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Based on core data, physical properties, well logging, gas testing, composition, and helium distribution from 13 newly drilled wells, in combination with regional tectonic evolution, depositional background, and typical gas reservoir profiles, helium-rich gas reservoirs geology characteristics and main controlling factors have been explored. The results show that three gas reservoirs formed from bottom to top, i.e., granite and metamorphite fractured reservoir in the basement, tight sandstone structural reservoirs in the Devonian, and huge continuous carbonate reservoirs in the Lower Carboniferous. Fractures developed on structural points control high-production, and gas is prominently displayed in low structural areas. The dry gas exhibits richness in helium, with decreasing helium content from the basement to the Carboniferous. The pre-Devonian large-scale granite and metamorphite basement serve as the primary helium source, while the high-gamma sandstones of the Devonian and the Carboniferous act as secondary sources. Fractures play a dual role, being filled up and conducting in two ways: gas from the Carboniferous source rocks migrates downward, and helium migrate upward. Thick interbedded gypsum-tight limestone on top of the Lower Carboniferous act as the regional caprock, effectively preserving the helium-rich gas reservoirs. [ABSTRACT FROM AUTHOR]

Based on core data, physical properties, well logging, gas testing, composition, and helium distribution from 13 newly drilled wells, in combination with regional tectonic evolution, depositional background, and typical gas reservoir profiles, helium-rich gas reservoirs geology characteristics and main controlling factors have been explored. The results show that three gas reservoirs formed from bottom to top, i.e., granite and metamorphite fractured reservoir in the basement, tight sandstone structural reservoirs in the Devonian, and huge continuous carbonate reservoirs in the Lower Carboniferous. Fractures developed on structural points control high-production, and gas is prominently displayed in low structural areas. The dry gas exhibits richness in helium, with decreasing helium content from the basement to the Carboniferous. The pre-Devonian large-scale granite and metamorphite basement serve as the primary helium source, while the high-gamma sandstones of the Devonian and the Carboniferous act as secondary sources. Fractures play a dual role, being filled up and conducting in two ways: gas from the Carboniferous source rocks migrates downward, and helium migrate upward. Thick interbedded gypsum-tight limestone on top of the Lower Carboniferous act as the regional caprock, effectively preserving the helium-rich gas reservoirs. [ABSTRACT FROM AUTHOR]

Shale oil production is vital for meeting the rising global energy demand, while primary recovery rates are poor due to the ultralow permeability. CO2 huff-n-puff can boost yields by enabling key enhanced oil recovery mechanisms. This review examines the recent research on mechanisms and formation factors influencing CO2 huff-n-puff performance in shale liquid reservoirs. During the soaking period, oil swelling, viscosity reduction and CO2-oil miscibility occur through molecular diffusion into shale nanopores. The main recovery mechanism during the puff period is depressurization with oil desorption and elastic energy release. The interplay between matrix permeability and fracture network directly determines the CO2 huff-n-puff performance. Nanopore confinement, wettability alterations, and heterogeneity also significantly impact the huff-n-puff processes, with controversial effects under certain conditions. This work provides an integrated discussion on the mechanistic insights and formation considerations essential for the advancement of CO2 huff-n-puff application in shale reservoirs. By synthesizing the recent research findings, we aim to spotlight the key challenges and opportunities in considering reservoirs for this process, thereby contributing to the advancement of CO2 huff-n-puff applications for enhanced oil recovery. [ABSTRACT FROM AUTHOR]

