Enrichment model and major controlling factors of below-source tight oil in Lower Cretaceous Fuyu reservoirs in northern Songliao Basin,NE China

Based on the geochemical,seismic,logging and drilling data,the Fuyu reservoirs of the Lower Cretaceous Quantou Formation in northern Songliao Basin are systematically studied in terms of the geological characteristics,the tight oil enrichment model and its major controlling factors.First,the Quantou Formation is overlaid by high-quality source rocks of the Upper Cretaceous Qingshankou Formation,with the development of nose structure around sag and the broad and continuous distribution of sand bodies.The reservoirs are tight on the whole.Second,the configuration of multiple elements,such as high-quality source rocks,reservoir rocks,fault,overpressure and structure,controls the tight oil enrichment in the Fuyu reservoirs.The source-reservoir combination controls the tight oil distribution pattern.The pressure difference between source and reservoir drives the charging of tight oil.The fault-sandbody transport system determines the migration and accumulation of oil and gas.The positive structure is the favorable place for tight oil enrichment,and the fault-horst zone is the key part of syncline area for tight oil exploration.Third,based on the source-reservoir relationship,transport mode,accumulation dynamics and other elements,three tight oil enrichment models are recognized in the Fuyu reservoirs:(1)vertical or lateral migration of hydrocarbon from source rocks to adjacent reservoir rocks,that is,driven by overpressure,hydrocarbon generated is migrated vertically or laterally to and accumulates in the adjacent reservoir rocks;(2)transport of hydrocarbon through faults between separated source and reservoirs,that is,driven by overpressure,hydrocarbon migrates downward through faults to the sandbodies that are separated from the source rocks;and(3)migration of hydrocarbon through faults and sandbodies between separated source and reservoirs,that is,driven by overpressure,hydrocarbon migrates downwards through faults to the reservoir rocks that are separated from the source rocks,and then migrates laterally through sandbodies.Fourth,the differences in oil source conditions,charging drive,fault distribution,sandbody and reservoir physical properties cause the differential enrichment of tight oil in the Fuyu reservoirs.Comprehensive analysis suggests that the Fuyu reservoir in the Qijia-Gulong Sag has good conditions for tight oil enrichment and has been less explored,and it is an important new zone for tight oil exploration in the future.

Crude oil cracking in deep reservoirs:A review of the controlling factors and estimation methods

The natural cracking of crude oils in deep reservoirs has gained great interest due to continuously increasing depth of petroleum exploration and exploitation.Complex oil compositions and surroundings as well as complicated geological evolutions make oil cracking in nature much more complex than industrial pyrolysis.So far,numerous studies,focused on this topic,have made considerable progress although there still exist some drawbacks.However,a comprehensive review on crude oil cracking is yet to be conducted.This article systematically reviews the controlling factors of oil cracking from six aspects,namely,oil compositions,temperature and time,pressure,water,minerals and solid organic matter.We compare previous experimental and modelling results and present new field cases.In the following,we evaluate the prevailing estimation methods for the extent of oil cracking,and elucidate other factors that may interfere with the application of these estimation methods.This review will be helpful for further investigations of crude oil cracking and provides a guide for estimation of the cracking extent of crude oils.

Factors Controlling Organic Matter Enrichment in Alkaline Lacustrine Source Rocks:A Case Study of the Late Paleozoic Fengcheng Formation in the Junggar Basin,NW China

The late Paleozoic Fengcheng Formation shale(LPF shale)in the Junggar Basin,NW China,is the oldest alkaline source rock discovered in the world,providing a unique perspective with which to explore organic matter(OM)enrichment in alkaline lake environments.Combined with the organic carbon isotope profile and paleoenvironmental proxies,this study reveals that the LPF shale was deposited in an arid climate with high salinity and a strong reducing environment,accompanied by frequent volcanic activity.High TOC values are concentrated in two intervals with frequent fluctuations in OM types.A negative excursion due to changes in sedimentary OM source is found in the δ^(13)C_(org) profile.The excursion corresponds to the OM enrichment interval and is accompanied by abnormally high values of Sr/Ba and Sr/Cu.This implies that the extreme arid climate has led to high salinity,resulting in strong reducibility and changes in paleontological assemblages,which in turn controlled the differential enrichment of OM.The Fengcheng Fm.high-quality source rocks are the result of the combined action of climatic events,volcanism,high-salinity water environment and superior hydrocarbon-generating organisms.The results provide new insights into the formation conditions of terrestrial alkaline high-quality source rocks and the factors controlling alkaline OM enrichment.

