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The Chang 73 sub-member of Triassic Yanchang Formation in the Ordos Basin was taken as an example and the lamina types and combinations, reservoir space features and shale oil enrichment patterns in organic-rich shale strata were investigated using core observation, thin section analysis, XRF element measurement, XRD analysis, SEM, high solution laser Raman spectroscopy analysis, and micro-FTIR spectroscopy analysis, etc. According to the mineral composition and thickness of the laminae, the Chang 73 organic-rich shales have four major types of laminae, tuff-rich lamina, organic-rich lamina, silt-grade feldspar-quartz lamina and clay lamina. They have two kinds of shale oil-bearing layers, “organic-rich lamina + silt-grade feldspar-quartz lamina” and “organic-rich lamina + tuff-rich lamina” layers. In the “organic-rich + silt-grade feldspar-quartz” laminae combination shale strata, oil was characterized by relative high maturation, and always filled in K-feldspar dissolution pores in the silt-grade feldspar-quartz laminae, forming oil generation, migration and accumulation process between laminae inside the organic shales. In the “organic-rich + tuff-rich lamina” binary laminae combination shale strata, however, the reservoir properties were poor in organic-rich shales, the oil maturation was relatively lower, and mainly accumulated in the intergranular pores of interbedded thin-layered sandstones. The oil generation, migration and accumulation mainly occurred between organic-rich shales and interbedded thin-layered sandstones.

Through reviewing the development history of tight oil and gas in China, summarizing theoretical understandings in exploration and development, and comparing the geological conditions and development technologies objectively in China and the United States, we clarified the progress and stage of tight oil and gas exploration and development in China, and envisaged the future development orientation of theory and technology, process methods and development policy. In nearly a decade, relying on the exploration and development practice, science and technology research and management innovation, huge breakthroughs have been made. The laws of formation, distribution and accumulation of tight oil and gas have been researched, the development theories such as “multi-stage pressure drop” and “man-made reservoirs” have been established, and several technology series have been innovated and integrated. These technology series include enrichment regions selection, well pattern deployment, single well production and recovery factor enhancement, and low cost development. As a result, both of reserves and production of tight oil and gas increase rapidly. However, limited by the sedimentary environment and tectonic background, compared with North America, China's tight oil and gas reservoirs are worse in continuity, more difficult to develop and poorer in economic efficiency. Moreover, there are still some gaps in reservoir identification accuracy and stimulating technology between China and North America. In the future, Chinese oil and gas companies should further improve the resource evaluation method, tackle key technologies such as high-precision 3D seismic interpretation, man-made reservoir, and intelligent engineering, innovate theories and technologies to enhance single well production and recovery rate, and actively endeavor to get the finance and tax subsidy on tight oil and gas. Key words: tight oil and gas, development history, theoretical and technological advancement, man-made oil and gas reservoir, enhancing recovery factor, development orientation

Fine-grained sedimentary formations occupy about two-thirds of the entire sedimentary sequences in nature. They have recently received great attention owing to the boom of shale oil and gas exploration. Unlike their coarse counterparts, detailed depositional features of fine-grained sedimentary rocks are quite difficult to recognize directly by visual inspection at core or hand specimen scales. A chemo-sedimentary facies analysis (CSFA) procedure has been developed by incorporating high-resolution (down to 30 μm) 2D elemental maps of rock slabs obtained by in situ XRF scanning with high-resolution optical images. The method enables one to unambiguously differentiate rock compositions and fine sedimentary structures and to generate core-scale chemo-sedimentary facies images. The 2D chemo-sedimentary facies map contains major elements with atomic numbers greater than 12, and some trace elements including Ba, V, Cr, Ni, Cu, Zn, Rb, Sr, Y, Zr, Nb, Mo, Th and U at individual pixels. The CSFA procedure was applied to study the Lucaogou Formation, a key shale oil producing interval in the Jimusaer Sag, Junggar Basin, NW China to illustrate three key advantages of CSFA in studying fine-grained sedimentary formations: (1) recognizing invisible sedimentary structures, (2) determining sediment provenances, (3) identifying subtle paleo environmental changes. The Lucaogou Formation has been characterized in unprecedented fine details with sedimentary heterogeneities at centimeter-millimeter scales. Seven chemo-sedimentary facies were delineated in the Lucaogou Formation. Among them, Facies CS-1 was deposited under a relatively deep, fresh to brackish water with strong energy and moist climate; Facies CS-2 was deposited under relatively shallow, brackish water with weak energy arid climate; Facies CS-6 was deposited under relatively shallow, brackish water, arid climate with intermittent hydrodynamic enhancement; Facies CS-7 was deposited in semi-deep, fresh water with strong energy and moist climate. Facies CS-1 and CS-7 have relatively high porosities. The dolomitic fragments in Facies CS-6 was originally deposited in Facies SC-2, but has higher porosities than that of Facies CS-2. Facies CS-6 and Facies CS-7 commonly occur together. The interval with well-developed. Facies CS-1, Facies CS-6 and Facies CS-7 are identified to be the most favorable exploration intervals in the Lucaogou Formation.

The organic-rich shale of the Chang 7 member is the most important source rock in the Ordos basin. The sedimentary environment and the controlling factors of organic matter enrichment, however, are still in contention. In this investigation, the Yishicun outcrop, located on the south margin of the Ordos basin, has been considered for the study. X-ray diffraction, polarizing microscopy, field emission scanning electron microscopy and cathodoluminescence (CL) were used to investigate the petrological features of the organic-rich shale. The content of volcanic ash and the diameter of pyrite framboid pseudocrystals were measured to illustrate the relationship between oxygen level, ash content and the enrichment of organic matter. It has been found that the diameter of pyrite framboid pseudocrystals has a strong correlation with the total organic carbon, demonstrating that the redox status degree of the water column has a positive impact on the enrichment of organic matter. Additionally, with an increase in the ash content, the content of organic matter increased at first and then decreased, and reached a maximum when the ash content was about 6%, illustrating that the ash input has a double effect on the enrichment of organic matter.

