Gas Generation Processes Research Articles

Chemical kinetics evaluation and its application of natural gas generation derived from the Yacheng Formation in the deep-water area of the Qiongdongnan Basin, China

The natural gas generation process is simulated by heating source rocks of the Yacheng Formation, including the onshore-offshore mudstone and coal with kerogens of Type II2-III in the Qiongdongnan Basin. The aim is to quantify the natural gas generation from the Yacheng Formation and to evaluate the geological prediction and kinetic parameters using an optimization procedure based on the basin modeling of the shallow-water area. For this, the hydrocarbons produced have been grouped into four classes (C1 C2 C3 and C4-6). The results show that the onset temperature of methane generation is predicted to occur at 110°C during the thermal history of sediments since 5.3 Ma by using data extrapolation. The hydrocarbon potential for ethane, propane and heavy gaseous hydrocarbons (C4-6) is found to be almost exhausted at geological temperature of 200°C when the transformation ratio (TR) is over 0.8, but for which methane is determined to be about 0.5 in the shallow-water area. In contrast, the end temperature of the methane generation in the deep-water area was over 300°C with a TR over 0.8. It plays an important role in the natural gas exploration of the deep-water basin and other basins in the broad ocean areas of China. Therefore, the natural gas exploration for the deep-water area in the Qiongdongnan Basin shall first aim at the structural traps in the Ledong, Lingshui and Beijiao sags, and in the forward direction of the structure around the sags, and then gradually develop toward the non-structural trap in the deep-water area basin of the broad ocean areas of China.

Methane clumped isotopes in the Songliao Basin (China): New insights into abiotic vs. biotic hydrocarbon formation

Abiotic hydrocarbon gas, typically generated in serpentinized ultramafic rocks and crystalline shields, has important implications for the deep biosphere, petroleum systems, the carbon cycle and astrobiology. Distinguishing abiotic gas (produced by chemical reactions like Sabatier synthesis) from biotic gas (produced from degradation of organic matter or microbial activity) is sometimes challenging because their isotopic and molecular composition may overlap. Abiotic gas has been recognized in numerous locations on the Earth, although there are no confirmed instances where it is the dominant source of commercially valuable quantities in reservoir rocks. The deep hydrocarbon reservoirs of the Xujiaweizi Depression in the Songliao Basin (China) have been considered to host significant amounts of abiotic methane. Here we report methane clumped-isotope values (Δ18) and the isotopic composition of C1–C3 alkanes, CO2 and helium of five gas samples collected from those Xujiaweizi deep reservoirs. Some geochemical features of these samples resemble previously suggested identifiers of abiotic gas (13C-enriched CH4; decrease in 13C/12C ratio with increasing carbon number for the C1–C4 alkanes; abundant, apparently non-biogenic CO2; and mantle-derived helium). However, combining these constraints with new measurements of the clumped-isotope composition of methane and careful consideration of the geological context, suggests that the Xujiaweizi depression gas is dominantly, if not exclusively, thermogenic and derived from over-mature source rocks, i.e., from catagenesis of buried organic matter at high temperatures. Methane formation temperatures suggested by clumped-isotopes (167–213 °C) are lower than magmatic gas generation processes and consistent with the maturity of local source rocks. Also, there are no geological conditions (e.g., serpentinized ultramafic rocks) that may lead to high production of H2 and thus abiotic production of CH4 via CO2 reduction. We propose that the Songliao gas is representative of an atypical type of thermogenic gas that can be mistaken for abiotic gas. Such gases may be encountered more frequently in future exploration of deep or over-mature petroleum systems.

Design of structure and simulation of the three-zone gasifier of dense layer of the inverted process

