Development characteristics and controlling factors of fractures in lacustrine shale and their geological significance for evaluating shale oil sweet spots in the third member of the Shahejie Formation in the Qikou Sag, Bohai Bay Basin

Natural fractures are critical for shale oil and gas enrichment and development. Due to the extremely high heterogeneity of shale, the factors controlling the formation of internal fractures, especially horizontal fractures, remain controversial. In this study, we integrate thin section analysis and microcomputed tomography(CT) data from several lacustrine shale samples from the third member(Es3) of the Shahejie Formation, Qikou Sag, Bohai Bay Basin, to assess the fractures in detail. The goal is to reveal the development characteristics, controlling factors, and geological significance for evaluating sweet spots in a shale oil play. The fractures in the Es3contain high-angle structural and horizontal bed-parallel fractures that are mostly shear and extensional. Various factors influence fracture development,including lithofacies, mineral composition, organic matter content, and the number of laminae. Structural fractures occur predominantly in siltstone, whereas bed-parallel fractures are abundant in laminated shale and layered mudstone. A higher quartz content results in higher shale brittleness, causing fractures, whereas the transformation between clay minerals contributes to the development of bedparallel fractures. Excess pore pressure due to hydrocarbon generation and expulsion during thermal advance can cause the formation of bed-parallel fractures. The density of the bed-parallel and structural fractures increases with the lamina density, and the bed-parallel fractures are more sensitive to the number of laminae. The fractures are critical storage spaces and flow conduits and are indicative of sweet spots. The laminated shale in the Es3with a high organic matter content contains natural fractures and is an organic-rich, liquid-rich, self-sourced shale play. Conversely, the siltstone, massive mudstone, and argillaceous carbonate lithofacies contain lower amounts of organic matter and do not have bed-parallel fractures. However, good reservoirs can form in these areas when structural fractures are present and the source, and storage spaces are separated.

Key geological factors governing sweet spots in the Wufeng-Longmaxi shales of the Sichuan Basin,China

Production performance of the Wufeng-Longmaxi shales varies significantly among Fuling,Weirong,and Wulong fields in the Sichuan Basin.Total organic carbon(TOC)content,mineralogy,and organic matter(OM)pore characteristics are investigated to identify key factors governing sweet spots.Siliceous shales with good preservation conditions in the Fuling Field exhibit large thickness,high TOC content and thin-section porosity(TSP),and well-developed OM macropores,thus high initial production and estimated ultimate recovery(EUR).Thin carbonate-containing siliceous shales with good preservation conditions in the Weirong Field feature medium-to-high TOC and well-developed OM macropores but low TSP,leading to high initial production but low EUR.Siliceous shales with poor preservation conditions in the Wulong Field are characterized by large thickness,high TOC,low TSP and poorly-developed OM macropores,causing low initial production and EUR.Both sedimentary and preservation conditions are intrinsic decisive factors of sweet spots,as they control the mineral composition,TOC,and OM macropore development.Deep-water shales in transgressive systems tracts(TSTs)exhibit better-developed OM macropores and greater TOC compared to highstand systems tracts(HSTs).OM macropores are most prevalent in siliceous shales,followed by carbonate-containing siliceous shales and then argillaceous shales.Furthermore,good preservation conditions are conducive to retain OM macropores with low pore aspect ratio(PAR).Comparison among the three fields shows that high-TOC silicious shales with good preservation conditions are the highest in TSP and EUR.Therefore,organic richness,lithofacies,and preservation conditions are the major factors which determine OM pore development,governing the sweet spots of the Wufeng-Longmaxi shales.

