Gas Accumulation Model Research Articles

Reservoir space type and storage capacity of Oligocene lacustrine carbonate in the Yingxi area of western Qaidam Basin, China

Over the past 2 years, a great progress in petroleum and gas exploration has been made in tight lacustrine carbonate rocks of E 3 2 in the Yingxi area. Many high-yield commercial oil flow wells with more than 1000 tons of oil production per day have been found, but their reservoir space type and storage capacity are the key issues highly concerned and disputed by many explorationists. According to the study on systemic petrology and mineralogy, analysis of physical property, study on capillary pressure curve, analysis of ultra-micro crystal morphology, analysis of micro-area geochemical composition, and other comprehensive analysis, and pore types of the E 3 2 reservoir are mainly intercrystalline pores of dolomite, and a small number of dissolution phenomena have been found. Therefore, large-scale oil and gas reservoirs can be formed. The analysis involves these key results and evidences: (1) under the microscope and scanning electron microscope, many intercrystalline pores are developed in dolomite rocks, presenting a “high-wide platform” phenomenon on the capillary pressure curves to imply “small bore—large quantity”. (2) The electron probe shows that the molecular formula of dolomite is Ca(Mg0.96Fe0.04)(CO3)2. In the penecontemporaneous dolomitization process, intercrystalline pores come from volume shrinkage when Mg2+ ions replace Ca2+ ions, and a small number of Fe2+ ions are involved in this chemical reaction. The 13.36% plane porosity can be formed in ideal conditions. (3) According to the statistical data, intercrystalline pores are enlarged by organic acids in several samples, no evidence is available for leaching dissolution of atmospheric water because of exposure, and many types of feldspar do not dissolve. The reservoir rocks are formed due to a high deposition rate. (4) According to the measured pore sizes and the values based on the chemical theory, the pore size is in the range of 0.4–1.07 μm. These pores can store all oil, gas, and symbiosis substances, which can flow or migrate in the pores because of the pore sizes. The pictures of fluorescence thin sections and the backscattered electron images of SEM show that these micro-pores are full of hydrocarbons, creating the oil and gas accumulation pattern of “containing oil for the whole sag” and “producing oil for each well” in the Yingxi area. Based on the oil and gas accumulation model, the Yingxi area is an integrated oil and gas field with the most potential and industrial value in Qaidam Basin. The research results are crucial to the petroleum and gas exploration of carbonates in a saline lacustrine basin.

Geological Factors and Reservoir Properties Affecting the Gas Content of Coal Seams in the Gujiao Area, Northwest Qinshui Basin, China

Coalbed methane (CBM) well drilling and logging data together with geological data were adopted to provide insights into controlling mechanism of gas content in major coal seams and establish gas accumulation models in the Gujiao area, Northwest Qinshui Basin, China. Gas content of targeted coals is various in the Gujiao area with their burial depth ranging from 295 to 859 m. Highly variable gas content of coals should be derived from the differences among tectonism, magmatism, hydrodynamism, and sedimentation. Gas content preserved in the Gujiao area is divided into two parts by the geological structure. Gas tends to accumulate in the groundwater stagnant zone with a total dissolved solids (TDS) value of 1300–1700 ppm due to water pressure in the Gujiao area. Reservoir properties including moisture content, minerals, and pore structure also significantly result in gas content variability. Subsequently, the gray correlation statistic method was adopted to determine the most important factors controlling gas content. Coal metamorphism and geological structure had marked control on gas content for the targeted coals. Finally, the favorable CBM exploitation areas were comprehensively evaluated in the Gujiao area. The results showed that the most favorable CBM exploitation areas were in the mid-south part of the Gujiao area (Block I).

Fuzzy fusion of geological and geophysical data for mapping hydrocarbon potential based on GIS

Effective fusion of multiple data, including geographical, geological, geophysical, geochemical and dynamic data for hydrocarbon potential mapping, involves both a fusion algorithm and a convenient modelling platform. In this study, fuzzy logic and a geographical information system (GIS) are used to fuse geological and geophysical interpretations in mapping the gas potential of the Kazakhstan Marsel Territory Carboniferous system based on the assumed gas-accumulation model. Non-linear membership functions are used to transform the input data, while the gamma operator is used to combine the multiple datasets. Finally, the Carboniferous system targets, the Visean (C 1 v) and Serpukhovian (C 1 sr) units, are mapped. Gas testing in situ validated our results.