During the Mesozoic, the T-J1 oil system of the Ordos Basin, whilst the degree of oil enrichment, main production layer, and source rock distribution exhibit strong regional differences, no systematic study has been conducted to investigate these differences. At this time, the total organic carbon abundance and vertical distribution of the eight long core wells in different areas of the basin within the Chang 7 member source layers were calculated by means of the ΔLogR method. According to the industrial oil well and the low production well, the favorable oil distribution areas of the Chang 8, Chang 7, and Chang 6 reservoirs are demarcated. The current study confirmed five distribution styles and strong regional differences in the longitudinal direction of source rocks. To be more specific, the Jiyuan area in the northwestern part of the lake basin is dominated by the bottom rich type and the full section rich type. The northeastern Shaanxi region is mainly dominated by the middle rich type and the top rich type. Meanwhile, the central area of the basin is mainly the interlayered type, and the southwestern Longdong region is mainly the bottom rich type. The comprehensive analysis of source rock type and oil favorable zone revealed that source rock type has a controlling effect on the crude oil distribution. The bottom rich type and full section rich type dominate the Jiyuan area and multiple layer oil production. In northern Shaanxi, the top rich type and middle rich type accumulate on the upper portion. Also, the Chang 6 reservoir was the main production layer. The bottom rich type of the Longdong area accumulates under the source, while the Chang 8 reservoir is the main production layer. The central parts of the lake basin are dominated by the interlayered type with multiple layers of production oil. The close relationship between the distribution pattern of source rocks and oil accumulation indicates an improvement on the distribution law of the continental lake with significance practical implication on the optimization of the field of near-source-in-source oil and gas exploration. Keywords: Ordos basin, Yanchang formation, Patterns of source rocks, Exploration direction

In unconventional petroleum reservoirs, such as shale gas, shale oil, tight oil, and tight gas reservoirs, the microscopic pore structure, namely, the size, geometry, distribution, and interconnected relations of the pores and throats of a shale rock, directly affects the porosity, storage, and permeability. Studies related to the microscopic pore structure of shale are considered important for evaluating shale resources and for elucidating their distribution characteristics; additionally, these studies aim to improve the productivity and recovery ratio of both oil and gas. Therefore, methods that can accurately characterize the microscopic pore structure of shale have received considerable attention. In this study, we used the electrodeposition method to fill the interconnected pores of a rock sheet with metal and then used selective dissolution of the rock portion of the rock sheet to obtain the metal complex of the pore-throat structure. The structure and morphology of the obtained metal complex, which represents the microscopic pore structure of the shale, are characterized by a scanning electron microscope (SEM). By combining electrochemical deposition and SEM images, we could directly observe the three-dimensional microstructure of the shale at a scale smaller than 50 nm with a large observation area. Additionally, the surface areas of the connected pores and throats of the shale were calculated.

Tight oil in China is mainly distributed in Mesozoic-Cenozoic continental petroliferous basins, either generally occurs in tight sandstone or tight carbonate reservoirs which are paragenetic to or in contact with lacustrine petroleum source rocks. By the end of 2016, the continental tight oil in China has established an annual production capacity of 1.55 million tons, and a cumulative oil production of 2.52 million tons. In 2016, the continental tight oil production in China was about 800,000 tons. By analyzing the characterization of the tight oil in Ordos Basin, Songliao Basin, Jungar Basin, the organic-rich shale in China were formed in multiple geological periods including the Permian, Triassic, Jurassic, Cretaceous, Neogene, and Paleogene, in freshwater, semi-saline to hyper-saline lacustrine depositional setting. They are generally tens of meters to several hundreds of meters thick, have total organic carbon (TOC) contents of 0.4–16%, and thermal maturity of 0.4–1.4%. Algal bloom as results of volcanism, low sedimentation rate, transgression, and stratification of water body are prominent factors controlling the formation of organic-rich shales. Tight oil reservoirs in China mainly include tight sandstone, tight carbonate rock, tight sedimentary tuff, etc., all of which are characterized by strong heterogeneity and poor petrophysical properties, with in situ porosity and permeability generally no more 12% and 1 × 10−3 μm2 respectively. They are also characterized by tight pore-throat systems of nano- to micro-scales, with pore throat diameters in the range of 80–1800 nm, and complex pore structures. The producing reservoirs have pressure coefficients between 0.7 and 1.8, covering under-pressure to over-pressure. Crude oil properties vary greatly with densities ranging from 0.75 to 0.92 g/cm3. It is believed that the abundance and effectiveness of hydrocarbon supply controls the formation of oil saturation, and tight oil “sweet spots” are generally in or near the areas with high hydrocarbon expulsion intensities. In addition, the quality of the source rock and the type of source reservoir control the distribution of the “sweet spots”, and storage space and movable fluid guarantee the formation of the “sweet spot” area of high oil saturation. “Sweet spots” are often developed in local mini-structural highs under a broad and gentle-sloped background. Continental tight oil in China has estimated geological resource of 14.66 × 109 tons with a technically recoverable resource of 1.45 × 109 tons. Tight oil is mainly distributed in Ordos Basin, Songliao Basin, Bohai Bay Basin, Junggar Basin, and Qaidam Basin, within the Mesozoic and Cenozoic formations. Overall China has a relatively large tight oil reserve and good exploration prospect.

Oil accumulation, being difficult and complicated, is an important issue in petroleum exploration researches. Authigenic minerals, such as carbonate cements and chlorite, can in certain reservoirs alter the wettability of some pore spaces from water-wet to oil-wet. Generally, these oil-wet pore spaces are favorable for oil accumulation. The alteration of reservoir wettability induced by authigenic minerals and the impacts on oil accumulation in tight sandstone reservoirs were investigated using a suite of mineralogical and geochemical characterization techniques, including thin section observation, SEM-EDS, XRD, QEMSEM, CL, quantitative grain fluorescence (QGF), fluorescence spectral analysis, contact angle measurement and sealed coring oil saturation testing on the fourth member of the Lower Cretaceous Quantou Formation (K1q4) in the southern Songliao Basin, China. The study shows that the tight sandstone reservoirs are compositionally immature with detrital grains distributed homogeneously. Quartz, carbonates and clay minerals that show heterogeneous distribution characteristics are the major authigenic minerals in some parts of the intergranular pores. The detrital mineral assemblage suggests that the reservoir rocks at deposition and before diagenesis had characterized by strong water-wet properties. With the development of authigenic minerals, carbonate cements and authigenic chlorite tend to alter the wettability of some parts of the existing pore spaces from water-wet to oil-wet. In the K1q4 sandstone reservoirs, oil prefers to accumulate in the cemented residual pore spaces around carbonate cements and chlorite. Reservoirs containing about 4–5% carbonate cements are suggested to be more preferable to oil accumulation. These reservoirs are mainly located between sandstone-mudstone interfaces and central parts of the sand bodies. Chlorites have mainly two effects: on one hand, chlorite alters the wettability of existing pore spaces and provides preferential accumulation sites for oil in the tight sandstone reservoirs, on the other hand it can reduce the adhesion of oil through forming “clay-oil flocs” and promote further migration. Consequently, the reservoirs with moderate amount of carbonate cements or chlorite always show relative high oil saturation in the K1q4 tight sandstones.

