Natural Gas Exploration Research Articles

New progress and prospect of Middle Permian natural gas exploration in the Sichuan Basin

In recent years, natural gas exploration in the deep marine carbonates of the Sichuan Basin has been strengthened continuously. And based on the research results in the regional geological setting, a great number of new geological understandings are obtained in terms of lithofacies palaogeography, reservoir genesis and hydrocarbon accumulation evolution. What's more, new progress of natural gas exploration is achieved in many strata and domains in the Sichuan Basin, e.g. Qixia Fm of Middle Permian, Maokou Fm and Emeishan basalt, and the prospect of natural gas exploration is promising. In this paper, the recent exploration achievements of Middle Permian natural gas in the Sichuan Basin were summarized. First, under the control of palaeogeomorphology of late Caledonian, marginal platform shoal and girdle intra-platform shoal that are distributed in a large scale were developed at the edge of western Sichuan sea basin and in the periphery of central Sichuan paleo-uplift during the sedimentation of Qixia Fm, and they are large in scale and extensive in distribution range. Dolomite reservoirs are developed with good gas bearing property. Second, two kinds of reservoir bodies are mainly developed in the Maokou Fm of the Sichuan Basin. One is the shoal body which is developed along the marginal platform and the intra-platform high belt under the control of facies belt superimposed with late dolomitization, and the other is the effective karst reservoir which is distributed at the karst slope belt under the action of large-scale erosion in the late Maokou period. Third, volcanic gas reservoirs are discovered in the areas of Chengdu and Jianyang, and their quality reservoirs are of volcanic eruption facies and characterized by great thickness, good physical properties and extensive distribution. The discovery of this new type of gas reservoir expands the exploration domain of Middle Permian natural gas in the Sichuan Basin. In conclusion, the improvement of the large-scale natural gas exploration potential in many strata and domains of Middle Permian in the Sichuan Basin promotes it to be one important replacement domain of the further increase of natural gas production and reserves in the Sichuan Basin.

Discovery of carbonate source rock gas reservoir and its petroleum geological implications: A case study of the gas reservoir in the first member of Middle Permian Maokou Formation in the Fuling area, Sichuan Basin

Carbonate rocks in the first member of the Middle Permian Maokou Formation in the Sichuan Basin (hereinafter “Mao 1 Member” for short) have been taken as a set of carbonate source rocks, and they have not been specifically studied from the aspects of reservoir evaluation and testing. By referring the exploration ideas of unconventional natural gas, the Mao 1 Member in Fuling area of southeastern Sichuan Basin has obtained industrial gas flow in development wells in recent years. In order to further clarify the natural gas exploration potential of the Mao 1 Member in this area, it is necessary to study its sedimentary characteristics, natural gas accumulation conditions and main control factors based on the data of field section measurement, systematic coring in drilling and laboratory testing. The following research results were obtained. First, the gas reservoir in the Mao 1 Member in the Fuling area is of source–reservoir integration, and its natural gas is mainly enriched in blackish gray marlite and nodular marlite. Second, its reservoir spaces are dominated by grain margin pores (fractures), diagenetic shrinkage pores (fractures), organic pores and fractures. Third, the pores are mostly in nanometer scale, and the main pore diameter is in the range of 5–50 nm, which is between shale reservoir and conventional reservoir, with strong heterogeneity. Fourth, the gas reservoir is characterized by source–reservoir coexistence, lithology controlling reservoir and extensive layered distribution, presenting two-stage differential hydrocarbon enrichment, namely intraformational near-source enrichment in the early stage and interformational blowdown adjustment in the late stage. Fifth, the development of blackish gray organic-rich fine marlite which is deposited with the episodic upwelling in the outer ramp facies belt is the foundation for the natural gas accumulation, the transformation of clay minerals controls the development of higher-quality reservoirs, good preservation conditions are the key to the natural gas accumulation, and fracture development is favorable for the enrichment and high yield of natural gas. In conclusion, the Mao 1 Member in this area is a special type of gas reservoir, i.e., carbonate source rock gas reservoir, which has greater potential of natural gas exploration and industrial gas flow have been obtained in several wells. The discovery of such type of gas reservoirs not only expands the domain of natural gas exploration in the Sichuan Basin, but provides the reference for the natural gas exploration in other areas of South China.