Quantitative and Comprehensive Prediction of Shale Oil Sweet Spots in Qingshankou Formation, Songliao Basin

The mud shale of Qingshankou Formation in Songliao Basin is the main rock source and contains rich shale oil resources. The successful development of shale oil depends on evaluating and optimizing the “sweet spots”. To accurately identify and optimize the favorable sweet spots of shale oil in Qingshankou Formation, Songliao Basin, the original logging data were preprocessed in this paper. Then the thin mud shale interlayer of Qingshankou Formation was identified effectively by using the processed logging data. Based on the artificial neural network method, the mineral content of mud shale in Qingshankou Formation was predicted. The lithofacies were identified according to the mineral and TOC content. Finally, a three-dimensional (3-D) model of total organic carbon (TOC), vitrinite reflectance (Ro), mineral content, and rock of Qingshankou Formation in Songliao Basin was established to evaluate and predict the favorable sweet spots of shale oil in the study area. The results show that there are a lot of calcareous and siliceous thin interlayers in Qingshankou Formation, and TOC content is generally between 2% and 3%. Ro is the highest in Gulong sag, followed by Sanzhao sag. The lithofacies mainly consists of felsic shale and mixed shale, mainly in the first member of Qingshankou Formation. Comprehensive analysis shows that shale oil development potential is enormous in the eastern part of Sanzhao Sag and the northern part of Gulong Sag.

Prestack inversion based on anisotropic Markov random field-maximum posterior probability inversion and its application to identify shale gas sweet spots

Economic shale gas production requires hydraulic fracture stimulation to increase the formation permeability. Hydraulic fracturing strongly depends on geomechanical parameters such as Young＇s modulus and Poisson＇s ratio. Fracture-prone sweet spots can be predicted by prestack inversion, which is an ill-posed problem; thus, regularization is needed to obtain unique and stable solutions. To characterize gas-bearing shale sedimentary bodies, elastic parameter variations are regarded as an anisotropic Markov random field. Bayesian statistics are adopted for transforming prestack inversion to the maximum posterior probability. Two energy functions for the lateral and vertical directions are used to describe the distribution, and the expectation-maximization algorithm is used to estimate the hyperparameters of the prior probability of elastic parameters. Finally, the inversion yields clear geological boundaries, high vertical resolution, and reasonable lateral continuity using the conjugate gradient method to minimize the objective function. Antinoise and imaging ability of the method were tested using synthetic and real data.

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%.

Intelligent prediction and integral analysis of shale oil and gas sweet spots

Shale reservoirs are characterized by low porosity and strong anisotropy. Conventional geophysical methods are far from perfect when it comes to the prediction of shale sweet spot locations, and even less reliable when attempting to delineate unconventional features of shale oil and gas. Based on some mathematical algorithms such as fuzzy mathematics, machine learning and multiple regression analysis, an effective workflow is proposed to allow intelligent prediction of sweet spots and comprehensive quantitative characterization of shale oil and gas reservoirs. This workflow can effectively combine multi-scale and multi-disciplinary data such as geology, well drilling, logging and seismic data. Following the maximum subordination and attribute optimization principle, we establish a machine learning model by adopting the support vector machine method to arrive at multi-attribute prediction of reservoir sweet spot location. Additionally, multiple regression analysis technology is applied to quantitatively predict a number of sweet spot attributes. The practical application of these methods to areas of interest shows high accuracy of sweet spot prediction, indicating that it is a good approach for describing the distribution of high-quality regions within shale reservoirs. Based on these sweet spot attributes, quantitative characterization of unconventional reservoirs can provide a reliable evaluation of shale reservoir potential.

Sweet spot prediction in tight sandstone reservoir based on well-bore rock physical simulation

To establish the relationship among reservoir characteristics and rock physical parameters,we construct the well-bore rock physical models firstly,considering the influence factors,such as mineral composition,shale content,porosity,fluid type and saturation.Then with analyzing the change rules of elastic parameters along with the above influence factors and the cross-plots among elastic parameters,the sensitive elastic parameters of tight sandstone reservoir are determined,and the rock physics template of sweet spot is constructed to guide pre-stack seismic inversion.The results show that velocity ratio and Poisson impedance are the most sensitive elastic parameters to indicate the lithologic and gas-bearing properties of sweet spot in tight sandstone reservoir.The high-quality sweet spot is characterized by the lower velocity ratio and Poisson impedance.Finally,the actual seismic data are selected to predict the sweet spots in tight sandstone gas reservoirs,so as to verify the validity of the rock physical simulation results.The significant consistency between the relative logging curves and inversion results in different wells implies that the utilization of well-bore rock physical simulation can guide the prediction of sweet spot in tight sandstone gas reservoirs.

