Reservoir Accumulation Research Articles

Research on application of a new bottom trap technology to catch particles rich in nutrients in a large shallow lake

The resuspension and sedimentation of particulate matter and the release of nutrients from sediment are important factors affecting the eutrophication of shallow lakes. The capture and removal of particles rich in nitrogen, phosphorus, and other nutrients at the bottom of lakes is of great significance for improving the management and eutrophication status of lakes. This study investigated the feasibility of applying lake bottom trap technology in seven different locations in Lake Chaohu, which is the fifth largest freshwater lake in China. The results showed that the trap in the western part of Lake Chaohu had the highest sedimentation rate and could capture most of the nutrients. The sedimentation rates were higher in spring and summer than in autumn and winter. The bottom trap effectively collected and preserved chlorophyll a, organic matter, total nitrogen, and total phosphorus. The trap per meter length (15–20 m wide) could catch 20.7–27.6 m3 of particles rich in nutrients with a water content of 50–70%, organic matter content of 281.9–375.8 kg, total nitrogen content of 24.5–32.6 kg, and total phosphorus content of 10.5–14 kg. The proposed bottom trap had little impact on the benthic organism system of the lake. The bottom trap technology used in this study could solve the problem of nitrogen, phosphorus, and algae accumulation in lakes and reservoirs, broaden the utility of lake hydrodynamics in environmental pollution control, and provide new ideas and strategies for the control and management of cumulative pollution in shallow lakes and reservoirs.

The Accumulation Characteristics of the Paleozoic Reservoir in the Central-Southern Ordos Basin Recorded by Organic Inclusions

The Permian tight clastic reservoir and Ordovician carbonate reservoir were developed in the central-southern Ordos Basin. This study investigated the fluid inclusion petrography, diagenetic fluid characteristics, formation process of natural gas reservoir, source rock characteristics, and reservoir accumulation characteristics of these Paleozoic strata by petrographic observations, scanning electron microscope imaging, fluid inclusion homogenization temperature, salinity, laser Raman spectrum, and gas chromatograph analyses. The results have suggested two phases of fluid inclusions in both the Permian sandstone and the Ordovician Majiagou Formation dolomite reservoirs, and the fluid inclusions recorded the history from the early thermal evolution of hydrocarbon generation to the formation, migration, and accumulation of natural gas. The early-phase inclusions show weak yellow fluorescence and recorded the early formation of liquid hydrocarbons, while the late-phase inclusions are nonfluorescent natural gas inclusions distributed in the late tectonic fractures and recorded the late accumulation of natural gas. The brine systems of the Permian and Ordovician fluid inclusions are, respectively, dominated by CaCl2-H2O and MgCl2-NaCl-H2O. The diagenetic fluids were in the ranges of medium-low temperature and moderate-low salinity. The natural gas hydrocarbon source rocks in the Ordos Basin include both the Permian coal-bearing rocks and the Ordovician carbonates. The process of the early-phase liquid hydrocarbon formation and migration into the reservoir corresponded to 220 Ma (Late Triassic). The late large-scale migration and accumulation of natural gas occurred at 100 Ma (early Late Cretaceous), which was close to the inclusion Rb/Sr isochron age of 89.18 Ma, indicating that the natural gas accumulation was related to the Yanshanian tectonic movement.

Geochemical Trends Reflecting Hydrocarbon Generation, Migration and Accumulation in Unconventional Reservoirs Based on Pyrolysis Data (on the Example of the Bazhenov Formation)

The current study is devoted to the determination and interpretation of geochemical trends reflecting hydrocarbon generation, migration and accumulation in unconventional reservoirs; the study is performed on the Bazhenov shale rock formation (Western Siberia, Russia). Results are based on more than 3000 Rock-Eval analyses of the samples from 34 wells drilled in the central part of the West Siberian petroleum basin, which is characterized by common marine sedimentation environments. Pyrolysis studies were carried out before and after the extraction of rocks by organic solvent. As a result, we have improved the accuracy of kerogen content and maturity determination and complemented the standard set of pyrolysis parameters with the content of heavy fraction of hydrocarbons. The data obtained for the wells from areas of different organic matter maturity was summarized in the form of cross-plots and diagrams reflecting geochemical evolution of the source rocks from the beginning to the end of the oil window. Interpretation of the obtained results revealed quantitative trends in the changes of generation potential, amount, and composition of generated hydrocarbons in rocks at different stages of oil generation process. The analysis of geochemical trends allowed us to improve approaches for the productivity evaluation of the formation and study the effect of organic matter maturity on distribution of productive intervals of different types.