Crude oil in unconventional shale systems, present as tight oil and shale oil, accumulates inside an oil kitchen in formations with coexistent sources and reservoirs. Organic matter present in oil shale is not yet mature and requires heating to convert it into crude oil. Oil exploration in shale systems involves the exploration of shale oil retained in source rocks and tight oil rich zones located near source rocks. Tight oil is a type of realistic unconventional oil resources in China. The marked increase in potential shale oil reserves, and exploration of these reserves, will result in a shale oil revolution similar to that experienced for shale gas. Based on a systematic comparison of geologic features of shale systems in the US and China, the geologic significance of the sweet spots in shale systems is proposed. This zone contains an abundance of unconventional oil in shale systems that can be explored and developed under current economic and technical conditions. The sweet spot zone refers to the zone in the tight oil rich zone which has industrial value within the scope of matured high-quality source rocks on the plane. The sweet spot interval refers to the high-productivity interval of tight oil which has industrial value through artificial stimulation. The main aim of oil exploration in shale formations is to identify the sweet spots. The distribution of the economic sweet spots in shale systems is evaluated by overlapping the geologic, engineering and economic sweet spots. Resource assessment techniques, the identification of logging data properties, high-resolution 3D seismic surveys, horizontal well production from well pads, and artificial reservoir development of sweet spots in oil-bearing shale formations can assist efficient development of oil. Globally, shale formations contain a significant volume of oil reserves. Currently, stimulated reservoir volume (SRV) techniques in horizontal wells in marine shale gas formations in the United States have average peak-productivity cycles of 10–15 years. To achieve commecial oil production in lacustrine shale systems in China, it is important to utilize large formation thicknesses and the high abundance of organic matter. In addition, the development of practical and economic techniques will result in an increase of productivity of tight oil and shale oil by 30–50 million tons, as well as the economic development of oil in lacustrine shale systems in China.

Lacustrine tight oil resources in China are significant exploration targets, similar to the marine tight oil resources in North America, while there are clear differences in terms of the forming environment, as well as reservoir properties of these two systems. In this study, the geological characteristics of lacustrine tight rocks are discussed, including the distribution, petrology, storage space, and movable fluid saturation. The tight rocks are consisted of tight sandstone, tight carbonate rock, tight peperite, and tight sedimentary tuff, in which intergranular pores and dissolved pores with the dimensions of 30–900 nm are mainly dominated by the storage space. The oil saturation typically exceeds 50% and the oil mobility is basically controlled by pore-throats with diameter less than 1 μm. These four types of reservoirs show different potentials for tight oil, in which tight sandstone plays in the Ordos Basin and the Songliao Basin are taken as the preferred targets into account for exploration and development. Although the distribution area, physical properties, and mobility of lacustrine tight oil in China are not as good as those of tight oil in North America, the lacustrine reservoirs are often stacked in multiple layers, resulting in a great thickness, and possess relatively high oil saturation, making them producible and commercially viable. Fluid properties are critical to effectively recovery of tight oil, and more comprehensive geological evaluation on fluid mobility should be undertaken.

Organic-matter-rich shales are the main target rocks for unconventional oil and gas exploration and development across the world. In China, shale-gas geological resources are estimated as approximately 110 × 1012 m3, with recoverable gas reserves of ca. 20 × 1012 m3. Recoverable shale-oil reserves are estimated as ca. 5 × 109 t. A total 35 important organic-matter-rich shale units have been recognized from Mesoproterozoic to Cenozoic strata across the entire China. These shales are categorized according to their origin under marine, marine–nonmarine transitional and lacustrine conditions. Shales of marine origin, with ca. 9 × 1012 m3 recoverable resources, dominate China's potential in terms of total volume of organic-carbon. Currently, the most favorable marine shales for oil and gas exploration are found in the Sichuan Basin within the lower Cambrian Qiongzhusi Formation and in the Wufeng-Longmaxi formations of uppermost Ordovician through lower Silurian. A fortuitous combination of of sea-level variations, of paleo-productivity, of tectonic activity causing development and migration of partially closed deep basin depocenters, and of sediment accumulation rates controlled the extensive deposition and distribution of organic-matter-rich shales in these Wufeng and Longmaxi formations. Organic-matter-rich shales in marine-nonmarine transitional facies associated with coal measures occur in North China within the Carboniferous and Permian, and in South China within the Permian. These Carboniferous-Permian organic-matter-rich shales are important source rocks for the gas fields in the Ordos and Sichuan Basins. Abundant organic-rich shales are also widely distributed within coal-bearing clastics and coal-measure shales of fluvial, lacustrine, and swamp facies in Upper Triassic to Middle Jurassic successions of many basins. Lacustrine organic-rich shales were deposited during the Permian through Neogene in various freshwater to saline lake settings. Lacustrine organic-matter-rich shales are the main oil source rocks in the Songliao, Bohai Bay, Ordos and Junggar basins. Lacustrine algae contributed to the rain of organic matter; and the preservation of organic matter and distribution of organic-rich shale was controlled by lake currents, water depth and oxygen-poor conditions, with enhanced preservation when buried by turbidity currents. Algal blooms were partly induced by trace nutrients from volcanic ash falls in all of these lacustrine basins. Seawater intrusion into the freshwater lake of the Songliao Basin promoted some episodes of black shales. Saline lacustrine basins, such as middle Permian Junggar Basin, contain organic-rich dolomite mudstone that mainly formed during hot climate conditions when the lakes had high salinity and stratified water columns that deprived the bottom waters of oxygen, thereby preserving massive amounts of organic matter. Laminated calcite-rich mudstone in the saline lacustrine settings formed in more brackish waters under stable warm conditions and weak biological activity. The modeling of the factors controlling the distribution of organic-matter-rich shales within China's basins is important for the exploration and development of unconventional oil and gas resources.