Application of Pore Evolution and Fracture Development Coupled Models in the Prediction of Reservoir "Sweet Spots" in Tight Sandstones

The Chang-63 reservoir in the Huaqing area has widely developed tight sandstone ＂thick sand layers, but not reservoirs characterized by rich in oil＂, and it is thus necessary to further study its oil and gas enrichment law. This study builds porosity and fracture development and evolution models in different deposition environments, through core observation, casting thin section, SEM, porosity and permeability analysis, burial history analysis, and ＂four-property-relationships＂ analysis.

Evaluation of sweet spots and horizontal-well-design technology for shale gas in the basin-margin transition zone of southeastern Chongqing,SW China

The shale gas accumulation conditions in the basin-margin transition zone of southeastern Chongqing in SW China are complex.In order to improve single-well productivity in this area,the geologic characteristics,major factors controlling the occurrence of sweet spots,and drilling/fracturing optimization were investigated in this study.The sweet spot evaluation system and criteria were established,and the horizontal-well-design technology was developed.The following three conclusions were drawn.First,the accumulation and high-productivity-oriented approaches for sweet spot evaluation are proposed and the criteria are established based on screened key indicators.Second,the horizontal well was designed based on:(1)the“six-map”method,to identify both the geology and engineering sweet spots for well locations;and(2)seismic attributes,to predict the development of fractures and cavities,and thus,avoid mud loss and improve the drilling efficiency.The target window,well-azimuth optimization,and the curvature were forecasted to improve the fracturing performances.Third,the Pingqiao anticline,Dongsheng anticline,Jinfo slope,and Wulong syncline were selected as Type I sweet spots.Currently,shale gas has been successfully discovered in the basin-margin transition zone and is being commercially developed.

Quantitative prediction of shale gas sweet spots based on seismic data in Lower Silurian Longmaxi Formation,Weiyuan area,Sichuan Basin,SW China

Sweet spots in the shale reservoirs of the Lower Silurian Longmaxi Formation in Weiyuan 201 Block of Sichuan Basin were predicted quantitatively using seismic data and fuzzy optimization method. First, based on seismic and rock physics analysis, the rock physics characteristics of the reservoirs were determined, and elastic parameters sensitive to shale reservoirs with high gas content were selected. Second, data volumes with high precision of the elastic parameters were obtained from pre-stack simultaneous inversion. The horizontal distribution of key parameters for shale gas evaluation were calculated based on the results of rock physics analysis. Then, the fuzzy evaluation equation was established by fuzzy optimization method with test and logging data of horizontal wells with similar operation conditions. key parameters affecting the productivity of horizontal wells were sorted out and the weights of them in the sweet spots quantitative prediction were worked out by fuzzy optimization to set up a sweet spots evaluation system. Three classes of shale gas reservoirs which including two kinds of sweet spots were predicted with the above procedure, and the sweet spots have been predicted quantitatively by combining the above prediction results with the testing production. The testing results of 7 verification wells proved the reliability of the prediction results.