Geometry and formation mechanism of tension gashes and their implication on the hydrocarbon accumulation in the deep-seated strata of sedimentary basin:A case from Shunnan area of Tarim Basin

With the theoretical and technological developments related to cratonic strike-slip faults,the Shuntuoguole Low Uplift in the Tarim Basin has attracted considerable attention recently.Affected by multi-stage tectonic movements,the strike-slip faults have controlled the distribution of hydrocarbon resources owing to the special fault characteristics and fault-related structures.In contrast,the kinematics and formation mechanism of strike-slip faults in buried sedimentary basins are difficult to investigate,limiting the discussion of these faults and hydrocarbon accumulation.In this study,we identified the characteristics of massive sigmoidal tension gashes(STGs)that formed in the Shunnan area of the Tarim Basin.High-resolution three-dimensional seismic data and attribute analyses were used to investigate their geometric and kinematic characteristics.Then,the stress state of each point of the STGs was calculated using seismic curvature attributes.Finally,the formation mechanism of the STGs and their roles in controlling hydrocarbon migration and accumulation were discussed.The results suggest that:(1)the STGs developed in the Shunnan area have a wide distribution,with a tensile fault arranged in an enéchelon pattern,showing an S-shaped bending.These STGs formed in multiple stages,and differential rotation occurred along the direction of strike-slip stress during formation.(2)Near the principal displacement zone of the strike-slip faults,the stress value of the STGs was higher,gradually decreasing at both ends.The shallow layer deformation was greater than the deep layer deformation.(3)STGs are critical for connecting source rocks,migrating oil and gas,sealing horizontally,and developing efficient reservoirs.This study not only provides seismic evidence for the formation and evolution of super large STGs,but also provides certain guidance for oil and gas exploration in this area.

Controlling factors of high-quality volcanic reservoirs of Yingcheng Formation in the Songnan gas field

Predicting high-quality volcanic reservoirs is one of the key issues for oil and gas exploration in the Songnan gas field.Core,seismic,and measurement data were used to study the lithologies,facies,reservoir porosity,and reservoir types of the volcanic rocks in the Songnan gas field.The primary controlling factors and characteristics of the high-quality volcanic reservoirs of the Yingcheng Formation in the Songnan gas field were investigated,including the volcanic eruptive stage,edifice,edifice facies,cooling unit,lithology,facies,and diagenesis.Stages with more volatile content can form more high-quality reservoirs.The effusive rhyolite,explosive tuff,and tuff lava that formed in the crater,near-crater,and proximal facies and in the high-volatility cooling units of large acidic-lava volcanic edifices are the most favorable locations for the development of the high-quality reservoirs in the Songnan gas field.Diagenesis dissolution,which is controlled by tectonic action,can increase the size of secondary pores in reservoirs.Studying the controlling factors of the high-quality reservoirs can provide a theoretical basis for the prediction and analysis of high-quality volcanic reservoirs.

Factors controlling the development of tight sandstone reservoirs in the Huagang Formation of the central inverted structural belt in Xihu sag, East China Sea Basin