Experts of LLC “New Energy Technologies” have developed gasifiers designs, with the implementation of the three-zone gasification method, which satisfy the following conditions: 1) the generated gas must be free from tar, soot and hydrocarbons, with a given ratio of CO/H2; 2) to use as the fuel source a wide range of low-grade low-value solid fuels, including biomass and various kinds of carbonaceous wastes; 3) have high reliability in operation, do not require qualified operating personnel, be relatively inexpensive to produce and use steam-air blowing instead of expensive steam-oxygen one; 4) the line of standard sizes should be sufficiently wide (with a single unit capacity of fuel from 1 to 50-70 MW). Two models of gas generators of the inverted gasification process with three combustion zones operating under pressure have been adopted for design: 1) gas generator with a remote combustion chamber type GOP-VKS (two-block version) and 2) a gas generator with a common combustion chamber of the GOP-OK type (single-block version), which is an almost ideal model for increasing the unit capacity.There have been worked out various schemes for the preparation of briquettes from practically the entire spectrum of low-grade fuel: high-ash and high-moisture coals, peat and biomass, including all types of waste - solid household waste, crop, livestock, poultry, etc. In the gas generators there are gasified the cylindrical briquettes with a diameter of 20-25 mm and a length of 25-35 mm.There have been developed a mathematical model and computer code for numerical simulation of synthesis gas generation processes in a gasifier of a dense layer of inverted process during a steam-air blast, including: continuity equations for the 8 gas phase components and for the solid phase; the equation of the heat balance for the entire heterogeneous system; the Darcy law equation (for porous media); equation of state for 8 components of the gas phase; equations for the rates of 3 gas-phase and 4 heterogeneous reactions; macro kinetics law of coke combustion; other equations and boundary conditions.

The biogeochemistry of gas generation from low-level nuclear waste: Modelling after 18 years study under in situ conditions

Gas generation from low- and intermediate-level nuclear waste (LILW) is an important process related to the safety of near surface and geological nuclear waste repositories. Generation of a gas phase has the potential to transport gaseous radionuclides such as 14C to the biosphere. Furthermore, pressurisation and gas phase formation can affect the engineered barrier system and water flow, which can affect the transport of radionuclides and other soluble contaminants in water. Microbial gas generation is coupled with a wider range of biogeochemical processes that are also relevant to the safety of LILW repositories through effects on pH, Eh and organic complexation. The large scale Gas Generation Experiment (GGE), located at the VLJ repository, Olkiluoto, Finland has been in operation for 18 years to study processes of gas generation from cellulose-containing low-level waste (LLW) from the Olkiluoto power plants. Here we provide an update of the experiment data set that records, over a period of 18 years, the neutralisation of the initial pH 10–11 conditions in the water filled regions of the experiment that were buffered by a concrete container, of relatively small mass, by acidity generated by degradation of cellulose LLW materials. The neutralisation of pH below pH 9 coincides with a doubling of the generation rate of a CH4-rich gas to around 1 m3 per year. Aqueous sulfide is also present at low levels (<3 × 10−6 M) at this time, consistent with equilibrium with mackinawite (FeS). Dissolved organic carbon attained a peak concentration of 8 mM during the early stages of the experiment, prior to the higher gas generation rate, but after 9 years declined. After 18 years the Eh of the tank water measured by a Pt electrode is consistent with that calculated for the S(-2)/S(6) and methanogenesis redox couples suggesting equilibration of redox processes. Carbonate concentration has increased steadily to around 15 mM. After 7 years CO2 gas and aqueous and solid carbonates are also close to equilibrium. The experimental data has been interpreted with the aid of a biogeochemical model that represents the coupled processes of organic waste degradation, pH buffering and microbial gas generation. The model simulates that the majority of the gas is generated by H2 and organic consuming methanogenic processes in the waste drums. The increase in CH4 generation is simulated to be the result of methanogenic processes that consume soluble cellulose degradation products that diffuse into the tank water from the waste drums. It is expected that under the steady state conditions now established, resulting from gas/liquid/solid equilibrium and diffusion processes, that gas generation should continue until the cellulose and steel materials are exhausted. The experimental data and biogeochemical model can be used to predict the rate of CH4-rich gas generation from the VLJ and other repositories with higher proportions of cementitious materials. In addition the study has shed further light on the role of microbial processes in affecting pH buffering of cementitious conditioned LILW.

