The Yetan1 well in Kedong tectonic belt, west Kunlun thrust belt in the front mountain of southwest area of Tarim Basin, has made a major breakthrough in the Permian Pusige Formation, marking the discovery of an important exploration successor system in southwest area of Tarim Basin. Based on the regional geological data and the field outcrop, drilling and seismic research, the reservoir-forming conditions of the Permian Pusige Formation sandstone around Kekeya are reunderstood. A new round of research holds that the upper member of Permian Pusige Formation has a semi-deep lacustrine facies source rock with an area of 1.0×10⁴km² and a maximum thickness of 800m. The organic matter abundance is 0.89%~1.5%. The source of organic matter is mainly bacteria and algae, and the type of organic matter is mainly type II. Hydrocarbon generation potential S₁+S₂ is 1.47-2.78mg/g. Due to the tectonic uplift of the Kunlun Mountains, the lower member of the Permian Pusige Formation in the periphery of Kekeya subdivision entered the continental sedimentary system, and the delta front and shoal shallow lake sand bar high-quality sandstone reservoirs developed in the lower submember II, withsand body thickness of 42.5-63m and sand-land ratio of 27%-47%, which are a set of widely distributed reservoirs. It is a high quality reservoir-cap association with lower submember I of Pusige Formation and upper mudstone member of Pusige Formation. According to the structural pattern analysis after drilling of Yetan 1 well, it is concluded that the Kekeya subperipheral developed two phases of Indosinian and Alpine tectonic deformation, Palaeozoic imbricated thrust, strong overthrust, trap rows and belts, and the further extrusion and uplift in the Alpine period were partially complicated, and the imbricated thrust belt formed is a favorable exploration area. According to the analysis of natural gas dryness coefficient of the Pusige Formation, the R_o is 1.31%-1.42%, which is similar to the maturity phase of the source rock, indicating that the oil and gas come from the foote overlying source rock and have undergone the accumulation process of early oil and late gas, and the accumulation has the characteristics of "hydrocarbon supply by the overlying foote, new layers produce and ancient layers storage, lower layers produce and upper layers storage, transport by fault ". Through re-comprehensive analysis, it is believed that the Permian Pusige Formation traps in this area are distributed in rows, and 9 traps are found, covering an area of 307.7km², with 286 billion square meters of natural gas resources and 143 million tons of condensate oil. Meanwhile, several favorable exploration targets such as PT1 well, PS1 well and PS2 well are selected and have great exploration prospects. It is expected to become an important replacement field for large-scale natural gas storage and production.

There are abundant high-rank coalbed methane resources in China, and the high-efficiency development is of great energy, security,and ecological significances. Taking the development practice of high-rank coalbed methane in southern Qinshui Basin as an example, five main problems and challenges in the early development process are systematically analyzed: (1) A lack of advanced theory for the development of high-rank coalbed methane; (2) Low accuracy of favorable area selection; (3) Insufficient adaptability of reservoir reconstruction technology; (4) Low efficiency and poor benefit of drainage-production control regime; (5) “Three difficulties” and “three highs” of the gathering and transportation system. PetroChina Huabei Oilfield Company has adhered to problem-oriented and target-oriented approach and integrated indoor research and field practice, forming a new concept and key technologies for the high-efficiency development of high-rank coalbed methane, which include a drainage development concept, and a development mode of single-lateral horizontal well casing + multistage fracturing; The technology for optimal selection of high-yield favorable areas has been researched, achieving the transformation from large-area overall production capacity construction to precise construction mode in the favorable area; The fracturing and reservoir stimulation technology for coalbed methane has been improved and updated, which supports to form multi-level effective fracture network; The optimal,fast and high-efficiency drainage-production control technology has been innovated; The low-pressure ring surface gathering and transportation technology has been developed to achieve high benefits. The field practice shows that the success rate of new constructed production capacity has increased from 37% to higher than 84%, the average daily gas production of a single well has increased by 1.6 times, the time reaching target production has decreased by more than 20%, and the cost of surface construction investment for new projects has reduced by 20%. In addition, the coalbed methane production breakthrough has been achieved in all types of reservoirs in Qinshui Basin, with an annual production capacity of 21×10⁸ m³ in coalbed methane field in the southern Qinshui Basin, which is it the largest coalbed methane field in China, greatly boosting the strategic development of coalbed methane.

