Large Gas Reservoirs Research Articles

Formation conditions of Sinian–Cambrian large lithologic gas reservoirs in the north slope area of central Sichuan Basin, SW China

Based on analyses of characteristics, hydrocarbon charging history and geological conditions for the formation of Sinian–Cambrian reservoirs in the north slope area of central Sichuan paleo-uplift, the natural gas origin, accumulation evolution, accumulation pattern and formation conditions of large lithologic gas reservoirs have been investigated. Through comprehensive analyses of natural gas composition, carbon and hydrogen isotopic compositions, fluid inclusions, reservoir bitumen, and geological conditions such as lithofacies paleogeography and beach body characterization, it is concluded that: (1) The natural gas in the Sinian–Cambrian of the north slope area is mainly oil cracking gas, and different contribution ratios of multiple sets of source rocks lead to different geochemical characteristics of natural gas in different reservoirs. (2) Although the both Sinian and Cambrian gas reservoirs in this area are lithologic gas reservoirs under monocline background, the former has normal-pressure and the latter has high-pressure. There are three types of source-reservoir-caprock combinations: single source with lower generation and upper reservoir, double sources with lower generation and upper reservoir or with side source and lateral reservoir, double sources with lower generation and upper reservoir or with upper generation and lower reservoir. The Permian–Triassic is the main generation period of oil, Early–Middle Jurassic is the main generation period of oil cracking gas and wet gas, and Late Jurassic–Cretaceous is the main generation period of dry gas. (3) The Sinian–Cambrian system of the north slope area has two favorable conditions for formation of large lithologic gas reservoirs, one is that the large scale beach facies reservoirs are located in the range of ancient oil reservoirs or near the source rocks, which is conducive to the “in-situ” large-scale accumulation of cracked gas in the paleo-oil reservoirs, the other is that the large scale mound-beach complex reservoirs and sealing layers of inter beach tight zones match effectively to form large lithologic traps under the slope background. The research results confirm that the north slope area has large multi-layer lithologic gas reservoirs with more than one trillion cubic meters of natural gas resources and great exploration potential.

BASS XXXVII: The Role of Radiative Feedback in the Growth and Obscuration Properties of Nearby Supermassive Black Holes

We study the relation between obscuration and supermassive black hole (SMBH) accretion using a large sample of hard X-ray selected active galactic nuclei (AGNs). We find a strong decrease in the fraction of obscured sources above the Eddington limit for dusty gas () confirming earlier results, and consistent with the radiation-regulated unification model. This also explains the difference in the Eddington ratio distribution functions (ERDFs) of type 1 and type 2 AGNs obtained by a recent study. The break in the ERDF of nearby AGNs is at . This corresponds to the λ Edd where AGNs transition from having most of their sky covered by obscuring material to being mostly devoid of absorbing material. A similar trend is observed for the luminosity function, which implies that most of the SMBH growth in the local universe happens when the AGN is covered by a large reservoir of gas and dust. These results could be explained with a radiation-regulated growth model, in which AGNs move in the N H–λ Edd plane during their life cycle. The growth episode starts with the AGN mostly unobscured and accreting at low λ Edd. As the SMBH is further fueled, λ Edd, N H and the covering factor increase, leading the AGN to be preferentially observed as obscured. Once λ Edd reaches the Eddington limit for dusty gas, the covering factor and N H rapidly decrease, leading the AGN to be typically observed as unobscured. As the remaining fuel is depleted, the SMBH goes back into a quiescent phase.

Metal content of the circumgalactic medium around star-forming galaxies at z ∼ 2.6 as revealed by the VIMOS Ultra-Deep Survey

