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.

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.

Discovery and Significance of the Triassic–Late Paleozoic Double-layered Basement in the Songliao Basin:Based on the Complete Coring Data from ICDP Borehole SK2

The Songliao Basin(SLB)covers an area of approximately 260,000 km2in northeastern Asia and preserves a continuous and complete Cretaceous terrestrial record(Wang et al.,2021).The region is the most important petroliferous sedimentary basin in China because of its continual annual oil and gas equivalent production of tens of millions of tons(ca.220–440 million barrels per year)since 1959.The SLB was previously thought to have developed on Hercynian basement and accumulated continuous sedimentary deposits during the Late Jurassic and Cretaceous(Wan et al.,2013;Wang et al.,2016).

Paleoenvironmental Evolution and Organic Matter Enrichment Genesis of the Late Turonian Black Shale in the Southern Songliao Basin,NE China

The Upper Cretaceous Qingshankou Formation black shales,deposited in the late Turonian(LTB shales),are the main source rocks of the Songliao Basin.The origins of organic matter enrichment of the shales is a contentious subject fuelling many ongoing debates.This study investigates the genesis of the organic matter-rich shale by using molecular geochemistry.The LTB shales can be divided into three sections.The SectionⅠshales were deposited in saline,stratified and anoxic water conditions,which are related to seawater incursion events.At least three episodic and periodic seawater incursion events were recognized during SectionⅠshale deposition.The SectionⅡshales deposited in brackish to fresh and deep lake-level conditions with high primary productivity,which are related to lake-level transgression.The SectionⅢshales were deposited under fresh and slightly oxidized water conditions,which are related to lake-level regression.Two organic matter enrichment models for the LTB shales are identified,that is,the seawater incursion model and the maximum lake-level transgression sedimentation model,which act on different shale sections,both playing significant roles in the enrichment of organic matter.

Quantitative evaluation model of shale oil adsorption:A case study of the first member of Cretaceous Qingshankou Formation in northern Songliao Basin,NE China

A quantitative evaluation model that integrates kerogen adsorption and clay pore adsorption of shale oil was proposed,and the evaluation charts of adsorption-swelling capacity of kerogen(Mk)and adsorbed oil capacity of clay minerals(Mc)were established,taking the 1st member of Cretaceous Qingshankou Formation in the northern Songliao Basin as an example.The model and charts were derived from swelling oil experiments performed on naturally evolved kerogens and adsorbed oil experiments on clays(separated from shale core samples).They were constructed on the basis of clarifying the control law of kerogen maturity evolution on its adsorption-swelling capacity,and considering the effect of both the clay pore surface area that occupied by adsorbed oil and formation temperature.The results are obtained in four aspects:(1)For the Qing 1 Member shale,with the increase of maturity,Mk decreases.Given Ro of 0.83%–1.65%,Mk is about 50–250 mg/g.(2)The clay in shale adsorbs asphaltene.Mc is 0.63 mg/m^(2),and about 15%of the clay pore surface is occupied by adsorbed oil.(3)In the low to medium maturity stages,the shale oil adsorption is controlled by organic matter.When Ro>1.3%,the shale oil adsorption capacity is contributed by clay pores.(4)The oil adsorption capacity evaluated on the surface at room temperature is 8%–22%(avg.15%)higher than that is held in the formations.The proposed evaluation model reveals the occurrence mechanisms of shale oils with different maturities,and provides a new insight for estimating the reserves of shale oil under formation temperature conditions.

Petroleum exploration breakthrough and geological significance in Cretaceous Yingcheng and Denglouku formations of Shuangcheng area, northern Songliao Basin, NE China

Based on drilling geological,geochemical,geophysical and production test data,the characteristics of source rocks,reservoir rocks and caprocks,as well as the process of hydrocarbon generation,trap evolution and oil accumulation of the oil-bearing assemblage composed of the Cretaceous Yingcheng Formation(K1yc)and Denglouku Formation(K1d)in the Shuangcheng area,northern Songliao Basin,NE China,were analyzed by using the research methods for petroleum systems.The source rocks mainly exist in K1yc,with the organic matters mainly originated from lower aquatic organisms and algae,and reaching the grade of high-quality source rock in terms of organic abundance.The crude oil,which is characterized by low density,high freezing point and high wax content,is believed to have generated by the K1yc source rocks.The reservoir rocks include K1d sandstones and K1yc glutenites.The mudstone in the fourth member of K1d serves as the direct caprock of the oil reservoir.The oil was generated during the period between Yaojia Formation and Nenjiang Formation,and then accumulated during the periods of Nenjiang Formation and Paleogene–Neogene.The traps evolved in three stages:the late Yingcheng Formation,the late Quantou Formation and the late Nenjiang Formation,forming structural and structural-lithologic reservoirs.It is concluded that good source-reservoir-caprock assemblage,late hydrocarbon charging,short migration distance and stable tectonic setting are favorable factors for the formation of oil reservoirs.