Syn-rift sedimentary evolution and hydrocarbon reservoir models in a graben rift sag, Songliao Basin, Northeast China

Identifying hydrocarbon reservoirs in lacustrine rift basins is a challenging task. This study uses seismic, well, and core data to analyse the sequence stratigraphy and describe the depositional systems and hydrocarbon reservoir models in the syn–rifted successions of Sujiatun sag in order to locate prospects in the Late Jurassic–Early Cretaceous continental rift basin. SQ2 is composed mainly of fan–delta and shallow lacustrine depositional systems. SQ3 and SQ4 consist of shallow lacustrine and semi–deep lacustrine depositional systems. SQ5 and SQ6 are composed of braid-delta and shallow lacustrine depositional systems, forming favourable reservoir sand bodies. Data obtained from thin section observation and physical property analysis show that the physical properties of braid-delta reservoirs are better than those of fan-delta reservoirs in terms of hydrocarbon accumulation. Geochemical analysis of the mudstones in SQ3 and SQ4 shows that they are excellent source rocks. Analysis of the conditions and characteristics under which the hydrocarbon reservoirs formed, indicate that Sujiatun sag models can be divided into six categories: a tilted fault–block hydrocarbon reservoir model, Syn trending fault–block hydrocarbon reservoir model, reverse trending fault hydrocarbon reservoir model, fault-lithologic hydrocarbon reservoir model, an updip pinch-out hydrocarbon reservoir model, and a lenticular lithologic hydrocarbon reservoir model. Research indicates that the distinctive depositional systems reflect three episodic syn–rifted stages. In particular, the rift climax influences the development of source rocks. Well-sorted sedimentary conditions and a large number of dissolution pores are key to improving a reservoir. The types and distribution of hydrocarbon reservoirs vary greatly over different structural stages and structural zones. Fault activity and a combination of different faults types, reservoirs, and source rocks jointly determine the enrichment degree of the hydrocarbon accumulation in different reservoirs. This study analyses the sedimentary evolution and hydrocarbon reservoirs within the syn–rifted successi to provide useful insight for oil and gas exploration in syn–rifted basins and may also be effective for use in areas in which wells or outcrop data are lacking.

Fault Mesh Petroleum Plays in the Donghetang Area, Tabei Uplift, Tarim Basin, Northwestern China, and Its Significance for Hydrocarbon Exploration

Abstract —The hydrocarbon formation mechanism and potential targets in clastic strata from the Tabei Uplift, Tarim Basin, are documented using the fault mesh petroleum plays theory, based on integrating seismic, well log, well core, and geochemical data. The reservoirs in the Donghetang area are typical allochthonous and far-source fault mesh petroleum plays. There are two sets of fault meshes in the study area: (1) the combination of the Donghe sandstone and Permian–Triassic strata and (2) the combination of the fourth and third formations in the Jurassic strata. The fault mesh petroleum play in the Jurassic is a secondary reservoir that originates from the Carboniferous Donghe sandstone reservoir adjustment based on source correlation. The fault mesh carrier systems show the fully connected, fault–unconformity–transient storage relay, fault–transient storage–unconformity relay, and transient storage–fault relay styles, according to the architecture of the fault mesh. Based on the characteristics of the fault mesh petroleum plays, the reservoirs are divided into three categories (upper-, inner-, and margin-transient storage styles) and 15 styles. Integrated analysis of the hydrocarbon generation and faulting time periods reveals that there were four periods of hydrocarbon charging, with the first three stages charging the reservoirs with oil and the last stage charging the reservoirs with gas. There are multiple stages of reservoir accumulation and adjustment in the fault mesh in the study area. These stages of fault mesh accumulation and adjustment are the main reason why the reservoir distribution multiple vertical units have different hydrocarbon properties. Fault-block and lithologic reservoirs related to the inner- and upper-transient storage styles are the main exploration targets in the clastic strata in the study area.

