China have been elaborated.By discussing the development of“source-reservoir-seal”controlled by the breakup and assembly of supercontinents and regional tectonic movements,and the mechanisms of petroleum generation and accumulation controlled by temperature-pressure system and fault conduit system,Both the South China and Tarim blocks passed through the intertropical convergence zone(ITCZ)of the low-latitude Hadley Cell twice during their drifts,and formed hydrocarbon source rocks with high quality.It is proposed that deep tectonic activities and surface climate evolution jointly controlled the types and stratigraphic positions of ultra-deep hydrocarbon source rocks,reservoirs,and seals in the Sichuan and Tarim basins,forming multiple petroleum systems in the Ediacaran-Cambrian,Cambrian-Ordovician,Cambrian-Permian and Permian-Triassic strata.The matching degree of source-reservoir-seal,the type of organic matter in source rocks,the deep thermal regime of basin,and the burial-uplift process across tectonic periods collectively control the entire process from the generation to the accumulation of oil and gas.Three types of oil and gas enrichment models are formed,including near-source accumulation in platform marginal zones,distant-source accumulation in high-energy beaches through faults,and three-dimensional accumulation in strike-slip fault zones,which ultimately result in the multi-layered natural gas enrichment in ultra-deep layers of the Sichuan Basin and co-enrichment of oil and gas in the ultra-deep layers of the Tarim Basin.

Multi-lump Kinetic Parameter Estimation and Simulation of Trickle-bed Reactor for Ultra-deep Hydrodesulfurization of Diesel

A three-lumping Langmuir-Hinshelwood kinetic model was established based on the structures and reactivities of sulfur compounds.This model described the ultra-deep hydrodesulfurization(UDHDS)performance of diesel,reducing sulfur content from 10000μg/g to less than 10μg/g,with experimental and predicted data showing a discrepancy of less than 10%.The diesel UDHDS reaction was simulated by combining the mass transfer,reaction kinetics model,and physical properties of diesel.The results showed how the concentrations of H2S,hydrogen,and sulfur in the gas,liquid,and solid phases varied along the reactor length.Moreover,the study discussed the effects of each process parameter and impurity concentrations(H2S,basic nitrogen and,non-basic nitrogen)on diesel UDHDS.

A Novel Fracturing Fluid with High-Temperature Resistance for Ultra-Deep Reservoirs

Ultra-deep reservoirs play an important role at present in fossil energy exploitation.Due to the related high temperature,high pressure,and high formation fracture pressure,however,methods for oil well stimulation do not produce satisfactory results when conventional fracturing fluids with a low pumping rate are used.In response to the above problem,a fracturing fluid with a density of 1.2~1.4 g/cm^(3)was developed by using Potassium formatted,hydroxypropyl guanidine gum and zirconium crosslinking agents.The fracturing fluid was tested and its ability to maintain a viscosity of 100 mPa.s over more than 60 min was verified under a shear rate of 1701/s and at a temperature of 175℃.This fluid has good sand-carrying performances,a low viscosity after breaking the rubber,and the residue content is less than 200 mg/L.Compared with ordinary reconstruction fluid,it can increase the density by 30%~40%and reduce the wellhead pressure of 8000 m level reconstruction wells.Moreover,the new fracturing fluid can significantly mitigate safety risks.

Research progress and development of deep and ultra-deep drilling fluid technology

