Reservoir Accumulation Research Articles

Effect of river damming on nutrient transport and transformation and its countermeasures

In recent decades, damming has become one of the most important anthropogenic activities for river regulation, and reservoirs have become hotspots for biogeochemical cycling. The construction of dams changes riverine hydrological conditions and alters the physical, chemical, and biological characteristics of rivers, eventually leading to significant variations in nutrient cycling. This review mainly explores the effects of river damming on nutrient transport and transformation, including i) nutrient (N, P, Si, and C) retention in reservoirs, ii) greenhouse gas (GHG) emissions, and iii) interactions between the nutrient stoichiometry ratio and the health of the reservoir ecosystem. The important drivers of nutrient transport and transformation, such as river connectivity, hydraulic residence time, hydropower development mode, microbial community variation, and anthropogenic pollution, have also been discussed. In addition, strategies to recover from the negative effects of damming on aquatic ecosystems are summarized and analyzed. To provide theoretical and scientific support for the ecological and environmental preservation of river-reservoir systems, future studies should focus on nutrient accumulation and GHG emissions in cascade reservoirs.

The effect of reservoir expansion from underground karst cave to surface reservoir on water quality in southwestern China.

Due to the excessive exploitation of traditional energy sources, the attention paid to water energy has increased in recent years. As an important means to effectively utilize water energy, reservoirs play an important role in drinking water, irrigation, flood control, and drought resistance. However, utilizing reservoirs often led to water quality issues resulting from the interaction of nutrients and hydrological conditions, especially due to the special structure of karst areas. Because of the change of hydrological conditions by the effect of dam construction, the dynamic of water quality will be more obvious in karst areas with a fast exchange of water and contaminants between underground and surface streams. In the present study, the change in water quality of a karst reservoir, the Muzhu Reservoir in the Houzhai Catchment, was studied. Long-term monitored datasets (1981-2002) and water quality datasets of more recent years were used to assess the effect on the water quality of reservoir expansion from the underground reservoir to the surface reservoir in a karst area. Long-term series datasets had shown that the hydro-chemistry type had been changed from HCO3-·SO42--Ca2+·Mg2+ type to HCO3--Ca2+ type in the short term after the reservoir's expansion. The chemical components of water originating from a rock background reduced markedly after the reservoir's expansion, whereas the content of the anthropogenic contribution in the water decreased after the expansion, except in April and May. Isotopic characteristics showed that δ15N-NO3- and δ18O-NO3- values were positively correlated before and after the reservoir expansion, but the slope of the linear regression before the expansion was 0.34, while the slope of the linear regression before the expansion was close to 0.7. This indicated that although denitrification and assimilation may occur simultaneously after the reservoir's expansion, the role of denitrification on nitrate removal decreased, which resulted in nitrate accumulation in the karst reservoir. The results highlighted that nitrate accumulation in karst reservoirs should be monitored to decrease nitrate concentration in the future.

Hydrocarbon accumulation mechanism in the far-source reservoirs of Dongsha Uplift of the Pearl River Mouth Basin, northern South China Sea

There are a lot of exploration examples of far-source reservoirs, but the description of the connotation and accumulation mechanism is very rare. The Dongsha Uplift is one of the major oil-producing areas in the eastern Pearl River Mouth Basin in the northern South China Sea. The uplift lacks generally hydrocarbon source rock and is far from the generative kitchen, making it a typical far-source oil and gas reservoir. The accumulation mechanism of crude oil in the reservoirs are still unknown. By systematically comparing biomarkers, nitrogenous compounds and isotopic characteristics of the crude oil and source rocks with those in the neighboring depressions, and combining with fracture, sand body, unconformity migration conduits and barrier conditions, this paper simulates oil and gas migration paths and formation time with Pathway and IES software, and analyzes oil and gas accumulation process from a dynamic perspective. We found that the deep lacustrine hydrocarbon source rocks of the Wenchang Formation in the H26 Sag of the Huizhou Depression reached peak oil production at the end of the Hanjiang Formation deposition. Influenced by the strong tectonic activities at the final deposition of the Hanjiang and Yuehai formations, two episodes of hydrocarbon charging occurred at the top of the Dongsha Uplift. The accumulation of hydrocarbons far from the source rocks in the Dongsha uplift is mainly controlled by the efficient carrier system at hydrocarbon generation period. The oil is mainly accumulated in L4-1 and L11-1 oil fields through the long-distance stepped migration mode. The spatial and temporal relationship between hydrocarbon generation of source rock and episode of fault activity are mainly responsible for accumulation in the far-source reservoirs of the Dongsha Uplift.