The Relationship between Diagenetic Evolution and Hydrocarbon Charge in Deep Tight Reservoirs: A Case Study from Kepingtage Formation of S9 Well Block in Tarim Basin

The Silurian Kepingtage Formation of Tarim Basin is an important deep clastic reservoir. Exploration practice shows that the reservoirs are generally characterized by extremely low porosity and permeability, exhibiting strong mineral composition and physical property heterogeneities. The study of heterogeneity and its effect on oil charge is of great significance to the understanding of oil migration and accumulation in such reservoirs and thus is crucial to oil exploration. Therefore, based on identification and quantitative statistics of thin sections, SEM, and homogenization temperature measurements, this study was designed to examine the relationship between diagenetic evolution and oil emplacement in the Silurian Kepingtage Formation in S9 Block in Tarim Basin. The control of differential diagenesis and physical property evolution on oil charge process are discussed. It is found that the sandstones with fine grain size and high content of ductile grains (>20%) have experienced strong mechanical compaction, the porosity decreases rapidly in the early stage of burial. The densification process has been completed before the first period of oil charge so that no oil was found in the reservoir. In contrast, sandstones with relatively coarse grain size and low ductile grain content have better porosity and permeability properties, which are generally oil-bearing. This kind of sandstone generally experienced two periods of oil charge. Apart from moderate compaction, multiple diagenesis events were also observed, including quartz cementation, calcite cementation, and dissolution. Before the early period of oil charge, the diagenesis events are dominated by moderate compaction, chlorite cementation, and dissolution, and the reservoir property is still high. It seems that the time of late period oil charge is relatively long. With the growth of multiple types of authigenic minerals such as quartz, calcite, and illite after or sometimes synchronously occurred with the late period oil charge, the porosity gradually decreases.

Turbidite Reservoirs in the Dongying Sag, Bohai Bay Basin: Interactions between Carbonate Cementation and Hydrocarbon Charge

AbstractTurbidite sandstones have become increasingly significant in hydrocarbon exploration. Carbonate cementation occur commonly in turbidite reservoirs developing within the Paleocene lacustrine basins, Northeastern China. This study utilizes core data, thin section data and production data to investigate the interaction between the carbonate cementation and hydrocarbon charge within turbidite reservoirs in the Niuzhuang Sub‐sag of the Dongying Sag, Bohai Bay Basin, East China. The results reveal that the carbonate cementation is mainly developed at the top and bottom of the turbidite sandbodies, and even forms carbonate cement shells. Three stages of hydrocarbon accumulation are identified based on fluid inclusion analysis: stage I (27.5–24.6 Ma), stage II (14.0–5.0 Ma), and stage III (5.0–0 Ma). The interaction between the carbonate cementation and hydrocarbon charge has significant controls on the formation of a turbidite reservoir. The temporal relations and intensity relations between the two factors should be considered significantly. Moreover, hydrocarbon charge during the early stage can inhibit the carbonate cementation, favoring the hydrocarbon accumulation in turbidite reservoirs. Many deep‐lacustrine turbidite sandbodies surrounded by source rocks with abnormal high pressure, are also favorable for hydrocarbon accumulation. These results suggest that some deeply buried turbidite sandbodies with similar geological settings have high potential for hydrocarbon exploration.