By means of thin section analysis, zircon U-Pb dating, scanning electron microscopy, electron probe, laser micro carbon and oxygen isotope analysis, the lithologic features, diagenetic environment evolution and controlling factors of the tight sandstone reservoirs in the Huagang Formation of Xihu sag, East China Sea Basin were comprehensively studied. The results show that: the sandstones of the Huagang Formation in the central inverted structural belt are poor in physical properties, dominated by feldspathic lithic quartz sandstone, high in quartz content, low in matrix, kaolinite and cement contents, and coarse in clastic grains;the acidic diagenetic environment formed by organic acids and meteoric water is vital for the formation of secondary pores in the reservoirs;and the development and distribution of the higher quality reservoirs in the tight sandstones of the Huagang Formation are controlled by sediment source, sedimentary facies belt, abnormal overpressure and diagenetic environment evolution. Sediment provenance and dominant sedimentary facies led to favorable initial physical properties of the sandstones in the Huagang Formation, which is the prerequisite for development of reservoirs with better quality later. Abnormal high pressure protected the primary pores, thus improving physical properties of the reservoirs in the Huagang Formation. Longitudinally, due to the difference in diagenetic environment evolution, the high-quality reservoirs in the Huagang Formation are concentrated in the sections formed in acidic diagenetic environment. Laterally, the high-quality reservoirs are concentrated in the lower section of the Huagang Formation with abnormal high pressure in the middle-northern part;but concentrated in the upper section of Huagang Formation shallower in burial depth in the middle-southern part.

Baoziwan-Majiashan Area of Jiyuan Oilfield Analysis of Reservoir Characteristics and Main Control Factors in Long 4 5 Section

Based on the sheet, scanning electron microscope and high pressure mercury analysis method, this paper takes Jiyuan oilfield-Ma Jia mountain district 4 5 sandstone reservoir as the research object, from the reservoir petrology, pore type and porosity, permeability, the system analyzed the reservoir characteristics and its control factors. The results show that the sandstone in the 4 5 section of Baoziwan-Majiashan area of Jiyuan oilfield is fine in size and high in filling content. The pore types were dominated by intergranular pores and dissolved pores, with a low face rate. The reservoir property is relatively poor, with mean porosity of 11.11% and mean permeability of 1.16 × 10−3 µm2. In the low porous, low otonic background, the development of relatively high pore hypertonic areas. Compaction and cementation should play a destructive role in reservoir properties, and dissolution should play a positive role in reservoir properties. Compaction adjusts the migration of clay minerals and miscellaneous bases in the original sediment in the study area, greatly reducing the porosity and permeability of the reservoir;the development of the cement cement, carbonate cementation and some quartz secondary compounds reduces the storage space;the dissolution effect, especially the secondary dissolution pores of the reservoir, which obviously improves the properties of the reservoir.

Controlling factors and models of shale oil enrichment in Lower Permian Fengcheng Formation,Mahu Sag,Junggar Basin,NW China

Based on the combination of core observation,experimental analysis and testingand geological analysis,the main controlling factors of shale oil enrichment in the Lower Permian Fengcheng Formation in the Mahu Sag of the Junggar Basin are clarified,and a shale oil enrichment model is established.The results show that the enrichment of shale oil in the Fengcheng Formation in the Mahu Sag is controlled by the organic abundance,organic type,reservoir capacity and the amount of migration hydrocarbon in shale.The abundance of organic matter provides the material basis for shale oil enrichment,and the shales containing typesⅠandⅡorganic matters have good oil content.The reservoir capacity controls shale oil enrichment.Macropores are the main space for shale oil enrichment in the Fengcheng Formation,and pore size and fracture scale directly control the degree of shale oil enrichment.The migration of hydrocarbons in shale affects shale oil enrichment.The shale that has expelled hydrocarbons has poor oil content,while the shale that has received hydrocarbons migrated from other strata has good oil content.Lithofacies reflect the hydrocarbon generation and storage capacity comprehensively.The laminated felsic shale,laminated lime-dolomitic shale and thick-layered felsic shale have good oil content,and they are favorable lithofacies for shale oil enrichment.Under the control of these factors,relative migration of hydrocarbons occurred within the Fengcheng shale,which leads to the the difference in the enrichment process of shale oil.Accordingly,the enrichment mode of shale oil in Fengcheng Formation is established as"in-situ enrichment"and"migration enrichment".By superimposing favorable lithofacies and main controlling factors of enrichment,the sweet spot of shale oil in the Fengcheng Formation can be selected which has great significance for the exploration and development of shale oil.