Elastic wave velocities of hydrate-bearing sands containing methane gas bubbles: Insights from CT-acoustic observation and theoretical analysis

The elastic wave velocities of hydrate-bearing sediments (HBS) are considerably affected by the content of free gas and hydrate. Although several existing models relate the free gas and hydrate saturation to acoustic velocities, the accuracy of these models is still uncertain because of the difficulty in determining the gas content. In this study, we acquired the gas volume fraction and acoustic velocity data of the hydrate-bearing sands through a X-ray computed micro-tomography (CT) and an ultrasonic apparatus, respectively. The acoustic velocities increase slowly at low hydrate saturation (Sh < 33%), whereas they increase rapidly when hydrate saturation exceeds 33%. Using the measured data, we verified three commonly used velocity models, namely, the Biot-Gassmann theory by Lee (BGTL), effective media theory (EMT), and simplified three-phase equation (STPE). The results obtained using the BGTL are consistent with the experimental results when hydrate saturation exceeds 45%, while the EMT-B model is more suitable for predicting P-wave velocity (Vp) when the hydrate saturation varies from 15% to 55%. The value of Vp calculated by the STPE model, without considering the effects of gas on velocity, is higher than the experimental value. We further applied a new method combining the Wood and Domenico equations to calculate the bulk modulus of HBS containing methane gas, which can improve the precision and applicability of the STPE model. Compared with the EMT and BGTL models, the modified STPE model is more suitable for predicting Vp of unconsolidated reservoirs with high porosity and permeability. These results provide a new method for further experimental research and a theoretical reference for accurately estimating hydrate saturation through logging data during natural gas exploration and development.

Distribution and possible sources of polycyclic aromatic hydrocarbons (PAHs) and metals in marine surface sediments off northern Mozambique

Liquid natural gas (LNG) exploration has started off the coast of northern Mozambique, in the Rovuma Basin, East Africa. In advance of gas production, we collected in 2018 over 100 samples of surface sediments from 40 locations in the pristine and exploration areas at water depths of 5–2000 m. We have determined the levels of hydrocarbons (total hydrocarbon contents (THC) and 49 individual PAHs), heavy metals, arsenic, grain size and total organic carbon. While sediment composition varied strongly from coarse sediment to high mud contents (<63 μm), background levels of hydrocarbons and metals were found in most samples. We found anthropogenic contamination at one site in Pemba harbor. We observed no petroleum-related contamination, including the Palma area with numerous exploration wells. Elevated concentrations of barium and THC at some locations in this area are attributed to drilling activities but are not considered to be of environmental concern.

The Cabo Delgado Insurgency in Mozambique: Origin, Ideology, Recruitment Strategies and, Social, Political and Economic Implications for Natural Gas and Oil Exploration

This paper reflects on the insurgency and terrorism in the northern province of Cabo Delgado in Mozambique by an alleged Islamic affiliated extremist group called Ansar al-Sunna. The paper closely investigates the origins of Ansar alSunna and its ideology including its links and possible supporters. It further delves into Ansar al-Sunna's recruitment strategies of its jihadists within and without the province under scrutiny and possibly internationally. The paper discusses the economic implications of the Ansar al-Sunna insurgency in light of the exploration and discovery of natural gas reserves in the province among other minerals that Cabo Delgado is endowed with as a province. Findings that emerge from the research show possible links of Ansar al-Sunna with ISIS of Iran and Iraq as well as Al Shabaab of Somalia among other terrorist groups including Boko Haram of Nigeria. Furthermore, the research shows that Ansar al-Sunna started as an extremist religious group in the province before it morphed into a fully-fledged terrorist outfit. The paper reveals that Ansar al-Sunna recruits from its adherent religious followers including forcefully recruiting the general populace in the province and that the insurgency pauses a serious threat to economic viability of Mozambique among other issues. The paper adopts a Desktop research methodology, which involves secondary data or that which can be collected without fieldwork. In the context of this research, it involved searching libraries, the internet, public and private media among others. This paper appealed to Human Needs Theory (HNT) as its theoretical framework.

