A set of organic-rich shale-dominated source rocks series with sandy deposits was developed during the heyday of lake basin development in the 7th Member of the Triassic Yanchang Formation （Chang 7 Member） in Ordos Basin. Over the years， aiming at the geological characteristics and exploration and development potential of this hydrocarbon source rock series， a number of theoretical studies and technical breakthroughs have been carried out. The research results show that： Shale oil in Chang 7 Member is mainly divided into two types： Interlayer and shale. The interlayer type can be further subdivided into gravity flow interlayer oil and delta front interlayer shale oil， and the shale type can be further subdivided into laminar shale oil and foliation shale oil. Gravity flow interlayer shale oil is highly charged by high quality source rocks in the middle of the lake basin. The sandy reservoir has high felsic content， good fracturing ability， numerous micron-size pores and good connectivity. Scale economy development has been realized in the Qingcheng Oilfield which was proved in the gravity flow interlayer shale oil. The thick sandstone reservoirs of delta front interlayer shale oil have good lateral continuity， lateral migration and accumulation of oil and gas， and large porosity of sandy reservoirs. Breakthroughs have been made in horizontal well tests， and it is a realistic backup field. The single sand body of laminar shale oil has thin thickness and high muddy content， which leads to low penetration rate of high-quality reservoirs， but high degree of oil and gas charging. The determination of “sweet spot” evaluation standard and the breakthrough of development technology are the next direction to be explored， and there is a huge amount of prospective resources. The foliation shale oil has high content of mud-scale felsic minerals， which is conducive to fracturing， reservoir space is small with porosity less than 2% and structure is complex， but hydrocarbon recovery and prediction resources are huge. At present， the risk exploration and in-situ conversion process of pure shale reservoirs are progressing steadily.
The exploration breakthrough of Chang 7 Member shale oil from sandstones in Ordos Basin has been made and scale effect has been realized， however， whether the mudstone and shale series have exploration potential and how to characterize the hydrocarbon occurrence states are still questions. Contrast between the shale oil and residual hydrocarbons shows that the test shale oil has high content of light and medium hydrocarbons （nC25-） but very low abundance of asphaltene， while the retained hydrocarbons has high concentrations of macromolecular compounds. Analytical tests and experimental studies suggest that the oily components have poor compatibility with asphaltene， their occurrence stats are distinct， the asphaltene components are mainly adsorbed by kerogen while the oily constituents occur in the pores and fractures as free hydrocarbons. Therefore， high contents of asphaltene will not affect the mobility of shale oil. Moreover， the method of separating and extracting free hydrocarbons from shale and mudstones is not only a rapid technique for shale oil resources evaluation， but also reveals that the free hydrocarbon contents of Chang 7 Member shale and mudstones are very high with an average content of 5 mg/g， showing a significant potential of shale oil exploration from mud shales.
A set of fine-grained sediments of organic-rich shale interbedded with thin layer silt and fine grained sandstones developed in the Chang 73 sub-member of the Ordos Basin. It has the characteristics of overall hydrocarbon generation and general oil-bearing. It is of great significance to clarify the hydrocarbon content， occurrence state and hydrocarbon components of different types of fine-grained sediments for the potential analysis and sweet spot selection of this type of shale oil resources. This study is based on the systemical core testing and analysis of the Chang 73 sub-member of Well CY1， movable hydrocarbon research on four fine-grained sedimentary rock types is carried out， including black shale， dark mudstone， siltstone and fine-grained sandstone， by using multi-granular multi-polar sequential extraction methods， and the movable hydrocarbon resources of shale oil in the Chang 73 sub-member is evaluated in the Ordos Basin. The research results show that the hydrocarbon content and its components of different types of fine-grained sediments under different particle size crushing conditions（1 cm3，0.5 cm3，150 mesh） are analyzed by multi-particle multi-polar sequential extraction. The unit extraction amount of hydrocarbon is fine-grained sandstone>black shale>siltstone>dark mudstone. The soluble organic matter extracted by step 1 and step 2 is mainly light-medium components and belongs to movable hydrocarbons. Soluble organic matter extracted by steps 3 and 4 is immobile hydrocarbon. Based on the above test methods， the movable hydrocarbon of black shale， dark mudstone， siltstone and fine sandstone in the Chang 73 sub-member are 3.35 mg/g， 1.45 mg/g， 3.28 mg/g and 4.48 mg/g， respectively. The movable hydrocarbon resources of 220 km2 shale oil in Cheng 80 well block in the Chang 73 sub-member were evaluated. The preliminary evaluation results were （0.37-0.51）×108 t. The distribution area of shale oil in the Chang 73 sub-member in the Ordos Basin is about 1.5×104 km2. Through analogy analysis， it is estimated that its movable hydrocarbon resources are （25-35）×108 t. This type of shale oil is expected to become a new replacement area for oil exploration in the basin.