Mixed deposits are sediments consisting of external clastic (epiclastic or terrigenous), intrabasinal components and pyroclastic components. The mixture, comprising variable amounts of the three components, is defined as “mixed sedimentary rocks”. The Permian Lucaogou Formation (P2l) in the Jimusar Sag of the Junggar Basin is a promising tight oil target in western China, the fine-grained mixed sedimentary rocks of which are rich in organic matter (OM) and two sweet spot intervals with relatively high porosity. However, the sediment composition, provenance and deposition environmental settings have not been studied in detail. In this study coupled chemostratigraphic-petrographic analysis were used to reconstruct their depositional environments. The results show that the fine-grained sedimentary rocks have three major sediment sources, external clastic input (terrigenous clastics), intrabasinal autochthonous to parautochthonous components (carbonates, siliceous skeletal debris and OM) and pyroclastic input. Main lithofacies include siltstone/fine sandstone, mudstone, dolomite and tuffite. The silt/sandstones were mainly sourced from rocks with calc-alkaline composition, while the tuffaceous sedimentary rocks were sourced from high-K calc-alkaline rocks. Elemental proxies suggest that the carbonate rocks were generally deposited under warm and arid conditions, whereas the fine-grained clastic sediments were deposited under relatively humid conditions. The muddy or silty tuffaceous mixed rocks were deposited under relatively reducing conditions compared with carbonates and sandstones. Variations of lithofacies and OM accumulation of different intervals reflect changing deposition environmental settings, and the frequently altered high TOC content rocks and good reservoirs are benefit for tight oil formation. The work may provide some useful insights and serve as a reference for studying other mixed fine-grained sedimentary rocks and tight oil plays in similar lacustrine basins elsewhere.

In order to better predict the petrophysical properties of the tight sandstone reservoirs in the Upper Triassic Yanchang Formation, Ordos Basin, China, the nature of the diagenetic system involved was investigated using a suite of petrographic and geochemical techniques including thin section and X-Ray Diffraction analysis, scanning electron microscopy, MAPS mineralogy, Cathodeluminescence, electron probe microanalysis and fluid inclusion analysis on a set of selected tight sandstone samples. The sandstones investigated are texturally mature but compositionally immature with an average framework grain composition of Q32F41L27. Authigenic quartz, calcite, and chlorite coatings are the major cements, while feldspars are partially dissolved. Two groups of quartz cement are present in the tight sandstone reservoirs. Smectite to illite conversion provided silica source for the first group of quartz overgrowth (Qo-I), while the silica source for the second quartz overgrowth (Qo-II) was mainly originated from feldspar dissolution. Calcite cements consist of two types, namely Ca-I and Ca-II. The Ca-I calcite cement contains no chlorite coatings and have higher concentrations of Fe2+ and Mg2+, whereas the Ca-II calcite cement developed over the chlorite coatings. Plagioclase dissolution appears to be always accompanied by calcite cementation, while K-feldspar albitization forms euhedral albite locally. The burial diagenesis processes were, in most cases, not episodic but occurred as slow adjustments in response to increased burial depths and temperatures. In all the studied diagenetic minerals, elemental distributions appear to be strictly constrained by the interaction of authigenic minerals involved, indicating that the chemical reactions during the diagenesis comply well with the general principle of mass balance. The diagenesis in the Upper Triassic Yanchang Formation tight sandstones was thus mainly developed in a closed geochemical system, where mineral dissolution and precipitation are approximately balanced, and diagenetic fluids were not affected by external sources significantly. Mechanical compaction has played a more important role in destroying the primary porosities of the Upper Triassic Yanchang Formation than the cementation.

Permeability is an essential petrophysical parameter for reservoir modeling, reservoir classification, and productivity prediction in tight sandstone reservoirs. In this study, multiple parameters are extracted from the mercury injection capillary pressure (MICP) curves and the degree of multicollinearity between these parameters is analyzed. The partial least squares regression (PLSR) method is used for establishing the permeability prediction model and the optimal number of latent variables of the model is determined by the leave-one-out cross-validation (LOOCV) method. A comparison of the existing empirical models, the permeability prediction model by ordinary least square (OLS) method, and the permeability prediction model by PLSR method based on the MICP curves indicates that the permeability prediction model by PLSR method is superior to the other models for tight sandstone reservoirs.

Based on the investigation of tight oil exploration and development in North America, the successful cases of tight oil exploration and development in North America are summarized. The geological differences between continental tight oil in China and marine tight oil in North America is analyzed to explore the technical strategies for the industrial development of continental tight oil in China. The experiences of large-scale exploration and profitable development of tight oil in North America can be taken as references from the following 6 perspectives, namely exploring new profitable strata in mature exploration areas, strengthening the economic evaluation of sweet spots and focusing on the investment for high-profitability sweet spots, optimizing the producing of tight oil reserves by means of repetitive fracturing and 3D fracturing, optimizing drilling and completion technologies to reduce the cost, adopting commodity hedging to ensure the sustainable profit, and strengthening other resources exploration to improve the profit of whole project. In light of the high abundance of tight oil in China, we can draw on successful experience from North America, four suggestions are proposed in sight of the geological setting of China's lacustrine tight oil: (1) Evaluating the potential of tight oil resources and optimizing the strategic area for tight oil exploration; (2) selecting “sweet spot zone” and “sweet spot interval” accurately for precise and high efficient development; (3) adopting advanced tight oil fracturing technology to realize economic development; (4) innovating management system to promote the large-scale profitable development of tight oil. Key words: continental tight oil, sweet-spot area, sweet-spot interval, profitable exploration and development, China, North America

By dissecting thirty-six representative oil-gas reservoirs and analyzing accumulation conditions of giant oil and gas provinces in four prototype basins, including the Songliao Rift and Depression Basin, the Mesozoic Ordos Depression Basin, the Bohai Bay Rift and Depression Basin, the Mesozoic Junggar Depression Basin, the West Sichuan Foreland Basin and the Tarim Craton Basin, the hydrocarbon plays classification scheme of lithologic reservoirs is established and improved from the perspective of oil-gas accumulation zone and exploration target. The results reveal the accumulation characteristics and controlling factors of lithologic reservoirs including three kinds of slopes, three kinds of sag centers, three kinds of hydrocarbon accumulation assemblages and multiple genetic types. Based on the understandings on reservoir formation mechanisms and enrichment regularities of different hydrocarbon provinces, six major assessment indexes, parameter system and grading standard of lithologic plays are established, including hydrocarbon source rock condition, transportation pathway, reservoir condition, reservoir-cap assemblage, fluid property, and time-space configuration etc. Through comprehensive analysis of burial history, hydrocarbon-generation history, digenesis history, tectonic history, trap-forming history and accumulation history, plays of lithologic reservoirs are evaluated and optimized. The results show that the middle-shallow layers in the slope zones of the Songliao and Bohai Bay Basins oil-rich sags, the Mesozoic in the Ordos Basin, the Permian-Jurassic in the Junggar Basin, etc. are the major oil exploration areas in the future, with oil resources of about 42×108 t–67×108 t; the deep volcanic rocks of the Songliao Basin, the Carboniferous of the Junggar Basin, the Cambrian of the Tarim and Sichuan Basins, the Permian-Triassic reefs and shoals within the platform and on the platform margin of the Sichuan Basin are the major gas exploration areas in the future, with gas resources of about 1.5×1012 m3−2.0×1012 m3. Key words: lithological reservoirs, lithological trap, play types, play assessment method, accumulation regularity, exploration area