Sweet spot evaluation and exploration practice of lacustrine shale oil of the second member of Kongdian Formation in Cangdong sag, Bohai Bay Basin

Based on core,thin section,X-ray diffraction,rock pyrolysis,CT scanning,nuclear magnetic resonance and oil testing data,the macro and micro components,sedimentary structure characteristics,of Paleogene Kong 2 Member in Cangdong sag of Huanghua depression and evaluation standard and method of shale oil reservoir were studied to sort out the best shale sections for shale oil horizontal wells.According to the dominant rock type,rhythmic structure and logging curve characteristics,four types of shale lithofacies were identified,namely,thin-layered dolomitic shale,lamellar mixed shale,lamellar felsic shale,and bedded dolomitic shale,and the Kong 21 sub-member was divided into four quasi-sequences,PS1 to PS4.The PS1 shale has a porosity higher than 6%,clay content of less than 20%,and S1 of less than 4 mg/g;the PS2 shale has well-developed laminar structure,larger pore and throat size,better connectivity of pores and throats,high contents of TOC and movable hydrocarbon,S1 of over 4 mg/g,clay content of less than 20%,and porosity of more than 4%;PS3 shale has S1 value higher than 6 mg/g and clay content of 20%-30%,and porosity of less than 4%;and PS4 shale has lower TOC content and low oil content.Shale oil reservoir classification criterion based on five parameters,free hydrocarbon content S1,shale rhythmic structure,clay content,TOC and porosity,was established.The evaluation method of shale oil sweet spot by using the weighted five parameters,and the evaluation index EI were proposed.Through comprehensive analysis,it is concluded that PS2 is best in quality and thus the dual geological and engineering sweet spot of shale oil,PS3 and PS1 come next,the former is more geologic sweet spot,the latter more engineering sweet spot,and PS4 is the poorest.Several vertical and horizontal wells drilled in the PS2 and PS3 sweet spots obtained high oil production.Among them,Well 1701 H has produced stably for 623 days,with cumulative production of over 10000 tons,showing bright exploration prospects of Kong 2 Member shale oil.

Dynamical Model with Application to the Analysis of the Sweet Spot on a Baseball Bat

In this paper, we analyze the effect on the ball-bat model of corking the bat, and investigate the relationship between the sweet spot and different materials. First and foremost, we develop a simple but effective theoretical model to give a rough estimate for the sweet spot of the bat. Second, we give a simplified abstract form of bat which facilitates the description and modeling study. Last but not least, according to different material caused rotary inertia and different recovery coefficient, researches on the different material hitting effect were made.

Comprehensive Evaluation of Geological and Engineering Sweet Spots of Shale Gas Reservoir: A Case Study of the Luzhou Block, Sichuan Basin

There is a huge amount of marine shale gas resources in the southern Sichuan Basin in China, and most of the resources are at the buried depth of 3500 - 4500 meters. At present, deep shale gas is in the early stage of exploration and development. In order to achieve large-scale efficient development, in addition to optimizing favorable blocks, it is also to identify the optimal target in the vertical direction combine geology, drilling, and fracturing. Therefore, Taking the Longmaxi formation shale in the Luzhou block as the research object, based on drilling, logging, and core experiment data, through single well and 3D geomechanical modeling methods, analyze the characteristics of organic matter abundance, porosity, pore pressure, collapse pressure, mineral composition and in-situ stress of different layers of shale in Longmaxi formation. Systematically summarized the main controlling factors of the “sweet spot” of deep shale gas and establish the comprehensive evaluation system of deep shale gas “sweet spots”, to clarify the optimal “sweet spots” of geology, drilling, and fracturing in the Longmaxi reservoir. Results show that the total organic carbon content, porosity, and gas saturation of the long111 layer are higher than other layers. The Long111 layer has a low collapse pressure and a high compressive strength, the risk of wellbore instability is relatively low. The stress difference coefficient of All layers is less than 0.3, and the brittleness index of the Long111 layer is 62.35%. A complex fracture network is easier to form after fracturing. The conclusion shows that the Long111 layer is the optimal reservoir section of the Longmaxi Formation. Ensure the drilled rate of the Long111 layer and maximize the length of the horizontal section can obtain higher production.