Despite the significant progress made in tight gas exploration and development in recent years, the understanding of the dynamic mechanisms of tight gas accumulation is still limited, and numerical simulation methods are lacking. In fact, the gap between theory and field application has become an obstacle to the development of tight gas exploration and development. This work sheds light on the dynamic mechanisms of hydrocarbon accumulation in tight formations from the aspect of capillary self-sealing theory by embedding calculation of pressure- and temperature-dependent capillary force in a pore network model. The microscale dynamic mechanisms are scaled up to the reservoir level by geological simulation, and the quantitative evaluation of reserves based on real geological sections is realized. From the results, several considerations are made to assist with resource assessment and sweet spot prediction. Firstly, the self-sealing effect of capillary in the micro-nano pore-throat system is at the core of tight sandstone gas accumulation theory; the hydrocarbon-generated expansion force is the driving force, and capillary force comprises the resistance. Furthermore, microscopic capillary force studies can be embedded into a pore network model and scaled up to a geological model using relative permeability curve and capillary force curve. Field application can be achieved by geological numerical simulations at the reservoir scale. Finally, high temperature and high pressure can reduce capillary pressure, which increases gas saturation and reserves. [ABSTRACT FROM AUTHOR]

Fossil resources are valuable wealth given to human beings by nature. Many mysteries related to them have been revealed such as the origin time and distribution area, but their vertical distribution depth has not been confirmed for people’s different understanding of their origin and the big depth variations from basin to basin. Geological and geochemical data of 13,634 source rock samples from 1,286 exploration wells in six representative petroliferous basins are examined to study their Active Source Rock Depth Limits (ASDL), defined in this study as the maximum burial depth of active source rocks beyond which the source rocks no longer generate or expel hydrocarbons and become inactive, to identify the maximum depth for fossil fuel resources distribution. Theoretically, the maximum depth for the ASDLs of fossil fuel resources ranges from 3,000 m to 16,000 m, while their thermal maturities (Ro) are almost the same with Ro≈3.5±0.5 %. A higher heat flow and more oil-prone kerogen are associated with a shallower ASDL. Active source rocks and the discovered 21.6 billion tons of reserves in six representative basins in China and 52,926 oil and gas reservoirs in the 1,186 basins over the world are found to be distributed above the ASDL, illustrating the universality of such kind of depth limit. The data are deposited in the repository of the PANGAEA database: https://doi.org/10.1594/PANGAEA.900865 (Pang et al., 2019). [ABSTRACT FROM AUTHOR]

The Liangshan and Qixia formations in the Sichuan Basin of central China were formed in the earlier middle Permian. Based on outcrop observation of the Changjianggou section at Shangsi, Guangyuan region and 3rd-order sequence division in typical drillings, one-dimensional spectrum analysis has been used to choose the better curve between the natural gamma ray spectrometry log(ln (Th/K)) in Well-Long17 and the gamma ray log(GR) in Well-Wujia1, respectively, for identifying Milankovitch cycles in Sequence PSQ1 which comprises the Liangshan and Qixia formations, and then to identify the variation in the Milankovitch cycle sequences. On this basis, the system tract and 4th-order sequence interfaces in Sequence PSQ1 were found via two-dimensional spectral analysis and digital filtering. Finally, a high-frequency sequence division program was established. Among these cycles, long eccentricity (413.0 ka) and short eccentricity (123.0 ka) are the most unambiguous, and they are separately the major control factors in forming 4th-order (parasequence sets) and 5th-order (parasequences) sequences, with the average thicknesses corresponding to the main cycles being 11.47 m and 3.32 m in Well-Long17, and 14.21 m and 3.79 m in Well-Wujia1, respectively. In other words, the deposition rate in the beach subfacies is faster than that of the inner ramp facies. The ln(Th/K) curve is more sensitive than the GR as the index of relatively ancient water depth in carbonate deposition. One-dimensional spectrum analysis of ln(Th/K) curve could distinguish the Milankovitch cycle sequences that arose from the Precession cycle (20.90 ka), with a much higher credibility. Sequence PSQ1 in Well-Long17 contains 10 4th-order sequences, and the growth span of Sequence PSQ1 consisting of the Liangshan and Qixia formations is about 4.13 Ma. The single deposition thickness of the long eccentricity cycle sequence has the characteristics of thinning and then thickening in the two-dimensional spectrum, which could be used to identify the system tract interface of the 3rd-order sequence. The precession sequence thickness remains stationary. As a result, the early deposition rate in the mid-Permian of the Sichuan basin was very slow, remaining nearly stationary, and this reflects a sustained depositional environment. Whole-rock carbon and oxygen isotope curves could also prove this point Milankovitch cycle sequence studies provide a basis for paleoenvironmental analysis and, as such, can be used to analyze ancient climate change, calculate deposition rate and deposition time, and carry out fine isochronous stratigraphic correlation. [ABSTRACT FROM AUTHOR]