The Mesozoic Cretaceous reservoirs with a burial depth of more than 4500 m (14,764 ft) in the Kuqa Depression of the Tarim Basin are rich in natural gas resources, categorized under the typical deep tight sandstone reservoirs. Based on the analysis of cores, borehole image logs, thin sections, and laboratory testing, this study comprehensively evaluated the effective fractures in deep tight sandstone reservoirs and illuminated the contribution of effective fractures to the properties of these reservoirs. Tectonic fractures are an important component in these reservoirs. More than 70% of these fractures are not filled by any minerals, and their apertures vary greatly, measured from a few to hundreds of microns. The influencing factors of the effective fractures in these reservoirs include tectonism, diagenesis, and in-situ stresses. The fractures that are formed at earlier tectonic movements are more susceptible to be filled with minerals. The diagenetic facies of strong cementation exhibit a higher abundance of mineral-filled fractures, while dissolution can cause fracture apertures to become larger and makes them more effective. The unfilled fractures, which are parallel to the maximum horizontal stress component, show larger apertures and therefore are more effective. Laboratory tests indicate that effective fractures contribute less to the reservoir storage space, with an average porosity contribution rate of 3.95%. However, the permeability of samples with microscopic effective fractures was found 1-2 orders of magnitude higher than those without such fractures, since these fractures are main pathways that can connect scattered pores to improve the effectiveness of storage space. Moreover, the macroscopic effective fractures with a smaller angle from the maximum horizontal stress component decide the dominant reservoir fluid flow direction. Consequently, effective fractures exhibit a leading contribution to the reservoir permeability.

Pore-throat size is a very crucial factor controlling the reservoir quality and oiliness of tight sandstones, which primarily affects rock-properties such as permeability and drainage capillary pressure. However, the wide range of size makes it difficult to understand their distribution characteristics as well as the specific controls on reservoir quality and oiliness. In order to better understand about pore-throat size distribution, petrographic, scanning electron microscopy (SEM), pressure-controlled mercury injection (PMI), rate-controlled mercury injection (RMI), quantitative grain fluorescence (QGF) and environmental scanning electron microscopy (ESEM) investigations under laboratory pressure conditions were performed on a suite of tight reservoir from the fourth member of the Lower Cretaceous Quantou Formation (K 1 q 4 ) in the southern Songliao Basin, China. The sandstones in this study showed different types of pore structures: intergranular pores, dissolution pores, pores within clay aggregates and even some pores related to micro fractures. The pore-throat sizes vary from nano- to micro-scale. The PMI technique views the pore-throat size ranging from 0.001 μm to 63 μm and revealed that the pore-throats with radius larger than 1.0 μm are rare and the pore-throat size distribution curves show evident fluctuations. RMI measurements indicated that the pore size distribution characteristics of the samples with different porosity and permeability values look similar. The throat size and pore throat radius ratio distribution curves had however significant differences. The overall pore-throat size distribution of the K 1 q 4 tight sandstones was obtained with the combination of the PMI and RMI methods. The permeability is mainly contributed by a small part of larger pore-throats (less than 30%) and the ratio of the smaller pore-throats in the samples increases with decreasing permeability. Although smaller pore-throats have negligible contribution on reservoir flow potential, they are very significant for the reservoir storage capacity. The pore-throats with average radius larger than 1.0 μm mainly exist in reservoirs with permeability higher than 0.1mD. When the permeability is lower than 0.1mD, the sandstones are mainly dominated by pore-throats with average radius from 0.1 μm to 1.0 μm. The ratio of different sized pore-throats controls the permeability of the tight sandstone reservoirs in different ways. We suggest that splitting or organizing key parameters defining permeability systematically into different classes or functions can enhance the ability of formulating predictive models about permeability in tight sandstone reservoirs. The PMI combined with QGF analyses indicate that oil emplacement mainly occurred in the pore-throats with radius larger than about 0.25–0.3 μm. This result is supported by the remnant oil micro-occurrence evidence observed by SEM and ESEM.

The Lower Cretaceous Quantou Formation in the southern Songliao Basin is the typical tight oil sandstone in China. For effective exploration, appraisal and production from such a tight oil sandstone, the diagenesis and reservoir quality must be thoroughly studied first. The tight oil sandstone has been examined by a variety of methods, including core and thin section observation, XRD, SEM, CL, fluorescence, electron probing analysis, fluid inclusion and isotope testing and quantitative determination of reservoir properties. The sandstones are mostly lithic arkoses and feldspathic litharenites with fine to medium grain size and moderate to good sorting. The sandstones are dominated by feldspar, quartz, and volcanic rock fragments showing various stages of disintegration. The reservoir properties are quite poor, with low porosity (average 8.54%) and permeability (average 0.493 mD), small pore-throat radius (average 0.206 μm) and high displacement pressure (mostly higher than 1 MPa). The tight sandstone reservoirs have undergone significant diagenetic alterations such as compaction, feldspar dissolution, quartz cementation, carbonate cementation (mainly ferrocalcite and ankerite) and clay mineral alteration. As to the onset time, the oil emplacement was prior to the carbonate cementation but posterior to the quartz cementation and feldspar dissolution. The smectite to illite reaction and pressure solution at stylolites provide a most important silica sources for quartz cementation. Carbonate cements increase towards interbedded mudstones. Mechanical compaction has played a more important role than cementation in destroying the reservoir quality of the K 1 q 4 sandstone reservoirs. Mixed-layer illite/smectite and illite reduced the porosity and permeability significantly, while chlorite preserved the porosity and permeability since it tends to be oil wet so that later carbonate cementation can be inhibited to some extent. It is likely that the oil emplacement occurred later than the tight rock formation (with the porosity close to 10%). However, thicker sandstone bodies (more than 2 m) constitute potential hydrocarbon reservoirs.