Rich shale oil resources are contained in the 7th Member of the Yanchang Formation （Chang 7 Member） in the Ordos Basin. The exploration and development of the Chang 7 Member sandstone interbedded shale oil has made substantial breakthroughs. The exploration and development potential of the shale oil， an important oil and gas replacement for Changqing Oilfield in the future， is huge. But its related geological basic research is relatively weak. This paper gives elaborations and thinkings on basic science issues， including： Characteristics of mud shale， the significance of shale oil exploration， the restoration of the formation of paleo-environment， the organic matter enrichment effect of tuff and the catalysis of hydrocarbon generation， etc. It is believed that continuous attention should be paid to related geological basic research， such as pure mud shale-type shale oil， and modern lacustrine semi-deep lake-deep lake facies sedimentation investigations can provide valuable ideas and methods for the study of mud shale formation， paleoenvironment restoration and formation mechanism. Meanwhile， we should pay close attention to the research on the catalysis of tuff on the hydrocarbon generation of source rocks and carry out a series of thermal simulation experiments on the catalysis of enriched elements in the tuff， providing important basic geological parameters for the exploration and development of Chang 7 Member pure mud shale-type shale oil.
The gravity flow-derived sandstone developed in the Chang 73 sub-member shale strata is a realistic replacement target for shale oil reserves and production in the Changqing Oilfield， but the research on the characteristics of the Chang 73 sub-member sandstone reservoir and shale oil enrichment controlling factors are weak. Based on the data from the Well CY1， the Chang 73 sub-member fully coring well， using X-ray diffraction， scanning electron microscopy， high-pressure mercury intrusion and nuclear magnetic resonance test data， combined with thin section and core data， an in-depth analysis of the macro and micro of the Chang 73 sub-member sandstone reservoir characteristics， and the controlling factors of storage capacity and oiliness were carried out. The results show that the Chang 73 sub-member mainly develops gray massive lithic feldspar， lithic feldspar and feldspar lithic siltstone， and a small amount of fine sandstone， which is characterized by typical low porosity and low permeability. The reservoir space is mainly composed of intergranular pores， feldspar dissolved pores， clay mineral intercrystalline pores， intragranular dissolved pores and micro-cracks， and widely developed micro-nano pore throats make it have a certain storage capacity； the micro-pore structure of the Chang 73 sub-member is mainly divided into three categories A， B， and C. The median pore throat radius and the micro pore throat sorting coefficient are the main parameters that control the median saturation pressure， the maximum mercury inlet saturation and the mercury removal efficiency， and determine the enrichment degree and seepage capacity of shale oil on the micro scale； the mineral composition， grain size， and micro-pore structure of the Chang 73 sub-member sandstone jointly control the storage capacity of the Chang 73 sub-member sandstone， and the pressurization of hydrocarbon generation promotes the hydrocarbon quality in the Chang 73 sub-member black shale. Hydrocarbons accumulate toward the adjacent sandstone with good seepage ability. Intergranular pores， feldspar dissolution pores and micro-cracks are the main storage spaces for free hydrocarbons. Clay minerals are the main storage spaces for adsorbed hydrocarbons. Finding sandstones with high felsic mineral content， low clay mineral content， and weak calcite cementation in the Chang 73 sub-member mud shale strata is the main goal of shale oil exploration and development； the porosity represented by type A and B pore structures is greater than 5%， and the sandstone with permeability higher than 0.05×10-3 μm2 is the dominant reservoir of the Chang 73 sub-member. The sandstone represented by type C pore structure is difficult to charge with oil and gas， and its physical properties and seepage capacity are poor， which is not conducive to the accumulation and production of shale oil.