Geochemical characteristics of the Paleozoic natural gas in the Yichuan-Huanglong area, southeastern margin of the Ordos Basin: Based on late gas generation mechanisms

As a new expansion area for natural gas exploration, the Yichuan-Huanglong area improves the complete evolutionary sequence of the Paleozoic natural gas in the eastern part of the Ordos Basin, but the geochemical characteristics and the late gas generation processes involved have not been discussed. By collecting and analyzing gas samples from existing wells, the following insights were obtained. The alkane components had a dichotic characteristic, the wetness of the O1m5–C2b natural gas (average: 3.85%) was relatively higher than that of the Permian natural gas (average: 0.88%). The carbon isotopic compositions exhibited partial reversal (δ13C-CH4 >δ13C-C2H6), and the Permian natural gas from the coal-measure strata had abnormally low δ13C-C2H6 values (<−28‰). The O1m5–C2b natural gas had characteristics typical of oil-associated gas, whereas the Permian natural gas had the mixed characteristics of coal-derived gas and oil-associated gas. The mixing and diffusion effects illuminated the isotopic anomalies of the alkane gas. According to the source rocks and the thermal maturity, mechanisms such as the depletion of 13C in the residual ethane, water-reforming, and the Fischer-Tropsch-like mechanism, and the demethylation of aromatic hydrocarbons also provided important information about the late gas generation of the Paleozoic natural gas in the Yichuan-Huanglong area.

Source and accumulation processes of natural gases in the Qixia Formation in the Northwestern Sichuan Basin, SW China

The Northwestern Sichuan Basin is a hot spot for natural gas exploration, and in recent years, many wells have obtained high-yield commercial gas flows in the Qixia Formation. However, due to the influence of foreland orogeny, the oil and gas in the Qixia Formation often have the characteristics of multisource filling, multistage accumulation and multistage transformation, which results in an unclear understanding of the gas source and the accumulation process. This paper uses the concept of sub-block research through carbon isotope and biomarkers analyses, to carry out gas-gas, gas-source and bitumen-source rock comparisons in different structural belts to determine the natural gas sources. On this basis, basin simulation and fluid inclusion analysis technology are used to determine the stage of hydrocarbon accumulation, and then the accumulation process is determined. Studies have shown that the gas source of the Qixia Formation are different in the different structural belts: (1) The natural gas from the Qixia Formation in the thrust nappe belt mainly originates from the Qiongzhusi Formation source rock; (2) The natural gas from the Qixia Formation in the hidden fault belt is a mixture of the source rock of the Qixia Formation and the source rock of the Qiongzhusi Formation; and (3) The natural gas from the Qixia Formation in the gentle belt mainly originates from the Middle Permian source rock and the Upper Permian Longtan Formation source rock. The Indosinian, Yanshania, and Himalayan movements all have great influence on the formation of the Qixia Formation reservoir in northwestern Sichuan, and the accumulation process of the different structural belts is quite different, but generally, the accumulation process of the different structural belts have experienced three stages: a primary oil pool stage, gas pool stage and reformation and definition stage.

Nonlinear 3D finite element analysis of a shear-wave vibrator-ground interaction system

Shear-wave vibrator is a type of equipment usually employed for natural gas exploration, consisting of a vibrating machine which applies dynamic loads to the surrounding soils in order to ease the extraction of the gas. Detailed numerical simulations of these devices are scarce in the literature. Thus, in this paper, a nonlinear 3D finite element numerical simulation has been conducted to investigate the main features of the dynamic response of the vibrator. Nonlinear aspects such as baseplates-soil interaction, soil plasticity, and non-reflect boundary conditions, usually neglected in previous simulations, are considered in this model. The stress distribution, plastic deformation, shear wave propagation, ground force and energy characteristics of the vibrator-ground system are analyzed in detail. Based on the achieved results, an evaluation criterion is developed to quantify the performance of the shear-wave vibrator. Moreover, a parametric analysis has been conducted and the effects of some system parameters, such as excitation frequency, different soil properties and embedded depth, are analyzed and discussed in detail. The results can provide a reference for operating or optimizing the design of shear-wave vibrators.

The Brazilian RBL: guarantees on the rights of the oil and natural gas exploration and production contract.