Volcanic reservoirs lacking a paleoweathering crust in Carboniferous deposits of the Junggar Basin, western China: Characteristics, controlling factors and hydrocarbon potential

The Junggar Basin is the basin with the richest exploration results of volcanic reservoirs among petroliferous basins in China. The oil test data and production data show there also exhibit good hydrocarbon displays in the Kebai Fault Zone where the paleo-weathering crust is not developed. In this study, we utilized core data and thin sections to investigate volcanic reservoir rocks lacking paleo-crusts of weathering in Carboniferous deposits in the western Junggar Basin, Northwestern China. The studied interval is dominated by seven types of volcanic rocks, two types of metamorphic rocks and three types of sedimentary rocks, which are grouped into six lithofacies. From northeast (NE) to southwest (SW) of study area, the dominant rock type changes from sedimentary rock, to pyroclastic rocks, tuffites, and then to welded pyroclastic rock, lavas, respectively volcanic rocks. And the corresponding dominant lithofacies changes from delta facies, to volcanic-sedimentary facies, explosive facies, and then to sub-volcanic facies, and effusive facies. Fault-induced fractures and dissolved pores are the dominant reservoir space types. Faults and lithology jointly played a significant role in controlling the macro-distribution, physical properties, and hydrocarbon accumulation in volcanic reservoirs. The geological setting controlled the influences of primary and secondary processes. This study also reveals that the volcanic lavas with high-angle fractures of effusive facies located in SW of the study area are favorable lithofacies for hydrocarbon accumulation. This study highlights the differences in volcanic reservoir rocks with and without paleo-crusts of weathering in terms of reservoir spaces, controlling factors and hydrocarbon accumulation. This study also reveals that volcanic reservoir lacking paleo-crust of weathering can have a high hydrocarbon potential.

Effectiveness of Oil Filling in Tight Sandstone Reservoirs of Yancheng Formation in Ordos Basin

The Ordos basin is one of the important oil-bearing basins in China, with abundant tight sandstone oil resources, wide distribution, and large thickness. It is the most realistic field of unconventional oil and gas exploration in China, and it is also an important oil and gas replacement resource at present. Oil accumulation in tight reservoirs is obviously different from conventional oil and gas accumulation, and the key lies in studying the effectiveness of oil filling in tight reservoirs. In order to solve this problem, this paper takes the tight sandstone reservoir of Member 7 of the Yancheng Formation in Ordos Basin as the research object, introduces the geological characteristics of this area after sorting out the previous research results, carries out the physical simulation experiment of oil filling, studies the relationship between the filling pressure and the lower limit of the filling throat, analyzes the effective accumulation space of the tight sandstone reservoir, defines the oil filling mechanism of the tight sandstone reservoir, and discusses the effectiveness and reservoir-forming effect of oil filling in different types of reservoirs. The results show that the rock types of tight sandstone reservoirs in the 7th member of the Yancheng Formation are mainly lithic feldspathic sandstone and feldspathic lithic sandstone, and the reservoirs have experienced strong compaction and carbonate cementation. The late iron-bearing carbonate cementation has further strengthened the degree of reservoir densification, and the reservoirs have been densified at the time of large-scale oil and gas filling. Through the simulation experiment of oil filling with natural cores with different physical properties, the relationship model between filling pressure and effective accumulation space of different types of tight sandstone reservoirs in Member 7 of the Yancheng Formation is established. With the change in filling pressure, the change trend and range of effective accumulation space of different types of tight sandstone reservoirs are obviously different. According to the relationship model between the filling pressure and the lower limit of the effective filling pore throat, the oil filling effectiveness of different types of tight sandstone reservoirs in the 7th member of the Yancheng Formation is determined. Class I and class II1 reservoirs are effective reservoirs for oil filling of tight sandstone reservoirs, which constitute the main oil-bearing section of tight sandstone reservoirs in member 7 of the Yancheng formation, while class II2 reservoirs are poor tight reservoirs, and class III reservoirs are basically oil-free, which are noneffective reservoirs for oil filling of tight sandstone reservoirs. The results provide theoretical data support for the next step of oil exploration and exploitation in tight sandstone reservoirs.