Diagenesis-porosity evolution and“sweet spot”distribution of low permeability reservoirs:A case study from Oligocene Zhuhai Formation in Wenchang A sag,Pear River Mouth Basin,northern South China Sea

The characteristics of low permeability reservoirs and distribution of sweet spots in the Oligocene Zhuhai Formation of Wenchang A sag, Pearl River Basin were investigated by core observation and thin section analysis. The study results show that there develop the fine, medium and coarse sandstone reservoirs of tidal flat–fan delta facies, which are of mostly low permeability and locally medium permeability. There are two kinds of pore evolution patterns: oil charging first and densification later, the reservoirs featuring this pattern are mainly in the third member of Zhuhai Formation between the south fault zone and the sixth fault zone, and the pattern of densification first and gas charging later is widespread across the study area. Strong compaction and local calcium cementation are the key factors causing low permeability of the reservoirs in the Zhuhai Formation. Thick and coarse grain sand sedimentary body is the precondition to form "sweet spot" reservoirs. Weak compaction and cementation, dissolution, early hydrocarbon filling and authigenic chlorite coating are the main factors controlling formation of "sweet spot" reservoir. It is predicted that there develop between the south fault and sixth fault zones the Class Ⅰ "sweet spot" in medium compaction zone, Class Ⅱ "sweet spot" in nearly strong compaction zone, Class Ⅲ "sweet spot" reservoir in the nearly strong to strong compaction zone with oil charging at early stage, and Class IV "sweet spot" reservoir in the strong compaction and authigenic chlorite coating protection zone in the sixth fault zone.

Multi-source genesis of continental carbonate-rich fine-grained sedimentary rocks and hydrocarbon sweet spots

This paper systematically discusses the multiple source characteristics and formation mechanisms of carbonate-rich fine-grained sedimentary rocks through the analysis of material source and rock formation.The hydrocarbon accumulation characteristics of carbonate-rich fine-grained sedimentary rocks are also summarized.The results show that the main reason for the enrichment of fine-grained carbonate materials in rift lake basins was the supply of multiple material sources,including terrestrial material input,formation of intrabasinal authigenic carbonate,volcanic-hydrothermal material feeding and mixed source.The development of carbonate bedrock in the provenance area controlled the filling scale of carbonate materials in rift lake basins.The volcanic-hydrothermal activity might provide an alkaline fluid to the lake basins to strengthen the material supply for the formation of carbonate crystals.Authigenic carbonate crystals induced by biological processes were the main source of long-term accumulation of fine-grained carbonate materials in the lake basins.Carbonate-rich fine-grained sedimentary rocks with multiple features were formed through the interaction of physical,biochemical and chemical processes during the deposition and post-deposition stages.The source and sedimentary origin of the fine-grained carbonate rock controlled the hydrocarbon accumulation in it.In the multi-source system,the types of"sweet spots"of continental shale oil and gas include endogenous type,terrigenous type,volcanic-hydrothermal type and mixed source type.

Analysis of tight oil accumulation conditions and prediction of sweet spots in Ordos Basin: A case study

Tight sandstone reservoirs are widely developed in the Mesozoic Yanchang Formation of the Ordos Basin,China.There is a lack of understanding on the sedimentary setting,source-reservoir relationship and oil accumulation conditions in this area.In this study,through the comprehensive analysis of the distri-bution of tight oil,we evaluated the properties and petrological features of reservoir,geochemical characteristics of source rocks,the source-reservoir relationship,as well as the trapping,preservation and accumulation conditions of tight oil in the Chang 7 Member,and predicted the sweet spots of tight oil in the study area.The results show that the Chang 7 Member is a typical low-porosity and ultra-low permeability reservoir with great tightness,small pore throat and high capillary pressure,and must have been of near-source accumulation.The source rocks are mainly developed in the Chang 7_(3) submember,and the reservoirs mainly occur in the Chang 7_(1) and Chang 7_(2) submembers,forming a combination mode of“lower source rock and upper reservoir”.Sandbodies with good connectivity and fractures being well developed in local areas are the main hydrocarbon transport systems.The abnormal high pressure caused by hydrocarbon generation and pressurization is the main driving force of tight oil accumulation.The mode of hydrocarbon transportation is dominated by the vertical or lateral migration from under-lying source rocks or adjacent source rocks to reservoirs within a short distance.Following the integrated evaluation of lithology,physical properties and oil saturation of reservoirs and geochemical character-istics of source rocks,we grouped the sweet spots of Chang 7 Member into three types:Type I,Type II and Type III.Among others,the Type I sweet spots are the best in terms of porosity,permeability and source rock thickness and hydrocarbon enrichment which should be the focus of oilfield development.This study lays an important foundation for the economic and efficient development of tight oil in the Chang 7 Member of Heshui area,and has important implications on tight sandstone reservoirs in other regions of Ordos Basin in China.