A comparative study on natural gas hydrate accumulation models at active and passive continental margins

The comparative study on natural gas hydrate accumulation models between active and passive continental margins as well as their controlling factors is of great significance to the guidance of natural gas hydrate exploration. Based on the data and research results of international typical active continental margin hydrate accumulation areas such as the Cascadia margin of the Northeast Pacific, the Nankai trough, etc. and passive continental margin areas like the Blake Ridge, the models of the gas hydrate accumulation system are summarized and numerically simulated, and a preliminary comparison of active and passive continental margin reservoir accumulation models was also carried out. The following results were obtained. (1) The active continental margin provides a driving force and channel for vertical gas migration, which induces deep free gas and in-situ biogas to migrate along the fault. The migration channels are mainly faults, fractures and slumps produced by subduction–accretion. (2) Coarse-grained turbidity sediments such as silt and sandy silt have good porosity and permeability. Moreover, the sediment thickness on the accretionary wedge is large, which provides a good storage space for hydrate accumulation. (3) Numerical simulations of the Blake Ridge, and Niger Delta hydrate accumulation show that the passive continental margin lacks the lateral stress caused by the subduction zone compared with the active continental margin. However, due to the plastic materials in the thick sedimentary layer, high-pressure fluids and volcanic activities outside the continental margin, vertical accretion and tensile stress are generated and the accumulation rate of diffusion-type hydrates mainly depends on the methane supply rate. (4) Organic matter content, gas production rate, geothermal gradient and sedimentation rate at the passive continental margin have different effects on the spatial distribution of hydrate content. Mud volcanoes or diapir structures provide an ideal place for the formation and occurrence of hydrates.

Nanotechnology approaches for global infectious diseases.

Infectious diseases are a major driver of morbidity and mortality globally. Treatment of malaria, tuberculosis and human immunodeficiency virus infection are particularly challenging, as indicated by the ongoing transmission and high mortality associated with these diseases. The formulation of new and existing drugs in nano-sized carriers promises to overcome several challenges associated with the treatment of these diseases, including low on-target bioavailability, sub-therapeutic drug accumulation in microbial sanctuaries and reservoirs, and low patient adherence due to drug-related toxicities and extended therapeutic regimens. Further, nanocarriers can be used for formulating vaccines, which represent a major weapon in our fight against infectious diseases. Here we review the current burden of infectious diseases with a focus on major drivers of morbidity and mortality. We then highlight how nanotechnology could aid in improving existing treatment modalities. We summarize our progress so far and outline potential future directions to maximize the impact of nanotechnology on the global population.

The future for Mediterranean wetlands: 50 key issues and 50 important conservation research questions.

Wetlands are critically important for biodiversity and human wellbeing, but face a range of challenges. This is especially true in the Mediterranean region, where wetlands support endemic and threatened species and remain integral to human societies, but have been severely degraded in recent decades. Here, in order to raise awareness of future challenges and opportunities for Mediterranean wetlands, and to inform proactive research and management, we identified (a) 50 key issues that might affect Mediterranean wetlands between 2020 and 2050, and (b) 50 important research questions that, if answered, would have the greatest impact on the conservation of Mediterranean wetlands between 2020 and 2050. We gathered ideas through an online survey and review of recent literature. A diverse assessment panel prioritised ideas through an iterative, anonymised, Delphi-like process of scoring, voting and discussion. The prioritised issues included some that are already well known but likely to have a large impact on Mediterranean wetlands in the next 30 years (e.g. the accumulation of dams and reservoirs, plastic pollution and weak governance), and some that are currently overlooked in the context of Mediterranean wetlands (e.g. increasing desalination capacity and development of antimicrobial resistance). Questions largely focused on how best to carry out conservation interventions, or understanding the impacts of threats to inform conservation decision-making. This analysis will support research, policy and practice related to environmental conservation and sustainable development in the Mediterranean, and provides a model for similar analyses elsewhere in the world.Supplementary InformationThe online version contains supplementary material available at 10.1007/s10113-020-01743-1.

A Study on the Heterogeneity Characteristics of Geological Controls on Coalbed Methane Accumulation in Gujiao Coalbed Methane Field, Xishan Coalfield, China