Accumulation mechanism and model of coal measure derived oil and gas in the deep reservoir of Huanghua Depression, Bohai Bay Basin, China

Hydrocarbon accumulation in coal measure strata is consist of several types which includes intra-source accumulation and adjusted accumulation, particularly in basins that had experienced multi-stage tectonic activities. The mechanism governing this accumulation is extremely complex. Using the Paleozoic petroleum system in the Huanghua Depression as an example, the mechanism of hydrocarbon accumulation in deep reservoirs can be revealed by means of core observation, geochemistry testing of coal measure source rock, thin section observation, fluid inclusions testing, and profile analysis of typical reservoirs, combined with the burial and thermal history of strata. Based on the regional tectonic, the mechanism of hydrocarbon accumulation in coal measures is defined and the accumulation model was established. The results show that the Paleozoic petroleum system in the Huanghua Depression has three different types of source-reservoir-cap assemblages: under-source, intra-source, and above-source assemblage. Regional tectonic evolution controlled the hydrocarbon generation process of source rocks, showing the hydrocarbon generation characteristics of “oil in the early stage and gas in the late stage.” The Middle Cretaceous is the first of two periods of hydrocarbon accumulation, during which stage occurred in low-maturity oil and a small amount of natural gas were mainly stored in intergranular pores and dissolution pores. The tectonic uplift in the Late Cretaceous caused the destruction of the ancient reservoir. The second accumulation stage occurred in the Middle Paleogene to the present. A large amount of natural gas and high maturity oil were mostly stored in dissolution pores and structural fractures. The hydrocarbon accumulation in Huanghua Depression was simultaneously regulated by reservoir reformation, tectonic activities, and source rock maturation. Based on the foregoing understanding, the hydrocarbon accumulation model of “oil and gas transfer station” in coal measures is put forward, that is, oil and gas are first accumulated in intra-source or near-source reservoirs, and are then adjusted from this “oil and gas transfer station” to other areas as a result of tectonic activity.

Helium enrichment mechanism of helium-rich gas reservoirs in central and western China: Degassing and accumulation from ancient groundwater

Helium, a coordinated resource in natural gas, is an essential and strategic resource, but research on its enrichment mechanism is relatively weak. This study investigated helium enrichment in major reservoirs in China using geochemical methods to address the challenging topic of how helium accumulates in helium-rich gas reservoirs. It concludes that helium is enriched in a few reservoirs which capture additional helium exsolution from ancient groundwater. Released dissolved helium from ancient groundwater into the reservoir is the main mechanism of helium accumulation in helium-rich gas reservoirs. This was the first time that the concept of “multiple sources of helium supply and primary sources of helium enrichment” was applied to guide the formation of helium-rich gas reservoirs. Moreover, the main sources of helium comes from the radioactive decay of uranium (U) and thorium (Th) in hydrocarbon rocks, reservoirs, and the decay of U and Th dissolved in groundwater from other rocks. When dissolved helium in ancient groundwater crosses free gas or gas reservoirs, the partial pressure of helium in the water is significantly higher than the partial pressure of helium in the free gas or gas reservoirs, proving Henry's Law which states that the helium in the water is almost completely exsolution into the gas reservoir, forming a helium-rich gas reservoir.