Context. The circumgalactic medium (CGM) is the location where the interplay between large-scale outflows and accretion onto galaxies occurs. Metals in different ionization states flowing between the circumgalactic and intergalactic mediums are affected by large galactic outflows and low-ionization state inflowing gas. Observational studies on their spatial distribution and their relation with galaxy properties may provide important constraints on models of galaxy formation and evolution. Aims. The main goal of this paper is to provide new insights into the spatial distribution of the circumgalactic of star-forming galaxies at 1.5 < z < 4.5 (⟨z⟩∼2.6) in the peak epoch of cosmic star formation activity in the Universe. We also look for possible correlations between the strength of the low- and high-ionization absorption features (LIS and HIS) and stellar mass, star formation rate, effective radius, and azimuthal angle ϕ that defines the location of the absorbing gas relative to the galaxy disc plane. Methods. The CGM has been primarily detected via the absorption features that it produces on the continuum spectrum of bright background sources. We selected a sample of 238 close pairs from the VIMOS Ultra Deep Survey to examine the spatial distribution of the gas located around star-forming galaxies and generate composite spectra by co-adding spectra of background galaxies that provide different sight-lines across the CGM of star-forming galaxies. Results. We detect LIS (C II and Si II) and HIS (Si IV, C IV) up to separations ⟨b⟩ = 172 kpc and 146 kpc. Beyond this separation, we do not detect any significant signal of CGM absorption in the background composite spectra. Our Lyα, LIS, and HIS rest-frame equivalent width (W0) radial profiles are at the upper envelope of the W0 measurements at lower redshifts, suggesting a potential redshift evolution for the CGM gas content producing these absorptions. We find a correlation between C II and C IV with star formation rate and stellar mass, as well as trends with galaxy size estimated by the effective radius and azimuthal angle. Galaxies with high star formation rate (log[SFR/(M⊙ yr−1)] > 1.5) and stellar mass (log[M⋆/M⊙] > 10.2) show stronger C IV absorptions compared with those low SFR (log[SFR/(M⊙ yr−1)] < 0.9) and low stellar mass (log[M⋆/M⊙] < 9.26). The latter population instead shows stronger C II absorption than their more massive or more star-forming counterparts. We compute the C II/C IVW0 line ratio that confirms the C II and C IV correlations with impact parameter, stellar mass, and star formation rate. We do not find any correlation with ϕ in agreement with other high-redshift studies and in contradiction to what is observed at low redshift where large-scale outflows along the minor axis forming bipolar outflows are detected. Conclusions. We find that the stronger C IV line absorptions in the outer regions of these star-forming galaxies could be explained by stronger outflows in galaxies with higher star formation rates and stellar masses that are capable of projecting the ionized gas up to large distances and/or by stronger UV ionizing radiation in these galaxies that is able to ionize the gas even at large distances. On the other hand, low-mass galaxies show stronger C II absorptions, suggesting larger reservoirs of cold gas that could be explained by a softer radiation field unable to ionize high-ionization state lines or by the galactic fountain scenario where metal-rich gas ejected from previous star formation episodes falls back to the galaxy. These large reservoirs of cold neutral gas around low-mass galaxies could be funnelled into the galaxies and eventually provide the necessary fuel to sustain star formation activity.

The strongest cool core in REXCESS: Missing X-ray cavities in RXC J2014.8–2430

We present a broad, multi-wavelength study of RXC J2014.8−2430, the most extreme cool-core cluster in the Representative XMM-Newton Cluster Structure Survey (REXCESS), using Chandra X-ray, Southern Astrophysical Research (SOAR) Telescope spectroscopic and narrow-band imaging, Atacama Large Millimeter/submillimeter Array (ALMA), Very Large Array, and Giant Metrewave Radio Telescope observations. While feedback from an active galactic nucleus (AGN) is thought to be the dominant mechanism by which a cooling flow is suppressed, the Chandra imaging observations surprisingly do not reveal the bi-lateral X-ray cavities one might expect to see in the intracluster medium (ICM) of an extreme cool core hosting a powerful radio source, though cavities commonly appear in many similar sources. We discuss the limits on the properties of putative radio bubbles associated with any undetected X-ray cavities. We place upper limits on any significant X-ray AGN in the brightest cluster galaxy (BCG) and show that the X-ray peak is offset from the central radio source, which exhibits a steep low-frequency radio spectrum indicative of electron ageing. The imaging and spectroscopy provided by SOAR reveal an extended, luminous optical emission-line source. From our narrow-band Hα imaging of the BCG, the central Hα peak is coincident with the radio observations, yet offset from the X-ray peak, consistent with sloshing found previously in this cluster. ALMA observations of the CO(1−0) emission reveal a large reservoir of molecular gas that traces the extended Hα emission in the direction of the cool core. We conclude either that the radio source and its cavities in the X-ray gas are nearly aligned along the line of sight, or that ram pressure induced by sloshing has significantly displaced the cool molecular gas feeding it, perhaps preempting the AGN feedback cycle. We argue that the sloshing near the core is likely subsonic, as expected, given the co-location of the Hα, CO(1−0), radio continuum, and stellar emission peaks and their proximity to the X-ray peak. Further, the X-ray emission from the core is strongly concentrated, as is the distribution of metals, indicating the cool core remains largely intact. Deeper Chandra observations will be crucial for definitively establishing the presence or lack of X-ray cavities, while X-ray micro-calorimetric observations from Athena could establish if the motion of the cold and warm gas is dominated by large-scale motions of the surrounding ICM.

Structural and Dynamical Analysis of the Quiescent Molecular Ridge in the Large Magellanic Cloud

We present a comparison of low-J 13CO and CS observations of four different regions in the LMC—the quiescent Molecular Ridge, 30 Doradus, N159, and N113, all at a resolution of ∼3 pc. The regions 30 Dor, N159, and N113 are actively forming massive stars, while the Molecular Ridge is forming almost no massive stars, despite its large reservoir of molecular gas and proximity to N159 and 30 Dor. We segment the emission from each region into hierarchical structures using dendrograms and analyze the sizes, masses, and line widths of these structures. We find that the Ridge has significantly lower kinetic energy at a given size scale and also lower surface densities than the other regions, resulting in higher virial parameters. This suggests that the Ridge is not forming massive stars as actively as the other regions because it has less dense gas and not because collapse is suppressed by excess kinetic energy. We also find that these physical conditions and energy balance vary significantly within the Ridge and that this variation appears only weakly correlated with distance from sites of massive-star formation such as R136 in 30 Dor, which is ∼1 kpc away. These variations also show only a weak correlation with local star formation activity within the clouds.