Origin and controlling factors of chlorite coatings—an example from the reservoir of T_3x Group of the Baojie area,Sichuan Basin,China

The study of the chlorite coatings always attracts scholars in China and other countries because the chlorite coatings play an important role in the preservation of residual primary pores in sandstone reservoirs.At present,the study of the origin and the controlling factors is relatively few.The occurrence,time of formation,genesis,controlling factors,and the mechanism of chlorite coatings inhibiting quartz overgrowths were studied in detail with thin section and SEM analysis.Samples were from the sandstone reservoirs of the T3x Group in the Baojie area,the transitional zone from the middle to the south of Sichuan Basin.The results indicate that the chlorite coatings on the walls of the pore spaces are oriented perpendicular to grain surfaces in the form of isopachous(even-thickness) grain-coating,while the chlorite coatings at the contacts between adjacent detrital grains are arranged with a preferred orientation tangential to the surface of detrital grains.The chlorite coatings were formed in the eogenetic stage.They were formed by recrystallization of Fe-rich clay films during the syndepositional period,and chlorite cements would be recrystallized after the coatings’ formation.The formation of chlorite coatings was mainly controlled by the depositional environment,provenance conditions,and diagenetic environment.The presence of chlorite coatings could result in the preservation of primary pores in deeply buried sandstone reservoirs by effectively inhibiting quartz overgrowths and the development of compaction and pressure solution.

Physical property characteristics of Yanchang Formation reservoir in the southwest of Ordos Basin and their controlling fac-tors： taking Chang 3 and Chang 4 ＋ 5 reservoirs in Longdong area as an example

分析鄂尔多斯盆地西南部陇东地区延长组储层物性特征及其控制因素，发现沉积和成岩控制储层物性。沉积微相带不同，储层物性就不同；成岩作用中压实-压溶作用、胶结作用对储层物性具有破坏作用，溶蚀作用是储层物性改善的关键因素。沉积微相带是控制油气藏储层展布的内在因素，成岩作用是储层形成过程中控制物性演化的外部因素。

Characteristics and Controlling Factors of Shale Oil Reservoir Spaces in the Bohai Bay Basin

The Cenozoic continental strata of the Bohai Bay Basin are rich in shale oil resources, and they contain various types of reservoir spaces that are controlled by complex factors. Using field emission scanning electron microscopy(FESEM), automatic mineral identification and characterization system(AMICS), CO2 and N2 gas adsorption, and focused ion beam scanning electron microscopy(FIB-SEM), the types of shale reservoir spaces in the Bohai Bay Basin are summarized, the spatial distribution and connectivity of the various types of pores are described in detail, the microscopic pore structures are characterized, and the key geological mechanisms affecting the formation and evolution of the reservoir spaces are determined. Three conclusions can be drawn in the present study. First, the shale reservoir spaces in the Bohai Bay Basin can be divided into three broad categories, including mineral matrix pores, organic matter pores, and micro fractures. Those spaces can be subdivided into seven categories and fourteen sub-categories based on the distribution and formation mechanisms of the pores. Second, the complex pore-throat structures of the shale reservoir can be divided into two types based on the shape of the adsorption hysteresis loop. The pore structures mainly include wedge-shaped, flat slit-shaped, and ink bottle-shaped pores. The mesopores and micropores are the main contributors to pore volume and specific surface area, respectively. The macropores provide a portion of the pore volume, but they do not significantly contribute to the specific surface area. Third, the factors controlling the development of microscopic pores in the shale are complex. The sedimentary environment determines the composition and structure of the shale and provides the material basis for pore development. Diagenesis controls the types and characteristics of the pores. In addition, the thermal evolution of the organic matter is closely related to inorganic diagenesis and drives the formation and evolution of the pores.