The research progress of deep and ultra-deep drilling fluid technology systematically reviewed,the key problems existing are analyzed,and the future development direction is proposed.In view of the high temperature,high pressure and high stress,fracture development,wellbore instability,drilling fluid lost circulation and other problems faced in the process of deep and ultra-deep complex oil and gas drilling,scholars have developed deep and ultra-deep high-temperature and high-salt resistant water-based drilling fluid technology,high-temperature resistant oil-based/synthetic drilling fluid technology,drilling fluid technology for reservoir protection and drilling fluid lost circulation control technology.However,there are still some key problems such as insufficient resistance to high temperature,high pressure and high stress,wellbore instability and serious lost circulation.Therefore,the development direction of deep and ultra-deep drilling fluid technology in the future is proposed:(1)The technology of high-temperature and high-salt resistant water-based drilling fluid should focus on improving high temperature stability,improving rheological properties,strengthening filtration control and improving compatibility with formation.(2)The technology of oil-based/synthetic drilling fluid resistant to high temperature should further study in the aspects of easily degradable environmental protection additives with low toxicity such as high temperature stabilizer,rheological regulator and related supporting technologies.(3)The drilling fluid technology for reservoir protection should be devoted to the development of new high-performance additives and materials,and further improve the real-time monitoring technology by introducing advanced sensor networks and artificial intelligence algorithms.(4)The lost circulation control of drilling fluid should pay more attention to the integration and application of intelligent technology,the research and application of high-performance plugging materials,the exploration of diversified plugging techniques and methods,and the improvement of environmental protection and production safety awareness.

Prediction Model of Drilling Costs for Ultra-Deep Wells Based on GA-BP Neural Network

Drilling costs of ultra-deepwell is the significant part of development investment,and accurate prediction of drilling costs plays an important role in reasonable budgeting and overall control of development cost.In order to improve the prediction accuracy of ultra-deep well drilling costs,the item and the dominant factors of drilling costs in Tarim oilfield are analyzed.Then,those factors of drilling costs are separated into categorical variables and numerous variables.Finally,a BP neural networkmodel with drilling costs as the output is established,and hyper-parameters(initial weights and bias)of the BP neural network is optimized by genetic algorithm(GA).Through training and validation of themodel,a reliable prediction model of ultra-deep well drilling costs is achieved.The average relative error between prediction and actual values is 3.26%.Compared with other models,the root mean square error is reduced by 25.38%.The prediction results of the proposed model are reliable,and the model is efficient,which can provide supporting for the drilling costs control and budget planning of ultra-deep wells.

Research progress and key issues of ultra-deep oil and gas exploration in China

Based on the recent oil and gas discoveries and geological understandings on the ultra-deep strata of sedimentary basins, the formation and occurrence of hydrocarbons in the ultra-deep strata were investigated with respect to the processes of basin formation, hydrocarbon generation, reservoir formation and hydrocarbon accumulation, and key issues in ultra-deep oil and gas exploration were discussed. The ultra-deep strata in China underwent two extensional-convergent cycles in the Meso-Neoproterozoic Era and the Early Paleozoic Era respectively, with the tectonic-sedimentary differentiation producing the spatially adjacent source-reservoir assemblages. There are diverse large-scale carbonate reservoirs such as mound-beach, dolomite, karst fracture-vug, fractured karst and faulted zone, as well as over-pressured clastic rock and fractured bedrock reservoirs. Hydrocarbons were accumulated in multiple stages, accompanied by adjusting and finalizing in the late stage. The distribution of hydrocarbons is controlled by high-energy beach zone, regional unconformity, paleo-high and large-scale fault zone. The ultra-deep strata endow oil and gas resources as 33% of the remaining total resources, suggesting an important successive domain for hydrocarbon development in China. The large-scale pool-forming geologic units and giant hydrocarbon enrichment zones in ultra-deep strata are key and promising prospects for delivering successive discoveries. The geological conditions and enrichment zone prediction of ultra-deep oil and gas are key issues of petroleum geology.

Theoretical Progress and Key Technologies of Onshore Ultra-Deep Oil/Gas Exploration