Commercial exploration and exploitation of coalbed methane (CBM) in Gujiao coalbed methane (CBM) field, Xishan coalfield, have rapidly increased in recent decades. The Gujiao CBM field has shown strong gas distribution heterogeneity, low gas content, and wide distribution of wells with low production. To better understand the geological controlling mechanism on gas distribution heterogeneity, the coal reservoir evolution history and CBM accumulation process have been studied on the base of numerical simulation work. The burial history of coal reservoir can be classified into six stages: shallowly buried stage; deeply buried stage; uplifting stage; short-term tectonic subsidence stage; large-scale uplifting stage; and sustaining uplifting and structural inversion stage. Mostly, coal seams have experienced two-time thermal metamorphisms with twice hydrocarbon-generation processes in this area, whereas in the southwest part, the coal seams in there suffered three-time thermal metamorphisms and hydrocarbon-generation processes. The critical tectonic events of the Indosinian, Yanshanian, and Himalayan orogenies affect different stages of the CBM reservoir accumulation evolution process. The Indosinian orogeny mainly controls the primary CBM generation. The Yanshanian orogeny dominates the second and third gas generation and migration processes. The Himalayan orogeny mainly affects the gas dissipation process and current CBM distribution heterogeneity.

Hydrothermal alteration and hydrocarbon accumulations in ultra-deep carbonate reservoirs along a strike-slip fault system, Tarim Basin, NW China

In recent years, petroleum exploration has extended from shallow to deep reservoirs in the globle petroliferous basins. In contrast to conventional carbonate reservoirs, the fractures and hydrotherm related karst reservoirs could mainly be developed in deep strata along the tectonic deformation and fault zone. The seismic imaging and tectonic analysis have been combined to identify the scale and features of the deep fault system in the Tarim Basin, NW China. It was found that the tectonic collision and movement generated a prevalent strike-slip fault system, which was conjugated and extended from paleo-uplift to the inside of the Manjar Sag (source kitchen) in the Tarim Basin. The micrograph of cores and images of well logging demonstrated that deep reservoirs are dominated by porosities, fractures, and hydrothermal minerals (e.g., fluorite, pyroxenite). Burial dissolution and thermohydrology along strike-slip fault have greatly contributed to the formation of deep carbonate reservoirs, meanwhile controled the accumulation and hydrocarbon properties in the deep carbonates reservoirs of Tarim Basin. The analysis of adamantane, dibenzothiophene, and carbon isotopes indicated that oil-cracking, thermochemical sulfate reduction (TSR), and gas-washing fractionation could be the main factors to control the sequential accumulation of various hydrocarbons around Manjar Sag. The formation and hydrocarbon accumulation of the deep carbonate reservoirs along this strike-slip fault system could provide important details regarding deep petroleum geological processes from both scientific and practical aspects. These findings could be applied to future studies and the exploration of deep potential marine reserves worldwide.

Evolution and Hydrocarbon Accumulation of the Cambrian-Ordovician Paleo-uplifts in the Southwest Tarim Depression, China

There is a close relationship between the evolution of the Cambrian–Ordovician paleo-uplifts and their hydrocarbon accumulation in the Southwest Tarim Depression. Using regional 2D seismic data interpretation, the structural morphology of the top of the Ordovician carbonate rocks in the Southwest Tarim Depression at different historical geologic stages is mapped. Combined with analysis of the discovered oil and gas reservoirs, the regularity of the hydrocarbon distribution in the Southwest Tarim Depression is investigated. Research shows that the evolution of the Cambrian-Ordovician paleo-uplifts in the Southwest Tarim Depression can be divided into four stages: (1) formation of the paleo-uplifts, (2) steady subsidence of the eastern paleo-uplift, (3) migration and adjustment of the western paleo-uplift and (4) strong subsidence and extinction of the paleo-uplifts. The Cambrian-Ordovician strata in the Southwest Tarim Depression experienced a seesaw-like structural reconstruction from an early north-dipping slope to a late south-dipping slope. During the structural reconstruction process, a pivot zone was located at the position of the current Maigaiti Slope, proving favorable for hydrocarbon accumulation and preservation. The fault zones in the eastern and western paleo-uplifts of the Maigaiti slope finalized during the Late Hercynian Period were favorable to preserving paleo-oil reservoirs. In contrast, the fault zones in the northern margin of the Maigaiti Slope, where the current tectonic uplift is located, were favorable for the accumulation of gas reservoirs.