Petroleum accumulation in the deeply buried reservoirs in the northern Dongying Depression, Bohai Bay Basin, China: New insights from fluid inclusions, natural gas geochemistry, and 1-D basin modeling

The deeply buried reservoirs (DBRs) from the Lijin, Shengtuo and Minfeng areas in the northern Dongying Depression of the Bohai Bay Basin, China exhibit various petroleum types (black oil-gas condensates) and pressure systems (normal pressure-overpressure) with high reservoir temperatures (154–185 °C). The pressure-volume-temperature-composition (PVTX) evolution of petroleum and the processes of petroleum accumulation were reconstructed using integrated data from fluid inclusions, stable carbon isotope data of natural gas and one-dimensional basin modeling to trace the petroleum accumulation histories.The results suggest that (1) the gas condensates in the Lijin area originated from the thermal cracking of highly mature kerogen in deeper formations. Two episodes of gas condensate charging, which were evidenced by the trapping of non-fluorescent gas condensate inclusions, occurred between 29-25.5 Ma and 8.6–5.0 Ma with strong overpressure (pressure coefficient, Pc = 1.68–1.70), resulting in the greatest contribution to the present-day gas condensate accumulation; (2) the early yellow fluorescent oil charge was responsible for the present-day black oil accumulation in well T764, while the late blue-white oil charge together with the latest kerogen cracked gas injection resulted in the present-day volatile oil accumulation in well T765; and (3) the various fluorescent colors (yellow, blue-white and blue) and the degree of bubble filling (Fv) (2.3–72.5%) of the oil inclusions in the Minfeng area show a wide range of thermal maturity (API gravity ranges from 30 to 50°), representing the charging of black oil to gas condensates. The presence of abundant blue-white fluorescent oil inclusions with high Grain-obtaining Oil Inclusion (GOI) values (35.8%, usually >5% in oil reservoirs) indicate that a paleo-oil accumulation with an approximate API gravity of 39–40° could have occurred before 25 Ma, and gas from oil cracking in deeper formations was injected into the paleo-oil reservoir from 2.8 Ma to 0 Ma, resulting in the present-day gas condensate oil accumulation. This oil and gas accumulation model results in three oil and gas distribution zones: 1) normal oil reservoirs at relatively shallow depth; 2) gas condensate reservoirs that originated from the mixture of oil cracking gas with a paleo-oil reservoir at intermediate depth; and 3) oil-cracked gas reservoirs at deeper depth.The retardation of organic matter maturation and oil cracking by high overpressure could have played an important role in the distribution of different origins of gas condensate accumulations in the Lijin and Minfeng areas. The application of oil and gas accumulation models in this study is not limited to the Dongying Depression and can be applied to other overpressured rift basins.

Origin of over-pressure in clastic rocks in Yuanba area, northeast Sichuan Basin, China

Abnormally high pressures are widely developed in clastic formations with burial depths greater than 3500 m in Yuanba area, where the top depth of over-pressure zone encompasses different formations in different locations. With increasing depths, well logs response in mudstone to over-pressure zone show that no significant anomalies occur in sonic and density logs, suggesting that the over-pressured mudstones are normally compacted. Multiple evidences from well-log responses to over-pressure, low porosity, normal geothermal gradient, low sedimentary rate and velocity-density crossplots show that over-pressures in mudrocks were created by gas generation and not related to the disequilibrium compaction. Based on diagenetic evolution analysis, stratigraphic framework and gas accumulation model, pressure transference and oil-to-gas cracking are expected to be the main causes of over-pressure in sandstone. Because of the favorable sealing condition and gas-bearing properties, lateral tectonic compression has also been an important factor for over-pressure development and preservation since early Cretaceous.

Giant gas discovery in the Precambrian deeply buried reservoirs in the Sichuan Basin, China: Implications for gas exploration in old cratonic basins