Coal pore size distributions controlled by the coalification process: An experimental study of coals from the Junggar, Ordos and Qinshui basins in China

Various sizes of pores in coal, which are generally formed by organic matter during the coalification process, have a direct influence on coalbed methane extraction. However, few studies have investigated the pore size distributions across the thermal evolution of coal from peat to anthracite. In this project, three series of coal samples collected from three key CBM development basins with graded vitrinite reflectance values (Ro), the eastern Junggar basin (Ro of approximately 0.5%), eastern Ordos basin (Ro of approximately 2.2%) and southern Qinshui basin (Ro of approximately 3.0%), were systematically characterized by optical observations, low-temperature nitrogen adsorption/desorption, and nuclear magnetic resonance (NMR) methods. The average pore radius calculated by the Brunauer-Emmett-Teller (BET) method shows that the low-rank (L) series (averaging 14.17nm) has values higher than either the middle-rank (M, 12.70nm) or high-rank (H, 12.66nm) samples. Bright and semi-bright coals (determined by the overall relative lustre and percentage of bright components) are generally distributed with relatively higher pore radii (averaging 16.86nm for all 3 series) than the semi-dull and dull coals (9.50nm). The range of pore sizes decreases as the coal rank increases, and the NMR transverse relaxation (T2) spectrum decreases from bi-modal and tri-modal (M and L series) to unimodal curves (H series). However, the pore surfaces and complexity inside the coal increase with the coal rank, with the fractal results showing a three-stage fitting slope of the H series compared with the M (two-stage) and L (one-stage) coals. The observations are generally caused by the L coals, which mainly include plant tissue pores, while the M series coals are characterized by circle-shaped tissue pores and gas pores. The H series of flattened tissue pores and more diverse gas pores are identified in the higher-rank coals. Combined with the thermogenic gas generation process of coal, three key transition points were recognized: (1) Ro of approximately 0.5%, transition of dehydration to bituminization with coals being much more compacted, shown as the >100nm range pores decreasing sharply; (2) Ro of approximately 1.2%, the beginning of the debituminization stage with the intensive generation of thermogenic gas, with pores ranging between 10 and 50nm increasing quickly; and (3) Ro of approximately 1.9%, coal being transformed into anthracite, becoming much more compacted with the induction of cleats/fractures, shown as another decrease in >100nm range pores but an increase in 50–100nm range pores. These observations could deepen the understanding of the complex pore size distribution differences between different coal ranks and the impact of the thermal evolution on the coal heterogeneity and its reservoir characteristics.

Generation of ultra-intense gamma-ray train by QED harmonics

In nonlinear media, photons may combine into a photon of energy and momentum of all those photons. This process, called harmonic generation, happens in nonlinear crystal, gas, and relativistic plasma. When the laser intensity reaches 1022 W/cm2, QED effects appear and play a significant role in the harmonic generation. In contrast to the gas and relativistic high-order harmonic generation processes, harmonics influenced by QED effects are usually not coherent because of the characteristic of random radiation, while the property of high intensity and ultra-short duration is conserved. In this work, the generation of high-order harmonics with QED effects is investigated by one- and two-dimensional particle-in-cell simulations. Studies have shown that interacting with a laser pulse with the intensity of I=5.35×1023 W/cm2, such harmonics can produce ultra-short gamma-ray train with periodic structures. The period of gamma-ray train is half of the laser period, and the peak intensity is 1.4×1022 W/cm2 from one-dimensional simulation when ions are considered immobile. This new harmonic production with QED effects are crucial to light-matter interaction in strong field and can be verified in experiments by 10 PW laser facilities in the near future.

Kinetics of oil-cracking for different types of marine oils from Tahe Oilfield, Tarim Basin, NW China

The C1–C5 gas and carbon isotope ratios generated during the cracking of heavy, normal and high-waxy marine oils from Tahe Oilfield, Tarim Basin, NW China, using a closed-gold tube system under high pressure were presented. The three types of oil tested resulted in a similar gas-generation process. The C2–C5 range initially increased its yield with the application of pyrolysis temperature and thereafter decreased, while the C1 yield increased with the same application. High-waxy oil had the highest C1–C5 yield of 510 mg/goil, whereas heavy oil had the lowest C1–C5 yield of 316 mg/goil. The δ13C1 value was low at first, but gradually became higher as the pyrolysis temperature increased. However, the δ13C2 and δ13C3 values gradually became higher when the temperature was greater than 420 °C. The kinetic parameters of the C1–C5 gas generation for the different types of marine oils were then calculated using KINETIC software. This calculation resulted in a frequency factor of about 1.78 × 1014 s−1, while the distribution of the activation energy of the C1–C5 gas mass generated was relatively narrow with a range from 56 to 66 kcal/mol. Among the three types of oil tested, heavy oil had the widest activation energy distribution and the lowest major frequency of activation energy. The maximum temperature at which oil could be preserved as a separate oil phase varied from about 178 °C at a slow geological heating rate to 206 °C at a fast geological heating rate. This result is based on the kinetic parameters determined and in combination with the fractional conversion (C) of oil to gas. Testing conducted at the volatile Middle Cambrian reservoir of well Zhongshen 1 in the Tazhong Uplift strongly supported this conclusion.