Two sets of coal seams are widely distributed in the Jurassic Xishanyao and Badaowan formations in Junggar Basin, and there are abundant coal measure gas resources. However, as a completely new field of natural gas, the level of basic research is low, and gas accumulation characteristics, enrichment laws and resource scale in the basin are unclear. In 2020, a risk exploration well CT1H was drilled to explore gas bearing property of deep coal rocks, and a maximum daily gas rate of 5.7×10⁴ m³/d was tested, as well as a steady gas rate of 2×10⁴ m³/d in trial production, confirming the high and steady production capacity of coal measure gas. A systematic study on coal rock petrological characteristics, reservoir performance, evolution characteristics and control factors for gas accumulation in coal measure strata has been conducted, which indicates that the thermal evolution degree of coal rocks is low, with a medium-low coal rank, and the coal reservoir in Xishanyao Formation is dominated by meso-macro pores, while that in Badaowan Formation is mainly micro-small pores; By combining with multiple factors controlling gas accumulation in coal measure strata, such as source rock, structure, coal reservoir, and abnormal gas logging shows, the potential fields of coal measure gas in the basin have been classified and evaluated. Two favorable exploration areas including Dinan-Baijiahai and Qigu areas have optimally been selected, and coal measure gas resources are more than 1×10¹² m³ with a burial depth of 2000-4000 m.

The deep coal measure gas is a new field of petroleum exploration in Turpan-Hami Basin, but the study level is relatively low. Based on coal rock testing and basic geological data, the distribution, coal quality, reservoir properties, macro and micro coal petrology, gas source,and isothermal adsorption characteristics of coal seams in the Middle Jurassic Xishanyao Formation in Taibei Sag in Turpan-Hami Basin have systematically been studied, which support to identify the gas-bearing property of coal measure reservoir and its influencing factors,propose accumulation and enrichment conditions for the deep coal measure gas, establish enrichment and accumulation patterns, and predict exploration potential in Taibei Sag. The results show that: (1) The coal seams in the deep Jurassic Xishanyao Formation in Taibei Sag have a wide distribution area and a great thickness, which are characterized by medium-low rank, primary structure, well-developed cleavages, low ash content, low water cut, and rich vitrinite. (2) The coal reservoir has high porosity and permeability, which is insignificantly affected by burial depth. The gas logging anomalies are common in coal seam interval, and the reservoir has moderate-good adsorption capacity, showing characteristics of coexistence of free gas and adsorbed gas, rapid gas breakthrough during trial production, and complete gas compositions.(3) After hydrocarbon supply and charging in coal measure strata in Taibei Sag, two hydrocarbon accumulation patterns were formed, i.e.,adjustment type in forward structural zone, and self-generation and self-storage type in slope and subsag areas. (4) There are abundant coal measure gas resources in Taibei Sag, with resource volume of nearly 3×10¹² m³ of the main coal seams in Xishanyao Formation. Based on the comprehensive evaluation of coal measure gas resource abundance, coal seam thickness, gas content, and preservation conditions, Wenjisang Structural Zone and Xiaocaohu South Slope are favorable areas for deep coal measure gas exploration.

