There are various types of gas reservoirs onshore in China, which are divided into four types based on their comprehensive geological conditions and exploration potential, i.e., the ancient carbonate gas reservoir, gas reservoir in foreland thrust belt, tight sandstone gas reservoir and bedrock-volcanic rock gas reservoir. By applying the advanced geological theory and the recent exploration progress, as well as the basic gas accumulation conditions, main controlling factors for gas enrichment of different types of gas reservoir in China are systematically summarized from the aspect of key geological factors controlling hydrocarbon accumulation, so as to guide onshore gas exploration in the next step. The results show that: The marine carbonate gas reservoir is controlled by high-quality source rocks, widely developed reservoir and large-scale trap; The gas reservoir in foreland thrust belt is controlled by high-quality source rocks, reservoir-cap rock assemblage and structural trap; The tight sandstone gas reservoir is controlled by tectonic settings, pattern of reservoir-cap rock assemblage and reservoir distribution; While the bedrock-volcanic rock gas reservoir is controlled by gas generation sag, reservoir type and gas transport system. In addition, the enrichment laws of large gas fields are summarized: (1) The macro distribution of gas reservoir is determined by highquality source rocks, and large-scale gas reservoirs are generally distributed in source kitchen and its periphery; (2) The high-quality reservoirs have diversified types and are widely distributed, which significantly control the enrichment of natural gas; (3) The multi-stage cap rock deposited in stable tectonic period is a key factor for multi-stage gas accumulation; (4) The favorable area for gas accumulation is controlled by the current tectonic framework. The results indicate that the key exploration zones of different types of gas reservoirs in the next step should be focused in Sichuan Basin, Ordos Basin and Tarim Basin.

In order to find out the gas occurrence conditions and controlling factors for high yield well of shallow shale gas in complex structural zone in the peripheral Sichuan Basin, expand exploration field of shale gas in mountainous area and achieve the high-efficiency development of shallow shale gas in southern China, PetroChina Zhejiang Oilfield Company has conducted reexamination on old wells,fracturing and testing on shallow shale gas wells in Zhaotong demonstration area, and obtained a new understanding that a “3D sealing box” is formed jointly by roof/floor cap rocks of shallow shale gas and self-sealing of shale reservoir. By considering the geological and surface conditions, an operation technology of integrated geology and engineering for shale gas in mountainous area has been implemented, which achieved a major discovery and commercial development of the first shallow shale gas field in China. The study results show that the sweet spots, gas enrichment and high production of shallow shale gas are controlled by “multi-factor coupling”, that is, shale gas accumulation is controlled by lithofacies, deformation intensity in the tectonic reconstruction period and sealing capability in the gas accumulation period.The shale burial depth, formation dip, denudation intensity and fluid potential jointly control the occurrence of shale gas. Specially, it is proposed that the “dynamic balance of shale gas accumulation and dispersion” is the key factor for the effective occurrence and enrichment of shallow shale gas of Taiyang gas field, which is complemented by slow gas dispersion owning to the shale self-sealing preservation and bed parallel super-short distance gas migration and accumulation. After the verification by field practice, three major countermeasures have innovatively been formed for high-efficiency development of shallow shale gas, including refinement of development and evaluation units of gas field, integrated drilling deployment and optimization, and personalized design of drilling platform, as well as the economical and practical technology series for drilling, fracturing and production.

Based on the analysis of reservoir in coal measure strata, geochemical characteristics of natural gas, as well as the test results of CBM wells, a new type of natural gas – coal measure gas of the Jurassic Badaowan Formation and Xishanyao Formation is proposed in this study,which is different from both coalbed methane and conventional gas, and characterized by coexistence of conventional and unconventional reservoirs, symbiosis of free gas and adsorbed gas, complementary accumulation and orderly distribution of self-generated and other-source gas. Aiming at this new exploration field, Well Caitan 1H has been deployed in Cai 31 faulted anticline structure in Baijiahai Bulge with the target layer of coal seam of the Jurassic Xishanyao Formation and great discovery of coal measure gas been achieved with the highest gas production of 5.7×104 m3/d and stable gas production of 2×104 m3/d during the well test, which enables a new breakthrough in geology understanding of natural gas, opens up a new field of natural gas exploration, guides a new direction of natural gas exploration, and has a major and far-reaching significance on gas exploration in coal bearing basins.