THE ROLE OF CLAY MINERALS IN HYDROCARBON GENERATION, MIGRATION AND ACCUMULATION

This study focuses on the understanding of the role of clay minerals in hydrocarbon generation, migration and accumulation. Clay minerals are involved in the formation of kerogen, catalytic cracking of kerogen into petroleum, migration of crude oil, and the continued change to hydrocarbon composition in petroleum reservoirs. In kerogen formation, clay minerals act as catalysts and sorbents to immobilize organic matter through ligand exchange, hydrophobic interactions and cation bridges by the mechanisms of Maillard reactions, polyphenol theory, selective preservation and adsorptive protection. Clay minerals also serve as catalysts in acid-catalyzed cracking of kerogen into petroleum hydrocarbon through Lewis and Bronsted acid sites on the clay surfaces. The amount and type of clay mineral affect the composition of the petroleum. Bronsted acidity of clay minerals is affected by the presence and state of interlayer water, and displacement of this water is a probable driver in crude oil migration from source rocks. During crude oil migration and accumulation in reservoirs, the composition of petroleum is continually modified by interaction with clay minerals. Clay minerals remain the most difficult of all earth materials to study and characterize. Their sheet structure results in features that can only be resolved at the sub-micron scale. They are also subtly variable in chemical composition (Fe, Mg, K, Al, et cetera) and can be confused with each other and other silicates. The recent innovative analytical tools and modern analysis techniques, example., micro- and nano-X-ray Computed Tomography (XCT), QEMSCAN (Automated Mineralogy and Petrography), FIB/SEM (Focused Ion Beam/Scanning Electron Microscope), EDS (Energy-dispersive X-ray spectroscopy), etc., have the capability of quantitative and qualitative characterizing nano-pore features and mineralogy of fine grained shale rocks, which create new era of studying clay minerals for facilitating unconventional (shale) reservoir exploration. Even though there have been reports about the application of clay minerals in the oil and gas exploration, surprisingly little work has been documented. Keyword: clay mineral, organic matter, generation, vitrinite, kerogen, montmorillonite.

Characterization of ultra-low permeability tight sandstone reservoir properties and criteria for hydrocarbon accumulation in Chang 6 member, Huaqing area, Ordos basin

How to accurately predict hydrocarbon enrichment and high-yield areas in ultra-low permeability sandstone reservoirs is a hot spot in petroleum geology. Taking the Chang 6 Member of the Yanchang Formation in the Huaqing Area, Ordos Basin as an example, the quantitative characterization and classification criteria of ultra-low permeability sandstone reservoir properties are systematically studied. The research results show that the seismic reflection characteristics of the Chang 6 Member show multi-stage wedge progradation characteristics, reflecting the sedimentary characteristics of the end of the delta front. The maturity of the minerals in the Chang 6 Member sandstones in the study area is very low, it indicates a low degree of transformation of sediments. Three types of sand bodies, namely sandy debris flow, turbidite, and slump rock sand bodies, were identified in the Chang 6 Member of the Yanchang Formation. The pore types of the Chang 6 Member in the study area include intergranular pores, dissolution pores (feldspar, detrital dissolution pores), and intercrystalline pores. Among them, the intergranular pores and feldspar dissolution pores are the most important storage spaces. The study also found that the test oil production is highly correlated with the cumulative thickness of the massive sandstone segments. Therefore, massive sandstone (sand debris flow) is the main contributor to oil well production, and its distribution directly controls the accumulation of oil reservoirs and the high production of oil wells. In addition, geological parameters affecting reservoir enrichment evaluation were analyzed, which include permeability, oil layer thickness, resistivity, and reservoir penetration rate. Finally, the evaluation criteria for hydrocarbon enrichment in the Chang 6 Member were formulated by the analysis of dominant facies, lithologic assemblages and electrical properties.

Major controlling factors and hydrocarbon accumulation models of large-scale lithologic reservoirs in shallow strata around the Bozhong sag, Bohai Bay Basin, China

Based on the practice of oil and gas exploration and the analysis of shallow lithologic reservoirs, combined with the allocation relationship and enrichment law of oil and gas accumulation factors, main controlling factors and models of hydrocarbon accumulation of large lithologic reservoirs in shallow strata around the Bozhong sag are summarized, and favorable exploration areas are proposed. The coupling of the four factors of “ridge-fault-sand-zone” is crucial for the hydrocarbon enrichment in the shallow lithologic reservoirs. The convergence intensity of deep convergence ridges is the basis for shallow oil and gas enrichment, the activity intensity of large fault cutting ridges and the thickness of cap rocks control the vertical migration ability of oil and gas, the coupling degree of large sand bodies and fault cutting ridges control large-scale oil and gas filling, the fault sealing ability of structural stress concentration zones affects the enrichment degree of lithologic oil and gas reservoirs. Three enrichment models including uplift convergence type, steep slope sand convergence type and depression uplift convergence type are established through the case study of lithologic reservoirs in shallow strata around the Bozhong sag.

