In order to further clarify the natural gas exploration potentials and distribution of favorable targets, analyze the outstanding problems and improve the exploration theories in onshore deep and ultra-deep formations in China, the exploration progresses and resources were reviewed, the geology, key factors on reservoir formation and potentials of natural gas resources were classified and evaluated. Our study shows that: (1) 10 foreland thrust belts and slopes (i.e. Kuqa, West Sichuan, South Junggar, etc.) developed in the piedmont zones of the superimposed basins in mid-western China are key exploration zones in foreland thrust belts; (2) the paleo-uplifts and slopes, intracratonic rifts and platform margin reef beaches in the Ordos Basin, Sichuan Basin and Tarim Basin developed on three major cratons (North China, Yangtze and Tarim) are important deep and ultra-deep marine carbonate rock zones; (3) the deep paleo-uplifts, slopes and bedrock buried hills in the Songliao Basin and the Bohai Bay Basin are deep and ultra-deep targets in Mesozoic-Cenozoic rifted basins; and (4) others like deep and ultradeep volcanic rocks are future exploration targets. More problems should be solved, and more basic researches should be conducted related to exploration of deep and ultra-deep natural gas in China.

The Ordos Basin has always been one of the key oil & gas exploration and development areas in China. And in the Ordos Basin, some important oil and gas fields have been discovered. As the exploration continues, the exploration objects turn to the areas with complex hydrocarbon accumulation conditions, e.g. tight zones, deep zones and basin peripheries, and the difficulty in searching for new large-scale monoblock reserves increases. The Changqing Oilfield Company’s new cognitions and progresses in the fields of tight sandstone gas and marine carbonate gas in recent years were summarized systematically, and the main geological characteristics and control factors of different types of gas reservoirs in the Ordos Basin were analyzed. Then, based on the distribution situations of oil and gas resources and the basic characteristics of newly discovered gas reservoirs, the “two-step” development strategy of Changqing Oilfield Company was put forward scientifically. In addition, the future exploration replacement fields of carbonate gas, tight sandstone gas and unconventional natural gas (tight sandstone gas excluded) were pointed out. Finally, in view of the problems in the process of exploration and development, the corresponding countermeasures were formulated systematically by combining with production practice closely. In this way, the geologic researches are strengthened, the working concept is transformed actively, the technical researches were reinforced and the exploration-development integration and the geology-engineering integration are promoted so as to realize the large-scale efficient exploration and development of natural gas and provide the experience and reference for the exploration and development of similar basins.

The Permian sandstone in the Wumaying buried hill in Huanghua depression showed high-yield oil and gas flows and good exploration prospects in the Upper Paleozoic. According to the analysis to the characteristics of the coal-bearing source rocks, reservoirs, traps and hydrocarbon accumulation in the Upper Paleozoic formations, dominant reservoir conditions and the geological significance of the Permian sandstone condensate gas reservoir were clarified in Huanghua depression. Our study shows that: (1) the Carboniferous-Permian coal-bearing strata have stable distribution, large hydrocarbon generating potentials and highly thermal evolution, which provide sufficient hydrocarbon sources; (2) the thick Permian lower Shihezi Formation sandstone reservoirs remain higher porosity even after deeply buried, which provide good space for accumulating hydrocarbon; (3) the thrust nappe structures include several large anticlines and faulted noses, which are favorable space for storing hydrocarbon; (4) the faulting activities tended to stop when the neotectonic movement took place in the Wumaying buried hill, so it is conducive to preserving hydrocarbon reservoirs. The discovery of the Permian sandstone condensate gas reservoir confirmed the huge hydrocarbon generating potential of the Carboniferous-Permian coal-bearing source rocks, pointed out key exploration targets in the Permian lower Shihezi Formation, extended effective exploration to the Paleozoic sandstone reservoir at 5000 m, and greatly expanded the exploration scope inside the buried hill in the Huanghua depression.