The natural gas resources are abundant in central Sichuan uplift zone, with proven reserve of more than one trillion cubic meters of the Sinian-Cambrian, gas discovery from the Middle Permian Qixia Formation, and commercial gas flows from Xixiangchi Formation,Maokou Formation, Xuanwuyan Formation and Changxing Formation. The geochemical characteristics of various gas reservoirs vary greatly.A large number of experimental data of natural gas composition, carbon isotope and hydrogen isotope are analyzed from newly drilled gas wells to systematically study the genesis and accumulation characteristics of gas reservoirs in central Sichuan uplift zone, so as to identify the key exploration areas with gas reserve of trillion cubic meters. The study results show that: (1) The gas reservoirs of the Sinian, Cambrian and Permian are dry gas mainly composed of hydrocarbon gas with drying coefficient of greater than 0.997, which is crude oil pyrolysis gas.The geochemical characteristics of C2H6 content, d13C2 and d2HCH4 of the Permian gas reservoir is similar with those of the Cambrian gas reservoir generated by source rock of the Lower Cambrian Qiongzhusi Formation. For example, gas reservoir of Qixia Formation in GS19 is characterized by high C2H6 content, light d13C2 and light d2HCH4, while gas reservoir of the Upper Sinian Dengying Formation has low C2H6 content, heavy d13C2 and light d2HCH4. The difference of C2H6 content and d13C2 is mainly related to maturity, and that of d2HCH4 is mainly affected by water salinity of the parent sediments. (2) The gas reservoirs in Anyue and Taihe areas are crude oil pyrolysis gas in different evolution stages, in which the former is mainly in the late stage, while the latter is in the early-late stage. The d13C2 of gas reservoir in Anyue area is heavier than that in Taihe area; (3) There are two types of accumulation pattern of the Sinian - Permian gas reservoirs, i.e., single-source and dual-source. The Cambrian gas reservoir and Permian Qixia gas reservoir in GS19 are single-source accumulation pattern, with source rocks of Qiongzhusi Formation and Longmaxi Formation, respectively. The Sinian Dengying gas reservoir and other Permian gas reservoirs are dual-source accumulation pattern, which are supplied by source rock of Qiongzhusi Formation, and supplemented by the Sinian and Permian source rocks respectively. The study indicates that the difference of geochemical characteristics of different gas reservoirs is mainly caused by the contribution ratio of source rocks of different geological ages. The superimposed and contiguous high-quality reservoir in Taihe area is closely adjacent to source rocks or within the range of paleo oil reservoir, which has a resource scale of one trillion cubic meters, showing great exploration potential.

In recent years, great breakthroughs have been made of the Middle Permian Qixia Formation in western Sichuan Basin, showing great exploration potential. A large number of well drilling, logging, and lab test data are studied to determine the key geological difficulties,such as the stages, evolution, main controlling factors and characteristics of gas accumulation of Qixia Formation in western Sichuan Basin, so as to provide technical support for the exploration in the near future. The study results show that: (1) The methane content and the gas drying coefficient of natural gas of Qixia Formation are more than 96% and 0.99 respectively, showing characteristics of high maturity, which is a typical dry gas and pyrolysis gas generated by the secondary cracking of crude oil; (2) The gas isotope of δ 13C1 ranges from -31.33‰ to -28.31‰,while that of δ 13C2 from -28.5‰ to -25.2‰. The gas-source correlation indicates that the natural gas of Qixia Formation is mainly a mixed type gas reservoir generated by source rock of the Cambrian Qiongzhusi Formation and supplemented by the Middle Permian, but no contribution from source rock of the Silurian Longmaxi Formation; (3) The identified fluid inclusions recorded four stages of hydrocarbon accumulation events. The first and second stages were paleo oil reservoir accumulation in the Middle-Late Triassic and the Late Triassic respectively; The third stage was paleo oil reservoir accumulation in the Early-Middle Jurassic; The fourth stage was paleo gas reservoir accumulation from the Late Jurassic to the Early Cretaceous; (4) The main controlling factors of hydrocarbon accumulation include the large-scale high-quality source rocks, high-quality reservoirs and structural-lithologic traps that supply accommodation for continuous gas accumulation; (5) The concealed front zone of the footwall of the nappe in Shuangyushi - Zhongba area in northwestern Sichuan Basin and Pingluoba - Daxingchang area in southwestern Sichuan Basin have superior conditions for gas accumulation, which are the most favorable areas for further exploration of Qixia Formation in western Sichuan Basin.