The Brazilian RBL: guarantees on the rights of the oil and natural gas exploration and production contract.

Process for assigning contracts for exploration and production of oil and natural gas - procedure and main aspects of ANP Resolution No. 785/2019

Process for assigning contracts for exploration and production of oil and natural gas - procedure and main aspects of ANP Resolution No. 785/2019

A new method for evaluating the unstable deliverability of gas wells in gas formation testing phase

Predicting the deliverability change laws of a gas well in the early stage is one of the technical difficulties in natural gas development. The commonly used steady seepage analysis methods have relatively large errors. And this problem cannot be solved effectively by the classical methods of production decline analysis and pressure transient well test analysis. To solve this problem, this paper did calculation based on the well test model of constant-pressure production, changed the previous approximate method of simplifying the calculation of the exponential integral function, and accurately calculated the analytical solution of absolute open flow potential of a gas well. In addition, the deliverability instability characteristics of different types of gas wells were quantitatively described by taking vertical wells in homogeneous reservoirs as the reference benchmark. Then, combined with the deliverability evaluation needs of a new well, a new practical method focusing on solving the problems of gas formation testing analysis was researched and developed, and also applied on site at some key wells in the hot spots of natural gas development in the Sichuan Basin. And the following research results were obtained. First, the accurate formula for the analytical solution of well test model significantly enhances the adaptability to short-term test conditions. Second, the newly established chart briefly reveals the quantitative relationship between the unstable deliverability characteristics of a gas well and the main influencing factors. Third, if the characteristic parameter of turbulence effect is unknown, it is necessary to perform iterative calculation of the variable skin factor when the well test model of constant-pressure production is used to analyze the change trend of the absolute open flow potential of a gas well. Fourth, by conducting comparative analysis on the calculation results of typical cases, the decline laws of the absolute open flow potential of different types of gas wells can be understood further. It is concluded that this new method is capable of improving the prediction accuracy of the unstable deliverability of gas wells. Therefore, it can be widely applied to the deliverability evaluation of gas wells in the exploration stage, the evaluation stage of early development and the commissioning stage of new development and production wells, which is conducive to the formulation of natural gas exploration and development decision.

Natural gas industry in China: Development situation and prospect

China's natural gas industry has entered a rapid development stage, and its supply, sales, storage and transport systems are continuously undergoing profound structural change. It is a major issue concerning national energy security to objectively grasp the current development situation and trend of natural gas industry and make scientific predictions. In this regard, through the analysis of global natural gas resources status, trade pattern and development trend, we reviewed the development process, current situation, trend and problems in natural gas industry in China, then made prospect analysis and judgment in terms of demand and supply, etc., and finally put forward some reasonable and sustainable strategic countermeasures and proposals. The following results were obtained. (1) The comprehensive energy elasticity coefficient method was adopted to predict that the natural gas consumption volume in 2030 will be about 650 billion cubic meters. (2) Affected by many factors such as energy supply side reform, industrialization and urbanization, the growth rate of domestic natural gas consumption is much higher than that of production and the development of natural gas industry still faces a series of problems. (3) It is necessary to strengthen the exploration and development of natural gas to ensure the increase of domestic natural gas storage and production. (4) Diversified import channels should be continuously expanded to disperse the risk of excessive growth of natural gas dependence on foreign countries. (5) The construction of national and dynamic interconnection system and hub should be accelerated. (6) China should rationalize the price mechanism of natural gas, gradually open the control on industrial and seasonal consumption, and strengthen the overall sales guidance in the national market.

Source of dolomitizing fluids and dolomitization model of the upper Permian Changxing and Lower Triassic Feixianguan formations, NE Sichuan Basin, China