There is a cross-cutting relationship between the E-W trending structures and the NE-trending structures in the northern Longmen-Micang Mountains region, which reflects possible regional tectonic transition and migration. Apatite fission track (AFT) analyses of 15 samples collected from this area yield apparent ages varying from 30.3±4.2 Ma to 111.7±9.0 Ma and confined-track-lengths ranging from 10.6±0.3 μm to 12.4±0.1 μm. Four specific groups were identified on the basis of the Track Age Spectrum Calculation (TASC) patterns, i.e., 143-112 Ma, 93.6-88 Ma, 42-40 Ma and ∼25.6 Ma. These age groups correspond to the spatial distributions of datasets and may represent four tectonic events. Together with the regional deformation patterns, the four age groups are interpreted to indicate tectonic superposition, transition and migration during the Meso-Cenozoic with the following possible order: (1) the Micang Mountains belt was dominated by the E-W trending structure during 143-112 Ma; (2) the contraction of the Longmen Mountains belt from the NW to the SE during 93.6-88 Ma led to the superposition of the NE-trending structures over the E-W trendinding structures; (3) dextral strike-slip shear dominated the Longmen Mountains belt at 42-40 Ma; (4) westward migration of the active tectonic belt occurred from 93.6-25.6 Ma in a break-back sequence in the northern Longmen Mountains belt. The Late Cenozoic tectonics in the northern Longmen Mountains belt are characterized by the dextral strike-slip shear and the occurrence of westward break-back sequence of deformations. As a result, north-south differences in deformations along the Longmen Mountains belt were intensified since the Miocene time and strains were mainly accumulated in the hinterland of the Longmen Mountains instead of being propagated to the foreland basin. [ABSTRACT FROM AUTHOR]

Abstract: The Kela-2 gas field, located in the Kuqa Depression of Tarim Basin, is the most productive gas field in China, based on per unit surface acreage. The gas trap is a subsurface anticline formed along an E–W trending thrust belt in front of the southern Tianshan Mountains. The gas pay zones are located in fine-grained sandstones of a fan delta front facies in the Lower Cretaceous Bashijiqik Formation (Klbs), and Eogene evaporitic rocks serve as regional cap rocks. The gases were derived primarily from over-mature Jurassic coal measures. Peak gas generation and final shaping of the Kela-2 structure occurred relatively late, in the late Himalayan orogeny (5–0Ma). Thrust faulting appears to have played a significant role in the hydrocarbon generation, migration, accumulation and preservation in this foreland basin. Detailed delineation of the Kela-2 gas field indicates that high-quality regional cap rocks are essential for the formation of the super-rich, abnormally pressured, giant gas fields, while buried anticlines under the overthrust zone are the favorable gas habitat. Late gas accumulation is considered to be decisive for the conservation of the giant gas fields. [Copyright &y& Elsevier]

Reports that the Lunnan area in the Tarim Basin has become an important onshore oil production base in China. Statement that the formation of the oil and gas pools in the low uplift of Lunnan has experienced a comparatively complex process of dynamics; Reference to the fact that, based on the hydrocarbon generation period of source rocks, the formation period of cap rocks and traps, the analysis of organic inclusion and the analysis of bitumen in the reservoir, this paper draws the conclusion that the low uplift area of Lunnan has experienced three pool formation periods: the Permian period, the Cretaceous-Early Tertiary period and the Late Tertiary-Quaternary period and two oil and gas reservoir adjustment periods: the Late Permian period and the Late Tertiary-Quaternary period; Indication of the comprehensive study that the large-scale Ordovician buried hill, formed in Early Hercynian, became the reservoir during the Permian period, because the Cambrian-Lower Ordovician oil was discharged laterally into the reservoir along the top of the Ordovician weathering crust from south to north; Claim that the reservoir experienced a complicated process—reconstruction in the end of Permian, adjustment in Cretaceous-Early Tertiary and re-discharging process in Late Tertiary-Quaternary, leading to the early original heavy oil reservoir of marine facies and the late original light oil reservoir and gas pool; Statement that carboniferous, Triassic and Jurassic oil and gas reservoirs result from upward adjustment and re-distribution of Ordovician oil and gas reservoirs; Reference to the fact that, of those results, the formation of Triassic-Jurassic oil and gas pools came under the influence of the northward-tilting structure; Vertical migration of the oil and gas sourcing from the different hydrocarbon source rock intervals into the base unconformity of Triassic system; Migration of the the oil and gas laterally from north to south and accumulated into th