The early study regarded that the carbonate of the first member of Maokou Formation (hereinafter referred to as Mao 1 member) is a set of high-quality source rock in Sichuan Basin, but ignored its potential as an unconventional reservoir similar to the shale. Recently,commercial gas flows have been achieved of Mao 1 member in several exploration wells, probably indicating a new exploration field. Five aspects of geological studies are conducted, including oil and gas bearing properties in wells and oil/gas distribution, gas source analysis and evaluation of source rock conditions, lithofacies types and distribution of favorable lithofacies zones, reservoir performance and its distribution, gas accumulation conditions and enrichment characteristics, by which the unconventional natural gas is considered to be enriched of marl source rock of Mao 1 member. It is continuously accumulated in a large area in structural highs, slope and syncline. The natural gas is mainly supplied by source rock of Mao 1 member itself, which is featured by high TOC and high degree of gas enrichment, indicating the characteristics of gas accumulation inside the source rock and in-situ gas enrichment. Four types of lithofacies are mainly developed in Mao 1 member, i.e., the augen limestone lithofacies, eyelid bearing augen limestone lithofacies, augen bearing eyelid limestone lithofacies and eyelid limestone lithofacies. The study of rock physical properties shows that the eyelid limestone is the most favorable facies with reservoir space of mainly organic nanopores and dolomite intercrystal micropores. The reservoir porosity and permeability are generally less than 10% and 1mD respectively, which is a typical tight reservoir of low porosity and low permeability. The reservoir sweet spots are continuously distributed in the a and c sub-members of Mao 1 member vertically, and widespread laterally in the whole basin. The reservoir has distinct characteristics of overpressure and good gas bearing property. Combined with burial depth and reservoir distribution, the favorable exploration zones are comprehensively evaluated, and three Class Ⅰ and four Class Ⅱ favorable areas selected. The Nanchuan-Hechuan area has the best exploration potential, where risk exploration well is suggested to be deployed and volume fracturing of horizontal well be implemented for large-scale gas production.

The Middle Jurassic Shaximiao Formation is a major exploration field for tight gas in Sichuan Basin, which has the characteristics of shallow burial depth, low investment and high benefits. At present, the core production area is focused in the Western Sichuan Basin. Therefore,it is urgent to systematically summarize the tight gas accumulation conditions of Shaximiao Formation at basin scale, so as to expand the exploration field. Based on the study of source rock, natural gas source and fault development of Shaximiao Formation, configurations of source rock and reservoir assemblage are analyzed in detail and gas accumulation patterns established of different regions in the basin, and favorable exploration targets are put forward. The results show that natural gas of the Middle Jurassic Shaximiao Formation in Sichuan Basin is jointly supplied by source rocks of the Upper Triassic Xujiahe Formation, Lower Jurassic Ziliujing Formation and Middle Jurassic Lianggaoshan Formation. From west to east, the natural gas is mainly coal type gas, oil type gas and mixed oil type gas-coal type gas. The gas accumulation patterns vary significantly among different regions due to the different geological conditions. For example, gas reservoir in Western Sichuan Basin is mainly a lower generation and upper storage type supplied by the Triassic source rock and communicated by reverse faults. In the Central Sichuan Basin, it is dominated by a self-generation and self-storage gas reservoir supplied by the internal Jurassic source rock and connected by normal faults. While in Eastern Sichuan Basin, it is mainly a self-generation and self-storage gas reservoir supplied by the Jurassic source rock and supplemented by a lower generation and upper storage type gas reservoir supplied by the Triassic source rock and communicated by normal faults. Among them, Shaximiao Formation in Western Sichuan Basin, northern part of Central Sichuan Basin and northern part of Eastern Sichuan Basin has relatively superior geological conditions for gas accumulation and relatively large exploration potential. At present, exploration degree is low in the northern part of Central Sichuan Basin, which is the key exploration area in the near future.