Dolostone reservoirs have been found in the Changxing (P2c) and Feixianguan (T1f) formations in both the east and west platform edges of the Kaijiang-Liangping trough; they are the main natural gas exploration targets in the NE Sichuan Basin. In this study, the sources of the dolomitizing fluids were determined, and a dolomitization model was established based on petrography, rare earth elements (REEs), major and trace element data, and 87Sr/86Sr ratios. The dolostone and limestone samples with ΣREE values of >5 ppm exhibit relatively flat PAAS-normalized REE patterns; their Zr, Sc, and Th concentrations increase with increasing ΣREE; and their Rb/Sr ratios are generally greater than 0.01, suggesting that these samples were likely contaminated by terrigenous components. Therefore, only the REEs and 87Sr/86Sr ratios of samples with ΣREE values of <5 ppm were selected to constrain the dolomitizing fluids. The PAAS-normalized REE patterns of the P2c–T1f dolostones, which are characterized by LREE depletion, negative Ce anomalies (Ce/Ce* of 0.70–0.85), and relatively high Y/Ho ratios (49.0–152.4), are similar to those of the unaltered coeval limestones, suggesting that the dolomitizing fluid was sourced from fluids similar to coeval seawater. The Ce/Ce* values of the dolostones are lower than those of the unaltered limestone, suggesting that the source of the dolomitizing fluids had more oxidizing conditions and most probably contained evaporated seawater from an evaporite platform. The Eu/Eu* values of the P2c dolostone in the west platform (means of 4.95 and 2.76 for the YB and XLC gas fields, respectively) are notably higher than the Eu/Eu* values of the P2c and T1f dolostones in the east platform (mean 1.29 and 1.39 in the PG gas field, respectively), suggesting that the P2c dolostone in the west platform was formed under more reducing conditions and at a deeper burial depth. The 87Sr/86Sr ratios of the T1f and P2c dolostones in the east platform (such as in the Puguang gas field) are similar to those of the co-existing early T1f limestone, while the 87Sr/86Sr ratios of the P2c dolostone in the west platform (such as in the Yuanba gas field) are higher than those of the coexisting P2c limestone but are close to those of the late T1f dolostone in the east platform. This indicates that the dolomitizing fluids were mainly from the early T1f and late T1f evaporated seawater on the east and west platforms, respectively. Driven by the density difference between the evaporated seawater and normal seawater, the evaporated seawater flowed downward and laterally into the limestone in the high-energy reefs and shoals through the original pores and the early selective dissolution pore system and/or through synsedimentary fractures. The continual seepage of evaporated seawater and the relatively high temperatures (50–60 °C) resulted in the large-scale shallow burial dolomitization in the NE Sichuan Basin.

Characteristics and petroleum geological significance of high-quality source rock in Gufeng Member of Middle Permian Maokou Formation in northern Sichuan Basin, China

Based on the comprehensive analysis of drilling and outcrop profile data, the stratigraphic boundary between Middle Permian and Upper Permian in northern Sichuan Basin is redefined systematically. It is confirmed that the source rock of carbonaceous and siliceous mudstone is widely developed in the Gufeng Member of Middle Permian Maokou Formation in the northern part of the basin, and its thickness is mainly between 5-40 m. The source rock in the Gufeng Member has large variation scope of TOC, mainly in the range of 0.5%-30%, and the average value of most profiles more than 2.0%. It is a set of high-quality source rock as a whole. Its major organic matter is type II. At present, its thermal maturity changes greatly, and it is in the mature-over mature stage. The deep-water sedimentary environment, strong reduction conditions and high paleo-productivity jointly controlled the development and distribution of high-quality source rock in the Gufeng Member under the background of extensional structure at the end of the Middle Permian. High-quality source rock in the Gufeng Member is of great significance to the exploration of marine conventional natural gas and shale gas in northern Sichuan Basin.

Hydrocarbon generation from confined pyrolysis of Xujiahe Formation coal in the northwest Sichuan Basin, China

The northwest depression of Sichuan Basin is an important gas-producing region. Previous studies have shown that large amount of gas was discovered in the Xujiahe Formaion. Several sets of source rocks from different formations including Xu1, Xu3 and Xu5 sections in Xujiahe Formation contribute to the gas resources. To investigate the gas generation potential of coal measure source rocks of Xujiahe Formation widely distributed in the Sichuan Basin. A coal sample from Xujiahe Formation was used in confined gold tube pyrolysis experiment and the activation energy and frequency factor of the coal sample were obtained. Based on the kinetic parameters of hydrocarbon generation, collected stratigraphic data, the burial, thermal and gas generation history of two wells in the northwest depression of Sichuan Basin were simulated by using Petromod I. Results showed that the activation energy of the coal sample was distributed in the range of 45-67 kcal/mol (main activation energy peak was58 kcal/mol) and the frequency factor was 4.7×1011s−1. For the two investigated wells, the gas generation started at 169-184 Ma, terminated at about 65 Ma. The well CHQ173 closer to the northwest depression entered the threshold of hydrocarbon generation earlier, was of deeper maximum buried depth and higher maximum thermal maturity and greater gas yield. The amount of expelled gas resulted from the uplift was small in this region. If combined with the tectonic evolution and sealing conditions of the study area, the gas generation history of Xujiahe formation would be of great significance for the exploration of natural gas in this area.