Robust computational approach to determine the safe mud weight window using well-log data from a large gas reservoir

A key parameter when drilling for gas and oil is to determine the safe mud weight window (SMWW) to ensure wellbore stability as part of a quantitative risk assessment (QRA). This study determines SMWW by predicting acceptable upper and lower limits of the bottom hole pressure window during over-balance drilling method. A novel machine learning method is developed to predict SMWW from ten well-log input variables subject to feature selection. 3389 data records from three South Pars gas field (Iran) wells include data from: uncorrected spectral gamma ray; potassium; thorium; uranium; photoelectric absorption factor; neutron porosity; bulk formation density; corrected gamma ray adjusted for uranium content; shear-wave velocity and compressional-wave velocity. Combinations of these well logs are tuned to provide predictions of the SMWW, measured in terms of subsurface pore and fracture pressures, using machine learning (ML) algorithms hybridized with optimizers. The ML algorithms assessed are multiple layer extreme learning machine (MELM) and least squares support vector machine (LSSVM), hybridized with genetic (GA) and particle swarm (PSO) optimizers. This new algorithm (MELM) incorporates special features that improve its prediction performance, speeds up its training, inhibits overfitting and involves less optimization in the model's construction. By combining MELM with PSO, its optimum control parameters are rapidly determined. The results reveal that the MELM-PSO combination provides the highest SMWW prediction accuracy of four models evaluated. For the testing subset MELM-PSO achieves high prediction performance of pore pressure (RMSE = 12.76 psi; R2 = 0.9948) and fracture pressure (RMSE = 15.71 psi; R2 = 0.9967). Furthermore, the model demonstrates that once trained with data from a few wells, it can be successfully applied to predict unseen data in other South Pars gas field wells. The findings of this study can provide a better understanding of how ML methods can be applied to accurately predict SMWW.

Formation mechanisms, potentials and exploration practices of large lithologic gas reservoirs in and around an intracratonic rift: Taking the Sinian—Cambrian of Sichuan Basin as an example

By examining structures, sediments, reservoirs and accumulation assemblages in the Deyang—Anyue rift and its surrounding area, four new understandings are obtained. First, during the initiation period of Deyang—Anyue rift, multiple groups of faults developed in the rift due to the effect of tensile force, bringing about multiple mound and shoal belts controlled by horsts in the second member of the Sinian Dengying Formation; in the development stage of the rift, the boundary faults of the rift controlled the development of mound and shoal belts at the platform margin in the fourth member of Dengying Formation; during the shrinkage period of the rift, platform margin grain shoals of the Cambrian Canglangpu Formation developed in the rift margin. Second, four sets of large-scale mound and shoal reservoirs in the second member of Dengying Formation, the fourth member of Dengying Formation, Canglangpu Formation and Longwangmiao Formation overlap with several sets of source rocks such as Qiongzhusi Formation source rocks and Dengying Formation argillaceous limestone or dolomite developed inside and outside the rift, forming good source-reservoir-cap rock combinations; the sealing of tight rock layers in the lateral and updip direction results in the formation model of large lithologic gas reservoirs of oil pool before gas, continuous charging and independent preservation of each gas reservoir. Third, six favorable exploration zones of large-scale lithologic gas reservoirs have been sorted out through comprehensive evaluation, namely, mound and shoal complex controlled by horsts in the northern part of the rift in the second member of Dengying Formation, isolated karst mound and shoal complex of the fourth member of Dengying Formation in the south of the rift, the superimposed area of multi-stage platform margin mounds and shoals of the second and fourth members of Dengying Formation and Canglangpu Formation in the north slope area, the platform margin mounds and shoals of the second and fourth members of Dengying Formation in the west side of the rift, the platform margin mound and shoal bodies of the fourth member of Dengying Formation in the south slope area, etc. Fourth, Well Pengtan-1 drilled on the mound and shoal complex controlled by horsts of the second member of Dengying Formation in the rift and Well Jiaotan-1 drilled on the platform margin mound and shoal complex of the North Slope have obtained high-yield gas flows in multiple target layers, marking the discovery of a new gas province with reserves of (2—3)×1012 m3. This has proved the huge exploration potential of large lithologic gas reservoir group related to intracratonic rift.