The “hothouse” Triassic climate provides a key window to study megamonsoon systems. However, high-quality paleoclimate proxy records are rarely reported from the Tethys realm, where the seasonal meridional migration of the intertropical convergence zone (ITCZ) should be the most remarkable. Here, continuous and well-dated proxy records are used to study monsoon activity in the northeastern Tethys from the Middle Triassic lacustrine sediments (Ch7 black shale, Yanchang Formation) in the Ordos Basin (China). The proxies, including natural gamma-ray intensity, magnetic susceptibility, and element concentrations and ratios (U, K, Zr/Al, Zr/Rb and Sr/Rb), document a strong precession signal in the sediments. We propose that the periodic deposition of the Ch7 black shale occurred in response to intensifications in a monsoon-driven climate enhanced by the early Ladinian warming event. Climate variability in the Ordos Basin (~25°N) reflects the harmonics of ~100 kyr, ~20 kyr and ~10 kyr cycles and is highly coherent with solar insolation at 9° latitude, suggesting that this monsoon system might have been modulated by tropical solar insolation. The dominance of tropical climate processes in this mid-latitude monsoon region can be attributed to the dramatic dynamics of the ITCZ and intense monsoon circulation in the Tethys realm.

According to porosity measured by high-pressure mercury intrusion experiments and helium, and analysis of oil saturation data of 30 samples of tight formations of Member 4 of Cretaceous Quantou Formation in Rangzijing slope zone, southern Songliao Basin, the lower limit of flowing porosity of tight oil and its controlling factors of the samples were determined. By conversion between capillary pressure in reservoir conditions and capillary pressure from high-pressure mercury intrusion experiments, flowing porosity in various injection pressures in reservoir condition can be calculated. By calculating the minimum flowing porosity of oil-bearing samples and the maximum flowing porosity of the samples without oil, it is confirmed that 3.2% is the lower limit of flowing porosity in the oil-bearing samples in the study area; and the corresponding injection pressure in reservoir conditions is 0.35 MPa. If the injection pressure is higher than 0.35 MPa, tight oil can effectively flow and accumulate. The flowing porosity of tight formations and pore-throat ratio are negatively related. When the connectivity of pore-throat in reservoirs becomes poorer, higher injection pressure is needed for flowing porosity to be 3.2%. The injection pressure needed for flowing porosity to be 3.2% have a negative correlation to reservoir quality coefficient. With the reservoir quality coefficient increasing, the injection pressure needed for flowing porosity to be 3.2% has a decreasing tendency. Based on the tight oil lower limit of flowing porosity and injection pressure in reservoir conditions, the discriminant chart of effective accumulation of tight oil was set up. Key words: southern Songliao Basin, Cretaceous, tight oil, lower limit of flowing porosity

The tight sandstones of the Cretaceous Quantou formation are the main exploration target for hydrocarbons in the southern Songliao basin. Authigenic quartz is a significant cementing material in these sandstones, significantly reducing porosity and permeability. For efficient predicting and extrapolating the petrophysical properties within these tight sandstones, the quartz cement and its origin need to be better understood. The tight sandstones have been examined by a variety of methods. The sandstones are mostly lithic arkoses and feldspathic litharenites, compositionally immature with an average framework composition of Q43F26L31, which are characterized by abundant volcanic rock fragments. Mixed-layer illite/smectite (I/S) ordered interstratified with R = 1 and R = 3 is the dominating clay mineral in the studied sandstone reservoirs. Two different types of quartz cementation modes, namely quartz grain overgrowth and pore-filling authigenic quartz, have been identified through petrographic observations, CL and SEM analysis. Homogenization temperatures of the aqueous fluid inclusions indicate that both quartz overgrowths and pore-filling authigenic quartz formed with a continuous process from about 70 °C to 130 °C. Sources for quartz cement produced are the conversion of volcanic fragments, smectite to illite reaction and pressure solution at micro stylolites. Potassium for the illitization of smectite has been sourced from K-feldspar dissolution and albitization. Silica sourced from K-feldspars dissolution and kaolinite to illite conversion is probably only minor amount and volumetrically insignificant. The internal supplied silica precipitate within a closed system where the transport mechanism is diffusion. The quartz cementation can destroy both porosity and permeability, but strengthen the rock framework and increase the rock brittleness effectively at the same time.

According to the study of the electron beam charging effects of oil-bearing samples from tight reservoirs, a “Two-step Cross-section Back Scattered Electron (BSE) Imaging Method” is put forward, revealing the distribution of residual oil (the oil without loss in the vacuum conditions) in the tight reservoir. With the injection of charging agent, the connectivity of the pore system can also be characterized by this method, and a novel quantitative method of evaluating the effectiveness of reservoir space is established. For the tight reservoir samples from the Permian Lucaogou Formation in the Jimusar sag in the Junggar Basin, which was taken as an example, the strong charging effects mainly result from the residual oil in pores. The oil from pyrolysis of organic matter was widely stored in the pores both inside and nearby organic matter, or existing in poor organic matter area. Due to the super-large analyzing area, the Two-step Cross-section BSE Imaging Method can solve the weak representation of analysis area caused by the heterogeneity. The method of injection of charging agent works well in revealing the connectivity of the pore system. The total plane porosity of oil-bearing samples from the tight reservoir of the Lucaogou Formation is 12.56%, and the connectivity of the whole pore system in the sample reaches as high as 90%. The effectiveness of the reservoir space is very high. Key words: electron beam charging effect, Two-step Cross-section BSE Imaging, effective reservoir space, pore connectivity, tight oil, Junggar Basin, Jimusar sag, Lucaogou Formation

Based on the development of sedimentology and its industrial application, research demands on the sedimentary reservoirs in the oil and gas exploration and development in the future are summarized, and the future key research fields and directions are put forward as well. Recently, sedimentology in China has achieved a number of accomplishments: Lithofacies paleogeography and a new model of sandbody in shallow water delta of depositional basins during significant tectonic movements have been built, expanding the fields of oil and gas exploration. New knowledge of deep-water sedimentary sandbody distribution has sustained significant discoveries in (ultra-)deep water area and lacustrine basin center. New cognition of (ultra-)deep reservoir mechanism extends the depth of oil and gas exploration and development. New progresses on development pattern of organic-rich shale and study of unconventional reservoirs make a series of major breakthroughs in unconventional oil and gas exploration and development. Multi-scale development geological modeling predicts the distribution of remaining oil effectively. New techniques and methods of sedimentary reservoirs provide a foundation for the development of the sedimentology theory and its industrial application. In the future development of sedimentology, it is necessary to make progress in traditional sedimentology, to innovate fine-grained sedimentology and unconventional reservoir geology and to provide a foundation for sedimentary prototype basin restoration, organic-rich shale area evaluation, favorable reservoir prediction and favorable target area optimization, promoting the continuous innovations on sedimentology. Key words: sedimentology, fine-grained sediment, (ultra-)deep reservoir, deep water sediment, unconventional reservoir, reservoir configuration

In exploration for tight oil, the content and saturation of hydrocarbon in the tight reservoir is a key factor for evaluating the reserve. Therefore, it is necessary to study the geological history of hydrocarbon accumulation and the tight oil charging process. However, kinetic models used for petroleum development are not applicable for petroleum exploration. In this study, a static resistance model is proposed after analyzing resistances in ultra-slow flow in porous media. Using this model, the discontinuous pattern of oil charging is reproduced through incompressible Navier-Stokes equations, the phase field method and the finite element method. This study also explains macroscopic percolation behavior with microscopic flow mechanisms and discusses some issues in ultra-slow flow in a micro/nano pore-throat network. The resistance analysis reveals that capillary resistance and dissipation resistance are dominant factors in the mechanism of oil accumulation in tight reservoirs. Numerical simulations show that pressure thresholds exist and result in discontinuous oil charging. Generally, it is proven that the static model is more applicable than kinetic models in describing oil accumulation in tight reservoirs.