Oil/gas exploration around the world has extended into deep and ultra-deep strata because it is increasingly difficult to find new large-scale oil/gas reservoirs in shallow–middle buried strata. In recent years, China has made remarkable achievements in oil/gas exploration in ultra-deep areas including carbonate and clastic reservoirs. Some (ultra) large-scale oil and gas fields have been discovered. The oil/gas accumulation mechanisms and key technologies of oil/gas reservoir exploration and development are summarized in this study in order to share China’s experiences. Ultra-deep oil/gas originates from numerous sources of hydrocarbons and multiphase charging. Liquid hydrocarbons can form in ultradeep layers due to low geothermal gradients or overpressures, and the natural gas composition in ultra-deep areas is complicated by the reactions between deep hydrocarbons, water, and rock or by the addition of mantle- or crust-sourced gases. These oils/gases are mainly stored in the original highenergy reef/shoal complexes or in sand body sediments. They usually have high original porosity. Secondary pores are often developed by dissolution, dolomitization, and fracturing in the late stage. The early pores have been preserved by retentive diageneses such as the early charging of hydrocarbons. Oil/gas accumulation in ultra-deep areas generally has the characteristics of near-source accumulation and sustained preservation. The effective exploration and development of ultra-deep oil/gas reservoirs depend on the support of key technologies. Use of the latest technologies such as seismic signal acquisition and processing, low porosity and permeability zone prediction, and gas–water identification has enabled the discovery of ultra-deep oil/gas resources. In addition, advanced technologies for drilling, completion, and oil/gas testing have ensured the effective development of these fields.

Zoned Zircon from Eclogite Lenses in Marbles from the Dabie-Sulu UHP Terrane,China: A Clear Record of Ultra-deep Subduction and Fast Exhumation

Eclogite lenses in marbles from the Dabie-Sulu ultrahigh-pressure （UHP） terrane are deeply subducted meta-sedimentary rocks. Zircons in these rocks have been used to constrain the ages of prograde and UHP metamorphism during subduction, and later retrograde metamorphism during exhumation. Inherited （detrital） and metamorphic zircons were distinguished on the basis of transmitted light microscopy, cathodoluminescence （CL） imaging, trace element contents and mineral inclusions. The distribution of mineral inclusions combined with CL imaging of the metamorphic zircon make it possible to relate zircon zones （domains） to different metamorphic stages. Domain 1 consists of rounded, oblong and spindly cores with dark-luminescent images, and contains quartz eclogite facies mineral inclusion assemblages, indicating formation under high-pressure （HP） metamorphic conditions of T = 571-668℃ and P =1.7-2.02 GPa. Domain 2 always surrounds domain 1 or occurs as rounded and spindly cores with white-luminescent images. It contains coesite eclogite facies mineral inclusion assemblages, indicating formation under UHP metamorphic conditions of T = 782-849℃ and P 〉 5.5 GPa. Domain 3, with gray-luminescent images, always surrounds domain 2 and occurs as the outermost zircon rim. It is characterized by low-pressure mineral inclusion assemblages, which are related to regional amphibolite facies retrograde metamorphism of T = 600- 710℃ and P = 0.7-1.2 GPa. The three metamorphic zircon domains have distinct ages; sample H1 from the Dabie terrane yielded SHRIMP ages of 245 ± 4 Ma for domain 1, 235 ± 3 Ma for domain 2 and 215± 6 Ma for domain 3, whereas sample H2 from the Sulu terrane yielded similar ages of 244 ± 4 Ma, 233 ± 4 Ma and 214 ± 5 Ma for Domains 1, 2 and 3, respectively. The mean ages of these zones suggest that subduction to UHP depths took place over 10-11 Ma and exhumation of the rocks occurred over a period of 19-20 Ma. Thus, subduction from - 55 km to 〉 160 km deep mantle depth took place at rates of approximately 9.5-10.5 km/Ma and exhumation from depths 〉160 km to the base of the crust at -30 km occurred at approximately 6.5 km/Ma. We propose a model for these rocks involving deep subduction of continental margin lithosphere followed by ultrafast exhumation driven by buoyancy forces after break-off of the UHP slab deep within the mantle.