The coal measure gas resources are abundant of the Permian Shanxi Formation of the Upper Paleozoic in Ordos Basin. Therefore, it is of great signifi cance to analyze the gas accumulation conditions in coal measure strata and predict the gas resource potential, so as to deepen the exploration of unconventional gas in Ordos Basin. By taking Shanxi Formation as the study object, the geological and geochemical characteristics of source rocks and the mineral composition, pore structure, physical properties of reservoirs are studied in details in this study based on core observation and stratigraphic characteristics in single well, as well as a series of lab test results such as SEM, XRD analysis and gas adsorption experiments. Then the gas accumulation conditions of coal measure strata are analyzed and the gas resource potential is predicted. The results show that: (1) The source rocks of Shanxi Formation are characterized by relatively thin thickness of a single layer,multiple layers, thick in the east and thin in the west, high TOC, organic matter types of Type Ⅱ2 and Ⅲ, high mature to over mature with R o ranging from 1.2%-2.6%, and great gas generation potential; (2) The sandstone reservoir of Shanxi Formation is mainly composed of lithic quartz sandstone and lithic sandstone, and the reservoir space is dominated by dissolution pores of lithiclast with the porosity of 2%-10%; The mineral composition of mudstone is mainly quartz and clay mineral, and the reservoir space types include organic pores and micro fractures,with complex pore throat structure; (3) The estimated gas resource of Shanxi Formation is about 306879.79×108m3 by using the Monte-Carlomethod, among which the 2nd member of Shanxi Formation has large gas resource potential of 177001.15×108m3.

The large-scale salt and gypsum rocks are deposited of the Ordovician Majiagou Formation in central and eastern Ordos Basin, with great thickness and superior sealing capacity. The underlying carbonates have always been a major field for natural gas exploration. Based on the analysis of regional tectonic sedimentary settings and sedimentary facies distribution, the characteristics of carbonate reservoir and gas accumulation of the target layer - the 4th member of Majiagou Formation (Ma 4 member) are studied, which is underlaid by subsalt caprock of the 5th member of Majiagou Formation (Ma 5 member). The results show that, controlled by the intra-platform sedimentary differentiation in the paleo uplift, the intercrystal pore type dolomite reservoir of beach facies is developed of Ma 4 member. In addition, the dissolution pore type dolomite reservoir of the 3rd member of Majiagou Formation (Ma 3 member) also has good reservoir performance. The weathering and denudation during the Caledonian resulted in a good spatial configuration between the dolomite reservoir of Ma 4 member and the source rocks of coal series in the Upper Paleozoic and marine source rocks of the Ordovician subsalt. Therefore, the gas reservoir has the characteristics of dual-source hydrocarbon supply and lithologic gas reservoir controlled by lithologic facies change. The detailed analysis of source rock – reservoir – caprock assemblages and the comparison of controlling factors for gas accumulation in different regions indicate that the salt diapir uplift zone of Ma 4 member of the Ordovician subsalt in the eastern basin and the lithologic facies change zone controlled by Wushenqi paleo uplift in the central basin are favorable targets for gas exploration. In addition,Ma 3 member and the 2nd member of Majiagou Formation (Ma 2 member) in deep formation also have promising prospects.