Structural characteristics and implications on oil/gas accumulation in north segment of the Longmenshan piedmont, northwestern Sichuan Basin, SW China

By integrating surface geology, seismic data, resistivity sections, and drilling data, the structural deformation characteristics of the frontier fault of thrust nappes were delineated in detail. The frontier fault of thrust nappes in northwest Scihuan Basin is a buried thrust fault with partial exposure in the Xiangshuichang—Jiangyou area, forming fault propagation folds in the hanging-wall and without presenting large-scale basin-ward displacement along the gypsum-salt layer of the Triassic Jialingjiang Formation to the Triassic Leikoupo Formation. The southwestern portion of the frontier fault of thrust nappes (southwest of Houba) forms fault bend folds with multiple ramps and flats, giving rise to the Zhongba anticline due to hanging-wall slip along the upper flat of the Jialingjiang Formation. In contrast, the northeastern portion of the frontier fault of thrust nappes (northeast of Houba) presents upward steepening geometry, leading to surface exposure of Cambrian in its hanging-wall. With the frontier fault of thrust nappes as the boundary between the Longmenshan Mountain and the Sichuan Basin, the imbricated structural belt in the hanging-wall thrusted strongly in the Indosinian orogeny and was reactivated in the Himalayan orogeny, while the piedmont buried structural belt in the footwall was formed in the Himalayan orogeny. In the footwall of the frontier fault of thrust nappes, the piedmont buried structural belt has good configuration of source rocks, reservoir rocks and cap rocks, presenting good potential to form large gas reservoirs. In comparison, the hanging-wall of the frontier fault of thrust nappes north of Chonghua has poor condition of oil/gas preservation due to the surface exposure of Triassic and deeper strata, while the fault blocks in the hanging-wall from Chonghua to Wudu, with Jurassic cover and thicker gypsum-salt layer of the Jialingjiang formation, has relative better oil/gas preservation conditions and thus potential of oil/gas accumulation. The frontier fault of thrust nappes is not only the boundary between the Longmenshan Mountain and the Sichuan Basin, but also the boundary of the oil/gas accumulation system in northwestern Sichuan Basin.

NOEMA observations support a recoiling black hole in 3C 186

3C 186 is a powerful radio-loud quasar (a quasi-stellar object) at the center of a cool-core cluster at z = 1.06. Previous studies have reported evidence for a projected spatial offset of ∼1″ between the isophotal center of the galaxy and the point-source quasi-stellar object (QSO) as well as a spectral shift of ∼2000 km s−1 between the narrow and broad line region of the system. In this work we report high-resolution molecular gas CO(4→3) observations of the system taken with the NOEMA interferometer. We clearly detect a large reservoir of molecular gas, MH2 ∼ 8 × 1010 M⊙, that is co-spatial with the host galaxy and likely associated with a rotating disk-like structure. We firmly confirm both the spatial offset of the galaxy’s gas reservoir with respect to the continuum emission of the QSO and the spectral offset with respect to the redshift of the broad line region. Our morphological and kinematical analysis confirms that the most likely scenario to explain the 3C 186 system is that the QSO is a kicked super-massive black hole (SMBH), which we believe may have resulted from a strong gravitational wave recoil as two SMBHs coalesced after the merger of their host galaxies.

Sinian hydrocarbon accumulation conditions and exploration potential at the northwest margin of the Yangtze region, China

Based on outcrop, drilling, logging and seismic data, the reservoir forming conditions, reservoir forming model and exploration potential of the ultra-deep Sinian Dengying Formation at the northwest margin of Yangtze craton region were examined. (1) This area is in craton rifting stage from Sinian to Early Cambrian, characterized by syn-sedimentary faults and rapid subsidence, significant sedimentary differences, and development of Dengying Formation platform margins on both sides of the rift. (2) The Sinian–Cambrian in this area has two sets of high-quality source rocks, Doushantuo Formation and Maidiping–Qiongzhusi Formation; of which, the latter has a thickness of 150–600 m and hydrocarbon generation intensity of (100–200)×108 m3/km2. (3) The mounds and shoals in the platform margin of Sinian Dengying Formation controlled by faults are thick and distributed in rows and zones; they are reformed by contemporaneous–quasi-contemporaneous and supergene karstification jointly, forming pore-type reservoirs with a thickness of 200–400 m. (4) The two sets of source rocks enter oil generation windows from Permian to Early Triassic, and the oil migrates a short distance to the lithologic traps of mounds and shoals to form a huge scale paleo-oil reservoir group; from Late Triassic to Jurassic, the oil in the paleo-oil reservoirs is cracked into gas, laying the foundation of present natural gas reservoirs. (5) The mound–shoal body at the platform margin of Dengying Formation and the two sets of high-quality source rocks combine into several types of favorable source-reservoir combinations, which, with the advantage of near-source and high-efficiency reservoir formation, and can form large lithologic gas reservoirs. The Mianyang–Jiange area is a potential large gas field with trillion cubic meters of reserves. According to seismic prediction, the Laoguanmiao structure in this area has the Deng-2 Member mound-shoal reservoir of about 1300 km2 making it a ultra-deep target worthy of exploration in the near future.