Research on Micro-Pore Structure and 3D Visual Characterization of Inter-Salt Shale Based on X-CT Imaging Digital Core Technology

Pore structure is the key factor affecting reservoir accumulation and enrichment behavior. Due to the complex mineral components and pore structure of shale oil reservoirs and strong heterogeneity, it is necessary to explore the micro-pore structure characteristics of inter-salt shale. In this study, in order to qualitatively and quantitatively analyze the pore structure of inter-salt shale reservoirs, as well as evaluate the mineral composition and its spatial distribution characteristics, three shale samples from the 10th cyclothem of the Eq3 (Eq34–10 cyclothem) inter-salt shale were selected to acquire 2D and 3D grayscale images by modular automated processing system (MAPS) and X-ray micro-computed tomography (Micro-CT), respectively. The color map of the inlaid characteristics of mineral aggregates was established by Quantitative Evaluation of Minerals by Scanning Electron Microscopy (QEMSCAN), and different mineral types in the grayscale image were determined. After that, the digital core technology was used to reconstruct the core in 3D, and the maximum sphere method was used to extract the pore network model, so as to realize the quantification of micron pore throats and the 3D visualization of inter-shale samples. Meanwhile, in order to compare the fractal characteristics of the pores of the samples, the two-dimensional and three-dimensional fractal dimensions of the three cores were calculated by combining the digital core technique with fractal theory. The study yielded several notable results: the pore structure of inter-salt shale reservoirs is complex and multi-scale, and the CT scanning digital core technology can effectively realize 3D visualization of rock microstructure without damage. The pore types of rock samples are mainly intergranular pores, interparticle pores, and dissolved pores, and the minerals are mainly dolomite, calcite, and glauberite. The micron pore throat radius of the rock sample is 0.5–13.9 μm, the distribution of coordination number is mainly in the range of 1–4, and the shape of the pore throat is mainly triangular and square. The pore space of inter-salt shale has suitable fractal characteristics, and the three-dimensional fractal dimension of the three cores is in the range of 2.41–2.49. In sum, this work used digital core technology to study the microscopic pore structure of inter-salt shale oil, establishing a basis for further understanding of the seepage characteristics and exploration and development of shale oil.

Two-Stage Differential Hydrocarbon Enrichment Mode of Maokou Formation in Southeastern Sichuan Basin, Southwestern China

Abstract Sichuan Basin is one of the most potential areas for natural gas exploration and development in China. The Maokou Formation in the basin is one of the important gas-bearing layers in southeastern Sichuan. In recent years, several exploration wells have obtained industrial gas flow in the first member of the Middle Permian Maokou Formation (hereinafter referred to as the Permian Mao-1 member of Maokou Formation), revealing that it may become a new field of oil and gas exploration in Sichuan Basin. Drilling and field survey results show that the shale of Maokou Formation in southeastern Sichuan contains eyeball-shaped limestone. Early studies suggest that the Permian Mao-1 member of Maokou Formation in Sichuan Basin is a set of high-quality carbonate source rocks, but ignoring its oil and gas exploration potential as an unconventional shale reservoir similar to the shale. The enrichment regularity of unconventional natural gas has not been studied from the perspective of source-internal accumulation. And there is a lack of analysis of oil and gas enrichment mode. In this study, we took the Permian Mao-1 member of Maokou Formation in southeastern Sichuan as the target layer. Through macroscopic outcrop observation and geochemical analysis and based on unconventional oil and gas enrichment theory, we carried out a study on natural gas enrichment mode of eyeball-shaped limestone of the Permian Mao-1 member of Maokou Formation in Sichuan Basin. The results show that the hydrocarbon enrichment pattern of the Maokou Formation in southeastern Sichuan is different from the accumulation and occurrence process of common unconventional shale gas reservoirs and conventional carbonate reservoirs. It is a special new hydrocarbon accumulation mode between the above two. According to the difference in the charging time of the hydrocarbon, the background of the reservoiring dynamics, and the occurrence state of oil and gas, we divide the two-stage differential enrichment mode of oil and gas, that is, “early intralayer near-source enrichment” and “late interlayer pressure relief adjustment.”