In Huanghua Depression, coal seams in Shanxi-Taiyuan formations have wide distribution area, great thickness, and large burial depth, showing good conditions for coal measure gas exploration. However, some geological parameters should urgently be clarified such as the main coal seam distribution, sealing capacity of cap rocks, gas bearing property, and resource amount. Based on drilling, wireline logging,mud logging and geochemical data from wells encountering coal seams in the depression, the preliminary assessment has been conducted on coal seam distribution, coal measure gas accumulation conditions, gas content, and resource amount in Taiyuan Formation. The study results indicate that two main coal accumulation areas of Shanxi-Taiyuan coal seams were developed in Kongdian-Guanjiabao and Botou-Yanshan areas in the southern and northern parts, with the main burial depth range of 1500-4500 m, multi set thin coal seams, single coal seam thickness of 0.5-6.2 m and mostly of 1-3 m, and cumulative thicknesses reaching up to 32.7 m; Due to deeply burying and volcanic heating, the vitrinite reflectance (R_o) of Taiyuan Formation coal measure rocks are generally greater than 0.85%, indicating a moderate thermal evolution stage;In the study area, there are three types of lithologic combinations between coal seam and cap rock, i.e., coal-mudstone, coal-limestone, and coal-sandstone, among which the extensively developed coal-mudstone combination provides favorable sealing conditions; By using methods such as multivariate regression analysis of well logging parameters, gas content of Ⅳ , Ⅴ , and Ⅵ coal groups in Taiyuan Formation has been predicted, and the preliminarily estimated coal measure gas resources of 1.1×10¹² m³. In addition, Wangguantun slope in Wumaying area has optimally been selected as a favorable orientation to study the coal measure gas accumulation pattern of “coexistence of conventional and unconventional reservoirs, and cooccurrence of adsorbed and free gas”.

In order to make clarify the favorable exploration of coal gas in deep Xujiaweizi fault depression, we systematical study the geological characteristics of coal gas reservoirs, such as genesis of coal gas, source, reservoir and cap-reservoir combination, by making full use of the data such as seismic, coal and geochemical. The result is that there are two kinds of coal named 1 and 2 coal, which is sediment in shallow lake and delta-front. They are generally reach the high-over-mature stage, with a thickness of 3～10m and average TOC of 29.65%.The coal gas is dominated by methane, and they are all coal-generated gas. The porosity and permeability of coal is 4.74% and 4.42mD. There are several different kinds of reservoir space such as tensile micro-fractures, primary plant tissue pores, pores and mineral pores are developed,which make the coal a favorable reservoir for coal gas in deep layer. There are two kinds of cape and three kinds of cap-reservoir combination in the coal gas, the top and bottom mudstone type and top mud and bottom sand type cap is the favorable gas cap-reservoir combination. The overpressure fluid seal box within the Shahezi Formation is a favorable condition for sealing coal gas at the same time. We selected the 1 coal in Songzhan Sag and 2 coal in Xuxi Sag as the favorable areas for coal gas enrichment area, based on various favorable reservoir-forming conditions, which is also the breakthrough direction of coal gas exploration in the future.