An estimated trillions of cubic meters of natural gas have recently been discovered in the Sinian dolomite reservoirs in the Sichuan Basin, China. The 20–26m thick reservoirs were found in deep structural–lithologic traps of 5000–5500m deep. The Sinian gas is thought to have been derived mainly from the Sinian source rocks, and partly from the Cambrian source rocks. The gas is thermally matured, with a dryness ratio (C1/∑C1–4) of 0.997–0.9998, C2H6 content <0.2%, and H2S content of 0.5–2.75%. The stable carbon isotope compositions (δ13C) of CH4 and C2H6 are similar in all the reservoirs, with values of −33.5‰ to −31.5‰ and −36.0‰ to −27.0‰ (PDB), respectively. The Sinian gas reservoirs are believed to have been accumulated by the following processes: initial paleo-oil accumulated during the Silurian–Devonian; gas accumulation after oil cracking of the paleo-oil accumulations during the Jurassic–Early Cretaceous; and the current gas accumulations formed by paleo-gas remigration and adjustment since the Late Cretaceous. This new gas accumulation model is consistent with large-scale gas accumulations for deeply buried paleocratonic basins worldwide, in which the source rocks initially underwent thermal oil generation from kerogen, followed by gas generation via oil cracking. The gas discovery in the Sinian dolomite reservoirs has led to a new wave of hydrocarbon gas exploration in the ancient cratonic basin containing the mostly thermally matured source rocks in China. We conducted a series of geochemical analyses of the H2S in the gases. The sulfur isotope compositions (δ34S) of gypsum, H2S, and secondary pyrite sampled from the Sinian Dengying Formation showed that H2S was generated by a moderate thermochemical sulfate reduction (TSR) during the Jurassic–Early Cretaceous periods. TSR was terminated by the depletion of SO42− in the formation water, resulting in a modest H2S content of 5–10% in the natural gases. Because the dolomites are rich in cations, a large quantity of H2S may have been bound to form metal sulfides. We infer that H2S contents of 0.8–10% are present widely in the deeply buried Sinian reservoirs. Therefore, measures should be taken to prevent H2S-related accidents during gas exploration in the deep strata.

Variable gas content, saturation, and accumulation characteristics of Weibei coalbed methane pilot-production field in the southeastern Ordos Basin, China

Using diverse geologic and geophysical data from recent exploration and development, and experimental results of analysis of gas content, gas capacity, and gas composition, this article discusses how geologic, structural, and hydrological factors determine the heterogeneous distribution of gas in the Weibei coalbed methane (CBM) field. The coal rank of the Pennsylvanian no. 5 coal seam is mainly low-volatile bituminous and semianthracite. The total gas content is 2.69 to 16.15 m3/t (95.00–570.33 scf/t), and gas saturation is 26.0% to 93.2%. Burial coalification followed by tectonically driven hydrothermal activity controls not only thermal maturity, but also the quality and quantity of thermogenic gas generated from the coal. Gas composition indicates that the CBM is dry and of dominantly thermogenic origin. The thermogenic gases have been altered by fractionation that may be related to subsurface water movement in the southern part of the study area. Three gas accumulation models are identified: (1) gas diffusion and long-distance migration of thermogenic gases to no-flow boundaries for sorption and minor conventional trapping, (2) hydrodynamic trapping of gas in structural lows, and (3) gas loss by hydrodynamic flushing. The first two models are applicable for the formation of two CBM enrichment areas in blocks B3 and B4, whereas the last model explains extremely low gas content and gas saturation in block B5. The variable gas content, saturation, and accumulation characteristics are mainly controlled by these gas accumulation models.

Accumulation Conditions of Upper Palaeozoic Coal-Derived Gas in Eastern Linqing Depression

The hydrocarbon exploration is presently going on well in the Bohai Bay Basin, where several coal-derived gas fields have been found in the Upper Palaeozoic. However, no significant breakthrough has been made in the exploration of Eastern Linqing Depression, which is located in the southwest of the Bohai Bay Basin. To improve the gas exploration efficiency, this study employed the petroleum system theory to analyze the accumulation elements and processes of coal-derived gas of Upper Palaeozoic in this area, and further summarized the natural gas accumulation model. Results showed that the source rocks of Upper Palaeozoic in Eastern Linqing Depression mainly consist of coal-measure strata of Benxi-Taiyuan Formations of Carboniferous, as well as Shanxi Formation and the Lower Shihezi Formation of Permian. Reservoirs are widely distributed from the bottom to the top, mainly consisting of carbonate rocks and sandstones. The carbonate rock reservoirs include the weathering crust of Ordovician and the dolomite within Ordovician, while sandstone reservoirs include the distributary channel sand bodies of delta plain and the meandering river channel sand bodies in Upper Palaeozoic, as well as sandstones of river and lacustrine in Mesozoic and Cenozoic. The whole region is covered by the semideep-deep lacustrine mudstone of Sha 3 member (Es3) of Palaeogene, which constitutes the regional cap rock in presence of many other localized cap rocks. There are four source-reservoir-cap assemblages including Upper Palaeozoic-generating and Mesozoic-preserving assemblage, Upper Palaeozoic-generating and Cenozoic-preserving assemblage, Upper Palaeozoic-generating and Upper Palaeozoic-preserving assemblage and Upper Palaeozoic-generating and Lower Palaeozoic-preserving assemblage. The source rock, reservoir and cap rocks match well in time and space. Tectonic evolution controlled the thermal evolution of the Upper Palaeozoic coal-measure source rocks, which experienced five hydrocarbon generation stages. Hydrocarbon generated from the coal strata of Upper Palaeozoic migrated to the surrounding traps through migration pathways consisting primarily of faults, cracks and unconformities, among which faults and unconformities played a major role. Multiple types of traps, mainly structural traps, were formed by the intense tectonic movements that historically occurred in this area. The trap formation stages match well with the hydrocarbon generation stages. By analyzing the accumulation conditions of coal-derived gas of Upper Palaeozoic, we identified four accumulation models and further predicted the favorable hydrocarbon accumulation zone in the study area.