Controlling Effect of Wave-Dominated Delta Sedimentary Facies on Unconventional Reservoirs: A Case Study of Pinghu Tectonic Belt in Xihu Sag, East China Sea Basin

In order to find the “sweet spots” of unconventional oil and gas from the Pinghu Formation in the Pinghu Tectonic Belt, we are committed to clarifying the development and distribution rules of coal-measure source rocks and tight reservoirs as well as the controlling factors. Using 3D seismic and logging data, combined with logging constraints of target lithology and pyrolysis experiments of source rock, "source and reservoir" research of uncoventional oil and gas was carried out in the Pinghu Tectonic Belt. The results show that two stages of regression and one stage of transgression occurred in the Pinghu Formation, resulting in river-controlled and wave-controlled delta-neritic facies dominated by sedimentary facies, source rocks developed in interdistributary bay and swamp microfacies, and tight sandstones in point bar and distributary channel microfacies are developed. The accumulation of coalbed methane and shale gas is controlled by the sedimentary facies and the degree of thermal evolution under the factors of burial depth. The accumulation of tight sandstone is closely related to the dominantly sedimentary facies and diagenetic modification of physical properties and faults. It is concluded that the structural type, type and distribution of faults, and depositional environment of the Pinghu Tectonic Belt in the Xihu Sag are the key factors controlling the development and accumulation of coal-measure source rocks and tight sandstone reservoirs. This understanding provides a clear direction for the deep exploration of unconventional oil and gas in the sag and provides a reference for finding reservoir “sweet spots.”

Wettability of Tight Sandstone Reservoir and Its Impacts on the Oil Migration and Accumulation: A Case Study of Shahejie Formation in Dongying Depression, Bohai Bay Basin

The migration and accumulation of oil in tight sandstone reservoirs are mainly controlled by capillary force. Due to the small pore radius and complex pore structure of tight sandstone reservoirs, the capillary force is very sensitive to wettability, so wettability significantly affects oil migration and accumulation. However, the study of oil migration and accumulation in tight sandstone reservoirs often needs to combine multiple methods, the process is complex, and the research methods of wettability are not uniform, so the mechanism of wettability affecting oil migration and accumulation is not clear. Taking the tight sandstone of the Shahejie Formation in the Dongying sag, Bohai Bay Basin, as the research object, the wettability characteristics of a tight sandstone reservoir and their influence on oil migration and accumulation were analyzed by means of a pore permeability test, XRD analysis, micro-CT experiment, contact angle tests, spontaneous imbibition experiments, and physical simulation experiments on oil migration and accumulation. The results show that the reservoir is of the water-wet type, and its wettability is affected by the mineral composition. Wettability in turn affects the spontaneous imbibition characteristics by controlling the capillary force. Oil migration in tight sandstone reservoirs is characterized by non-Darcy flow, the oil is in the non-wetting phase and subject to capillary resistance. The key parameters to describe the oil migration and accumulation characteristics include the kickoff pressure gradient, the critical pressure gradient, and ultimate oil saturation. Wettability affects oil migration characteristics by controlling the capillary force. The more oil-wet the reservoir is, the more favourable it is to oil migration and oil accumulation and therefore the higher the reservoir’s ultimate oil saturation is.

LA‐ICP‐MS U–Pb dating of calcite cement in Upper Triassic tight‐gas sandstone reservoirs, western Sichuan Basin, SW China

AbstractThe absolute timing of carbonate cement formations can provide insights into porosity evolution and hydrocarbon accumulation in reservoirs. In this study, scanning electron microscopy‐based cathodoluminescence (SEM‐CL), quantitative mineralogical analysis and laser ablation inductively coupled plasma‐mass spectrometry U–Pb dating were performed on calcite cements from sandstones in the fourth member of the Upper Triassic Xujiahe Formation in the western Sichuan Basin. Two samples yielded U–Pb ages of 149.5 ± 31.1 Ma and 190.0 ± 180.5 Ma (2σ), suggesting late diagenetic precipitation of calcite cements. The backscattered electron and SEM‐CL images indicated that the cements precipitated after feldspar dissolution. Combined with palaeo‐temperature evolution, these results suggest that the timing of calcite cement precipitation occurred between the Late Jurassic to Early Cretaceous. Rapid accumulation of sediment in the western basin in response to the Yanshanian Orogeny may have caused stratum overpressure, resulting in geofluid activity and calcite precipitation in the Xu4 sandstone.

