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Based on the production curves, changes in hydrocarbon composition and quantities over time, and production systems from key trial production wells in lacustrine shale oil areas in China, fine fraction cutting experiments and molecular dynamics numerical simulations were conducted to investigate the effects of changes in shale oil composition on macroscopic fluidity. The concept of “component flow” for shale oil was proposed, and the formation mechanism and conditions of component flow were discussed. The research reveals findings in four aspects. First, a miscible state of light, medium and heavy hydrocarbons form within micropores/nanopores of underground shale according to similarity and intermiscibility principles, which make components with poor fluidity suspended as molecular aggregates in light and medium hydrocarbon solvents, such as heavy hydrocarbons, thereby decreasing shale oil viscosity and enhancing fluidity and outflows. Second, small-molecule aromatic hydrocarbons act as carriers for component flow, and the higher the content of gaseous and light hydrocarbons, the more conducive it is to inhibit the formation of larger aggregates of heavy components such as resin and asphalt, thus increasing their plastic deformation ability and bringing about better component flow efficiency. Third, higher formation temperatures reduce the viscosity of heavy hydrocarbon components, such as wax, thereby improving their fluidity. Fourth, preservation conditions, formation energy, and production system play important roles in controlling the content of light hydrocarbon components, outflow rate, and forming stable “component flow”, which are crucial factors for the optimal compatibility and maximum flow rate of multi-component hydrocarbons in shale oil. The component flow of underground shale oil is significant for improving single-well production and the cumulative ultimate recovery of shale oil.

The geological characteristics and production practices of the major middle- and high-maturity shale oil exploration areas in China are analyzed. Combined with laboratory results, it is clear that three essential conditions, i.e. economic initial production, commercial cumulative oil production of single well, and large-scale recoverable reserves confirmed by the testing production, determine whether the continental shale oil can be put into large-scale commercial development. The quantity and quality of movable hydrocarbons are confirmed to be crucial to economic development of shale oil, and focuses in evaluation of shale oil enrichment area/interval. The evaluation indexes of movable hydrocarbon enrichment include: (1) the material basis for forming retained hydrocarbon, including TOC>2% (preferentially 3%–4%), and type I–II1 kerogens; (2) the mobility of retained hydrocarbon, which is closely related to the hydrocarbon composition and flow behaviors of light/heavy components, and can be evaluated from the perspectives of thermal maturity (Ro), gas-oil ratio (GOR), crude oil density, quality of hydrocarbon components, preservation conditions; and (3) the reservoir characteristics associated with the engineering reconstruction, including the main pore throat distribution zone, reservoir physical properties (including fractures), lamellation feature and diagenetic stage, etc. Accordingly, 13 evaluation indexes in three categories and their reference values are established. The evaluation indicates that the light shale oil zones in the Gulong Sag of Songliao Basin have the most favorable enrichment conditions of movable hydrocarbons, followed by light oil and black oil zones, containing 20.8×108 t light oil resources in reservoirs with Ro>1.2%, pressure coefficient greater than 1.4, effective porosity greater than 6%, crude oil density less than 0.82 g/cm3, and GOR>100 m3/m3. The shale oil in the Gulong Sag can be explored and developed separately by the categories (resource sweet spot, engineering sweet spot, and tight oil sweet spot) depending on shale oil flowability. The Gulong Sag is the most promising area to achieve large-scale breakthrough and production of continental shale oil in China.