Recently, major gas discoveries have been made in the near-source lower combination of the Carboniferous-Permian in wells SX16 and SX18 in the peripheral West Well Pen 1 Sag in Junggar Basin, showing prospects of a large gas production area. However, there is a lack of systematic study on gas distribution rule and favorable exploration areas of the Carboniferous-Permian gas reservoirs in the study area, which restricts the understanding of gas accumulation law and exploration deployment in the near future. As a result, by using seismic, logging, core, thin section and geochemical experimental data, hydrocarbon accumulation conditions such as source rock, reservoir and transport system have systematically been analyzed, and the hydrocarbon accumulation patterns of three major oil-bearing layer groups in the Carboniferous-Permian have been established. In addition, the further exploration orientation has been proposed. The study results show that: (1) Two sets of highquality source rocks are observed in Fengcheng Formation and Lower Wuerhe Formation, with large thickness of 80-200 m, burial depth of greater than 7000 m, high thermal evolution degree, and R_o of greater than 1.72%, which have reached the stage of large-scale gas generation and have gas source conditions for the formation of large and medium-sized gas fields. (2) Three sets of large-scale reservoirs were developed, providing basis for high-yield gas production. The Carboniferous reservoir is dominated by volcanic rocks, with high-porosity stomatal overflow volcanic rocks and explosive volcanic breccias developed, good physical properties after weathering and fracture transformation,and the maximum porosity of more than 20%; In Fengcheng Formation, the conventional glutenite, unconventional dolomitic tight sandstone, and unconventional dolomitic shale are orderly distributed, with an average porosity of lower than 8% but large distribution area of more than 2600 km²; The sand bodies in the first member of Upper Wuerhe Formation are overlapped and contiguous in the sag area, forming lithologic trap groups. (3) The stereoscopic transport system was formed by matching of deep and large fault system in the Hercynian and unconformity surface, which was conducive to the large-scale gas accumulation in the lower combination. (4) The three major oil-bearing layers showed different hydrocarbon accumulation patterns, among which the Carboniferous was dominated by “upper source rock and lower reservoir, and a large-span connection of source rock and reservoir”, it was coexistence of unconventional and conventional gas in source rock in Fengcheng Formation, and it was a large-scale stratigraphic-lithologic trap type gas reservoir in Upper Wuerhe Formation. It is concluded that the gas accumulation conditions are good in the Carboniferous-Permian in West Well Pen 1 Sag and its peripheral area, showing great potential for gas exploration. The favorable exploration orientations include the Carboniferous structural gas reservoir near source rock area in the nose uplift zone, the conventional glutenite gas reservoir, unconventional tight gas and shale gas in Fengcheng Formation, and the first member of Upper Wuerhe Formation in the sag area.

The coal measure strata in the Upper Permian Longtan Formation serve as important source rock in Sichuan Basin, and good oil and gas displays have commonly been observed during well drilling. However, the previous study work only focused at the level of source rock evaluation, and there was a lack of evaluation on geological conditions of coal measure gas. Based on the comprehensive analysis of core measurement data and well drilling data of Longtan Formation in Da’an area in Western Chongqing, the distribution characteristics, source rock characteristics, reservoir properties, gas bearing property, and preservation conditions have been studied, which support to determine the geological conditions and exploration prospects of coal measure gas in Longtan Formation in Da’an area in Western Chongqing. The study results show that Da’an area was located in the favorable sedimentary zone during the deposition of Longtan Formation coal measure strata,with wide distribution area, thin single layer thickness, and concentrated coal accumulation area; The coal rock is dominated by primary structure, medium-low ash content, and well developed cleats and pores; The coal rock has good gas bearing property, high free gas content,high porosity, high formation pressure and high gas content; The roof and floor of coal rock are mainly composed of mudstone, showing good preservation conditions. The comprehensive evaluation indicates that the coal measure reservoir in Longtan Formation in Da’an area has good porosity and permeability, high gas content, and moderate burial depth, which shows great exploration prospects.