The onshore exploration realm has been continuously expanded to (ultra-) deep oil and gas recently in China. New comprehension and significant breakthroughs have been made in understanding generation and preservation conditions, reservoirs formation mechanisms, exploration potential, petroleum resources assessments, and exploration engineering technology of deep oil and gas. Deep oil and gas reservoirs include clastic, carbonate and volcanic settings. The temperature of deep oil can be up to 295 °C. Long term shallow burial and rapid deep burial at late stages help preserve the porosity in deep clastic rocks, and dissolution and fracturing effects improve their reservoir properties. Affected by faulting, hydrothermal karst processes, dolomitization and early oil and gas injection, carbonate rocks have good reservoir properties even at depths of 8 000 m. Controlled by tectonism, volcanism, diagenesis and diagenetic reconstruction during supergene and burial stages, primary and secondary weathering types of reservoirs develop deep volcanic reservoirs. Deep oil and gas resources in China are distributed mainly within three main practical areas of carbonate, clastic and volcanic areas. Dominated by gas, some of the more productive areas include the Tarim, Ordos, Sichuan, Junggar, Songliao, Santanghu and Bohai Bay basins. Deep oil and gas exploration in China has entered an age of breakthrough and discovery. Relevant engineering technology, such as ultra deep well drilling and ultra high temperature drilling fluid techniques have facilitated the ability to find (ultra-) deep oil and gas. Key words: deep oil and gas, reservoir formation mechanism, resources potential, exploration engineering technology

Global petroleum exploration is currently undergoing a strategic shift from conventional to unconventional hydrocarbon resources. Unconventional hydrocarbons in tight reservoirs show characteristics distinct from those of conventional hydrocarbon sources hosted in structural and stratigraphic traps. The characteristic features include the following: a hydrocarbon source and reservoir coexist; porosity and permeability are ultra-low; nano-pore throats are widely distributed; hydrocarbon-bearing reservoir bodies are continuously distributed; there is no obvious trap boundary; buoyancy and hydrodynamics have only a minor effect, and Darcy's law does not apply; phase separation is poor; there is no uniform oil–gas–water interface or pressure system; and oil or gas saturation varies. Examples of unconventional hydrocarbon accumulations are the Mesozoic tight sandstone oil province and the Upper Paleozoic tight sandstone gas province in the Ordos Basin, north-central China. Generally, continuous hydrocarbon accumulation over a large area is a distinguishing characteristic of unconventional hydrocarbon sources. Because of the great potential of unconventional petroleum resources, it is believed that research on such resources will be at the forefront of the future development of petroleum geology.

Seismic diagenetic facies is an important control on reservoir quality. This study investigated the feasibility of predicting sandstone diagenetic facies using conventional 3D seismic data by seismic sedimentology and calibrated by laboratory core-analysis data in the Qijia area of the Songliao Basin. Three core issues are related to seismic characterization of diagenetic facies, including how to correlate stratigraphic and diagenetic units from seismic data, how to evaluate the relationship between core-based diagenetic facies and seismic attributes, and how to find an effective way of mapping seismic diagenetic facies. Well- and seismic-based, high-resolution sequence analysis and seismic stratal slicing provide reservoir-scale (20 m) seismic representations of diagenetic units. Core-based analyses of sandstone diagenetic processes and diagenetic sequences reveal the kind of diagenesis that most influences reservoir quality. An investigation of reservoir parameters and acoustic rock properties further reveals the link between diagenetic facies and impedance, leading to a recognition of calcite cementation as the process that can be detected by seismic data. A seismic-based lithology cube (e.g., 90°-phased seismic volume) provides the amplitude (impedance) signal for detection of diagenetic facies. Stratal slices made from the seismic-based lithology cube are then used to interpret depositional facies and systems. Eventually a seismic diagenetic-facies map is generated through analysis of the relationships between depositional facies, impedance, and diagenetic facies. A case study of clay- and calcite-cemented sandstone in the Qijia area shows that although still in its infancy, seismic detection of sandstone diagenetic facies using conventional seismic data is definitely feasible. Key words: seismic diagenetic facies, seismic detection of diagenetic facies, seismic sedimentology, diagenesis, sandstone

Multi-scale (nano-to-micro) three-dimensional CT imaging was used to characterize the distribution and texture of micro-scale pore throats in tight sandstone reservoirs of the Triassic Yanchang Formation, Ordos Basin. First, the low-resolution Micro-CT was used to reflect the micro-pore texture of the core column with a diameter of 2.54 cm. Then, some samples with a diameter of 65 μm was derived from different areas according the different characteristics of micro-pore texture of the core scanned by low-resolution Micro-CT and scanned by high-resolution Nano-CT. Thus, a three-dimensional texture model of nano-scale micro-pores was reestablished and the permebility and porosity data of the sample could be obtained. On a micrometer scale, the size of the micro-pores varies, and their diameters range from 5.4 to 26.0 μm. The micro-pores are isolated, locally in the shape of a band. On a nanometer scale, the quantity of nanoscale micropores increases, the diameter of which ranges from 0.4 to 1.5 μm. The pore throats are arranged in the shape of tube and ball inside or on the surface of mineral particles(crystals). The ball-shaped micropores in nanoscale, often isolated in the three-dimensional space, show the poor connectivity and consequently act as the reservoir space. By contrast, the tube-shaped micropores in nanoscale show certain connectivity with micro-scale tube-shaped micropores and adjacent isolated ball-shaped micropores in nanoscale. Therefore, these tube-shaped micropores in nanoscale serve as throats and pores. Based on the calcution, the permeability of the samples is 0.843×10−3 μm2 and porosity is 10%. Key words: 3D X-ray CT, tight sandstone reservoirs, micro pore throat, nano-micro scale