Geological risk characterization of traps and migration pathways for gas in an inverted salt basin

Inversion of the Lower Saxony Basin produced large-scale broad antiforms, hosting Germany's largest gas accumulations in stacked Permian and Triassic reservoir systems. Locally, however, several exploration and production wells failed in the upper Permian Zechstein-2-Carbonte (Ca2) due to missing reservoir (hiatus) or formation-water wet conditions despite of drilling the topmost position of the trap structure. Seismic interpretation indicates lateral shear of the structural topmost Ca2 reservoir by oblique inversion-related collision with the overlying rigid sandstone unit of the Triassic Bunter Formation (Fm.), triggering a halokinetic redistribution of Permian Zechstein (Z2-Z4) salt in between. Lateral shear was enabled along a thin Zechstein (Z1) salt decollement, underlying the Ca2 reservoir. This ultimately led to a full detachment of large intra-salt Ca2 allochthonous gas reservoirs (‘stringers’), leaving behind areas of Ca2 autochthonous absence (‘bald highs’). These kinematics triggered hydraulic connection and gas leakage from both, the ‘bald high’ margins of the autochthonous Ca2 and the Ca2 ‘stringers’ into the sandstone reservoir of the Bunter Fm. Methane-rich fluid inclusions in sub-horizontal tectonic veins and from within vertical stylolites sampled from cores of wells, which tested the Ca2 reservoir to be formation water-wet, proof gas leakage as a result of inversion. Identification criteria for gas leakage and reservoir presence have been established from this study to frame geological subsurface scenarios for pre-drill risk assessments of inverted settings. The generalized conclusion from this study is that the topmost position of structural highs in inverted salt basins may be actually exposed to the highest geological risk.

Experimental Study on Stimulation of Horizontal Wells Filled with Film-Coated Gravel in Deep-Sea Bottom-Water Gas Reservoirs.

Large gas reservoirs such as Yacheng and Lingshen have been discovered in the exploration of deep-sea gas reservoirs in the South China Sea in recent years. However, the existence of bottom-water and sufficient water energy in deep-sea gas reservoirs seriously affects the development effect of gas reservoirs. In order to effectively control water development of deep-seabed water gas reservoirs, it is proposed to fill water-resistant and breathable coated gravel outside horizontal wells for mining. Based on the preparation of modified coated gravel, this paper studies the resistance of coated gravel to friction damage, temperature damage, acid damage, pressure damage, and water-resistant performance of coated gravel. The results show that (1) the coated gravel coating has good resistance to friction erosion and acidification corrosion. The upper limit of temperature resistance is 250°, and the pressure resistance is 90.4 MPa. (2) The water resistance of the film-coated gravel packing layer decreases with the increase of packing layer permeability. (3) The film-coated gravel pack can reduce the water-phase flow ability of reservoirs, delay the bottom water ridging speed, increase the waterless gas development time of gas wells, and improve the final recovery of bottom-water gas reservoirs. The coated gravel has permeability and water resistance properties and can be used to voluntarily select water plugging in the coated gravel pack. This technology can develop a new technical idea for water control of deep-seabed water gas reservoirs.

Unveiling the main sequence to starburst transition region with a sample of intermediate redshift luminous infrared galaxies

ABSTRACT We present a CO(3−2) study of four systems composed of six (ultra) luminous infrared galaxies (U/LIRGs), located at 0.28 <z < 0.44, that straddle the transition region between regular star-forming galaxies and starbursts. These galaxies benefit from previous multiwavelength analysis allowing in depth exploration of an understudied population of U/LIRGs at a time when the universe is experiencing a rapid decline in star formation rate density. We detect CO(3−2) emission in four targets and these galaxies fall between the loci of regular star-forming galaxies and starbursts on the Kennicutt–Schmidtt relation. Compared to low luminosity LIRGs and high luminosity ULIRGs at similar redshifts, we find they all have similar molecular gas budgets with the difference in their star formation rates (SFR) driven by the star formation efficiency (SFE). This suggests that at these redshifts large molecular gas reservoirs must coincide with an increased SFE to transition a galaxy into the starburst regime. We studied the structure and kinematics and found our four detections are either interacting or have disturbed morphology which may be driving the SFE. One of the CO(3−2) non-detections has a strong continuum detection, and has been previously observed in H α, suggesting an unusual interstellar medium for a ULIRG. We conclude that our sample of transitioning U/LIRGs fill the gap between regular star-forming galaxies and starbursts, suggest a continuous change in SFE between these two populations and the increased SFE may be driven by morphology and differing stages of interaction.