Analysing the Potential Impact of Climate Change on the Hydrological Regime of the Upper Benue River Basin (North Cameroon)

In this study, we analyse the climate variability in the Upper Benue basin and assess its potential impact on the hydrology regime under two different greenhouse gas emission scenarios. The hydrological regime of the basin is more vulnerable to climate variability, especially precipitation and temperature. Observed hydroclimatic data (1950-2015) was analysed using a statistical approach. The potential impact of future climate change on the hydrological regime is quantified using the GR2M model and two climate models: HadGEM2-ES and MIROC5 from CMIP5 under RCP 4.5 and RCP 8.5 greenhouse gas emission scenarios. The main result shows that precipitation varies significantly according to the geographical location and time in the Upper Benue basin. The trend analysis of climatic parameters shows a decrease in annual average precipitation across the study area at a rate of -0.568 mm/year which represents about 37 mm/year over the time 1950-2015 compared to the 1961-1990 reference period. An increase of 0.7°C in mean temperature and 14% of PET are also observed according to the same reference period. The two climate models predict a warming of the basin of about 2°C for both RCP 4.5 and 8.5 scenarios and an increase in precipitation between 1% and 10% between 2015 and 2100. Similarly, the average annual flow is projected to increase by about +2% to +10% in the future for both RCP 4.5 and 8.5 scenarios between 2015 and 2100. Therefore, it is primordial to develop adaptation and mitigation measures to manage efficiently the availability of water resources.

3D reservoir characterization of the Mangahewa Formation,Mangahewa Field,Taranaki Basin,New Zealand

The Mangahewa Formation is the primary reservoir target in the Mangahewa Field in the Taranaki Basin,New Zealand.This formation is distinguished by its marginal marine substantial tight-sand reservoir,having thickness exceeding 800 m.The aim of this study is to assess the reservoir properties of the Mangahewa Formation through 3D reservoir modeling,employing 3D seismic data,core data,and well data from the Mangahewa Field.Utilizing variance attributes,the faults and horizons have been identified successfully within the field.The majority of the interpreted faults exhibit dip angles exceeding 60°,with a maximum displacement of 118 m.To detect direct hydrocarbon indicators,root-mean-square amplitude seismic attribute,envelope,and generalized spectral decomposition techniques have been employed.Subsequently,four lithofacies,comprising 78.3%sandstone,9.2%siltstone,9.5%claystone,and 3.0%coal have been established by utilizing the Sequential Indicator Simulation(SIS)algorithm to create a lithofacies model.A property model has been generated using the Sequential Gaussian Simulation(SGS)algorithm.Petrophysical evaluation indicates that the Mangahewa Formation exhibits reservoir qualities ranging from fair to good,with porosity levels between 8%and 11%,permeability averaging up to 10 mD,variable shale volumes,and hydrocarbon saturation in the range of 40%-50%.This study's methodologies and findings can serve as a valuable foundation for similar investigations in other tightsand gas fields located in different regions.

Pore Connectivity of Deep Lacustrine Shale and its Effect on Gas-bearing Characteristics in the Songliao Basin:Implications from Continental Scientific Drilling

The lacustrine shale of deep Shahezi Formation in the Songliao basin has great gas potential,but its pore evolution,heterogeneity,and connectivity characteristics remain unclear.In this work,total organic carbon analysis,rock pyrolysis,X-ray diffraction field emission scanning electron microscopy,the particle and crack analysis system software,low-temperature nitrogen adsorption experiment,fractal theory,high-pressure mercury injection experiment and nuclear magnetic resonance experiment were used to study the Shahezi shale from Well SK-2.The result indicated that the organic pores in Shahezi shale are not developed,and the intergranular and intragranular pores are mainly formed by illitedominated clay.As the burial depth increases,the pore size and slit-shaped pores formed by clay decrease,and dissolved pores in the feldspar and carbonate minerals and dissolved fractures in the quartz increase.The pore evolution is affected by clay,compaction,and high-temperature corrosion.Based on the pore structure characteristics reflected by the pore size distribution and pore structure parameters obtained by multiple experimental methods,the pore development and evolution are divided into three stages.During stageⅠandⅡ,the pore heterogeneity of the shale reservoirs increases with the depth,the physical properties and pore connectivity deteriorate,but the gas-bearing property is good.In stageⅢ,the pore heterogeneity is the highest,its gas generation and storage capacity are low,but the increase of micro-fractures makes pore connectivity and gas-bearing better.