Hetianhe is a big carbonate gas field which is found and demonstrated in the period of "Chinese National Ninth 5-Year Plan". The proved reserve of Hetianhe gas field is over 600x10&sup8; m³. Its main producing layers are Carboniferous bioclastic limestone and Ordovician carbonate composed of buried hill. The former is stratified gas pool with water around its side, and the latter is massive gas pool with water in its bottom. The gases in the gas pools belong to dry gases with normal temperature and pressure systems. Based on the correlation of gas and source rock, the gases are mainly generated from Cambrian source rocks. According to the researches on source rock and structure evolution, and the observations on the thin section to reservoir bitumen and the studies on homogenization temperature of fluid inclusions, the gas pool has been identified and divided into three formation periods. The first is Late Caledonian when the oil generated from the Cambrian source rocks and migrated along faults, as a form of liquid facies into Ordovician carbonate reservoir and accumulated there. After that, the crust uplifted, the oil reservoir had been destroyed. The second is Late Hercynian when condensate gases generated from the Cambrian source rocks and migrated into Ordovician reservoir, as a form of liquid facies. Since the fractures had reached P strata, so the trap might have a real poor preservation condition, and the large-scale gas pool formation had not happened. The third gas reservoir formation period occurred in Himalaya. The fractures on both sides of Hetianhe gas field developed violently under the forces of compression, and thus the present fault horst formed. The dry gases generated from Cambrian source rocks and migrated upwards as the form of gas facies into Ordovician and Carboniferous reservoirs, and the large gas pool as discovered at present was formed finally.

Reports that the Kela-2 gas field is located in the center of Kelasu structural belt in Kuqa Depression. Fact that this trap is one of a series of traps in the folded belts which are distributed in a string of pearls in the dual structure; Statement that the primary gasbearing layers are sandstone of Lower Cretaceous K&sub1;bs while the secondary layers are dolomite member and glutenite member of Lower Tertiary E&sub1-2;km and sandstone of Lower Cretaceous K&sub1;b; Reference to the fact that the main component of natural gas is methane whose content is higher than 97%; Characterization by dry gas whose source rock is Jurassic coal measure; Formation of the Kela-2 structural trap during the Xiyu period, becoming a reservoir in the late time; Formation of late and the thick seal rock of Lower Tertiary gipsmantle in the reservoir as the avail reason why the giant Kela-2 gas field has been well kept; Conclusion that the abnormal high pressure of the Kela-2 gas field results from the strong structural compression in the northern part during the Xiyu period.

Reports that the Kuqa depression is a foreland basin developed with Mesozoic-Triassic-Jurassic coal-bearing formation. Indication of the research results of the coal-derived hydrocarbon foreland basins in Kuqa depression that the coal-bearing formation can be the rich sources for generating gas because of their thickness and rich source rocks with gas-generating predominant kerogen; Statement that although the foreland thrust belt mainly acting in compression is very complicated, integral large structural traps can be formed; Claim that the thrust belt can act as the passage for communication with deep source rocks; Development of the high quality gypsolish and gypseous mudstone cap rock in the upper formation as the key for the formation of the large gas field; Dependence of the late formation of reservoirs in the large gas fields on the hydrocarbon-generating history controlled by the foreland basin and the developing process of foreland thrust belt.