Study on Hydrocarbon Accumulation Periods Based on Fluid Inclusions and Diagenetic Sequence of the Subsalt Carbonate Reservoirs in the Amu Darya Right Bank Block

In order to provide paleofluid evidence of hydrocarbon accumulation periods in the Amu Darya Right Bank Block, microexperiments and simulations related to the Middle-Upper Jurassic Callovian-Oxfordian carbonate reservoirs were performed. On the basis of petrographic observation, the diagenetic stages were divided by cathodoluminescence, and the entrapment stages of fluid inclusions were divided by laser Raman experiment and UV epifluorescence. The hydrocarbon generation (expulsion) curve and burial (thermal) history curve of source rocks were simulated by using real drilling data coupled with geochemical parameters of source rocks, such as total organic carbon (TOC) and vitrinite reflectance ( R o ). The above results were integrated with microthermometry of fluid inclusions by inference the timing of hydrocarbon migration into the carbonate reservoirs. The horizon-flattening technique was used to process the measured seismic profile and restore the structural evolution profile. Four diagenetic periods and three hydrocarbon accumulation periods were identified. (i) For Syntaxial stage, the fluid captured by the overgrowing cement around particles is mainly seawater; (ii) for (Early) Mesogenetic burial stage, the calcite cements began to capture hydrocarbon fluids and show yellow fluorescence under UV illumination; (iii) for (Late) Mesogenetic burial stage, two sets of cleavage fissures developed in massive calcite cements, and oil inclusions with green fluorescence were entrapped in the crystal; (iv) for Telogenetic burial stage, blue fluorescent inclusions along with hydrocarbon gas inclusions developed in dully luminescent calcite veins. Based on the accurate division of hydrocarbon migration and charging stages, combined with the structural evolution history of the traps, the hydrocarbon accumulation model was established. Because two of the three sets of source rocks are of marine origin, resulting in the lack of vitrinite in the kerogen of those source rocks, there may be some deviation between the measured value of R o and the real value. Some systematic errors may occur in the thermal history and hydrocarbon generation (expulsion) history of the two sets of source rocks. Due to the limitations of seismic horizon-flattening technique—such as the inability to accurately recover the inclined strata thickness and horizontal expansion of strata—the final shape of the evolution process of structural profile may also deviate from the real state in geological history. The accumulation model established in this study was based upon the fluid inclusion experiments, which can effectively characterize the forming process of large condensate gas reservoirs in the Amu Darya Right Bank Block and quantify the timing of hydrocarbon charging. However, the hydrocarbon migration and accumulation model does not take the oil-source correlation into account, but only the relationship between the mature state of source rocks and the timing of hydrocarbon charging into the reservoirs. Subsequent research needs to conduct refined oil-source correlation to reveal the relationship between gas, condensate, source rocks, and recently discovered crude oil and more strictly constrain and modify the accumulation model, so as to finally disclose the origin of the crude oil and oil reservoir forming process in the Amu Darya Right Bank Block, evaluate the future exploration potential, and point out the direction of various hydrocarbon resources (condensate gas and crude oil).

Reservoir Forming Conditions and Enrichment Law of Tight Sandstone Gas in Upper Paleozoic in Ordos Basin

With the popularization of natural gas and the requirement of environmental protection, the development and utilization of natural gas are particularly important. The status of natural gas in China’s oil and gas exploration and development is constantly improving, and the country pays more and more attention to the exploitation and utilization of natural gas. The tight sandstone of Upper Paleozoic in Ordos Basin has the characteristics of low porosity, low permeability, and large area of concealed gas reservoirs. With the increasing demand for natural gas in China, systematic analysis and research on the geological conditions and enrichment law of large area gas reservoirs of tight sandstone of Upper Paleozoic in Ordos Basin to guide exploration and production also pose a great challenge to the current level of natural gas exploration and development in China. In order to solve this problem, this paper takes the Upper Paleozoic tight sandstone in Hangjinqi area in the north of Ordos Basin as the research object, summarizes the natural gas accumulation conditions of tight sandstone according to the geological characteristics of Ordos Basin, determines the main control factors of gas accumulation by superposition method in combination with the distribution characteristics of natural gas, simulates the dynamic process of gas accumulation, establishes the gas accumulation model, analyzes the conditions of gas accumulation and enrichment by statistical methods, summarizes the accumulation and enrichment rules of natural gas, and predicts the favorable exploration areas for natural gas. The results show that the relationship between hydrocarbon source rocks, traps, migration and transportation, and other reservoir-forming conditions and the distribution characteristics of natural gas is summarized, and the main controlling factors of reservoir-forming are determined by superposition method. We simulate the evolution process of faults, strata, oil, gas, and water flows in the study area and establish the dynamic reservoir-forming process of natural gas. By combining the reserve data with the reservoir source distance, reservoir facies type, and gas filling times, the reservoir-forming and enrichment conditions are studied by statistical analysis method, and the reservoir-forming and enrichment rules are summarized by combining the main controlling factors. The enrichment of natural gas is controlled by the development degree of source rocks, favorable tight sandstone combination, confluence channel, and local structure. According to the law of accumulation and enrichment, the division standards of favorable areas in different horizons are established, and the favorable exploration areas in the study area are predicted. The results provide theoretical data support for natural gas exploration in the study area.