Hydrocarbon Generation in the Lacustrine Mudstones of the Wenchang Formation in the Baiyun Sag of the Pearl River Mouth Basin, Northern South China Sea

The lacustrine mudstones of the Eocene Wenchang Formation are the primary source rocks of natural gas in the Baiyun Sag of the Pearl River Mouth Basin, China. The hydrocarbon generation kinetic parameters and the characteristics of lacustrine mudstones sampled from the Eocene Wenchang Formation at well LF13-2-1 were investigated on the basis of a thermocompression simulation experiment in a closed system. In addition, combined with the burial history and paleothermal history, the gas generation process of the Eocene Wenchang Formation source rocks in the Baiyun Sag was investigated through the hydrocarbon generation equations established from parameters of hydrocarbon generation kinetics. The gas generation process of the Eocene Wenchang Formation lacustrine mudstones has three thermal evolutionary stages: the middle mature stage (39–30 Ma), the late mature stage (30–23.5 Ma), and the main gas generation stage (23.5–0.0 Ma). The main hydrocarbon generation stage in the central area of the Baiyun Sag occurred from 24 Ma to 22 Ma, while the main hydrocarbon generation stage in the eastern and northern areas occurred at ∼15 Ma.

Formation of low permeability reservoirs and gas accumulation process in the Daniudi Gas Field, Northeast Ordos Basin, China

The Daniudi Gas Field is a typical large-scale coal-generated wet gas field located in the northeastern Ordos Basin that contains multiple Upper Paleozoic gas-bearing layers and considerable reserves of gas. Based on integrated analysis of reservoir petrology, carbonate cement C–O isotope, geochemistry of source rocks and HC gas and numerical basin modeling, a comprehensive study focusing on the formation of low permeability reservoirs and gas generation process uncovers a different gas accumulation scene in Daniudi Gas Field. The gas accumulation discovered was controlled by the reservoir permeability reduction and gas generation process, and can be divided into two distinct stages by the low permeability reservoir formation time: before the low permeability reservoir formation, the less matured gas was driven by buoyancy, migrated laterally towards NE and then accumulated in NE favorable traps during Late Triassic to early Early Cretaceous; after the low permeability reservoir formation, highly matured gas was driven by excessive pressure, migrated vertically and accumulated in-situ or near the gas-generating centers during early to late Early Cretaceous. The coupling relationship between reservoir diagenetic evolution and gas generation process controlled on gas accumulation of the Daniudi Gas Field. This study will aid in understanding the gas accumulation process and planning further E&D of the Upper Paleozoic super-imposed gas layers in the whole Ordos Basin and other similar super-imposed low permeability gas layer basins.

Experimental study on the gas generation processes of lacustrine and marine shales in North China: Source implications for shale gas

Using a high pressure, semi-closed thermal simulation system, two organic-rich lacustrine and marine immature shales from North China were investigated to probe the dynamic processes and potential for the gas generation throughout the oil-gas window. The results identified at least two processes during gas generation, as revealed by changes of the gas dry coefficient (C1/ΣC1-5%) and carbon isotopic composition of methane (δ13Cmethane). The first process is associated with kerogen thermal degradation and occurs during the main phase of oil generation, showing an increasing trend of the dry coefficient and δ13Cmethane. The second process begins during the late phase of oil generation, accompanied by increasing yields of wet gas components and 13C depleted methane. Due to the lack of aliphatic carbon in kerogen in the highly to over mature stage, as demonstrated by FTIR analyses, the second process is mainly related to the cracking of kerogen-generated bitumen which is retained in the shale.The hydrogen content, a key parameter for petroleum generation, is then used to quantitatively evaluate the gas generation potential from kerogen thermal degradation and retained-bitumen cracking. Mass balance calculations show that kerogen-generated bitumen that is retained in lacustrine and marine shales is estimated to contribute 30% of the total gas yields at the stage of 1.5–2.0% Ro. This result is consistent with the theoretical calculations from Xia et al. (2013) and has important implications for the shale gas sources, and further the shale gas resource assessments before exploration in shale-gas petroleum systems.