Subaqueous volcanic eruptive facies,facies model and its reservoir significance in a continental lacustrine basin:A case from the Cretaceous in Chaganhua area of southern Songliao Basin,NE China

The conventional lithofacies and facies model of subaerial and marine pyroclastic rocks cannot reflect the characteristics of subaqueous volcanic edifice in lacustrine basins.In order to solve this problem,the lithofacies of subaqueous eruptive pyroclastic rocks is discussed and the facies model is established by taking the tuff cone of Cretaceous Huoshiling Formation in the Chaganhua area of the Changling fault depression,Songliao Basin as the research object.The results indicate that the subaqueous eruptive pyroclastic rocks in the Songliao Basin can be divided into two facies and four subfacies.The two facies are the subaqueous explosive facies and the volcanic sedimentary facies that is formed during the eruption interval.The subaqueous explosive facies can be further divided into three subfacies:gas-supported hot pyroclastic flow subfacies,water-laid density current subfacies and subaqueous fallout subfacies.The volcanic sedimentary facies consists of pyroclastic sedimentary rocks containing terrigenous clast subfacies.A typical facies model of the tuff cone that is formed by subaqueous eruptions in the Songliao Basin was established.The tuff cone is generally composed of multiple subaqueous eruption depositional units and can be divided into two facies associations:near-source facies association and far-source facies association.The complete vertical succession of one depositional unit of the near-source facies association is composed of pyroclastic sedimentary rocks containing terrigenous clast subfacies,gas-supported hot pyroclastic flow subfacies,water-laid density current subfacies and subaqueous fallout subfacies from bottom to top.The depositional unit of the far-source facies association is dominated by the subaqueous fallout subfacies and contains several thin interlayered deposits of the water-laid density current subfacies.The gas-supported hot pyroclastic flow subfacies and the pyroclastic sedimentary rocks containing terrigenous clast subfacies are favorable subaqueous eruptive facies for reservoirs in continental lacustrine basins.

Rare earth elemental and Sr isotopic evidence for seawater intrusion event of the Songliao Basin 91 million years ago

Petrogenesis of lacustrine dolostone is closely related with paleo-lake water conditions.Here we report the high spatial-resolution petrographic and geochemical results of a lacustrine dolomite nodule from the Qingshankou Formation,the Songliao Basin.Sedimentary and elemental signatures confirm the protogenetic origin of this nodule and its effectiveness in recording geochemical characteristics of paleo-lake water during dolomitization.The low Y/Ho ratios,middle rare earth element(MREE)enrichment and subtle positive Eu anomalies within the nodule indicate a fresh water source.However,the Sr isotope values in the core of the nodule(0.7076-0.7080)are close to contemporaneous seawater(0.7074),yet different from the modern river(0.7120)and the host black shale(0.7100).On the premise of excluding the influence of hydrothermal fluids,the significantly low strontium isotope values of the lacustrine dolomite might be caused by seawater interference during dolomitization.Our findings demonstrate that lacustrine dolomite within black shales is not only a faithful tracer of diagenetic water environment,but also a novel and easily identified mineralogical evidence for episodic seawater intrusion event(91 Ma)in the Songliao Basin,which supplements other paleontological and geochemical evidence.

Gulong shale oil enrichment mechanism and orderly distribution of conventional–unconventional oils in the Cretaceous Qingshankou Formation,Songliao Basin,NE China