Characteristics of hydrocarbon migration and accumulation in the Lower Jurassic reservoirs in the Tugerming area of the eastern Kuqa Depression, Tarim Basin

The hydrocarbon migration and accumulation process in the Kuqa Depression are complex. In recent years, the Jurassic strata have recorded hydrocarbon discoveries in the Tugerming area of the eastern Kuqa Depression. However, it is a new exploration area, and the characteristics of hydrocarbon accumulation are unclear, restricting oil and gas exploration and development. In our study, the deeply-buried and shallow-buried Jurassic strata in the Tugerming area are the research objects. Quantitative fluorescence technique and the observation of fluid inclusions are implemented on the selected samples of Jurassic reservoirs to determine the stages and times of hydrocarbon charging. Gas chromatography-mass spectrometry analysis for saturated hydrocarbons is used to identify the origin of organic matter in oils. Combined analytical data with the reconstructed histories of burial, thermal evolution and tectonic evolution, allow simulating the hydrocarbon migration and accumulation process in the Tugerming area. The results show that two categories of hydrocarbon accumulation mechanisms (Burnham and Sweeney, 1989). The first type is acted as the type of “self-generation and self-storage and late accumulation”. Three stages of hydrocarbon charging have occurred in the deeply-buried Lower Jurassic reservoirs, represented by two wells (Tudong 2 and Tuge 4). The charging time is the early-middle stage of Miocene Kangcun Formation (18–9 Ma), forming the yellow or yellow-green fluorescent oil inclusions and paleo-oil reservoir. During the sedimentary period of the late Kangcun Formation to the early Kuqa Formation (9–5 Ma), the reservoir experienced massive condensate oil migration, corresponding to blue-white fluorescent oil inclusions. Large-scale natural gas was expelled into the reservoirs since the deposition stage of Kuqa Formation (5–0 Ma), and then accumulated in the trap controlled by the late Himalayan movement, forming the current gas reservoir. Moreover, the hydrocarbons are generated from Jurassic coal measure source rocks in the Well Tudong 2 based on the characteristics of biomarkers (Burruss et al., 1991). The second type is characterized by “lower source rock and upper reservoir and accumulation in the late or recent period”, developed in the shallow-buried Lower Jurassic reservoirs containing two wells (Tuxi 1 and Mingnan 1). In this case, there are two stages of crude oil charging: mature oil charging in the early-medium Miocene (18–12 Ma) and condensate oil migration in the late Miocene (8–5 Ma). The strata suffered denudation due to the stratigraphic uplifting in the orogenic movement, resulting in the leakage of the early oil reservoir. In both categories, it suggests that the Lower Jurassic reservoirs in the Tugerming area have the favorable potential for oil and gas exploration. The understanding of hydrocarbon migration and accumulation in the Lower Jurassic reservoirs in the Kuqa Depression of Tarim Basin can contribute to the knowledge of the potential to form large oil and gas reservoirs Tugerming Area, and reservoirs of basins with a similar tectonic settings, such as the Jungar Basin.

Hydrocarbon Accumulation Process and Mechanism in the Lower Jurassic Reservoir in the Eastern Kuqa Depression, Tarim Basin, Northwest China: A Case Study of Well Tudong 2 in the Tugerming Area.

The Lower Jurassic reservoir has recently made a significant breakthrough in petroleum exploration in the Tugerming area of the eastern Kuqa depression, Tarim Basin, northwest China. Therefore, it is significantly essential to simulate the hydrocarbon charging process and analyze the accumulation mechanism in the study area. In our study, we mainly combine three geochemical techniques to identify the origin of crude oils and fluid inclusion characteristics in the target well (Tudong 2), including gas chromatography–mass spectrometry (GC–MS), comprehensive two-dimensional GC-time-of-flight MS (GC × GC–TOFMS), fluid inclusion analysis, and quantitative fluorescence technique. Combined comprehensive experiments with a set of burial history and tectonic thermal evolution history realize the reconstruction of the hydrocarbon charging process in the Yangxia formation. The results show that Jurassic coal-bearing source rocks are primary hydrocarbon sources, and there are three hydrocarbon charging events in the Lower Jurassic reservoir. First, the mature oils have expelled into the reservoir during the early-middle period (15–10 Ma) of the Miocene Kangcun Formation, forming yellow fluorescent oil inclusions and most of the quantitative grain fluorescence (QGF) indexes exceed 4. Second, numerous condensate oils have charged into the reservoir in the period of late Miocene Kangcun Formation-early Kuqa Formation (9–6 Ma), accompanied by blue-white oil inclusions and QGF on extract intensity greater than 4. Finally, vast natural gas has accumulated in the reservoir since Kuqa Formation (5–0 Ma), resulting in the adjustment of ancient reservoir and residual bitumen. Interestingly, we find that the hydrocarbon accumulation mechanism is characterized by self-generation and self-storage in the Jurassic Yangxia Formation, and the study area has prominent characteristics of late accumulation. Thus, the evidence obtained from our analysis suggests favorable geological conditions for the formation of a large oil and gas reservoir in the eastern Kuqa Depression.