Coal pyrolysis characteristics by TG–MS and its late gas generation potential

As natural gas production from deep strata becomes increasingly successful in recent years, the evaluation on gas generation potential from OM (organic matter) in the deep formations is attracting increasing attention recently, especially the late gas generation potential. In this paper, a TG–MS (thermogravimetric analysis–mass spectrometry) device was applied to investigate the process and characteristics of gas generation of HCs (hydrocarbons) and non-HCs (non-hydrocarbons) during the pyrolysis processes of coal from Songliao Basin, and its late gas generation potential. The TG–MS experimental results show that: (1) during coal pyrolysis with heating rate of 10 °C/min, methane generation started at ∼300 °C, reached its peak generation at 480 °C, and ended at ∼850 °C, which was much higher than the ending generation temperatures of other HCs; (2) when investigating the methane generation characteristics through the gas generation curve by ion fragments, more accurate results are obtained with mass to charge ratio m/z = 15 than currently used m/z = 16; (3) this coal has a high late gas generation potential, with 17% additional methane generated at late generation stage after 600 °C (equivalent to a vitrinite reflectance: Ro ⩾ 2.5%), and the methane generation ended at a maturity level of 5.3% (Ro). After calibration with coal pyrolysis results, a kinetic model has been established and applied successfully to estimate gas generation timing and illustrate gas generation mechanisms of the Shahezi Formation in the Xujiaweizi Fault Depression. Based on the results derived from the proposed kinetic model, the gas generation timing for the Shahezi Formation was between ∼115 and 50 Ma, and methane appeared to be generated steadily without any major generation peak indentified during this period and the gas generation lasted for ∼65 Ma.

Kinetics and model of gas generation of source rocks in the deepwater area, Qiongdongnan Basin

In order to investigate the hydrocarbon generation process and gas potentials of source rocks in deepwater area of the Qiongdongnan Basin, kinetic parameters of gas generation (activation energy distribution and frequency factor) of the Yacheng Formation source rocks (coal and neritic mudstones) was determined by thermal simulation experiments in the closed system and the specific KINETICS Software. The results show that the activation energy (E a) distribution of C1–C5 generation ranges from 50 to 74 kcal/mol with a frequency factor of 2.4×1015 s−1 for the neritic mudstone and the E a distribution of C1–C5 generation ranges from 49 to 73 kcal/mol with a frequency factor of 8.92×1013 s−1 for the coal. On the basis of these kinetic parameters and combined with the data of sedimentary burial and paleothermal histories, the gas generation model of the Yacheng Formation source rocks closer to geological condition was worked out, indicating its main gas generation stage at R o (vitrinite reflectance) of 1.25%–2.8%. Meanwhile, the gas generation process of the source rocks of different structural locations (central part, southern slope and south low uplift) in the Lingshui Sag was simulated. Among them, the gas generation of the Yacheng Formation source rocks in the central part and the southern slope of the sag entered the main gas window at 10 and 5 Ma respectively and the peak gas generation in the southern slope occurred at 3 Ma. The very late peak gas generation and the relatively large gas potential indices (GPI: 20×108–60×108 m3/km2) would provide favorable conditions for the accumulation of large natural gas reserves in the deepwater area.

Simulation of Gas Generation from the Paleogene Enping Formation in the Baiyun Sag in the Deepwater Area of the Pearl River Mouth Basin, the South China Sea

The deepwater area of the Baiyun Sag is the primary region of deepwater oil and gas exploration in China. Since the breakthrough made in natural gas exploration in the structure of the LW3-1 gas field, explorers have given significant attention to the conditions and process of gas generation and to the accumulation and preservation of natural gas. The Paleogene Enping Formation provides one of the two sets of major source rocks in the Baiyun Sag. We selected three samples of source rocks from the Enping Formation, performed a thermocompression simulation experiment, calculated the hydrocarbon generation kinetic parameters, and simulated single-well gas generation to explore the process of gas generation in the gas-generating center of the Enping Formation in the Baiyun Sag. The major conclusions included the following: (1) The lacustrine dark mudstone and the carbonaceous mudstone from the Enping Formation in well HZ08-1-1 have hydrocarbon yields of 383.7 versus 307.3 kg/t of total organic carbon (TOC) and gas yields of 370.1 versus 302.9 kg/t of TOC. (2) The thermal evolution of the Enping Formation in the Baiyun Sag can be divided into three evolutionary stages. (3) Finally, the source rocks of the Enping Formation have a gas generation peak from approximately 17.5 to 8 Ma.