Numerical study on the effect of reservoir heterogeneity and gas supply on hydrate accumulation in subsea shallow formations

Seepage-type gas hydrate accumulation in subsea shallow formations involves complicated thermo-hydro-solid coupling processes and matching problem between various accumulation elements. The formation physical properties control local natural gas migration pathway and thus the final reservoir characteristics of hydrates. In this paper, a novel mixed-flux model for gas hydrate accumulation is established and then used to simulate the process of methane gas migration into the shallow stratum to form a hydrate reservoir. The effects of reservoir heterogeneity and gas source conditions on the distribution of pore fluid and hydrate accumulation are examined. The simulation results show that reservoir heterogeneity is conducive to the retention and lateral migration of CH4 in a hydrate stability zone. CH4 can contact more pore water to form a large hydrate reserve, but the formed hydrate is often dispersed. Low-permeability layers enhance the trapping of CH4 and form a uniform and large hydrate saturation. Besides, gas source conditions have an important impact on the hydrate accumulation in reservoirs. Large gas flux, small pore water flux, continuous gas supply, high content of heavy components in natural gas, and numerous gas source points contribute to large amounts of hydrates generation in a certain time period. The presented work will deepen our understanding of the controls of natural gas hydrate systems in subea shallow formations.

Hydraulic response and stability of a reservoir slope with landslide potential under the combined effect of rainfall and water level fluctuation

The Xinhua slope of Dagangshan Reservoir is taken as an example to study the potential for reservoir landslides under the combined effects of rainfall and water level fluctuations. Through field investigation, monitoring analysis by global navigation satellite system and numerical simulation, the hydraulic response and stability of the accumulated deposits on the Xinhua slope were evaluated. Three occurrences of large deformations were recorded by real-time monitoring. By comparing the deformation records with the hydrological records, it was found that the first one was caused by water storage, and the last two were due to the coupling effect of rainfall and water level drawdown. Deformation of the reservoir overburden cannot simply be thought of as the sliding of a single block (geology survey showed deep sliding surfaces with depths of 50–70 m), but shallow sliding (less than 5 m) is also much likely to occur revealed by a supporting evidence from deformation records. In cases where hydrostatic pressure and transient osmotic forces are favorable for shear resistance, softening may play a major role in the process of impoundment leading to deformation. The deformation pattern of the slope deposits changes as it enters operation. A drawdown of water level during flood season accelerates the deformation, while a following rise of water level inhibits this acceleration. More extensive monitoring (i.e. pore water pressure sensor and inclinometer) and more in-depth analysis are needed in the future, and this article is a synthesis of limit equilibrium and infinite slope analysis to better understand the deformation patterns and response mechanisms of reservoir accumulation.

ОСОБЕННОСТИ НАКОПЛЕНИЯ ФОСФОРА В ДОННЫХ ОТЛОЖЕНИЯХ ИВАНЬКОВСКОГО ВОДОХРАНИЛИЩА

The distribution of phosphorus in the bottom sediments of the Ivankovo reservoir, the forms of phosphorus and its content in different size fractions of the sediment were studied. It is shown that the main part of phosphorus is in mineral form and is contained in the clay fraction of sediment, which increases the risk of secondary pollution of water with phosphorus, due to its flow from bottom sediments in suspended form in the active dynamic zones of the reservoir. The estimation of the diffusion input of soluble forms of phosphorus from the bottom sediments of the reservoir accumulation zones was performed.

Factors of gas accumulations formation in oil-bearing sediments and in casing annulus of wells

Gas component study is one of the important tasks of petroleum geology. Gas component can exist in various forms in sedimentary rocks. Of great interest is nitrogen, the gaseous accumulations of which are formed in oil-bearing strata, causing complications during the oilfield development. The problem of abnormal nitrogen accumulations had great relevance in the fields of the Volga-Ural petroleum province, which is one of the long-term developed with a large stock of wells for various purposes. This article discusses possible sources of gaseous nitrogen and the reasons for its accumulations in oil-bearing reservoirs. The main purpose of the article is to clarify the reasons for the gaseous nitrogen and its deposits formation. The main patterns of the areal distribution of nitrogen gas accumulations in oil-bearing strata are revealed on the basis of field, hydrogeological, geological and geophysical researches data analysis. It has been established that during the gas caps formation, the source of gaseous nitrogen is its dissolved compounds in groundwater and oil, biochemical decomposition of which leads to the dissolved molecular nitrogen accumulation in a liquid medium. The release of free gaseous nitrogen and the formation of its accumulations is associated with the decompression of formation waters for natural (geological) or man-made reasons (hydrocarbons extraction). Disturbance of the natural hydrodynamic regime in oil-bearing formations leads to the release of gaseous nitrogen and the formation of its accumulations under favorable conditions (the presence of reservoirs, structures and impermeable rocks in the top of the formation).