Carbonate fine lithology prediction using amplitude attribute of PS-wave

The Lower Permian Maokou Formation and Qixia Formation in central part of Sichuan Basin have huge natural gas exploration potential. For the Maokou Formation, there is argillaceous limestone in the limestone background, and thin dolomite reservoirs are developed. The ability to use conventional PP-wave data to predict lithology and dolomite reservoirs is insufficient. In PP-wave seismic section, the response of argillaceous limestone and dolomite in the limestone background is not obvious, which limits the prediction accuracy of lithology. Making full use of high-precision matched PP-wave and PS-wave seismic data, we find a special phenomenon in the Maokou Formation that PP-wave usually shows weak amplitude and poor continuity while PS-wave shows stronger amplitude and better continuity. Aimed at the particular phenomenon, through multi-wave seismic forward modelling, log lithology interpretation, cross plot of P-impedance with S-impedance and seismic section comparison of PP-wave and PS-wave, it is clear that the weak peak of PP-wave and strong peak of PS-wave reflects weak and subtle change in lithology, which shows the existence of argillaceous limestone in limestone background. And the gas-bearing dolomite reservoirs are characterized by low frequency and weak amplitude on PP-wave, while show weak peak feature on PS-wave. Compared with PP-wave, the favorable lithology (dolomite) distribution predicted by maximum peak amplitude of PS-wave is more consistent with reservoir distribution and origin of hydrothermal-karst-controlled dolomite. Result of lithology prediction by PS-wave is reasonable and reliable for Maokou Formation.

Exploration domains and technological breakthrough directions of natural gas in SINOPEC exploratory areas, Sichuan Basin, China

Natural gas exploration in SINOPEC's exploratory areas in the Sichuan Basin has achieved rapid development. Since discovering the Xinchang gas field, natural gas exploration has expanded gradually, with both reserves and production growing fast. The deepening understanding of geological theory and crucial technology's progress is the main driving force for the natural gas industry's significant development. Considering the rich natural gas resources, various types of promising targets, exploration degree of 16% of conventional natural gas, and the initial stage of shale gas exploration, the HCZ model predicted natural gas exploration. Additionally, it indicated that the shale gas in the SINOPEC exploratory areas in the Sichuan Basin would consistently have rapid development with proven reserves increasing by (160–200) × 109m3/a. In the future, the development strategy is to vigorously develop shale gas, steadily promote marine conventional natural gas, and continually research continental tight sandstone gas. The following tasks need to be strengthened for efficient exploration of the three resource types: prediction of deep and ultra-deep reef-shoal reservoirs, weathered crust reservoirs, and technological breakthrough of efficient drilling and fracturing test of ultra-deep wells; the technological breakthrough of geological-logging-seismic integrated sweets pot prediction of tight sandstones and low-cost multi-layer fracturing of vertical wells; the technological breakthrough of increasing-yield and reducing-cost drilling, completion, and fracturing of deep marine shale gas; resources potential evaluation and adaptive exploration and development technologies preparation of continental and transitional shale gas.