Optimum Design and Global Analysis of Flexible Jumper for An Innovative Subsurface Production System in Ultra-Deep Water

The study focuses on the flexible jumper issue of Subsurface Tension Leg Production （STLP） system concept, which is considered as a competing alternative system to support well completion devices and rigid risers in ultra-deep water for offshore petroleum production. The paper presents analytical and numerical approaches for the optimum design and global analysis of the flexible jumper. Criteria using catenary concept are developed to define the critical length for optimum design. Based on the criteria, detailed hydrodynamic analyses including quasi-static analysis, modal analysis, and dynamic analysis are performed. Modal analysis with respect to the quasi-static analysis shows that the existence of resonant modes requires special consideration. The results of dynamic analysis confirm the effectiveness of the de-coupled effect from the jumper on STLP system. The approaches developed in the study also have wide application prospect in reference to the optimum design and analysis of any Hybrid Riser （HR） concept.

Parametric Study on the Behavior of An Innovative Subsurface Tension Leg Platform in Ultra-Deep Water

This study focuses on a new technology of Subsurface Tension Leg Platform （STLP）, which utilizes the shallow- water rated well completion equipment and technology for the development of large oil and gas fields in ultra-deep water （UDW）. Thus, the STLP concept offers attractive advantages over conventional field development concepts. STLP is basically a pre-installed Subsurface Sea-star Platform （SSP）, which supports rigid risers and shallow-water rated well completion equipment. The paper details the results of the parametric study on the behavior of STLP at a water depth of 3000 m. At first, a general description of the STLP configuration and working principle is introduced. Then, the numerical models for the global analysis of the STLP in waves and current are presented. After that, extensive parametric studies are carried out with regarding to SSP/tethers system analysis, global dynamic analysis and riser interference analysis. Critical points are addressed on the mooring pattern and riser arrangement under the influence of ocean current, to ensure that the requirements on SSP stability and riser interference are well satisfied. Finally, conclusions and discussions are made. The results indicate that STLP is a competitive well and riser solution in up to 3000 m water depth for offshore petroleum production.

Parametric Dimensional Analysis on the Structural Response of An Innovative Subsurface Tension Leg Platform in Ultra-Deep Water

The innovative Subsurface Tension Leg Platform(STLP), which is designed to be located below Mean Water Level(M.W.L) to minimize direct wave loading and mitigate the effect of strong surface currents, is considered as a competitive alternative system to support shallow-water rated well completion equipment and rigid risers for large ultra-deep water oil field development. A detailed description of the design philosophy of STLP has been published in the series of papers and patents. Nonetheless, design uncertainties arise as limited understanding of various parameters effects on the structural response of STLP, pertaining to the environmental loading, structural properties and hydrodynamic characteristics. This paper focuses on providing quantitative methodology on how each parameter affects the structural response of STLP, which will facilitate establishing the unique design criteria as regards to STLP. Firstly, the entire list of dimensionless groups of input and output parameters is proposed based on VaschyBuckingham theory. Then, numerical models are built and a series of numerical tests are carried out for validating the obtained dimensionless groups. On this basis, the calculation results of a great quantity of parametric studies on the structural response of STLP are presented and discussed in detail. Further, empirical formulae for predicting STLP response are derived through nonlinear regression analysis. Finally, conclusions and discussions are made. It has been demonstrated that the study provides a methodology for better control of key parameters and lays the foundation for optimal design of STLP. The obtained conclusions also have wide ranging applicability in reference to the engineering design and design analysis aspects of deepwater buoy supporting installations, such as Grouped SLOR or TLR system.

Evolution of diagenetic fluid of ultra-deep Cretaceous Bashijiqike Formation in Kuqa depression