Through the study of organic matter enrichment,hydrocarbon generation and accumulation process of black shale of the Cretaceous Qingshankou Formation in the Songliao Basin,the enrichment mechanism of Gulong shale oil and the distribution of conventional–unconventional oil are revealed.The Songliao Basin is a huge interior lake basin formed in the Early Cretaceous under the control of the subduction and retreat of the western Pacific plate and the massive horizontal displacement of the Tanlu Fault Zone in Northeast China.During the deposition of the Qingshankou Formation,strong terrestrial hydrological cycle led to the lake level rise of the ancient Songliao Basin and the input of a large amount of nutrients,resulting in planktonic bacteria and algae flourish.Intermittent seawater intrusion events promoted the formation of salinization stratification and anoxic environment in the lake,which were beneficial to the enrichment of organic matters.Biomarkers analysis confirms that the biogenic organic matter of planktonic bacteria and algae modified by microorganisms plays an important role in the formation of high-quality source rocks with high oil generation capability.There are four favorable conditions for the enrichment of light shale oil in the Qingshankou Formation of the Gulong Sag,Songliao Basin:the moderate organic matter abundance and high oil potential provide sufficient material basis for oil enrichment;high degree of thermal evolution makes shale oil have high GOR and good mobility;low hydrocarbon expulsion efficiency leads to a high content of retained hydrocarbons in the source rock;and the confinement effect of intra-layer cement in the high maturity stage induces the efficient accumulation of light shale oil.The restoration of hydrocarbon accumulation process suggests that liquid hydrocarbons generated in the early(low–medium maturity)stage of the Qingshankou Formation source rocks accumulated in placanticline and slope after long-distance secondary migration,forming high-quality conventional and tight oil reservoirs.Light oil generated in the late(medium–high maturity)stage accumulated in situ,forming about 15 billion tons of Gulong shale oil resources,which finally enabled the orderly distribution of conventional–unconventional oils that are contiguous horizontally and superposed vertically within the basin,showing a complete pattern of“whole petroleum system”with conventional oil,tight oil and shale oil in sequence.

Whole petroleum system and hydrocarbon accumulation model in shallow and medium strata in northern Songliao Basin,NE China

Based on the oil and gas exploration practice in the Songliao Basin,combined with the latest exploration and development data such as seismic,well logging and geochemistry,the basic geological conditions,oil and gas types and distribution characteristics,reservoir-forming dynamics,source-reservoir relationship and hydrocarbon accumulation model of the whole petroleum system in shallow and medium strata in the northern part of Songliao Basin are systematically studied.The shallow-medium strata in northern Songliao Basin have the conditions for the formation of whole petroleum system,with sufficient oil and gas sources,diverse reservoir types and well-developed transport system,forming a whole petroleum system centered on the source rocks of the Cretaceous Qingshankou Formation.Different types of oil and gas resources in the whole petroleum system are correlated with each other in terms of depositional system,lithologic association and physical property changes,and they,to a certain extent,have created the spatial framework with orderly symbiosis of shallow-medium conventional oil reservoirs,tight oil reservoirs and shale oil reservoirs in northern Songliao Basin.Vertically,the resources are endowed as conventional oil above source,shale oil/tight oil within source,and tight oil below source.Horizontally,conventional oil,tight oil,interlayer-type shale oil,and pure shale-type shale oil are developed in an orderly way,from the margin of the basin to the center of the depression.Three hydrocarbon accumulation models are recognized for the whole petroleum system in northern Songliao Basin,namely,buoyancy-driven charging of conventional oil above source,retention of shale oil within source,and pressure differential-driven charging of tight oil below source.

Influencing mechanism and hydrogeological implications of water level fluctuation of lakes in the northern Qaidam Basin,Qinghai-Tibet Plateau

The Qaidam Basin(QB)is a concentrated distribution area and chemical industrial bases of salt lakes in China.Lakes in the QB have been expanding during the past 20 years.Rapid lake expansion resulted in some considerable scientific issues on the protection of salt lake resources and infrastructure,and monitoring of hydrological processes at the lake-basin scale.Although the spatial-temporal trends of lake changes on the Qinghai-Tibet Plateau(QTP)have been well documented,the underlying influencing mechanism and hydrogeological implications of rapid lake changes in the QB are not well understood.Three lakes in the northern QB were selected to investigate lake water level fluctuations on different time scales based on extensive in-situ monitoring and satellite observations.The influencing mechanism and hydrogeological implications of rapid changes of terminal lakes were discussed in combination with the reported increasing precipitation rate and mass balance of glaciers in the northern QTP.Results reveal the following:(1)the fluctuation pattern of Sugan Lake was asynchronous and out of phase with that of Xiao Qaidam and Toson lakes during the monitoring period;(2)Sugan Lake water rose gradually,and the rise interval was from late April to early July.In contrast,Xiao Qaidam and Toson lakes took on a rapid and steep rise,and the rise intervalwas from late July to September;(3)the influencing mechanisms for rapid lake fluctuations are controlled by different factors:glacier and snow melting with increasing temperature for Sugan Lake and increasing precipitation for Xiao Qaidam and Toson lakes;(4)in accordance with different intervals and influencing mechanisms of rapid lake expansions in the QB,hydrological risk precaution of lakes and corresponding river catchments was conducted in different parts of the basin.This study provided an important scientific basis for assessing the hydrological process and hydrological risk precaution,and protection of salt lake resources along with rapid lake expansions in the arid area.