The degree of oil and gas discovery varies greatly in Junggar Basin and the proven rate of natural gas is low. Till now, the Carboniferous volcanic rock has the largest natural gas reserves of the basin, however, it shows complex gas accumulation characteristics indicated by exploration practice. Based on the core, thin section observation and lab test data of wells in Kelameili Gas Field, Wucaiwan Gas fi eld, Fukang Sag and Dibei Bulge, characteristics of the main source rocks of section b of Songkalsu Formation are studied, and the gas accumulation characteristics and exploration potential of the Carboniferous volcanic rocks are analyzed by combining with the studies on reservoir properties and gas sources. The results show that the source rocks of the Carboniferous are widely deposited in Wucaiwan Sag, Dishuiquan Bulge and Dongdaohaizi Sag, with an average TOC of 1.58%, organic matter type of mainly Type Ⅲ, and mostly in high mature stage.In addition, volcanic activity has promoted the hydrocarbon generation capacity of organic matter. High quality reservoirs are dominated by volcanic rocks of explosive and overfl ow facies, with pores and fractures developed, forming a good reservoir-caprock assemblage with the overlying the Permian or internal the Carboniferous tight layers. The gas accumulation conditions in the deep sag area are favorable for forming largescale gas reservoirs. The exploration potential is promising in Dinan Bulge, Dibei Bulge, Beisantai Bulge and Cheguai area.

In recent years, major breakthroughs have been made in natural gas exploration in Bozhong Depression in Bohai Bay Basin, where is an important area for natural gas exploration in the future. The gas reservoir characteristics and the main controlling factors of natural gas accumulation are systematically summarized, and natural gas resource potential and exploration targets are identified in Bozhong Depression in Bohai Bay Basin by using 3D seismic, wireline logging, mud logging, and geochemical lab test data. The results show that the gas reservoirs in Bozhong Depression are mainly accumulated in the Archean metamorphic rocks buried hills, the Lower Paleozoic carbonate buried hills and the Cenozoic Paleogene glutenite reservoirs. The CO2 content of natural gas reservoirs varies greatly. The condensate gas is characterized by low CO2 content, while the dry gas is high CO2 content. The formation of large natural gas reservoir is jointly controlled by multiple factors. The late rapid maturation and high intensity gas generation of mixed source rocks are the material basis, and the multi type, large-scale and high-quality reservoir is the key to the formation of large-scale gas reservoir. The thick and over pressured mudstone caprock is the guarantee for the formation of large and medium-sized natural gas reservoir. The study indicates that buried hill and  the Cenozoic glutenite reservoir covered by thick and over pressured mudstone caprock are the favorable exploration targets in Bozhong Depression.

The main reservoir of lithologic gas reservoir of He 8 member of the Pernian Lower Shihezi Formation is multi-stage superimposed sand bodies in Su 14 well block of Sulige Gas field, which are deposited in braided river sedimentary system. The understanding of the superposition relationship of sand bodies is crucial, and how to formulate a reasonable working system is challenging for the efficient development of the gas field. By analyzing the superposition of sandstone reservoir of the gas field, the distribution relationship models between various types of production wells and superimposed sand bodies are established, and reasonable working systems are proposed for vertical wells and horizontal wells with different superposition models of sand bodies by numerical simulation method. The results show that the production of vertical well is mainly affected by the lateral superposition of sand bodies, and the vertical superposition of sand bodies has little effect; While for horizontal well, both vertical and lateral superposition of sand bodies have significant impact on the production;Sand body superposition is the main reason for the decline of gas production in the early stage of Sulige Gas field; The working system of intermittent production will better release the formation energy, delay gas well decline and play a good role in peak shaving. The study provides technical support for the efficient development of Sulige Gas field and achieves good application results, which can be used as a reference for the gas field development of similar type of reservoir.