Quantitative Analysis of Cenozoic Extension in the Qiongdongnan Basin, South China Sea: Insight on Tectonic Control for Hydrocarbon Reservoir Accumulation and Formation

Cenozoic extension rates were calculated based on 20 seismic profiles across the Qiongdongnan Basin, South China Sea. The results confirmed that the Cenozoic rifting in the Qiongdongnan Basin exhibited multistage extension and spatiotemporal variation. In terms of the N–S striking seismic profiles, the structural forms of the northern and southern sags of the basin were characterized by narrow half grabens, while the structure at the center sag of the basin was characterized by wide and gentle grabens. The fault strikes in the west of the basin were mainly northeast–southwest trending, whereas those in the east of the basin changed from northeast–southwest trending to nearly east–west trending. The extension rate in the east sag was higher than that in the west area. The extension rate in the middle part was lower relative to the east and west sags. This was because the rifting was controlled by the distribution of the main boundary fault along the basin. Temporally, the Cenozoic extension could be divided into three periods: Eocene, Oligocene, and Miocene. The amount of stretching in the three extension stages was unevenly distributed in the entire basin. The maximum was mainly in the Oligocene Lingshui and Yacheng Formations. The Oligocene extension occurred in the entire basin, and the Eocene extension was limited to the Ledong and Changchang sags. Significant fault activity could be observed during the deposition period of the Yacheng and Lingshui Formations and could be attributed to strong extensional activity. The rifting tectonics controlled the distribution of source rocks and oil-generating window as well as hydrocarbon generation, reservoir formation, and accumulation.

Managing reservoir sedimentation through coordinated operation of a transboundary system of reservoirs in the Mekong

Sedimentation is one of the major challenges for the long-term sustainable operation of a hydropower reservoir. Trapping of sediment reduces its storage capacity and consequently diminishes hydropower production. Here first the development of a REServoir Sediment MANagement routine (ResSMan) and its integration into a hydrological model, the Soil and Water Assessment Tool (SWAT) is presented. ResSMan has functions to determine sediment accumulation in multiple reservoirs, and its impacts on the storage capacity and hydropower production under user-specified operation policies. It allows to compute the restoration of storage capacity due to the removal of sediment by flushing, i.e. the removal of sediment from a reservoir by passing water and sediment through the low-level outlets, and sluicing, i.e. passing sediment before suspended sediment solids have settled down in the reservoir. The capabilities (flushing and sluicing) of ResSMan were evaluated through a comparison with the Sediment Simulation Screening (SedSim) model, a well-tested sediment management simulation model and the resulting R2 values of 0.99 validated its capabilities. Subsequently, ResSMan was applied to assess and manage reservoir sedimentation using different management strategies in a complex system of 19 reservoirs (12 currently existing) in the Sesan and Srepok (2S) basin of the Mekong River for 2021–2120. The unregulated mean annual sediment yield at the outlet of the 2S basin was estimated as 7.24 million tonnes/year (Mt/y) and it will be reduced to 0.11 Mt/y due to the operation of the 19 reservoirs. In total 924 Mt of sediment will accumulate in these 19 reservoirs over 100 years, resulting in an average trapping efficiency of 74% (ranging from 11% to 97%). System-wide sediment management coordination simulations demonstrated that bi-annually and 5-yearly flushing of alternate reservoirs are effective options for efficiently releasing sediment in the 2S basin. However, the use of frequent flushing (annual or bi-annual) may be more favourable to minimize adverse impacts due to release of higher sediment loads on downstream ecosystems. The analysis is an initial step towards the coordination of sediment management plans and policies for the multi-reservoirs system in the Mekong basin.

Characteristics and Accumulation Model of Silurian Tight Sandstone Gas Reservoir: A Case Study in the Shajingzi Belt, Northwest Tarim Basin, China

The first industrial gas flow has been recently achieved in the Shajingzi Belt, northwest Tarim Basin, China, although that area is always characterized as a monoclinic structure background, typical tight bituminous sandstones, and poor hydrocarbon accumulation conditions. Based on drilling, oil testing, geophysics, geological, and geochemical data, a comprehensive research including reservoir description, hydrocarbon tracing, and accumulation history analysis in the Shajingzi Belt was carefully conducted to establish a meaningful Silurian hydrocarbon accumulation model and discover potential areas in this study. It is found that 1) the Silurian reservoirs in the Shajingzi Belt mainly bear gas and could be defined as the typical tight sandstone reservoir with ultralow porosity generally less than 10% and low-permeability distributed between 0.1 and 10 mD, respectively; 2) petroleum substantially comes from the Cambrian–Ordovician source rocks, especially in the Awati Sag, possessing the dominant contribution from the shales in Cambrian Yuertusi Formation; 3) synthetically, three major hydrocarbon accumulation periods can be determined in the Shajingzi Belt, namely, the Late Caledonian, the Late Hercynian–Early Indosinian, and the middle Himalayan, respectively; 4) the evolution of Shajingzi Fault system apparently dominated the formation of various Silurian structural–lithologic traps in the monoclinic structure background and efficiently connected the deep source rocks in the Awati Sag, especially during Himalayan period when the current tight sandstone gas reservoirs were formed. This research could find its benefit for exploration in similar basin–range junction areas.