Tight gas sandstone (tight gas) reservoirs have been widely discovered in a number of basins in China including the Sichuan, Ordos, Turpan-Hami (Tuha), Songliao, Junggar, Tarim, Chuxiong and East China Sea basins. Genetically they are of either primary or secondary accumulations. The tight gas accumulations are mostly associated with coal strata. The reservoir rocks are mainly lithic arkosic sandstones and feldspathic lithic arenite. They are characterized by low compositional maturities, low cement contents and moderate petrological textural maturities. The diagenetic evolution of the reservoirs is generally at a high level, reaching the mid-late diagenesis stages. Reservoir porosities are dominated by secondary intragranular dissolution porosity and intergranular dissolution porosity with subordinate primary residual intergranular porosity. The pore throats are sheet like or bending-flake like in geometry and generally have poor connectivity. The tight gas reservoirs have poor petrophysical properties and show strong heterogeneities. Locally, some moderately porous and low permeable or low porous and moderately permeable sandstone reservoirs are present. Compaction and cementation appear to be key factors contributing to the formation of the tight gas sandstone reservoirs. Based on the synthesis of a vast data set obtained from some known tight sandstone gas accumulations from (1) the upper Palaeozoic sandstones in the Ordos Basin, (2) the Triassic Xujiahe Formation in the Sichuan Basin, (3) the Jurassic sandstones from the foothill belt in the Tuha Basin and (4) the Cretaceous deep reservoirs in the Songliao Basin, the criteria for recognizing tight gas sandstone reservoirs in China can be established as an in situ permeability of less than or equal to 0.1 mD and with no natural commercial production.

This study summarizes the research experiences of non-marine seismic sedimentology in recent years in China and uses Qijia area, Songliao Basin, as a template to establish general guidelines for seismic sedimentology. Basic data sets include stacked 3D seismic volumes, 2D regional seismic lines, data for regional geologic settings, and well data. The workflow emphasizes the integration of seismic and geologic interpretations and balanced use of seismic sedimentology, sequence stratigraphy and seismic stratigraphy. Basic steps include well-to-seismic tie for the establishment of sequence framework, wavelet-phase adjustment, picking of geologic-time parallel seismic events, seismic resolution analysis, petrophysical analysis, selection of seismic attributes, stratal slicing, seismic depositional facies analysis, and applications to exploration and development. Expected maps range from key interpreted well-seismic sections, flattened relative geologic-time sections, stratal slices, and depositional facies maps, etc. The workflow is applied in the study of the Cretaceous Qingshankou Formation in the Qijia area, Songliao Basin, which can be used as a reference for seismic sedimentologic study in non-marine basins, especially in postrift depression-type basins in China. Key words: non-marine postrift basin, seismic sedimentology, guidance, workflow, thin bed, shallow-water delta

By comparison of the types, geological characteristics and exploration technologies of conventional and unconventional hydrocarbon, this paper proposes the concept of “nano-hydrocarbon” and regard nano-hydrocarbon as the development direction of oil and gas industry in the future. Nano-hydrocarbon refers to the research and production, by nano-technology, of oil and gas accumulated in the reservoir system of nano-sized pore-throats. It is mainly distributed in source rocks and the neighbouring tight reservoirs and includes shale oil, shale gas, coal-bed methane, tight sandstone oil and gas, tight limestone oil and so on, with nano-sized diameter of pore-throats in reservoirs. Oil, gas and water in nano-sized pore-throats exhibit poor percolation and phase separation, and are mainly driven by ultra-pressure, thus existing pervasively and continuously in the coexisting tight source and reservoir rocks. China's petroliferous basins develop multiple series such as coexisting tight source and reservoir, carbonate fractures and cavities, volcanic fractures and cavities, and metamorphic rock fractures. Among the series, the first type is located in the center or on the slopes of the basins, where nano-hydrocarbons are accumulated extensively within or near sources and are dominant potential sources. With accumulations within coexisting source and reservoir in the Ordos Basin and Sichuan Basin as examples, the method of “two lines and one area” to prospect continuous-type hydrocarbon accumulation is proposed, i.e. the top and bottom boundaries of a set of coexisting source and reservoir and the boundaries of hydrocarbon accumulation as lines, and “sweet spot” distributing core area as the main exploration target. Key words: nano-hydrocarbon, nano-technology, tight hydrocarbon within coexisting source and reservoir, unconventional hydrocarbon, tight oil, tight gas, shale gas, shale oil, continuous hydrocarbon accumulation

Based on the controlling of diagenesis and diagenetic facies on reservoir development, this article discusses the formation, classification, and evaluation of diagenetic facies and its application and significance in petroleum exploration. For constructive diagenetic facies such as clastic rock, carbonate rock, and igneous rock, eight kinds of genetic mechanisms are developed, including dissolution of organic acid, dolomitization, and so on. Nine constructive diagenetic facies and seven destructive diagenetic facies are classified. A naming scheme for diagenetic facies is proposed reflecting lithology, diagenesis, porosity, and permeability, that is, “porosity and permeability level + rock type + diagenesis type”. Diagenetic facies are evaluated synthetically and quantitatively on the basis of sedimentary facies, log facies, seismic facies, rock cores, and thin sections, and “four steps” and “superposition of three charts” are put forward as the methods of evaluation and mapping. Favored reservoirs, “sweet spots”, and lithostratigraphic traps can be predicted according to the distribution of different types of diagenetic facies. The development of the study on diagenetic facies is also discussed in the article. Key words: diagenetic facies, formation mechanism, classification and naming, evaluation method, petroleum exploration, diagenetic study

Volcanic hydrocarbon exploration in China has experienced three phases: accidental discovery, local prospecting, and all-round exploration. There are mainly Carboniferous-Permian, Jurassic-Cretaceous, Paleogene-Neogene volcanic rocks and lava, pyroclastics, and karst reservoirs in the oil- and gas-bearing basins in China. Volcanic rocks cannot generate organic hydrocarbons, and the combination of volcanic rocks, source rocks, and seals are the key controlling factor of the primary lava plays. The near-source play is most favorable for hydrocarbon accumulation. Distribution of oil and gas is controlled predominantly by the hydrocarbon generating center. The play requires communication with faults or unconformities. Near-source plays are in the faulted basins in eastern China. Structural-lithologic hydrocarbon reservoirs are formed in the higher place of faults and lithologic hydrocarbon reservoirs are formed on the slope. Two types of plays are developed in central and western China. The near-source play is most favorable for the formation of large stratigraphic hydrocarbon reservoirs. Key words: volcanic reservoir, eruption environment, volcano-deposition tectonic sequences, controlling factors