Geological characteristics and high production control factors of shale gas reservoirs in Silurian Longmaxi Formation, southern Sichuan Basin, SW China

Marine shale gas resources have great potential in the south of the Sichuan Basin in China.At present,the high-quality shale gas resources at depth of 2000–3500 m are under effective development,and strategic breakthroughs have been made in deeper shale gas resources at depth of 3500–4500 m.To promote the effective production of shale gas in this area,this study examines key factors controlling high shale gas production and presents the next exploration direction in the southern Sichuan Basin based on summarizing the geological understandings from the Lower Silurian Longmaxi Formation shale gas exploration combined with the latest results of geological evaluation.The results show that:(1)The relative sea depth in marine shelf sedimentary environment controls the development and distribution of reservoirs.In the relatively deep water area in deep-water shelf,grade-I reservoirs with a larger continuous thickness develop.The relative depth of sea in marine shelf sedimentary environment can be determined by redox conditions.The research shows that the uranium to thorium mass ratio greater than 1.25 indicates relatively deep water in anoxic reduction environment,and the uranium to thorium mass ratio of 0.75–1.25 indicates semi-deep water in weak reduction and weak oxidation environment,and the uranium to thorium mass ratio less than 0.75 indicates relatively shallow water in strong oxidation environment.(2)The propped fractures in shale reservoirs subject to fracturing treatment are generally 10–12 m high,if grade-I reservoirs are more than 10 m in continuous thickness,then all the propped section would be high-quality reserves;in this case,the longer the continuous thickness of penetrated grade-I reservoirs,the higher the production will be.(3)The shale gas reservoirs at 3500–4500 m depth in southern Sichuan are characterized by high formation pressure,high pressure coefficient,well preserved pores,good pore structure and high proportion of free gas,making them the most favorable new field for shale gas exploration;and the pressure coefficient greater than 1.2 is a necessary condition for shale gas wells to obtain high production.(4)High production wells in the deep shale gas reservoirs are those in areas where Long11-Long13 sub-beds are more than 10 m thick,with 1500 m long horizontal section,grade-I reservoirs penetration rate of over 90%,and fractured by dense cutting+high intensity sand injection+large displacement+large liquid volume.(5)The relatively deep-water area in the deep-water shelf and the area at depth of 3500–4500 m well overlap in the southern Sichuan,and the overlapping area is the most favorable shale gas exploration and development zones in the southern Sichuan in the future.With advancement in theory and technology,annual shale gas production in the southern Sichuan is expected to reach 450×108 m3.

Major controlling factors and hydrocarbon accumulation models of large-scale lithologic reservoirs in shallow strata around the Bozhong sag,Bohai Bay Basin,China

Based on the practice of oil and gas exploration and the analysis of shallow lithologic reservoirs,combined with the allocation relationship and enrichment law of oil and gas accumulation factors,main controlling factors and models of hydrocarbon accumulation of large lithologic reservoirs in shallow strata around the Bozhong sag are summarized,and favorable exploration areas are proposed.The coupling of the four factors of“ridge-fault-sand-zone”is crucial for the hydrocarbon enrichment in the shallow lithologic reservoirs.The convergence intensity of deep convergence ridges is the basis for shallow oil and gas enrichment,the activity intensity of large fault cutting ridges and the thickness of cap rocks control the vertical migration ability of oil and gas,the coupling degree of large sand bodies and fault cutting ridges control large-scale oil and gas filling,the fault sealing ability of structural stress concentration zones affects the enrichment degree of lithologic oil and gas reservoirs.Three enrichment models including uplift convergence type,steep slope sand convergence type and depression uplift convergence type are established through the case study of lithologic reservoirs in shallow strata around the Bozhong sag.

Controlling Factors Affecting Pressure Differential Development of the Upper Paleozoic Gas Reservoir in the Sulige and Yulin Area of the Ordos Basin

The origin and distribution of Upper Paleozoic reservoirs pressure in mid-northem Ordos Basin has attracted widespread attention in recent years.The reservoir pressure of Sulige area shows characteristics of basically subnormal pressure,and the reservoir pressure of Yulin area shows characteristics of basically normal hydrostatic pressure.In the sense of structural location,

Subsection and superposition method for reservoir formation damage evaluation of complex-structure wells