Diagenetic fluid types of the Cretaceous Bashijiqike formation are restored based on the analysis of petrographic,electron microprobe composition,inclusions homogenization temperature,salinity and vapor composition and laser carbon and oxygen isotope of diagenetic mineral,and regional geological background.Diagenetic fluid evolution sequence is analyzed on this basis.The crystalline dolomite cement has a low concerntration of Sr,high concerntration of Mn and higher carbon isotope,showing that the crystalline dolomite is affected by meteoric fresh water,associated with the tectonic uplift of late Cretaceous.Similarδ13CPDB,negative transfer ofδ18OPDB and the differentiation of the concerntration of Fe and Mn indicate that the diagenetic fluid of the vein dolomite cement is homologous with the diagenetic fluid of the crystalline dolomite cement,temperature and depth are the dominant factors of differential precipitation between these two carbonate cements.Anhydrite cements have high concerntration of Na,extremely low concerntration of Fe and Mn contents.Based on these data,anhydrite cements can be thought to be related to the alkaline fluid overlying gypsum-salt layer produced by dehydration.The barite vein has abnormally high concerntration of Sr,ultra-high homogenization temperature and high-density gas hydrocarbon inclusions,which is speculated to be the forward fluid by intrusion of late natural gas.Coexistence of methane inclusions with CO2 gas proves existence of acid water during the accumulation of natural gas in the late stages.Therefore,the alkaline environment and associated diagenesis between the meteoric fresh water in epidiagentic stage and carbonic acid in the late diagenesis have dominated the process of diagenesis and reservoir,the secondary porosity and fracture zone formed by gas accumulation is a favorable play for the exploration of ultra-deep reservoirs.

Geological conditions, reservoir evolution and favorable exploration directions of marine ultra-deep oil and gas in China

By analyzing the structural background,petroleum geological conditions,and typical regional(paleo) oil and gas reservoirs in marine ultra-deep oil and gas regions in China,this paper reveals the evolution processes of the marine ultra-deep oil and gas reservoirs and the key controlling factors of accumulation.The marine ultra-deep oil and gas resources in China are buried at depth of greater than 6000 m,and are mainly distributed in the Precambrian and Lower Paleozoic strata in the Sichuan,Tarim and Ordos cratonic basins.The development of marine ultra-deep source rocks in China is controlled by cratonic rifts and cratonic depressions with the background of global supercontinent breakup-convergence cycles.The source rocks in Sichuan Basin have the most developed strata,followed by Tarim Basin,and the development strata and scale of Ordos Basin needs to be further confirmed.The marine ultra-deep reservoir in China is dominated by carbonate rocks,and the reservoir performance is controlled by high-energy sedimentary environment in the early stage,superimposed corrosion and fracture in the later stage.The regional caprocks are dominated by gypsum salt rocks,shale,and tight carbonate rock.The ultra-deep oil and gas fields in China have generally experienced two stages of oil-reservoir forming,cracking(or partial cracking) of paleo-oil reservoirs,and late finalization of cracked gas(or highly mature to over mature oil and gas).The oil and gas accumulation is controlled by static and dynamic geological elements jointly.Major hydrocarbon generation center,high quality and large-scale reservoir resulted from karstification of high energy facies belt,thick gypsum rock or shale caprock,and stable trapping and preservation conditions are the key factors for accumulation of ultra-deep oil and gas.We propose three favorable exploration directions,i.e.the areas around intracratonic rift and intracratonic depression,and craton margin.

Practice and theoretical and technical progress in exploration and development of Shunbei ultra-deep carbonate oil and gas field,Tarim Basin,NW China

In this review on the exploration and development process of the Shunbei ultra-deep carbonate oil and gas field in the Tarim Basin, the progress of exploration and development technologies during the National 13th Five-Year Plan of China has been summarized systematically, giving important guidance for the exploration and development of ultra-deep marine carbonate reservoirs in China and abroad. Through analyzing the primary geological factors of “hydrocarbon generation-reservoir formation-hydrocarbon accumulation” of ancient and superposed basin comprehensively and dynamically, we point out that because the Lower Cambrian Yuertusi Formation high-quality source rocks have been located in a low-temperature environment for a long time, they were capable of generating hydrocarbon continuously in late stage, providing ideal geological conditions for massive liquid hydrocarbon accumulation in ultra-deep layers. In addition, strike-slip faults developed in tectonically stable areas have strong control on reservoir formation and hydrocarbon accumulation in this region. With these understandings, the exploration focus shifted from the two paleo-uplifts located in the north and the south to the Shuntuoguole lower uplift located in between and achieved major hydrocarbon discoveries. Through continuing improvement of seismic exploration technologies for ultra-deep carbonates in desert, integrated technologies including seismic acquisition in ultra-deep carbonates,seismic imaging of strike-slip faults and the associated cavity-fracture systems, detailed structural interpretation of strike-slip faults, characterization and quantitative description of fault-controlled cavities and fractures, description of fault-controlled traps and target optimization have been established. Geology-engineering integration including well trajectory optimization,high efficiency drilling, completion and reservoir reformation technologies has provided important support for exploration and development of the Shunbei oil and gas field.