Types and genesis of sweet spots in the tight sandstone gas reservoirs:Insights from the Xujiahe Formation,northern Sichuan Basin,China

Through comprehensively applying geological and geophysical data,as well as core and thin section observation,the characteristics of reservoirs and fractures in the second member of the Xujiahe Formation(hereinafter referred to as Xu2 Member)in the Yuanba area,northern Sichuan Basin,were studied.Combined with the analysis of the main controlling factors of production capacity,the types and characteristics of the sweet spots in the tight sandstone gas reservoir were determined.The evaluation standards and geological models of the sweet spots were established.The results are as follows:(1)There are bedding-parallel fracture-,fault-induced fracture-,and pore-dominated sweet spots in the tight sandstone gas reservoirs of the Xu2 Member.(2)The bedding parallel fracture-dominated sweet spots have developed in quartz sandstones with well-developed horizontal fractures and micro-fractures.They are characterized by high permeability and high gas output during production tests.This kind of sweet spots is thin and shows a limited distribution.Their logging responses show extremely low gamma-ray(GR)values and medium-high AC values.Moreover,the bedding parallel fracture-dominated sweet spots can be mapped using seismic methods.(3)The fault-induced fracture-dominated sweet spots have welldeveloped medium-and high-angle shear fractures.Their logging responses show an increase in peaks of AC values and total hydrocarbon content and a decrease in resistivity.Seismically,the areas with welldeveloped fault-induced fracture-dominated sweet spots can be effectively mapped using the properties such as seismic entropy and maximum likelihood.(4)The pore-dominated sweet spots are developed in medium-grained feldspathic litharenites with good reservoir properties.They are thick and widely distributed.(5)These three types of sweet spots are mainly determined by sedimentation,diagenesis,and tectonism.The bedding parallel fracture-dominated sweet spots are distributed in beachbar quartz sandstones on the top of the 1st sand layer group in the Xu2 Member,which develops in a shore-shallow lake environment.The fault-induced fracture-dominated sweet spots mainly occur near faults.They are increasingly developed in areas closer to faults.The pore-dominated sweet spots are primarily distributed in the 2nd and 3rd sand layer groups,which lie in the development areas of distributary channels near provenances at western Yuanba area.Based on the geological and seismic data,a comprehensive evaluation standard for these three types of sweet spots of the tight sandstone reservoirs in the Xu2 Member has been established,which,on the one hand,lays the foundation for the development and evaluation of the gas reservoir,and on the other hand,deepens the understanding of sweet spot in the tight sandstone gas reservoirs.