Kinds of complex-structure wells can effectively improve production,which are widely used.However,in the process of drilling and completion,complex-structure wells with long drilling cycle and large exposed area of reservoir can lead to the fact that reservoir near wellbore is more vulnerable to the working fluid invasion,resulting in more serious formation damage.In order to quantitatively describe the reservoir formation damage in the construction of complex-structure well,taking the inclined well section as the research object,the coordinate transformation method and conformal transformation method are given according to the flow characteristics of reservoir near wellbore in anisotropic reservoir.Then the local skin factor in orthogonal plane of wellbore is deduced.Considering the un-even distribution of local skin factor along the wellbore,the oscillation decreasing model and empirical equation model of damage zone radius distribution along the wellbore direction are established and then the total skin factor model of the whole well is superimposed to realize the reservoir damage evaluation of complex-structure wells.Combining the skin factor model with the production model,the production of complex-structure wells can be predicted more accurately.The two field application cases show that the accuracy of the model can be more than 90%,which can also fully reflect the invasion characteristics of drilling and completion fluid in any well section of complex-structure wells in anisotropic reservoir,so as to further provide guidance for the scientific establish-ment of reservoir production system.

Geological characteristics and models of fault-foldfracture body in deep tight sandstone of the second member of Upper Triassic Xujiahe Formation in Xinchang structural belt of Sichuan Basin,SW China

In the second member of the Upper Triassic Xujiahe Formation(T_(3)x_(2))in the Xinchang area,western Sichuan Basin,only a low percent of reserves has been recovered,and the geological model of gas reservoir sweet spot remains unclear.Based on a large number of core,field outcrop,test and logging-seismic data,the T_(3)x_(2) gas reservoir in the Xinchang area is examined.The concept of fault-fold-fracture body(FFFB)is proposed,and its types are recognized.The main factors controlling fracture development are identified,and the geological models of FFFB are established.FFFB refers to faults,folds and associated fractures reservoirs.According to the characteristics and genesis,FFFBs can be divided into three types:fault-fracture body,fold-fracture body,and fault-fold body.In the hanging wall of the fault,the closer to the fault,the more developed the effective fractures;the greater the fold amplitude and the closer to the fold hinge plane,the more developed the effective fractures.Two types of geological models of FFFB are established:fault-fold fracture,and matrix storage and permeability.The former can be divided into two subtypes:network fracture,and single structural fracture,and the later can be divided into three subtypes:bedding fracture,low permeability pore,and extremely low permeability pore.The process for evaluating favorable FFFB zones was formed to define favorable development targets and support the well deployment for purpose of high production.The study results provide a reference for the exploration and development of deep tight sandstone oil and gas reservoirs in China.

Study of the Seepage Mechanism in Thick Heterogeneous Gas Reservoirs

The seepage mechanism plays a crucial role in low-permeability gas reservoirs.Compared with conventional gas reservoirs,low-permeability sandstone gas reservoirs are characterized by low porosity,low permeability,strong heterogeneity,and high water saturation.Moreover,their percolation mechanisms are more complex.The present work describes a series of experiments conducted considering low-permeability sandstone cores under pressuredepletion conditions(from the Xihu Depression in the East China Sea Basin).It is shown that the threshold pressure gradient of a low-permeability gas reservoir in thick layers is positively correlated with water saturation and negatively correlated with permeability and porosity.The reservoir stress sensitivity is related to permeability and rock composition.Stress sensitivity is generally low when permeability is high or in the early stage of gas reservoir development.It is also shown that in sand conglomerates,especially the more sparsely filled parts,the interstitial materials among the conglomerates can be rapidly dislodged from the skeleton particles under stress.This material can therefore disperse,migrate,and block the pore throat producing serious,stress-sensitive damage.