The major enrichment type of shale oil in the Chang 73 shale of Upper Triassic Yanchang Formation in the Ordos Basin is unknown. This paper analyzes the organic matter transformation ratio, hydrocarbon expulsion efficiency and roof/floor sealing conditions of the Chang 73 shale, and evaluates the major enrichment type of shale oil in this interval. The average organic matter transformation ratio of the Chang 73 shale is about 45%; in other words, more than 50% of the organic matters have not transformed to hydrocarbons, and the lower the maturity, the greater the proportion of untransformed organic matters. The cumulative hydrocarbon expulsion efficiency of the transformed hydrocarbon is 27.5% on average, and the total proportion of untransformed organic matters plus retained hydrocarbons is greater than 70%. The relative hydrocarbon expulsion efficiency of the Chang 73 shale is 60% on average, that is, about 40% of hydrocarbons retain in the shale. The Chang 73 shale corresponds to Chang 71+2 and Chang 8 sandstones as the roof and floor, respectively, and is further overlaid by Chang 6 shale, where extensive low porosity and low permeability–tight oil reservoirs have formed in the parts with relatively good porosity and permeability. Moreover, the Chang 73 shale is tested to be in a negative pressure system (the pressure coefficient of 0.80–0.85). Therefore, the roof/floor sealing conditions of the Chang 73 shale are poor. The retained hydrocarbons appear mostly in absorbed status, with low mobility. It is concluded that the medium–high mature shale oil is not the major enrichment type of shale oil in the Chang 73 shale, but there may be enrichment opportunity for shale oil with good mobility in the areas where the sealing conditions are good without faults and fractures and oil reservoirs are formed off Chang 71+2, Chang 6 and Chang 8. Furthermore, low–medium mature shale oil is believed to have great potential and is the major enrichment type of shale oil in the Chang 73 shale. It is recommended to prepare relevant in-situ conversion technologies by pilot test and figure out the resource availability and distribution.

The Cambrian Xixiangchi Formation is an important succeeding stratum for gas exploration[JP] in the Lower Paleozoic of the Sichuan Basin, for which, the exploration degree is low, and the main controlling factors of large-scale[JP] reservoirs have not been well understood. Based on core, logging, seismic data and combined with regional tectonic evolution analysis, the original sedimentary structure of the shoal dolograinstone reservoir in the Cambrian[JP] Xixiangchi Formation were restored in this paper, and the characteristics, controlling factors and development patterns were further discussed. Results show that: (1) The rock types of shoal dolograinstone karst reservoirs mainly include dolograinstone, dolopackstone, granulated dolostone, residual granular dolomite and grain dolomite. It appeared to be developed in a variety of sedimentary micro-facies assemblages, such as shoal-shoal superimposition, flat-shoal-flat, lagoon-beach-lagoon and flat-beach-lagoon. The main reservoir space are intergranular (dissolution) holes, intergranular (dissolution) pores and vugs, and a small number[JP] of (dissolution)fractures were also developed. The reservoir porosity ranges from 3% to 6% with a highest value greater than 10%. (2) The dolograinstone shoal facies dolostone of the Xixiangchi Formation was controlled by the sea level changes and synsedimentary paleouplift. A large number of dissolution pores were formed after multiple exposure in the contemporaneous and quasi contemporaneous periods. Among them, the Yunan Movement at the end of the Xixiangchi period has a wide range of distribution and extensive exposure in the whole basin, which may further transform the middle and upper reservoirs of the Xixiangchi Formation. The Duyun and Guangxi movements might transform the central Sichuan paleouplift and its surrounding reservoirs. (3) The favorable reservoir distribution areas of Xixiangchi Formation in the Sichuan Basin were predicted to develop mainly type Ⅱ and Ⅲ reservoirs. Among them, the scale reservoir distribution area of the slope belt around the paleo-uplift is more than 10 000 km2[JP], which is expected to be the most favorable exploration area at present.

Based on the comparison of basic geological conditions and enrichment characteristics of shale oil plays, the heterogeneity of source and reservoir conditions and differential enrichment of medium-high maturity continental shale oil plays in China have been confirmed. (1) Compared with the homogeneous geological settings and wide distribution of marine shale oil strata in North America, the continental medium and high maturity shale oil plays in China are significantly different in geological conditions generally; continental multi-cyclic tectonic evolution forms multiple types of lake basins in multi-stages, providing sites for large-scale development of continental shale oil, and giving rise to large scale high-quality source rocks, multiple types of reservoirs, and diverse source-reservoir combinations with significant heterogeneity. (2) The differences in sedimentary water environments lead to the heterogeneity in lithology, lithofacies, and organic material types of source rocks; the differences in material source supply and sedimentary facies belt result in reservoirs of different lithologies, including argillaceous and transition rocks, and tight siltstone, and complex source-reservoir combination types. (3) The heterogeneity of the source rock controls the differentiation of hydrocarbon generation and expulsion, the diverse reservoir types make reservoir performance different and the source-reservoir configurations complex, and these two factors ultimately make the shale oil enrichment patterns different. Among them, the hydrocarbon generation and expulsion capacity of high-quality source rocks affect the degree of shale oil enrichment. Freshwater hydrocarbon source rocks with TOC larger than 2.5% and saline hydrocarbon source rocks with TOC of 2% to 10% have a high content of retained hydrocarbons and are favorable. (4) High-abundance organic shale is the basis for the enrichment of shale oil inside the source. In addition to being retained in shale, liquid hydrocarbons migrate along laminae, diagenetic fractures, and thin sandy layers, and then accumulate in laminae of argillaceous siltstone, siltstone, and argillaceous dolomite, and dolomitic siltstone suites, etc. with low organic matter abundance in the shale strata, resulting in differences in enrichment pattern.