Practice and development suggestions of hydraulic fracturing technology in the Gulong shale oil reservoirs of Songliao Basin, NE China

This paper reviews the multiple rounds of upgrades of the hydraulic fracturing technology used in the Gulong shale oil reservoirs and gives suggestions about stimulation technology development in relation to the production performance of Gulong shale oil wells.Under the control of high-density bedding fractures,fracturing in the Gulong shale results in a complex fracture morphology,yet with highly suppressed fracture height and length.Hydraulic fracturing fails to generate artificial fractures with sufficient lengths and heights,which is a main restraint on the effective stimulation in the Gulong shale oil reservoirs.In this regard,the fracturing design shall follow the strategy of"controlling near-wellbore complex fractures and maximizing the extension of main fractures"Increasing the proportions of guar gum fracturing fluids,reducing perforation clusters within one fracturing stage,raising pump rates and appropriately exploiting stress interference are conducive to fracture propagation and lead to a considerably expanded stimulated reservoir volume(SRV).The upgraded main hydraulic fracturing technology is much more applicable to the Gulong shale oil reservoirs.It accelerates the oil production with a low flowback rate and lifts oil cut during the initial production of well groups,which both help to improve well production.It is suggested to optimize the hydraulic fracturing technology in six aspects,namely,suppressing propagation of near-wellbore microfractures,improving the pumping scheme of CO_(2),managing the perforating density,enhancing multi-proppant combination,reviewing well pattern/spacing,and discreetly applying fiber-assisted injection,so as to improve the SRv,the distal fracture complexity and the long-term fracture conductivity.

Identification, evolution and geological indications of solid bitumen in shales: A case study of the first member of Cretaceous Qingshankou Formation in Songliao Basin, NE China

On the basis of sorting out current understanding of solid bitumen (SB) in shales and taking organic-rich shales in the first member of the Cretaceous Qingshankou Formation in the Songliao Basin as an example, the definition, classification, occurrence and evolution path of SB are systemtically studied, and the indicative significance of SB reflectance (Rob) on maturity and its influence on the development of reservoir space are discussed and summarized. The results show that the difference of primary maceral types is primarily responsible for the different evolution paths of SB. Most of the pre-oil bitumen is in-situ SB with only a small amount being of migrated SB, while most of the post-oil bitumen and pyrobitumen are migrated SB. From the immature to early oil maturity stage, bituminite, vitrinite, and inertinite can be distinguished from SB based on their optical characteristics under reflected light, and alginite can be differentiated from SB by their fluorescence characteristics. Under scanning electron microscope, in-situ SB and migrated SB can be effectively identified. Rob increases linearly with increasing vitrinite reflectance (Ro), as a result of a decrease of aliphatic structure and the enhancement of aromatization of SB. Within the oil window three types of secondary pores may develop in SB, including modified mineral pores, devolatilization cracks and bubble holes. At a high maturity stage spongy pores may develop in pyrobitumen. Scanning electron microscopy combined with in-situ SEM-Raman spectroscopy can further reveal the structral information of different types of SB, thus providing crucial data for understanding for understanding OM migration paths, dynamics, and distances at micro-scale.

Layered injection technology for chemical flooding of Class-Ⅲ oil reservoirs in the Daqing Oil Fields complex,Songliao Basin,Northeast China

The Class-Ⅲ oil reservoirs of Lasaxing oilfield in the Daqing Oil Fields complex have geological oil reserves of 1.86 billion tonnes,an oil recovery of 39%,with remaining reserves accounting for more than 45%of the total geological reserves of the oilfield.Therefore,they have considerable potential for future oil production.The current layered injection technologies fail to achieve effective control over the low single-layer injection rates since they can only produce low throttle differential pressure under low injection rates(5-20 m^(3)/d).In this study,a symmetrically-structured double-offset-hole injection allocator and a novel throttling component were developed.Their spatial layout was constructed and mechanical parameters were optimized using finite element analysis,which allows for expanding the flow rate range at low injection rates.According to experimental results,the throttle differential pressure increased from 0.2 MPa to 0.8 MPa at an injection rate of 5 m^(3)/d,and the range of the single-layer flow rates expanded from 20-70 m^(3)/d to 5-70 m3/d.The field test results show that the effective production of oil layers with medium and low permeability was achieved and that the ratio of producing oil layer thickness to the total reservoir thickness increased by 9.7%on average.Therefore,this study provides valuable technical support for the effective chemical-flooding-based development of Class-Ⅲ oil reservoirs.