Variation and mechanisms of clastic reservoir quality in the Paleogene Shahejie Formation of the Dongying Sag, Bohai Bay Basin, China

Reservoir quality varies greatly in the Shahejie Formation in the Dongying Sag. It is essential to analyze the variation and mechanisms of reservoir quality for determining the controlling factors based on cores, porosity measurements and fluid inclusion techniques and so on. The sandstones in the fluvial, （fan） delta-front have the best reservoir quality due to the depositional conditions mechanically controlling the petrology configuration and the primary porosity, and chemically influencing the diagenesis and development of secondary pores. The activity of the boundary faults and the sedimentary facies dominate the variation of reservoir quality in different areas and intervals. The reservoir quality varies with the position of sandstone beds in different vertical models of sandstone and mudstone. This mainly arose from the strong cementation or strong dissolution in the sandstone caused by the diagenesis evolution of adjacent mudstone. With higher oil saturation reservoir quality is better because the hydrocarbon charge favors dissolution and restricts cementation. Diagenesis, depositional conditions and tectonic setting are the key controls of reservoir quality in the Shahejie Formation of the Dongying Sag.

Necessity and feasibility of improving the residual resistance factor of polymer flooding in heavy oil reservoirs

The efficiency of water flooding in heavy oil reservoirs would be improved by increasing the viscosity of the displacing phase, but the sweep efficiency is not of significance due to the low mobility of the vicious oil. On the basis of mobility control theory, increasing the residual resistance factor not only reduces the water-oil mobility ratio but also decreases the requirement for viscosity enhancement of the polymer solution. The residual resistance factor caused by hydrophobic associating polymer solution is higher than that caused by polyacrylamide solution in brine containing high concentrations of calcium and magnesium ions. The results of numerical simulations show that the polymer flooding efficiency improved by increasing the residual resistance factor is far better than that by only increasing solution viscosity. The recovery factor of heavy oil reservoirs （70 mPa·s） can be enhanced by hydrophobic associating polymer solution of high residual resistance factor （more than 3） and high effective viscosity （24 mPa·s）. Therefore, increasing the residual resistance factor of the polymer solution not only decreases the requirement for the viscosity of polymer solution injected into heavy oil reservoirs but also is favorable to enhanced oil recovery during polymer flooding.

Characteristics of Shale Reservoir and Sweet Spot Identification of the Lower Cambrian Niutitang Formation in Northwestern Hunan Province, China

The accumulation and productivity of shale gas are mainly controlled by the characteristics of shale reservoirs;study of these characteristics forms the basis for the shale gas exploitation of the Lower Cambrian Niutitang Formation(Fm),Southern China.In this study,core observation and lithology study were conducted along with X-ray diffraction(XRD)and electronic scanning microscopy(SEM)examinations and liquid nitrogen(N2)adsorption/desorption and CH4 isothermal adsorption experiments for several exploration wells in northwestern Hunan Province,China.The results show that one or two intervals with high-quality source rocks(TOC>2 wt%)were deposited in the deep-shelf facies.The source rocks,which were mainly composed of carbonaceous shales and siliceous shales,had high quartz contents(>40 wt%)and low clay mineral(<30 wt%,mainly illites)and carbonate mineral(<20 wt%)contents.The SEM observations and liquid nitrogen(N2)adsorption/desorption experiments showed that the shale is tight,and nanoscale pores and microscale fractures are well developed.BJH volume(VBJH)of shale ranged from 2.144×10^-3 to 20.07×10^-3 cm^3/g,with an average of 11.752×10^-3 cm3/g.Pores mainly consisted of opened and interconnected mesopores(2–50 nm in diameter)or macropores(>50 nm in diameter).The shale reservoir has strong adsorption capacity for CH4.The Langmuir volume(VL)varied from 1.63 to 7.39 cm^3/g,with an average of 3.95 cm^3/g.The characteristics of shale reservoir are controlled by several factors:(1)A deep muddy continental shelf is the most favorable environment for the development of shale reservoirs,which is controlled by the development of basic materials.(2)The storage capacity of the shale reservoir is positively related to the TOC contents and plastic minerals and negatively related to cement minerals.(3)High maturity or overmaturity leads to the growth of organic pores and microfractures,thereby improving the reservoir storage capacity.It can be deduced that the high percentage of residual gas in Niutitang Fm results from the strong reservoir storage capacity of adsorbed gas.Two layers of sweet spots with strong storage capacity of free gas,and they are characterized by the relatively high TOC contents ranging from 4 wt%to 8 wt%.