The lithology and lithofacies assemblage of shale are highly complex and heterogeneous in the continental shale formations due to rapid changes in the sedimentary environment and source material, complicating the evaluation of shale oil enrichment areas, such as the member 3 of the Shahejie Formation in the Qikou sag, Bohaibay Basin, China. We used core observations and descriptions of well F39 × 1 and performed X-ray diffraction, scanning electron microscopy, nitrogen adsorption analysis, and nuclear magnetic resonance analysis to investigate the shale lithofacies characteristics and types in member 3 of the Shahejie Formation, and their effects on shale oil enrichment. The results showed the following. (1) The lithofacies are divided into four types according to the shale’s laminar structure, lithological characteristics, mineral composition, and organic matter content: thin laminar shale, thick laminar shale, massive mudstone, and argillaceous siltstone. These are divided into six subcategories. Each lithofacies has thin vertical layers. (2) The thin and thick laminar shale layers have favorable conditions for shale oil enrichment, such as a high total organic carbon content (TOC) (1.1–1.6%), many micropores (with the diameter of 0.5–2 µm) and fissures, a high residual hydrocarbon content (1.0–2.3 mg/g), and a good source-reservoir relationship, making them suitable for shale oil exploration. (3) The degree of lamina development influences the organic matter and residual hydrocarbon contents, the number of micropores, and the degree of shale oil enrichment. The semi-deep and deep lake facies are favorable areas for shale oil development.

The Chang 7 member in Ordos Basin is an important shale oil exploration layer, with new shale-oil discoveries in recent years. The Chang 73 submember is rich in organic shale, which is the main source rock of shale oil in the Yanchang Formation. In order to clarify the lamina structure, composition, types, and distribution characteristics in Chang 73 submember and its influence on organic matter enrichment, a full coring well in Chang 73 submember located in deep-lacustrine facies is selected to obtain intensive systematic core samples. Core observation, thin section identification, X-ray fluorescence element analysis, X-diffraction analysis, scanning electron microscope, electron probe, rock pyrolysis, and other techniques are performed to systematically analyze the morphology, structure, thickness, mineral compositions, and organic matter content of the shale lamina in the Chang 73 submember. Five types of lamina are identified: silty felsic lamina (SF), tuffaceous lamina (TF), organic-rich clay lamina (ORC), organic-bearing clay lamina (OBC), and homogeneous clay lamina (HC), which are further subdivided into eight subtypes. The lamina types change greatly vertically in the Chang 73 submember, in which the lower part is mainly silty felsic lamina, organic-rich clay, and tuffaceous lamina, the middle part is mainly organic rich and organic clay lamina and organic-bearing clay lamina, and the upper part is mainly homogeneous clay lamina and a small amount of silty felsic organic-bearing clay lamina and organic-bearing clay lamina. Different laminae show various organic matter types, organic matter content (TOC), and organic matter occurrence states which can be divided into four occurrence types. The TOC in organic-rich clay lamina and part of homogeneous clay lamina is high, while that of silty felsic lamina is lower. The relationship between shale lamina and organic matter enrichment is established according to the correlation analysis of laminar characteristics, mineral content, and organic matter content. Among them, the organic-rich lamina is richest of TOC and is a favorable “sweet point” for shale oil exploration.