中国石油陆上中-高成熟度页岩油勘探现状、进展与未来思考

中国石油天然气集团公司(简称中国石油)矿权区陆相页岩油地质资源丰富,“十三五”资源评价,中国石油陆上中-高熟页岩油(简称页岩油)地质资源量201×108t,占中国页岩油地质资源总量的71%。鄂尔多斯盆地延长组7段、松辽盆地青山口组和准噶尔盆地芦草沟组等重点层系的产量显著增长,中国石油页岩油年产量从2010年的2.5×10^(4)t增至2023年的391.6×10^(4)t,勘探潜力巨大。研究表明:近年来由于陆相不同类型页岩油具有地质特征非均质性强、富集高产因素明显差异的特点,其效益勘探存在地质认识与勘探技术的双重挑战。特别是:(1)夹层型页岩油虽在鄂尔多斯盆地长71+2亚段(延长组7段1+2亚段)实现规模勘探,但勘探靶体钻遇率变化大,深湖区细粒沉积体系研究薄弱,靶体空间展布刻画技术的准确性仍需提升;(2)混积型页岩油地层垂向厚度大、岩性频繁变化、发育多套地质甜点,在柴达木盆地、渤海湾盆地虽然实现了勘探突破,但垂向地质甜点产量差异明显,页岩油高产主控因素不清,主力靶体评价优选技术、方法仍需完善;(3)页岩型页岩油在松辽盆地青山口组实现了古龙页岩油勘探突破,但陆相湖盆不同类型页岩生、排烃差异明显,页岩原位滞留烃特征变化大,地质-工程一体化靶体评价技术、方法仍需深化研究。总体上中国石油页岩油勘探开发仍处于快速推进阶段,未来应:(1)加强淡水湖盆深湖区多类型砂体成因机制研究,实现薄层浊积砂体等夹层型页岩油效益勘探;(2)加强咸化湖盆富碳酸盐混积型页岩油源-储组合评价,优选主体靶体实现混积型页岩油高效勘探;(3)加强淡水、咸化湖盆优质源岩生、排烃差异性评价,优选最佳靶体,实现不同湖盆页岩型页岩油地质-工程一体化精细勘探。

基于储层甜点特征的致密砂砾岩油气藏压裂增产技术研究

中拐凸起南斜坡二叠系佳木河组二段储层为一套含浊沸石的致密砂砾岩。早期,在有利构造上钻探的直井产量均较低,2014年钻探该区第一口水平井用于提产,但因未能有效控制缝高,致深部水体锥进过快,长期试采效果差。近年来,地震地质综合研究发现,储层存在局部甜点。因下部并无隔层,压裂时需严格控制裂缝下延。通过对储层甜点特征进行梳理,综合评价了各断块甜点的优质程度,为试油段优选和新井的部署提供依据。为了实现控缝高压裂,提高单井产量和试采效果,开展人工隔层导流能力室内实验、完成人工隔层支撑剂粒径的优选,进行压裂数值模拟研究、确定了研究区形成有效厚度人工隔板的临界施工参数。研究认为,前置液阶段,改变液体黏度和排量、铺置有效人工隔层能够抑制水力裂缝向下扩展;携砂液阶段,加入一定浓度纤维,可进一步提升支撑剂对上部优质储层的支撑。这种组合控缝高工艺技术投入该区压裂实施后,有效控制了裂缝下延高度,甜点层段得到充分改造,试油及后续试采期间维持了较高油气产能,压裂提产效果明显。

基于核磁共振测井的页岩油产能分析及甜点评价

吉木萨尔凹陷芦草沟组页岩油水平井产能差异大,各井产水率区别明显,其主控因素尚不清楚,现有甜点分类标准无法满足该区页岩油精细开发的需要,基于核磁共振测井截止值的含油性及可动性解释难以精细识别页岩油甜点。以核磁共振测井和实验室核磁共振测试为基础,基于分频处理、分流体核磁共振测井孔隙结构表征、弹性能排油模拟等技术,精细表征页岩油层中不同类型流体分布;分别刻画油水赋存孔径,建立可动油量评价模型,对流体赋存、孔径分布、可动油量等进行定量表征;结合单井试油和生产数据,明确水平井产能控制因素。研究表明:大孔轻质组分占比、可动油孔隙度与水平井产能的相关性明显优于孔隙度、含油饱和度和核磁可动油孔隙度;水影响指数则反映地层水对页岩油流动的影响程度,该值越小,相同可动油孔隙度下水平井产能越高、含水率越低。以大孔轻质组分占比、水影响指数和可动油孔隙度为指标,将页岩油油层划分为3类,由Ⅰ类至Ⅲ类油层,日产油量快速减小,含水率明显升高,可作为芦草沟组页岩油甜点精细评价的依据。