Non-circular flows in HIghMass galaxies in a test of the late accretion hypothesis

ABSTRACT We present H α velocity maps for the HIghMass galaxies UGC 7899, UGC 8475, UGC 9037, and UGC 9334, obtained with the SITELLE Imaging Fourier Transform Spectrometer on the Canada–France–Hawaii Telescope, to search for kinematic signatures of late gas accretion to explain their large atomic gas reservoirs. The maps for UGC 7899, UGC 9037, and UGC 9334 are amenable to disc-wide radial flow searches with the DiskFit algorithm, and those for UGC 7899 and UGC 9037 are also amenable to inner-disc kinematic analyses. We find no evidence for outer disc radial flows down to $\bar{V}_{\mathrm r} \sim 20 \ \mathrm{km\, s}^{-1}$ in UGC 9037 and UGC 9334, but hints of such flows in UGC 7899. Conversely, we find clear signatures of inner (r ≲ 5 kpc) non-circularities in UGC 7899 and UGC 9037 that can be modelled as either bisymmetric (which could be produced by a bar) or radial flows. Comparing these models to the structure implied by photometric disc–bulge–bar decompositions, we favour inner radial flows in UGC 7899 and an inner bar in UGC 9037. With hints of outer disc radial flows and an outer disc warp, UGC 7899 is the best candidate for late accretion among the galaxies examined, but additional modelling is required to disentangle potential degeneracies between these signatures in H i and H α velocity maps. Our search provides only weak constraints on hot-mode accretion models that could explain the unusually high H i content of HIghMass galaxies.

APEX-SEPIA660 Early Science: gas at densities above 107 cm−3 towards OMC-1

Context. The star-formation rates and stellar densities found in young massive clusters suggest that these stellar systems originate from gas at densities of n(H2) > 106 cm−3. Until today, however, the physical characterization of this ultra high density material remains largely unconstrained in observations. Aims. We investigate the density properties of the star-forming gas in the OMC-1 region located in the vicinity of the Orion Nebula Cluster (ONC). Methods. We mapped the molecular emission at 652 GHz in OMC-1 as part of the APEX-SEPIA660 Early Science. Results. We detect bright and extended N2H+ (J = 7–6) line emission along the entire OMC-1 region. Comparisons with previous ALMA data of the (J = 1–0) transition and radiative transfer models indicate that the line intensities observed in this N2H+ (7–6) line are produced by large mass reservoirs of gas at densities n(H2) > 107 cm−3. Conclusions. The first detection of this N2H+ (7–6) line at parsec-scales demonstrates the extreme density conditions of the star-forming gas in young massive clusters such as the ONC. Our results highlight the unique combination of sensitivity and mapping capabilities of the new SEPIA660 receiver for the study of the ISM properties at high frequencies.

Sedimentary geochemical records and their depositional environmental significance during Cambrian-Ordovician transition in well YD 2, eastern Tarim Basin, NW China

In the eastern part of Tarim Basin, thick carbonate rocks were developed in the Upper Cambrian, while thick grey-black mudstone in the Lower Ordovician. Due to the complex tectonic and geological conditions in the basin, there are still many uncertainties in the study of the sedimentary environment, post-diagenesis and hydrocarbon accumulation conditions during the Cambrian-Ordovician transition. In this paper, 24 drilling core samples of the key strata of the Upper Cambrian-Lower Ordovician in the well YD2 were selected as the main research objects. Their mineral composition, major and trace elements (including REE) were analyzed. The results showed that the proportion of carbonate-siliciclastic sources in the rocks was controlled by the sedimentary environment, especially seawater depth. The oxic-suboxic sedimentary environment is indicated by the redox-sensitive elements and other trace elements (especially REE). This is consistent with the mineralogical features of the samples and the relatively low TOC. The elements Mn, Cr, V, Ni, on the whole, show a trend of first increasing, then decreasing, and repeated occurrence, indicating the sea level fluctuation cycle. In addition, according to the Ba/Sr and Mn/Sr ratios, the upper Cambrian carbonate rocks were normal marine sediments with weak post-diagenesis. Quartz, dolomite and calcite veins filling the fissures of the samples may have formed in the stage of intense uplift denudation, which was unfavourable to the formation of large oil and gas reservoirs.