In order to explore the oil and gas exploration potential and controlling factors of the thick and tight Mesozoic breccia in the Xinglongtai buried hill in Liaohe depression, and guide optimization of Mesozoic exploration targets, fine reconstruction of geological data were carried out and controlling factors were analyzed on the Mesozoic reservoir according to core, debris, well logging and geochemical data. Our study indicates that: (1) 30 kinds of rocks in two types are developed in Mesozoic, Xinglongtai buried hill, of which the granitic breccia (the breccias here include granitic breccia and mixed breccia) is the best reservoir with space composed of structural fractures and dissolved pores; (2) the Mesozoic breccia reservoir is a layered fractured reservoir controlled by lithology and structure, and the overlapped zone of thick and homogeneous breccia with large faults is the favorable reservoir zone. After establishing lithology and reservoir identification charts based on the rock minerals, well logging and geochemical data, the lithology and reservoir profiles of 135 development wells were reconstructed, and reservoirs in 110 development wells were reinterpreted into classes I and II. Finally, profitable Mesozoic development was carried out by following the idea "conduct well test to define the controllable oil-bearing areas of old wells, and drill new wells to establish productivity", and 10 wells delivered industrial oil and gas flows. The Mesozoic reservoir in the Xinglongtai buried hill has been proved reserves at 108 ton scale, and in 2018, over 40 million tons of controlled petroleum geological reserves was reported.

Slope zone is a significant area for prospecting oil and gas, whereas there is no commercial discovery in the slope zone of southern Liaoxi uplift through many years of exploration. Using a large number of core, well logging, seismic as well as laboratory data, critical questions constricting the oil and gas exploration in the slope zone of the Liaoxi uplift are systematically analyzed. The study result reveals that: (1) According to the concealed strike-slip fault interpretation model, many large-scale structural trap groups were reconfirmed in southern Liaoxi uplift. (2) The thick high-quality reservoir of the mixed beach bar in the Shahejie Formation breaks through the traditional understanding of thin-layer reservoir, laying a foundation for large-scale oil and gas reserves discovery in this formation. (3) Structural-lithological process controlled the development of high-quality reservoir in Mesozoic and formed the porous-fractured reservoir in LD25-A structure. (4) The over-pressure sealing and efficient transporting pathway both decide the convergence and accumulation of oil and gas in the slope zone. These innovative geological understandings have guided the discovery of LD25-A light oil field with high yield and high abundance, contributed to the breakthrough in the slope zone of the Liaoxi uplift, and have important guiding significance for prospecting the slope zone in the Bohai Bay Basin.

Several giant gas fields (Puguang, Huanglongchang, etc.) have been found in eastern Kaijiang-Liangping ocean trough, Sichuan Basin, which is an important exploration domain of reef beach. In order to further clarify the development characteristics of platform margin reefs in the Changxing Formation in the eastern side of the ocean trough, based on drilling and seismic data, the development characteristics of platform margin belt and platform margin reefs on the eastern side of the trough were studied on the basis of the external outline, internal shape and plane distribution of platform margin reefs. The study results indicate: (1) According to slope angle difference, the platform margin in the eastern Kaijiang-Liangping ocean trough can be divided into steep slope type and gentle slope type. (2) It has sectional feature on plane. (3) There are two development patterns (retrogradation pattern in gentle slope, superposition pattern in steep slope), and the retrogradation pattern in gentle slope is the dominant. (4) The development of platform margin reefs is mainly controlled by sea level change, multiple-level synsedimentary faults and regional tension. (5) The reef development on the gentle slope platform margin experienced four evolution stages: micro-palaeogeomorphic formation stage (Wujiaping), gentle slope beach development stage (Chang 1 stage), weak rimmed platform margin reef beach development stage (early Chang 2 stage), multi-row reef beach development stage on rimmed platform margin (middle Chang 2-Chang 3 stage). The multi-row reefs in the gentle slope of the Changxing Formation in the eastern Kaijiang-Liangping ocean trough have wide distribution area and favorable hydrocarbon accumulation conditions. It is an important area for natural gas exploration.