Reports that there is a gas-rich and well-charged petroleum system in the Kuqa Depression where Triassic and Jurassic source rocks play important roles. Distribution in an area of more than 10000 km² and with a thickness of up to 1000 m; Composition of dark mudstones, carbonaceous mudstones and coal seams containing 6%, 40% and 90% of TOC, respectively, mainly of humic organic matter; Statement that, as high-quality regional cap rocks, the Neogene and Eogene gypsum rocks and gypseous mudstones matched well with the underlying Neogene and Cretaceous-Eogene sandstones; Formation of the most favorable reservoir-seal assemblages in the Kuqa Depression; Formation of the Jurassic sandstones and mudstones into another favorable reservoir-seal assemblage; Shaping of the traps late in the fold-thrust belt, mainly fixed in the Tertiary-Quaternary, where ten structure styles have been distinguished; Reference to tge fact that these traps spread as a zone in N-S, are scattered like a segmental line in W-E and show tier-styled vertically; Statement that the best traps are gypsum-salt covered fault-bend anticlines related to the passive roof duplex; Characterization of this petroleum system by late accumulation; Distribution in the early Himalayan Movement of an accumulation of mainly gas condensate and oil in the outer circular region of the kitchen; Claim that in the middle and late Himalayan the gas accumulations mainly formed and were distributed in the inner circular region near the kitchen; Statement that the overpressure of gas pools is common and is formed by seal capacity of thick gypsum layers, extensive tectonic compression and large uplift; Suggestion that the well-preserved anticline traps underlying the high-quality regional cap rocks of the Tertiary gypsum rocks and gypseous mudstones are the main targets for the discovery of giant and medium-sized gas fields; Importance of the above conclusions for the petroleum geology theory and the exploratio

Reports that, based on the theory of plate tectonics, combining with the isotopic dating of ophiolite, igneous and volcanics, geochemical tests rare earth element analyze and seismic interpretation, this paper studies the pre-Carboniferous tectonics and sedimentary formation of the south edge of the Tarim Basin and proves that there exists the Kunlun Ocean under tensional tectonics during the Sinian and Cambrian in the south edge of the Tarim Plate. Statement that, due to the collision orogenesis, there formed the peripheral foreland basin in the south edge of Tarim; Reference to the fact that Upper Silurian and Devonian molasses sedimentary system superposed on the Sinian and Middle Silurian passive margin flysch sedimentary system and formed the bivariate structure of the foreland basin; Identification, based on the field geology and seismic interpretation, that the formation of the Silurian and Devonian have the character of half deposit which shows thick in the south area and thin in the north, and the pre-Carboniferous thrust compression tectonics remained in the foreland thrust belt, which further demonstrates that there existed the Silurian and Devonian peripheral foreland basin on the south edge of the Tarim Basin.

Reports that, analyzing the characteristics of the Tethyan tectonic field, the authors think that the Tethyan tectonic field underwent three evolutional stages: closing of Paleo-Tethys and rifting of Neo-Tethys from early Permian to late Triassic, subduetion of Neo-Tethys and collision between the Indian plate and the Eurasia plate from Jurassic to early of low Tertiary, and collision between the Arab plate and the Eurasia plate and the A-type subduction of Indian plate from late of low Tertiary to the present. Suggestion that, combining the evolution of the Tethyan orogenic belt with the characteristics of the Northern Margin basin, the sedimentary and tectonic characteristics and types of the Northern Margin basin are controlled by the formation and evolution of the Tethyan orogenic belt and the ingression of Tethys; Statement that the evolution of Northern Margin basin can be divided into three development stages: back-arc foreland basin from late Permian to Triassic, the back-arc fault subsidence and depression from Jurassic to the early of low Tertiary, and the reactive foreland basin from the late of low Tertiary to the present; Conclusion that the Northern Margin basin in the Tethyan tectonic field is an important region for natural gas accumulation, and the Tarim Basin is a part of this region.