Key oil accumulation periods of ultra-deep fault-controlled oil reservoir in northern Tarim Basin, NW China

A giant fault-controlled oilfield has been found in the ultra-deep(greater than 6000 m) Ordovician carbonate strata in the northern Tarim Basin. It is of great significance for hydrocarbon accumulation study and oil exploitation to determine the key oil accumulation periods. Based on detailed petrographic analysis, fluid inclusion association(FIA) in calcite samples filling in fractures from 12 wells were analyzed, and key accumulation periods of the strike-slip fault-controlled oilfield was studied by combining oil generation periods of the source rocks, formation periods of the fault and traps, and the fluid inclusion data.(1) There are multiple types of FIA, among them, two types of oil inclusions, the type with yellow fluorescence from the depression area and the type with yellow-green fluorescence from the uplift area with different maturities indicate two oil charging stages.(2) The homogenization temperature of the brine inclusions in FIA is mostly affected by temperature rises, and the minimum temperature of brine inclusions symbiotic with oil inclusions is closer to the reservoir temperature during its forming period.(3) FIA with yellow fluorescence all have homogenization temperatures below 50 ℃, while the FIA with yellow-green fluorescence have homogenization temperatures of 70–90 ℃ tested, suggesting two oil accumulation stages in Middle-Late Caledonian and Late Hercynian.(4) The Middle-Late Ordovician is the key formation period of the strike-slip fault, fracture-cave reservoir and trap there.(5) The oil generation peak of the main source rock of the Lower Cambrian is in the Late Ordovician, and the oil accumulation stage is mainly the Late Ordovician in the depression area, but is mainly the Early Permian in the uplift area. The key oil accumulation period of the strike-slip fault-controlled reservoirs is the Late Caledonian, the depression area has preserved the primary oil reservoirs formed in the Caledonian, while the uplift area has secondary oil reservoirs adjusted from the depression area during the Late Hercynian. Oil reservoir preservation conditions are the key factor for oil enrichment in the strike-slip fault zone of northern Tarim, and the Aman transition zone in the depression is richer in oil and gas and has greater potential for exploration and development.

Techniques for improving fracture-controlled stimulated reservoir volume in ultra-deep fractured tight reservoirs: A case study of Kuqa piedmont clastic reservoirs, Tarim Basin, NW China

Based on analysis of the reasons for low efficiency and low production after fracturing of some wells in the ultra-deep fractured tight reservoirs of the Kuqa piedmont zone, Tarim Basin and the matching relationship between the in-situ stress field and natural fractures, technological methods for creating complex fracture networks are proposed. Through theoretical study and large-scale physical simulation experiments, the mechanical conditions for forming complex fracture network in the Kuqa piedmont ultra-deep reservoirs are determined. The effectiveness of temporary plugging and diversion, and multi-stage fracturing to activate natural fractures and consequently realize multi-stage diversion is verified. The coupling effect of hydraulic fractures and natural fractures activating each other and resulting in "fracture swarms" is observed. These insights provide theoretical support for improving fracture-controlled stimulated reservoir volume(FSRV) in ultra-deep tight reservoirs. In addition, following the concept of volume fracturing technology and based on the results of fracture conductivity experiments of different processes, fracturing technologies such as multi-stage fracture-network acid fracturing, "multi-stage temporary plugging + secondary fracturing", fracturing of multiple small layers by vertically softness-and-hardness-oriented subdivision, and weighted-fluid refracturing are proposed to increase the FSRV. New environment-friendly weighted-fluid with low cost and new fracturing fluid system with low viscosity and high proppant-carrying capacity are also developed. These techniques have achieved remarkable results in field application.