Based on the comprehensive analysis of outcrop, drilling and seismic data, a Middle Permian trough is discovered in the Guangyuan-Bazhong area of the Sichuan Basin. And it is an extensional trough which is formed under the effect of Hercynian tension along the northwestern margin of Upper Yangtze, and its geological-seismic characteristics are obvious. It is extended in the direction of northwest, with a width of 70-120 km and an area of about 2?04 km2. According to the distribution of its central line, it is named as Guangyuan-Bazhong extensional trough. This extensional trough controls the distribution of the Middle Permian hydrocarbon source rocks. Inside the extensional trough are deepwater undercompensation sediments. It is filled with argillaceous rocks of deepwater shelf facies with the thickness of 10-30 m and its TOC ranges from 2% to 9%, so it is classified as high-quality hydrocarbon source rock. It also controls the formation and distribution of high-quality reservoirs in the platform margin belt of Maokou Formation, Middle Permian. The sediments of platform margin beach facies are developed on both sides of the extensional trough. It is indicated from the 3D seismic data interpretation that the platform margin belt covers an area of 3000 km2 with a width of 8-10 km. Owing to the diagenetic reformation (such as karstification), large-scale high-quality reservoirs can be formed. The discovery of Guangyuan-Bazhong extensional trough changes the traditional understanding that the Permian tectonic sedimentary differentiation in the Sichuan Basin is formed in the Changxing Period. The high-quality source rocks inside the extensional trough and the large-scale reservoirs of beach facies in the periphery constitute the source-reservoir configuration relation of side generation and lateral preservation, they are the favorable reservoir-forming assemblages and the importation exploration direction of the Maokou Formation.

The He 8th member in the Ordos Basin is a typical field explored for tight sandstone gas. Studies have been carried out to figure out the microscopic characteristics and forming mechanisms of the tight sandstone gas reservoir. After observation of cores, a variety of experiments were carried out, such as cast thin sections, field emission scanning electron microscope, CT scanning and constant-rate mercury injection. It is concluded that the tight sandstones in the He 8th member are quartz sandstone and lithic quartz sandstone, which are moderate to coarse, moderately sorted, angular and subangular, point or line contact among grains and filled with various cements. The reservoir space is composed of residual intergranular pores, dissolution pores, micro- and nano-pores and fractures, and micro- and nan-pores contribute over 90% to the reservoir porosity. Throats are contracted necks, curved sheets and tubes. They are thin and commonly connected. The average throat radius is well correlated to permeability. Sedimentation and sedimentary structure control the difference in original reservoir physical properties. The central sandstone in the braided river and the delta distributary channel have better physical properties than those of the channel sandstone in the alluvial fan. Late strong compaction tightened the reservoir, cementation damaged the pore structures, and strong dissolution improved the reservoir properties to some extent.

Tight oil and gas is an important exploration field in the Dibei section of the structural belt of northern Kuqa depression, but the overall exploration degree is low. In order to investigate the source of tight oil and gas in the Dibei section and ascertain its exploration potential, oil/gas-source correction was carried out after geochemical characteristics of hydrocarbon source rocks in different series of strata were analyzed. It is shown that the oil and gas in the tight reservoirs of Ahe Formation of Lower Jurassic is mainly contributed by the Triassic source rocks. Then, based on the test results of homogeneous temperature of hydrocarbon fluid inclusion in the Ahe Formation and the analysis results of single well thermal history and burial history, it is determined that there are hydrocarbon charging characteristics of three periods in the Ahe Formation. Combined with the dynamic evolution analysis on the hydrocarbon accumulation process in the Dibei section, it is shown that the hydrocarbon in the Dibei section has the characteristics of early oil and late gas. Finally, the early and late hydrocarbon accumulation models were established. It is ascertained that the distribution of tight gas reservoirs is under the joint control of the effective expulsion intensity of source rocks and the fracture development degree of tight reservoirs. What’s more, it is predicted that the favorable exploration area of tight gas reservoirs in Ahe Formation is located along the northern margin of Dibei.

The western depressed area of the Qaidam Basin is an important zone for oil and gas exploration, with unique Paleogene-Neogene source rocks in plateau saline lake facies. In recent years, important discoveries have been made in the middle-deep layers of Nanyishan, Huangguayao, Kaitemilike, Yingzhong and Zahaquan, etc., and many natural gas reservoirs have been discovered. However, the source of natural gas in this area is controversial. Whether there is mixed deep Jurassic coal-type gas affects the understanding of natural gas and reservoir-forming model in this area and the next exploration direction. On the basis of comprehensive analysis of geochemical characteristics of a large number of natural gas samples, the geochemical differences of natural gas between Paleogene-Neogene in western Qaidam Basin and Jurassic in the northern margin of the Qaidam Basin, and the geochemical characteristics of associated crude oil and regional geological conditions were studied, and the following three cognitions were obtained: (1) Unlike other regions, carbon isotope of ethane cannot be used as the main index to identify the type of high mature saline lacustrine natural gas in western Qaidam Basin. (2) The natural gas in western Qaidam Basin mainly comes from the saline lacustrine source rocks of the upper member of the Lower Ganchaigou Formation (E3 2) and the Upper Ganchaigou Formation (N1). They are mainly oil-type gas, and there is almost no mixed Jurassic coal-type gas. (3) Gas accumulation in western Qaidam Basin is characterized by near source, with short transverse migration distance and late vertical adjustment. The areas with deep burial and high maturity of source rocks, such as Yingdong, Huangguayao-Kaitemilike and Youdunzi, are important exploration areas in the future.