Well Shuangyushi 1 and Well Nanchong l deployed in the NW and central Sichuan Basin have obtained a high-yield industrial gas flow in the dolomite and karst reservoirs of the Middle Permian Maokou Formation, showing good exploration prospects of the Maokou Formation. In order to identify the sequence stratigraphic features of the Maokou Formation, its sequence stratigraphy was divided and a unified sequence stratigraphic framework applicable for the entire basin was established to analyze the stratigraphic denudation features within the sequence framework by using the spectral curve trend attribute analysis, together with drilling and outcrop data. On this basis, the controls of sequence on source rocks and reservoirs were analyzed. In particular, the Maokou Formation was divided into two third-order sequences – SQ1 and SQ2. SQ1 was composed of members Mao 1 Member and Mao 3, while SQ2 was composed of Mao 4 Member. Sequence stratigraphic correlation indicated that the Maokou Formation within the basin had experienced erosion to varying extent, forming “three intense and two weak” denuded regions, among which, the upper part of SQ2 was slightly denuded in the two weak denuded regions (SW Sichuan Basin and locally Eastern Sichuan Basin), while SQ2 was denuded out in the three intense denuded regions (Southern Sichuan Basin–Central Sichuan Basin, NE and NW Sichuan Basin). The development of source rocks and reservoirs within sequence stratigraphic framework was significantly affected by sequence boundary; the grain banks that can form effective reservoir were predominately distributed in SQ1 highstand systems tract (HST), while effective source rocks were predominately distributed in SQ1 transgressive system tract (TST). It is concluded that the sequence division method is objective and reasonable, which can effectively guide oil and gas exploration in this region.

Both Sulige gas field in the Ordos Basin and Xujiahe gas field in Central Sichuan Basin are large low-porosity and low-permeability gas fields composed of conventional gas reservoirs and unconventional gas accumulations, representing typical cases of low-grade natural gas accumulations in China. Studies show there are both similarities and differences between these two fields in terms of forming conditions, mechanism and distribution. The similarities include: (1) large-scale development of accumulation conditions and close source-reservoir contacts, providing basis for large-scale accumulations; (2) large-scale energy storage during source kitchen burial and large-scale gas expulsion during uplift, providing important conditions for large-scale natural gas accumulation; (3) coexisting volume flow and diffusion flow, giving rise to the co-existence of conventional gas reservoirs and unconventional gas accumulations; and (4) clustered lithologic gas accumulations on a large scale. The differences lie in: (1) different source-reservoir-seal assemblages, giving birth to two kinds of accumulations, i.e. large area accumulations and large-scale patchy accumulations; (2) different source kitchen continuity and gas supply potentials, leading to different enriched gas-bearing layer series and different abundances. Timely understanding of the similarities and differences between large-scale natural gas accumulations with low porosity and permeability in these two basins would be conducive to avoid risks and improve exploration success ratio and benefits. Key words: accumulation conditions, accumulation mechanisms, similarities and differences, Sulige, Ordos Basin, Sichuan Basin

This paper analyzes the large-scale accumulation conditions and distribution characteristics of medium-low abundance hydrocarbon resources in China. Large-scale development of accumulation elements and their change in scale are the material basis of large scale oil and gas accumulation, determining the regional nature of oil and gas distribution. Liquid hydrocarbon dispersed in marine source rocks being cracked to form a large volume of gas and coal measure source rocks expelling gas during uplift are two important factors for the formation of large-scale hydrocarbon accumulation, which control the scale of source rocks that enter the main gas-generating stage. Volume flow and diffusive flow are the main migration-accumulation mechanism for the large-scale hydrocarbon accumulation, which ensures the sufficiency of hydrocarbon supply. Pancake, layer-like, and cluster are three main accumulation forms of large-scale hydrocarbon accumulation, which ensure the scale of hydrocarbon accumulation. Middle to low abundance hydrocarbon resources are characterized by near-source distribution, main-body play, late accumulation stage and single accumulation type. The periclinal area of palaeo-highs in marine craton basins, the lower slopes and sags in an intra-continental depression basin, and the gentle slopes of foreland basins are the most likely areas for the development of large-scale hydrocarbon accumulation, and they have two types of accumulation, large area and large scope. The proposal of the large-scale accumulation of middle to low abundance hydrocarbon resources in China improves the hydrocarbon discovering potential in middle to deep layers of superimposed basins and in the lower slopes and sags in depression basins, enlarges the exploration scale, and extends the hydrocarbon exploration from local second-order structure zones to the whole basin with the main source rock as the center, and from middle layers to deep, even super-deep, layers. Key words: middle-low abundance hydrocarbon resources, large-scale hydrocarbon accumulation, condition, distribution characteristics, exploration field, superimposed basin, depressed basin, onshore China