Reports that using the modern tectonic geology theories and methods such as the plate tectonic analysis, the paleo-structure analysis, the structural-lithofacies analysis, and the fault related fold and petroleum system, and combining with the seismic data, well drilling data and the circumferential field geology, study on the structural characteristics and petroleum prospect in the Tarim Basin has been carried out. Results showing that the Tarim Basin is a large superimposition and combination basin with continental crustal basement, composed of a Paleozoic craton and Meso-Cenozoic foreland basins; Characteristics of the basin, including that the kernel part of the basin is the marine facies Paleozoic craton, superimposed 4 continental facies foreland basins; Statement that though the scale of the paleozoic craton of the Tarim Basin is relatively small, the structure is steady; Reference to the petroleum prospect of the Paleozoic craton: multiphase pool-generation and accumulation controlled by ancient uplift; Report that the Meso-Cenozoic foreland basins in the Tarim Basin, which are distributed on the cratonic circumference and are a long-term subsidence, turned into rejuvenated foreland basins after the Meso-Cenozoic period; Petroleum prospects, including coal-bed generating hydrocarbon, abundant natural gas, pool-generation in later and recent periods, the oil and gas distribution controlled by the foreland thrust belt; Statement that the structural characteristics of Tarim provide it with a superimposition and combination petroleum system of multiple resources, multiple reservoirs and multiphase pool-generation; Report that the oil and gas exploration prospect covers two large fields: the Paleozoic craton and the Meso-Cenozoic foreland thrust belt.

In northeastern China, Cenozoic ‘reverse faults’ can be found in most basins. It was proposed that these faults were the products of inversion tectonics during a Cenozoic regional compressional event. This explanation is inconsistent with regional structures that are dominated by normal faults on seismic profile. As a typical example, the Rongxintun fault in northeastern Liaohe basin is systemically analyzed on the basis of deformation style, sedimentary character, and systemic mapping using seismic data. We conclude that the Rongxintun fault was related to right-lateral strike slipping of the Yannan main fault during the late Paleogene when the Dongying deposition occurred, and it was a secondary right-lateral/reverse fault (P Plane Fault) of the Yannan strike-slip fault system. The fold was a fault related fold formed as a result of movement along the Rongxintun fault. According to this analysis of the Rongxintun fault, we conclude that the Cenozoic ‘reverse fault’ in northeastern China was a secondary right-lateral/reverse fault of a strike-slip fault system and did not represent a compressional event. This kind of mechanism of ‘reverse faulting’ is consistent with the fact that there was large-scale strike-slip faulting in northeastern China during the Cenozoic. [Copyright &y& Elsevier]

Volcanic rocks are distributed widely in China, which are important exploration targets. By analyzing many discovered volcanic hydrocarbon reservoirs all over the world, the authors summarized the geologic characteristics of the formation of volcanic hydrocarbon reservoirs in China, and gave further exploration directions and advices. (1) There are mainly Carboniferous-Permian, Jurassic-Cretaceous, Paleogene-Neogene volcanic rocks in oil- and gas-bearing basins in China, which are mainly distributed in the Junggar Basin, Songliao Basin, Bohai Bay Basin, etc. There are mainly intermediate rocks and acidic rocks in east China, and intermediate rocks and basic rocks in west China. They primarily develop in intracontinental rift settings and island are environments. (2) Porefissure reservoirs are distributed widely in basins, which are volcanic rocks mainly in explosive and effusive facies. (3) Volcanic hydrocarbon reservoirs are chiefly near-source lithostratigraphic hydrocarbon reservoirs, and the oil and gas accumulation is predominantly controlled by lithotypes, faults and structural positions. (4) Deep-seated oil and gas reservoirs in the Songliao Basin and Carboniferous volcanic hydrocarbon reservoirs in the Junggar Basin are potential giant volcanic gas provinces, the volcanic hydrocarbon reservoirs in the Bohai Bay Basin and Santanghu Basin are favorable for oil and gas reserves increase, and volcanic rocks in the Turpan Basin, Sichuan Basin, Tarim Basin have exploration potentiality. (5) The technology series of oil and gas exploration in volcanic rocks have been preliminarily formed. [ABSTRACT FROM AUTHOR]