Controls of sandstone architecture on hydrocarbon accumulation in a shallow-water delta from the Jurassic Shaximiao Formation of the western Sichuan Basin in China

The thin but widely distributed sandstones in the shallow-water deltas have the potential to hold large deposits of natural gas. The fluvial sand bodies formed from river channels with different sedimentary facies and sandstone architectures have remarkable heterogeneities. The results of this investigation suggest that the sandstone architectures of river channels have a significant impact on the hydrocarbon accumulation and production of sandstone reservoirs. Information regarding the sandstone origin and architecture of shallow-water deltas, especially of the fingered bar type, is still very limited. This investigation focused on the channel sandstones in the shallow-water delta of the Jurassic Shaximiao Formation on the eastern slope of the western Sichuan Basin in China. Based on data from cores and logs of sandstones, seven types of lithofacies and four sedimentary facies were identified. Sand-rich sedimentary facies, including distributary channels, mouth bars, and overbanks, have merged to form four types of compound sandbodies corresponding to the fourth-order architectural unit in the architectural classification. Four types of compound sand bodies were classified, including CS1 (variable distributary channel sand body), CS2 (diverting channel and residual overbank sand body), CS3 (diverting channel and residual mouth bar sand body), and CS4 (terminal channel and residual mouth bar sand body). Relatively good physical properties occur in the sandstone architectures of types CS3 and CS4, with an average porosity of 11.06% and an average permeability of 0.97 mD. Sandstone architectures from the Jurassic Shaximiao Formation of the study area were controlled by water depth and climate, tectonic subsidence, distance from provenance, erosion effect, and channel sinuosity and discharge. With strong lateral erosion, a shallow-water delta would have more sandstone architectures of types CS3 and CS4 in the high sinuosity of channels far from the provenance. Furthermore, sandstone architectures of types CS3 and CS4 are prone to form in connected channel sandbodies with relatively high contents of natural gas of approximately 4.48 × 104 m3/d. The results of this investigation are beneficial to improve the understanding of the characteristics of sandstone distributions in shallow-water deltas, especially in fingered bar types, and to guide oil and gas explorations in shallow-water delta systems.

Terrigenous organic carbon drives methane dynamics in cascade reservoirs in the upper Yangtze China

Methane (CH4) emissions from freshwaters to the atmosphere have a profound impact on global atmospheric greenhouse gas (GHG) concentrations. Anthropogenic footprints such as dam construction and reservoir operation significantly changed the fate and transport of CH4 in freshwaters. The source of particulate organic carbon (POC) in reservoirs is a critical factor controlling CH4 production and emissions. However, little is known of how reservoir operation mediates the transport of POC and regulates CH4 accumulation in cascade hydroelectric reservoirs. Here, spatial and temporal variations in POC and CH4 were explored in the Xiluodu (XLD) and Xiangjiaba (XJB) reservoirs which are deep valley cascade reservoirs located in the main channel of the upper Yangtze River. Based on the δ13C-POC and N/C mole ratio of particulate organic matter, the results of multi-endmember stable isotope mixing models by a Bayesian model showed that terrigenous POC and autochthonous POC accounted for approximately 55% ± 18% and 43% ± 19% (SD, n = 179) of POC, respectively. Together with other hydrological and environmental parameters, we found that the input of terrigenous POC was dominantly influenced by water level variations and flow regulation due to reservoir operation. The cumulative effect of POC caused by cascade dams was not apparent. Terrigenous POC were more likely to drive CH4 accumulation in our study. Evident low level of CH4 in both reservoirs were likely affected by low sedimentation of POC and microbial CH4 oxidation. We hope our study could provide a conceptual framework for further modeling of CH4 dynamics in cascade reservoirs.