Rich shale oil resources are developed in the Chang 7 Member of the Mesozoic Triassic Yanchang Formation in northern Shaanxi area， Ordos Basin. This paper mainly uses core， thin section， well logging and other data， focusing on high-precision pore analysis techniques such as scanning electron microscopy， micro-CT， two-dimensional FIB-SEM testing， etc. to finely portray the micro-features and to discuss its main controlling factors. The pores of the Chang 7 shale oil reservoir in the study area are mainly micro-pores with a radius of 2.0-50 μm and a throat radius of 0.3-13 μm. Numerous clustered pore-throat units are formed by micro-nano-level and micro-level throat connecting micro-level pores. The porosity is distributed between 3.0% and 13.0%， and the average porosity is 7.0%. The permeability is between 0.02×10-3 μm2 and 0.30×10-3 μm2， the average permeability is 0.15×10-3 μm2. The mercury injection curve is characterized by low displacement pressure， high mercury removal efficiency， and high proportion of thick throat. The Chang 7 Member shale oil reservoir in the study area has fine grain size and high content of plastic components. The main factors that cause its pores to decrease are the strong early compaction and the strong cementation of clay minerals and carbonates. The dissolution and the protective effect of the chlorite film have a certain pore-enhancing effect. The potential for Type I exploration and development of Chang 7 Member shale in northern Shaanxi area is huge. Compared with Qingcheng area， the class Ⅰ multi-stage superimposed sandstone reservoir of shale oil has great exploration and development potential in northern Shaanxi area.
A great breakthrough has been made in the exploration of shale oil in the 7th Member of Yanchang Formation （Chang 7 Member） in Longdong area of the Ordos Basin. The practice shows that there is rich shale oil in the Chang 7 Member in the northern Shaanxi area， which has become a new target for the unconventional oil exploration of the basin. In this paper， the quality， type， reservoir， interlayer and source-reservoir configuration of source rocks are quantitatively characterized. The results show that： （1） Black shale （TOC=13.81%） and dark mudstone （TOC=3.74%） are widely distributed， the thin layers of sandstone associated with them constitute favorable geological conditions for the formation of shale oil. （2） The mean ratio of shale oil-sand is 20.1% and the mean thickness of single sand body is 3.8 m in the Chang 7 Member of the northern Shaanxi area. The source reservoir of the Chang 7 Member is typical shale oil， and the reservoir dessert is mainly “sandwich type”， the effective allocation of source and reservoir provides guarantee for the formation of shale oil. （3） Comprehensive study of the integration of geological engineering shows that the favorable reservoir， interlayer and fracture scale control the distribution law of shale oil "sweet spot" and the high productivity. Based on the analysis of the main controlling factors of shale oil enrichment， the evaluation criteria of the “sweet spot area” of the Chang 7 Member in the northern Shaanxi area were established， with a total of 21 “sweet spots” evaluated.
Pyrite is widely developed in shale reservoirs. It is of great significance in the identification of sedimentary environment and the analysis of shale oil and gas enrichment. Combined with core observation， scanning electron microscopy identification， X-ray diffraction analysis and other methods， taking the shale samples in Chang 73 sub-member of Yanchang Formation in the Ordos Basin as an example， macro and micro characteristics analysis of pyrite is carried out. At the same time， its significance on environmental evolution and shale oil enrichment is discussed. The results show that there is a large amount of pyrite in the Chang 73 sub-member shale， with an average content approximately of 16%， up to 50%. There are mainly nodules and clumps sedimentary pyrite in core. Three types of pyrite including strawberry-like shapes， automorphic crystals， and metasomatic filling organisms are observed. Strawberry-like pyrite has two external forms， one with obvious spherical outline， and the other as clusters and contiguous aggregates， which are the products of the sedimentary period. The self-shaped crystals are mainly octahedrons and cubes that are isolated or distributed in the layer. They are formed in the early diagenesis. Pyrite metasomatism can be seen filling the algae framework locally， which was formed in the late diagenetic stage. The average particle size of pyrite in Chang 73 sub-member is 6.5-12.3 μm， and the standard deviation is 1.8-5.3 μm， indicating the characteristics of oxygen content in the water body during the sedimentary period. The content of pyrite is directly proportional to the abundance of organic matter， reflecting that pyrite is of great significance for the enrichment of organic matter and hydrocarbons. Strawberry-like shaped pyrite crystallites have a protective effect on the development of internal organic matter pores. Pyrite has a two-way influence on the physical properties of the reservoir， the pore-forming characteristics of different types of pyrite should be discussed in detail.
Based on the core data of Chang 73 sub-member mud shale in Zhidan area， through the experimental test and analysis of organic carbon， rock pyrolysis， biomarker compounds， trace elements and organic matter group components， and using a variety of organic geochemical parameters， the formation environment， parent material type， maturity and oil content of Chang 73 sub-member mud shale in the study area were comprehensively identified. The results show that it is a weak oxidation-reduction sedimentary environment， and the parent material has the mixed source characteristics of aquatic organisms and higher plants in Chang 73 sub-member mud shale of the study area. Its TOC is avg. 4.65%， hydrocarbon generation potential （S1+S2） is avg. 20.86 mg/g， pyrolysis Tmax is avg. 447 ℃， and the maturity is high. The hydrocarbon component content is high （avg. 52%）， the asphaltene content is low （avg. 3.48%）， and the average saturated/aromatic hydrocarbon ratio is large （avg. 4.2） in the asphalt "A" composition of Chang 7 Member. The carbon number of saturated hydrocarbons is distributed between nC13-nC33， and the main peak carbon is mainly nC15 and nC16 of Chang 73 sub-member mud shale in the study area， which is biased towards low molecular weight alkanes.