Qingdong Sag is a typical marginal sag in Bohai Bay Basin, with characteristics of small area and shallow burial depth of source rock. The unclear understanding of hydrocarbon generation and evolution mechanisms, favorable hydrocarbon accumulation combinations and hydrocarbon accumulation pattern near source rock restricted the petroleum exploration in the sag. Based on analysis of regional tectonic settings, and combined with study on geochemical experiment, rock pyrolysis, thin section, fluid inclusion and thermal evolution, hydrocarbon accumulation conditions and exploration potential in Qingdong Sag has been re analyzed. The study results show that the upper sub-member of the fourth member and the lower sub-member of the third member of Shahejie Formation served as the main source rocks in Qingdong Sag,and the source rock deposited in saline lake basin in the fourth member of Shahejie Formation showed evolution characteristics of early mature and early hydrocarbon generation; Influenced by high heat flow in Tanlu Strike Slip Fault Zone, the depth threshold of hydrocarbon generation was shallower, and there were two hydrocarbon generation mechanisms of normal pyrolysis and immature-low mature at shallow burial depth;The regional oil and source rock correlation shows that crude oil in the northern fault horst zone was supplied by the mature oil in the northern sub-sag area and low mature oil in the middle sub-sag; During the deposition period from the fourth member to the third member of Shahejie Formation, the sediments supply continuously increased, and the deposition range of beach bar expanded and changed to bar sand deposits in the late stage; The reservoir quality in the third member of Shahejie Formation gradually improved upwards, and the thin and widespread sand bodies provided favorable pathway for hydrocarbon migration; Due to the structural inversion at the end of the Late Paleogene, the southern part of the northern fault horst zone uplifted from an early sub-sag, and local structural high points in the northern part changed to the southern part; Driven by the fluid potential in hydrocarbon accumulation period, oil and gas migrated from north and south directions towards structural high parts along faults and sand bodies in a stepped manner. The hydrocarbon accumulation pattern near source rock shows “hydrocarbon supply by two sub-sags, reservoir controlled by faults, and hydrocarbon migration and accumulation in a stepped manner” in fault horst zone in the northern Qingdong Sag, which is typical in the marginal sag in Bohai Bay Basin, showing excellent exploration potential.

The deep coal measure gas is one of the key exploration and production targets in the field of unconventional oil and gas exploration,which has gradually grown to be a major source of natural gas resources in China. Compared with medium-shallow coalbed methane (CBM)reservoirs, the deep coal measure gas reservoirs are characterized by more complex gas occurrence and geological characteristics. For example,in B block in Ordos Basin, the deep coal measure gas shows higher gas content, better coal structure, poorer reservoir physical properties, and higher reservoir temperature, pressure, and formation water salinity than that in middle-shallow formations. The geological characteristics of deep CBM have systematically been analyzed, which support to summarize the typical logging response characteristics of coal seams, such as low density, low GR, high neutron, high acoustic time difference, and high resistivity, and achieve the identification of coal reservoir; After analyzing the different logging response characteristics of various coal structures, some logging parameters have optimally been selected to establish coal structure factors applicable for the target area and classify coal structure types; Based on experimental results and combined with logging response characteristics of target interval, a porosity logging interpretation model has been established by using variable matrix parameters of coal rocks; Based on the experimental data, the correlation between logging response and macerals has been analyzed, as well
as that between macerals, which supports to construct a proximate component logging evaluation model in the target area; After conducting geological quality parameters evaluation such as coal seam identification, maceral calculation, and coal structure classification, the conventional and NMR-isothermal adsorption combined gas content evaluation technology has been developed to calculate both free gas and adsorbed gas in deep coal reservoirs, providing guarantees for the calculation of deep coal measure gas resources and reserves; Furthermore, with the aim of further improving the exploration efficiency and development benefits of deep coal measure gas, logging suggestions and technical research directions for the evaluation of deep coal reservoir have been proposed, so as to meet the goal of high-quality development of the deep coal measure gas industry.

The deep coal measure gas is a new field of CBM exploration, and the scientific estimation of deep coal measure gas reserves is a new challenge. By summarizing the exploration and development achievements in Daning-Jixian block, and analyzing gas accumulation characteristics, occurrence pattern, gas production laws and performance of deep coal measure gas, the unique occurrence characteristics of adsorbed gas + free gas, gas production mechanism of free gas → free gas + adsorbed gas → adsorbed gas, and its significant differences from shallow and medium-shallow coal measure gas have been studied. Furthermore, the inadaptability of the current reserve estimation standards for deep coal measure gas has been pointed out in terms of estimation methods, unit division, estimation parameters, threshold for calculation,and data collection. Based on the above research, the following suggestions are put forward for estimating deep CBM reserves. The volume and volumetric methods can be used to estimate reserves, and the appropriate method should be selected based on the proportion of free gas;The reserve calculation units for free gas should be divided separately; The lower limit for coal gangue thickness deduction is 0.3-0.5 m; When R_o is less than 1.0%, the lower limit for net coal seam thickness should be greater than 1.0 m; When R_o is larger than 1.0%, the lower limit for net coal seam thickness should be greater than 0.8 m; The lower limit for single well gas production should be determined by well depth range and well type; In the early exploration stage, the recovery factor of vertical wells was 30%-45%, while that of horizontal wells was 35%-55%.This study provides technical references for the exploration and development, as well as reserve estimation of deep coal measure gas.