Natural gas exploration and discovery of hydrocarbons in the Xujiahe Formation in the Sichuan Basin are mainly concentrated in the Xu-2, 4 and 6 Members. The discovered gas reservoirs are characterized by a low filling degree, high water saturation in lower intervals, low levels of produced reserves and a limited number of gas-bearing reservoirs. The Xu-1, 3 and 5 Members are identified as source rocks. Based on the study of natural gas accumulations and several gas production wells in the Xu-1, 3 and 5 Members, this paper proposes that large scale sand bodies are developed in the Xu-1, 3 and 5 Members in the northwest, southwest and central parts of the Sichuan Basin. In each member, the favorable gas bearing area is about 5 000-11 000 km2, the reservoir rocks have good continuity with single layer extensions of up to 15-20 km. It is believed that these sand bodies have favorable conditions for gas accumulation within source kitchens to form lithologic or structural-lithologic gas reservoirs and have better accumulation potential and higher gas-bearing abundance than that of the Xu-2, 4 and 6 Members. Exploration practices have revealed that economic gas reserves may be generated. It is suggested that more efforts be made to study the gas accumulation conditions in the Xu-1, 3 and 5 Members (especially the Xu-3 and Xu-5 Members) and more exploration work be conducted in these strata. 摘要: ：目前四川盆地须家河组天然气藏勘探和发现主要集中于须二、四、六段，气藏总体表现出充满度不高、低部位含水饱和度偏高、储量动用比例偏低、储集层含气范围有限的特点，须一、三、五段则被作为烃源岩。在对须一、三、五段进行成藏条件研究和已知出气点解剖分析的基础上，提出川西北、川西南和川中部分地区须一、三、五段发育规模较大的砂体，各段有利含气面积约5 000～11 000 km2，储集层连续性较好，单层延伸长度达15～20 km，具备源内成藏的有利条件，可形成岩性或构造-岩性圈闭气藏，比须二、四、六段的成藏潜力和丰度更好，经勘探可形成工业规模的天然气储量，今后应该加强上述地区须一、三、五段源内天然气成藏研究与勘探力度，尤其是须三和须五段。图8表4参19 Key words: accumulation inside source, favorable play, Xujiahe Formation, Sichuan Basin

Low-to-medium maturity lacustrine shale oil resources have enormous potential and are projected to play a crucial role in the massive scale-up of crude oil production in China in the near future. The in-situ conversion process is currently the only effective means of utilizing this resource. Nevertheless, significant scientific challenges and technological bottlenecks still exist. Under this circumstance, the National Natural Science Foundation of China approved an integrated project of the Enterprise Innovation and Development Joint Fund titled "The Mechanism of Low-to-medium Maturity Lacustrine Shale Oil Resource Formation and its in-situ Conversion and Exploitation". This project aims to systematically investigate the entire process of in-situ conversion for low-to-medium maturity shale oil resources and lay a solid scientific and technological foundation for advancing the smooth implementation of on-site pilot trials. This paper presents the latest progress in this field and summarizes the existing scientific and technological challenges that need to be addressed. With the foundational support of the above project, our research team has made significant progress in several fields, including the formation mechanisms of organic matter superrich shale, low-to-medium maturity shale oil enrichment area evaluation, heat and mass transfer dynamics, coupled fluid field and hydrocarbon expulsion efficiency, exploitation methods, among others. Despite these theoretical advances, several major challenges were identified, which help to further focus on the critical scientific issues, determine the in-situ conversion technique-developing direction, and formulate a feasible implementation plan for future resource utilization. [ABSTRACT FROM AUTHOR]