Shale oil in Ordos Basin has the characteristics of low pressure coefficient， low brittleness index and vertical multi-interlayer. Horizontal well and volume fracturing technology can greatly increase the production of single well， but it is difficult to achieve economic and effective development under low oil price. On the basis of big data's field practice in the basin， a quantitative evaluation method of volume fracturing effect was established， which puts forward the volume fracturing transformation strategy and the direction of the next project. On the basis of the establishment of a new sectional classification evaluation standard for the comprehensive geological engineering quality of horizontal wells and the fine classification of reservoir types， and based on the test results of fluid production profiles of 112 sections of nine horizontal wells， it was concluded that the fracturing segments of I and II reservoirs account for 85.2%， and the output accounts for 96.4%， which was the main productivity contribution section. The fracturing segments of III reservoirs account for 14.8%， and the output accounts for only 3.6%， with the lowest contribution. Therefore， priority is given to fracturing type I and II reservoirs， and selective fracturing of III reservoirs. The main factors affecting productivity are reservoir length， fluid injection strength， fracture density， brittleness index， sand addition strength， permeability， discharge， porosity， horizontal stress difference and oil saturation. The material basis of reservoir is the first condition to obtain high productivity， and increasing the sweep volume of fracture net is an important way to maximize the productivity of unconventional oil and gas. The research results can provide a scientific basis for the optimal design of volume fracturing of shale oil horizontal wells and effectively promote the scale benefit development of shale oil.
Shale oil reservoir in Chang 7 Member， Ordos Basin， is characterized with tight reservoir rocks， complex pore structure， and low formation pressure. Decades of developing practices have proved that large-scale volumetric fracturing in horizontal wells with long lateral length is effective in the basin， compared to previously drilled directional wells and short horizontal wells. However， the reservoir underground is stress-sensitive， and the large-scale volumetric fracturing will result in a huge change in reservoir properties and strata energy. Thus water saturation during initial production period and individual well production will be affected. Through improved stress sensitive experiment， it has been clear that the compacted reservoir of shale oil has more strong stress sensitivity compared with the original matrix reservoir. The permeability variation law under stress sensitivity is obtained. Numerical simulation and field statistics are used to optimize the reasonable flowback intensity and flowback time of shale oil horizontal wells in Ordos Basin.
Chang 7 Member shale oil reservoir in Ordos Basin， China is continental and shows characteristics of low porosity and permeability with poor reservoir connectivity. Oil displacement efficiency of this type is low， because water breaks through underground during water flooding. Shale oil in the basin has been developed by large-scale fracturing to increase water-oil contact thus to improve oil recovery rate. Oil recovered by water imbibition was proved to be effective， and both development practices and indoor experiments showed that shale oil recovered by water imbibition accounts for 15%-40% of the total， providing a new method for oil displacement in shale oil reservoir. In this study， open-boundary core system was used to quantitatively study the impacts of pore radius， interfacial tension and permeability on oil recovery by water imbibition underground. Indoor experiments showed that shale oil produced from pores with radius less than 10μm accounts for 56%-80% of the total； shale oil recovered by water imbibition peaks when interfacial tension is 1.18 mN/m； core permeability is positively correlated with imbibition recovery when interfacial tension is less than 2 mN/m， while the two are not significantly correlated when interfacial tension is higher than 4mN/m.
Previous development practices in Chang 7 Member shale oil reservoir have proved that individual well production in Qingcheng Oilfield is related to several factors in both geology and engineering， such as physical properties of a reservoir， lateral length， fracturing scale etc. However， the most decisive factors among those are not clear. This paper is outlined to study the dominating factors relating to individual well production， based on geological parameters， fracturing construction data， and production data. Productivity influencing factors are quantitatively ordered using gray correlation analysis method and random forest algorithm. Research shows that the most decisive factors affecting individual well production of a horizontal well are porosity， oil saturation， brittleness index， effective lateral length， number of fracturing sections， sand volume pumped into a single fracturing section， and the amount of fracturing fluid pumped underground. Thus， well displacement， lateral length and fracturing parameters are optimized. This study provides a guidance to the development of shale oil reservoir considering low oil prices nowadays.
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