Geochemistry and origins of hydrogen-containing natural gases in deep Songliao Basin,China:Insights from continental scientific drilling

The different reservoirs in deep Songliao Basin have non-homogeneous lithologies and include multiple layers with a high content of hydrogen gas.The gas composition and stable isotope characteristics vary significantly,but the origin analysis of different gas types has previously been weak.Based on the geochemical parameters of gas samples from different depths and the analysis of geological settings,this research covers the diverse origins of natural gas in different strata.The gas components are mainly methane with a small amount of C_(2+),and non-hydrocarbon gases,including nitrogen(N_(2)),hydrogen(H_(2)),carbon dioxide(CO_(2)),and helium(He).At greater depth,the carbon isotope of methane becomes heavier,and the hydrogen isotope points to a lacustrine sedimentary environment.With increasing depth,the origins of N_(2)and CO_(2)change gradually from a mixture of organic and inorganic to inorganic.The origins of hydrogen gas are complex and include organic sources,water radiolysis,water-rock(Fe^(2+)-containing minerals)reactions,and mantle-derived.The shales of Denglouku and Shahezi Formations,as source rocks,provide the premise for generation and occurrence of organic gas.Furthermore,the deep faults and fluid activities in Basement Formation control the generation and migration of mantle-derived gas.The discovery of a high content of H_(2)in study area not only reveals the organic and inorganic association of natural-gas generation,but also provides a scientific basis for the exploration of deep hydrogen-rich gas.

Distribution and genesis of the anomalously high porosity zones in the middle-shallow horizons of the northern Songliao Basin

High-quality reservoirs occur in the middle-shallow horizons of the northern Songliao Basin. The distribution and genesis of their anomalously high porosity zones were studied using measured porosities, examination of ordinary thin sections and blue epoxy resin-impregnated thin sections and by SEM. The results show that there are three anomalously high porosity zones at the depth of 450-900 m, 1,300-1,900 m, 2,050-2,350 m respectively, named zone i, zone ii and zone iii from top to bottom. Horizontally, zone i and zone ii are distributed all over the basin except in the Southeastern Uplift, while zone iii is only distributed in the Central Depression. Zone i was formed by meteoric water leaching as well as organic acid dissolution. Zones ii and iii were dominantly formed by organic acid dissolution. Additionally, clay mineral transformation generating H＋ and hydrocarbon emplacement retarding the cementation of sandstones are also important for the formation of zones ii and iii.

Discovery of nano organo-clay complex pore-fractures in shale and its scientific significance:A case study of Cretaceous Qingshankou Formation shale,Songliao Basin,NE China

A new pore type,nano-scale organo-clay complex pore-fracture was first discovered based on argon ion polishing-field emission scanning electron microscopy,energy dispersive spectroscopy and three-dimensional reconstruction by focused ion-scanning electron in combination with analysis of TOC,R_(o)values,X-ray diffraction etc.in the Cretaceous Qingshankou Formation shale in the Songliao Basin,NE China.Such pore characteristics and evolution study show that:(1)Organo-clay complex pore-fractures are developed in the shale matrix and in the form of spongy and reticular aggregates.Different from circular or oval organic pores discovered in other shales,a single organo-clay complex pore is square,rectangular,rhombic or slaty,with the pore diameter generally less than 200 nm.(2)With thermal maturity increasing,the elements(C,Si,Al,O,Mg,Fe,etc.)in organo-clay complex change accordingly,showing that organic matter shrinkage due to hydrocarbon generation and clay mineral transformation both affect organo-clay complex pore-fracture formation.(3)At high thermal maturity,the Qingshankou Formation shale is dominated by nano-scale organo-clay complex pore-fractures with the percentage reaching more than 70%of total pore space.The spatial connectivity of organo-clay complex pore-fractures is significantly better than that of organic pores.It is suggested that organo-complex pore-fractures are the main pore space of laminar shale at high thermal maturity and are the main oil and gas accumulation space in the core area of continental shale oil.The discovery of nano-scale organo-clay complex pore-fractures changes the conventional view that inorganic pores are the main reservoir space and has scientific significance for the study of shale oil formation and accumulation laws.