In view of the difficulty in logging evaluation of the interlayered type shale oil sweet spots, a quantitative evaluation method has been proposed for the coupling relationship of source rock quality parameters, reservoir quality parameters, and their spatial distance, achieving the production capacity classification of the interlayered type shale oil and the determination of the lower limit standard for sweet spots with commercial oil flows after calibrated by well test data, which is reliable confirmed by multiple wells. Based on geochemical experimental data of core samples, a regional model for qualitative TOC calculation has been constructed by using logging data, and the effective source rock interval in the seventh member of Yanchang Formation in Wuqi area has been determined by referring to the relevant standards. The porosity-permeability relationship curve has been used to calculate the reservoir quality factor, and the vertical shortest distance between reservoir interval to be tested and effective source rock has been determined. Finally, a formula for calculating source rock and reservoir coupling coefficient that reflects the coupling relationship between the tested interval and the adjacent main source rock has been established by combining with reservoir quality factor, average TOC and effective source rock thickness. After calibrated by well test results, the lower limit of coupling coefficient in an oilfield or a well area with commercial oil flows has been determined, which enables to evaluate intervals to be tested in new wells and avoid ineffective well testing, improving success rate of unconventional oil and gas well test and providing decision basis for the optimization of capacity construction.

For oil and gas reservoirs with complex types and in new fields, the determination of the key parameters for reserve evaluation has a great significance and is also the difficulty and focus of exploration and development. At present, the mainstream practice in the world is to establish reliable technologies for conducting reserve evaluation. In China, the main exploration and development technologies are focused, but there is little research on reliable technology for reserve evaluation. In recent years, domestic companies have successively made major oil and gas breakthroughs in deep formations with a depth of nearly 10000 m and deep-sea area with a water depth of more than 500 m. The reserve evaluation of ultra-deep large-scale carbonate fractured-cavity type oil and gas reservoirs and deep-sea oil and gas reservoirs discovered in the exploration and evaluation stages faces problems such as limited data, investment constraint and low degree of well control. As a result, how to select or establish reliable technology to conduct reserve evaluation has become the research focus in this field. By starting from the definition and applicability of reliable technology, and focusing on challenges such as great exploration investment, inability to deploy exploration wells in a large area, and the complexity, particularity and uncertainty in the process of reserve evaluation for deep fractured-cavity type oil and gas reservoirs and deep-sea oil and gas reservoirs, research on the applicable conditions and scope of reliable technology in selecting key reserve evaluation parameters has been conducted. The typical case study shows that reliable technology such as amplitude-offset (AVO),direct hydrocarbon indicator (DHI) and pressure system method enables to identify oil-water contact of deep-sea oil and gas reservoirs, with a success rate of 92% in four blocks, which supports to reduce the investment of exploration and evaluation and the risk of development. For the deep fractured-cavity type carbonate oil and gas reservoir, seismic sensitive attributes analysis technology has been applied to determine oil and gas bearing boundary and area, which has high reliability in reservoir prediction confirmed by venting and lost circulation during drilling,logging interpretation results and production performance, with the coincidence rate of 84% between seismic reservoir prediction and actual drilling results, resulting in closer reserve evaluation results to the actual oil and gas reservoirs, ensuring the rationality of reserve evaluation results in the case of few wells, and promoting the application of reliable technology in the field of reserve evaluation.