Gas source and exploration direction of the Middle Triassic Leikoupo Formation in the Sichuan Basin, China

Natural gas exploration in the Middle Triassic Leikoupo Formation (T2l) in the Sichuan Basin of China has achieved a remarkable breakthrough in recent years and thus attracts wide attention. However, there is no consensus on the gas source. The genetic identification and gas–source correlation based on the geochemical characteristics of natural gas indicate that the natural gas in the Leikoupo Formation is mainly from the underlying Upper Permian Longtan/Wujiaping Formation (P3l/P3w) source rocks (Moxi). Nevertheless, it has assistance (Zhongba) or hydrocarbon supply condition (Chuanxi) brought by the source rocks in the 1st Member of the Upper Triassic Xujiahe Formation (T3x), or mainly from the T3x source rocks with a specific contribution by the T2l source rocks (Yuanba and Longgang). The T2l gas pools display three types of source-reservoir assemblages, and they suggest different exploration directions. The P3l source rocks serve as main source rocks that can constitute the lower-generation and upper-accumulation pattern. The positive structures around the deep faults connecting the P3l and T2l are the most favorable areas to explore large-scale T2l gas pools. The source rocks at the bottom of T3x and the reservoirs at the top of the T2l can constitute the side-generation and side-accumulation pattern, which is favorably developed in the residual karst hills or the local tectonic high position of the karst slope. The T2l can constitute self-generation and self-accumulation pattern, which assists in the formation of gas pools.

Assessment of deep demersal fish fauna diversity of the Colombian Caribbean Sea

We compiled georeferenced records of deep demersal fishes from the Colombian Caribbean Sea in order to assess the level of survey coverage and geographic completeness of species richness inventories at a scale of 15 minutes by 15 minutes cells, in view of threats from fishing and oil and natural gas exploration. We identified a rich fauna with a minimum of 362 species registered. Areas with elevated observed and predicted species richness were identified. Survey coverage and geographic richness completeness resulted in being deficient with no cell reaching the status of well-sampled spatial unit and 83% of the Colombian Caribbean exclusive economic zone bottoms unexplored, in particular regarding depths beyond 1000 m. A plea is made for renewed survey efforts with a focus on the protection of the Colombian Caribbean deep-sea biota.

Genetic types, origins, and accumulation process of natural gas from the southwestern Junggar Basin: New implications for natural gas exploration potential

Although the Junggar Basin is one of the four largest petroliferous basins in China, the proved reserves of natural gas is primarily concentrated in the eastern and southern parts of the basin and only account for 9.0% of the total resources. In this study, four types of natural gas are identified in the southwestern Junggar Basin based on their molecular composition, stable carbon isotopes, and low molecular weight hydrocarbons. Type I gas is coal-type gas generated from highly mature–overmature humic source rocks of the deeply buried Carboniferous (C)–Lower Permian Jiamuhe Formation (P1j) in the Shawan Sag. Type II gas is derived from lacustrine source rocks of the Lower Permian Fengcheng Formation (P1f), which can be further divided into the two subtypes IIA and IIB. The former is highly mature oil-type gas from the Shawan Sag, while the latter is low-mature oil-type gas from the Mahu Sag. Type III gas is a mixture of Types I and II gases. Type IV gas is secondary microbial gas formed by the biodegradation of oils. The ethane and propane are systematically affected by biodegradation, and the selective biodegradation of propane is most notable. Based on the results of reservoir fluid inclusions and the reconstructed burial and thermal histories, two stages of oil and gas charging events and one stage of oil and gas adjustment are identified in the study area. The first stage of oil and gas charging event occurred from the Late Triassic to the Early Jurassic in terms of the homogenization temperatures of ~70–100 °C of hydrocarbon inclusions. During this stage, hydrocarbons generated from P1j/C and P1f source rocks in the Shawan Sag migrated along faults and unconformities to structural highs and accumulated in reservoirs. In the Late Jurassic, a large-scale uplifting event occurred, and the early natural gas was extensively lost and the oils suffered severe biodegradation, leading to the formation of a large amount of secondary microbial methane. The second charging event occurred during the Early Cretaceous based on the homogenization temperature of hydrocarbon inclusions (~100–125 °C). At this stage, the study area began to subside again and large amounts of highly mature coal-type and oil-type gases were generated from the P1f and P1j/C source rocks, which migrated to structural highs to form new gas pools. These results confirm that there are two sets of effective gas source rocks i.e., C/P1j and P1f, in the southwestern Junggar Basin, revealing a new field for natural gas exploration. The results of this study represent a case study regarding the gas–source correlation and restoration of the accumulation process under complex geological conditions and improve the understanding of the natural gas exploration potential in the Junggar Basin.