Testing the basin-centered gas accumulation model using fluid inclusion observations: Southern Piceance Basin, Colorado

The Upper Cretaceous Mesaverde Group in the Piceance Basin, Colorado, is considered a continuous basin-centered gas accumulation in which gas charge of the low-permeability sandstone occurs under high pore-fluid pressure in response to gas generation. High gas pressure favors formation of pervasive systems of opening-mode fractures. This view contrasts with thatofothermodelsoflow-permeabilitygasreservoirsinwhich gas migrates by buoyant drive and accumulates in conventional traps, with fractures an incidental attribute of these reservoirs. We tested the aspects of the basin-centered gas accumulation model as it applies to the Piceance Basin by determining the timing of fracturegrowth and associated temperature,pressure, and fluid-composition conditions using microthermometry and Raman microspectrometry of fluid inclusions trapped in fracture cement that formed during fracture growth. Trapping temperatures of methane-saturated aqueous fluid inclusions record systematic temperature trends that increase from approximately 140 to 185°C and then decrease to approximately 158°C over time, which indicates fracture growth during maximum burial conditions. Calculated pore-fluid pressures for methanerich aqueous inclusions of 55 to 110 MPa (7977–15,954 psi) indicate fracture growth under near-lithostatic pressure conditions consistent with fracture growth during active gas maturation and charge. Lack of systematic pore-fluid–pressure trends

Forming mechanism of hydrocarbon reservoirs in Yingshan Formation of Yuqi block in Akekule arch, Tarim Basin

The reservoir conditions, oil and gas charge history and accumulation phases were studied for Yingshan Formation of Yuqi block, and an oil and gas accumulation model was established by using the techniques of reservoir prediction, fluorescence thin section and fluid inclusion analysis under the guidance of the theories of oil and gas accumulation. The results indicate that the main rock types in Yingshan Formation are micrite and calcarenite. The carbonate reservoirs are of cave, fracture-pore and fracture types, and their physical properties are intermediate; there are at least four oil/gas charges, i.e. late Hercynian, Yanshanian, early Himalayan and middle Himalayan (Cenozoic). The most important charge periods are late Hercynian, early Himalayan and middle Himalayan; the oil and gas accumulation model is self source—lateral expulsion of hydrocarbon—multistage accumulation, or hydrocarbon sourced from and preserved in the same old rocks—long term expulsion of hydrocarbon—multistage accumulation.

Gas accumulation models and characteristics of gas fields in China

Due to different accumulation conditions between natural gas and oil, as well as the complexity of geological settings of the main gas bearing basins in China, accumulation of natural gas in Chinese gas fields is a complex process. In terms of timing of gas charging and entrapment, gas fields in China can be grouped into four types: fields with super late (Neogene–Quaternary) gas generation, migration and entrapment; fields with late (Paleogene–Neogene) gas generation, migration and entrapment; fields characterized by late (Tertiary–Quaternary) secondary accumulation of gas generated and accumulated earlier (primarily Mesozoic); and fields with early (primarily Mesozoic) gas generation, migration and entrapment. The first three types are all finalized during the Himalayan period. The Himalayan tectonic movements have, therefore, significant effects on the formation and distribution characteristics of natural gas pools in China. Currently, we are focusing our gas exploration in three major areas: the foreland basins in west–central China, three cratonic superimposed basins (Sichuan, Ordos and Tarim) and offshore rift basins. In these gas-rich basins, the final gas charging and entrapment took place in late and super late stages.

The axisymmetric vibrations of a cylindrical incompressible liquid column with a gas bubble

The axisymmetric vibrations of an ideal incompressible liquid column in a rigid circular cylindrical vessel with a spherical gas bubble pulsating near the position of dynamic equilibrium are considered. The boundary-value problem for the liquid velocity potential and the equations for the vibrations of the gas bubble are solved under the conditions on the free surface, sidewall, and the boundary of the gas body. For the case of small amplitudes, the resonance frequencies of the system are determined, and the pressure field in the liquid column is constructed. The results are compared with data known for the gas-accumulation model, data obtained without allowance for the boundedness of the liquid, and experimental data.