In order to study the formation and distribution of the dominant Permian reservoirs in Huanghua depression, geological data (including drilling, geological logging, well logging and seismic) were investigated based on actual drilled cores and sidewall cores, and the characteristics and controlling factors of the tight Permian reservoirs were analyzed in the study. It is concluded that the formation and distribution of the dominant Permian reservoirs are controlled by Indo-Chinese palaeo-structures; the Permian rocks are mainly arkose and quartz sandstone; and the reservoir space is composed of secondary intergranular and intragranular dissolution pores, moldic pores and fractures. The low-porosity and extra-low-permeability reservoirs can be classified into porous and fractured reservoirs, and their distribution depends on the Indo-Chinese palaeo-structures. The buried hill in the Qikou sag is located in the Indo-Chinese palaeo-uplift area where the palaeo-uplift and its slope have been denuded intensively, so that it is easy to develop porous reservoirs. The buried hill in the Cangdong sag had been influenced by strong Indo-Chinese thrust nappe activities, so that fractures are developed, and it is possible to form fractured reservoirs.

The 3D geological model established using existing hydrocarbon migration simulation software cannot reflect the spatial superimposition relationships of sandstone and mudstone, so it is hard to accurately and effectively simulate actual hydrocarbon migration process. Taking BZ 19-6 oil and gas field and its surrounding area in Bohai Sea as a case, a 3D geological model was established from individual wells and constrained by seismic attributes, and then the mudstone percent of the source fault was calculated and taken as the quantitative evaluation standard of fault transporting capacity, and finally 3D hydrocarbon migration was simulated. It is concluded that deep hydrocarbon accumulation is mainly under the joint control of sandstone and geomorphically low-potential zones, and middle and shallow hydrocarbon accumulation is affected by fault activity and mudstone percent of fault plane. The high-precision hydrocarbon migration simulation successfully reproduced the process of hydrocarbon accumulation in BZ 19-6 structure and presents good application prospect.

In terms of complex geological conditions, different stimulation effects on stimulation and multiple flow stages during the production, a dynamic model was established for shale gas reservoirs in Weiyuan Block. Then, controlling factors on the estimated ultimate recovery (EUR) was studied from the aspects of geological, technological and production conditions, and quantitatively evaluated by means of Monte Carlo method and principal component analysis. Finally, 5 principal components were determined, which comprehensively represent 98.31% information of 22 original indictors. Considering the static and dynamic data of over 60 production wells, the factors influencing the EUR are the drill-in rate and fracturing effect of Long1-1a. Precise geosteering to increase the drill-in rate, new technology and process are important or improving stimulated reservoir volume (SRV).

In order to realize geology-engineering integration on the development of Weiyuan shale gas field, base on the geological and reservoir characteristics, the influences of reservoir characteristics on fracturing stimulation and production test were analyzed, and then a static model for estimating the rate of production test was established. Our study shows that, in wellblock Weiyuan 202, the shut-in pressure decreases with the increase of shale brittleness index and the decrease of horizontal stress difference coefficient. Fracturing operation on pad Weiyuan 202H2 shows that the closer to the fracture zone, the lower the shut-in pressure is. It is concluded that the critical drilling meeting efficiency rate and critical stimulated section of a horizontal shale gas well are 80% and 1250 m, respectively, in Weiyuan block. Finally, a regression model was established for estimating the testing production in Wellblock Weiyuan 202. The error between calculated production and actual production is small. Prediction of testing production is used to evaluate early well production and provides theoretical and practical basic data for geosteering while drilling and fracturing operation.