Genesis and accumulation process of deep natural gas in the Altun foreland on the northern margin of the Qaidam Basin

Deep natural gas in the northern margin of the Qaidam Basin is characterized by low exploration degree and large resource potential, but the genesis of deep gas reservoirs is complex. In particular, Jurassic source rocks do not develop in west of Altun foreland. The mechanism of deep accumulation is unknown, which restricts the exploration of deep gas reservoirs. Using gas components, isotopes and other data, combined with the geological characteristics of the Altun foreland, this study analyzes the genesis and source of deep gas in this area, and establishes the accumulation procesess of the deep gas reservoirs. Analysis shows that the gas in the Altun foreland is dominated by coal-type gas with the characteristics of oil cracking gas in deep reservoirs. The Jurassic source rocks in the Qaidam Basin are mainly lacustrine mudstones, with black mudstones and oil shale developed, which have the material basis for forming large oil reservoirs. The deep reservoir has experienced the process of long-term deep burial and high-temperature cracking, and has the conditions to form oil cracked gas. Accumulation in deep bedrock gas reservoirs is characterized by early oil filling, later high-temperature cracking into gas, and late adjustment. Reservoirs formed in early stage can be deeply cracked into gas reservoir under high temperature in the process of deep burial. The ancient uplifts along the basin margin and large anticline traps in the late tectonic zone are favorable exploration areas for deep gas. The exploration area for natural gas in the Qaidam Basin has been expanded by the discovery of deep oil cracking gas in the Altun foreland, which has crucial significance for guiding and intensifying deep gas exploration in the Qaidam Basin.

Tectonic Fracture Formation and Distribution in Ultradeep Marine Carbonate Gas Reservoirs: A Case Study of the Maokou Formation in the Jiulongshan Gas Field, Sichuan Basin, Southwest China

Tectonic fractures are the key factors affecting hydrocarbon migration and accumulation in ultradeep marine carbonate gas reservoirs. Taking the Maokou Formation in the Jiulongshan Gas Field as an ...

Geochemical studies on high-efficiency hydrocarbon expulsion of Triassic lacustrine shale in the Ordos Basin, China: implications for tight and shale oil accumulation

The organic-rich sediments developed at the bottom of Chang7 Member of the Upper Triassic Yanchang formation, which can be divided into oil shale and black mudstones, are dominate source rocks for low permeability and tight oil reservoirs in Mesozoic petroleum system of Ordos Basin. The hydrocarbon expulsion characteristics of the organic-rich sediments have been investigated by geology and geochemistry data. By comparison with the geochemical parameters of crude oil samples and mudstone extracts, Chang7 shale has many geochemical evidences on high rates of hydrocarbon expulsion. Lower conversion rate of chloroform extracted bitumen “A”, especially for the samples with TOC higher than 10%; lower productivity indexes and lower hydrogen indexes reducing with TOC increasing; lower ratios of saturates/aromatics but higher contents of polar components for residual bitumen “A”; heavier stable carbon isotopes of residual hydrocarbons. It should be noted that the source rock evaluation and oil-source correlation would be misunderstood based on these abnormal geochemical data caused by high-efficiency hydrocarbon expulsion. Calculation for rock samples have shown that the expelled rates of the shale samples averaging 70% are significantly higher than those of the mudstone samples. Chang 7 shale with large amounts of hydrocarbons generated and expelled is critical for hydrocarbon accumulation in low-permeability and tight oil reservoirs. Also, Chang7 organic-rich rocks is an important exploration field of shale oil in the future.