GPS-based crustal deformations in Azerbaijan and their influence on seismicity and mud volcanism

Using Shen’s method (Shen et al., 1996), deformations of the Earth’s crust in Azerbaijan were studied based on GPS measurements. For estimating the rate of deformation, we used the field of velocity vectors for Azerbaijan, Iran, Georgia, and Armenia that were derived from GPS measurements during 1998–2012. It is established that compression is observable along the Greater Caucasus, in Gobustan, the Kura depression, Nakhchyvan Autonomous Republic, and adjacent areas of Iran. The axes of compression/contraction of the crust in the Greater Caucasus region are oriented in the S-NE direction. The maximum strain rate (approximately 200 × 10−9 per annum) is documented in the zone of mud volcanism at the SHIK site (Shykhlar), which is marked by a sharp change in the direction of the compression axes (SW-NE). It is revealed that the deformation field also includes the zones where strain rates are very low approximating 5 × 10−9 per annum. These zones include the Caspian-Guba and northern Gobustan areas, characterized by extensive development of mud volcanism. The extension zones are confined to the Lesser Caucasus and are revealed in the Gedabek (GEDA) and Shusha (SHOU) areas, as well as in the zone located between the DAMO and PIRM sites (Iran), where the deformation rate amounts to 100 × 10−9 per annum. It is concluded that the predominant factor responsible for the eruption of mud volcanoes is the intensity of gas-generation processes in the earth’s interior, while deformation processes play the role of a trigger. The zone of the epicenters of strong earthquakes is correlated to the gradient zone in the crustal strain rates.

Расширение уровня информативности метода термогазовой хроматографии

This article presents the results of studies of a deep sorbed gas, which were obtained during the experimental work. The analysis of gas from rock samples of En-Yakhinskaya borehole was made using the method of thermo gas chromatography (ТGC). Analyses are performed at three temperatures: 90, 200, 300 degrees Celsius. Generaliztion of the results of study allowed making conclusion that the method thermo gas chromatography has proven to be capable to establish the peculiarities of the process of gas generation.

Evidence of gas permeation in electrostatically painted steel cylinders

The process of gas generation, entrapment and subsequent permeation through coatings applied on the surface of metals for corrosion protection purposes is the cause for the development of a defect known as “fish-scaling”. Particularly, in operations like enamelling or painting of metals, this phenomenon affects strongly the surface quality of the items and the development of further processing operations. In this work, the reasons that promote the development of defects in electrostatically painted steel during the manufacture route of fire extinguishers are analysed. The surface and cross section analyses of the samples conducted by optical and scanning electron microscopy suggest that the degradation of the coating is caused by gas migration from the metal-coating interface to the surface of the cylinders. Water used during cleaning and descaling operations acted as a potential gas source being hydrogen the element responsible for the formation of both, structural defects in the coating and cracks in the steel substrate. The degradation mechanisms of the steel and the coatings are found and explained in terms of manufacturing process of the cylinders.

Numerical modeling of hydrocarbon generation in the Douglas Formation of the Athabasca basin (Canada) and implications for unconformity-related uranium mineralization

The Proterozoic Athabasca Basin in Canada is known for its world-class, high-grade uranium deposits developed near the unconformity between the basin and the basement. The Douglas Formation in the upper part of the preserved strata in the basin contains total organic carbon (TOC) of up to 3.56wt.%. Previous studies of organic matter maturation and hydrocarbon inclusions indicate that oil and gas have been generated from this formation, and that some hydrocarbons found in the unconformity-related uranium deposits were derived from the Douglas Formation. This study aims to evaluate how the oil and gas generation processes may have affected the fluid overpressure development in the basin, and whether or not the hydrocarbons generated in the Douglas Formation could migrate downward to stratigraphically lower intervals and eventually to the sites of mineralization near the unconformities. We carried out a series of numerical experiments to examine fluid overpressures, flow directions, temperatures, and oil and gas generation processes using a two-dimensional conceptual model derived from a geologic cross-section from the basin center to the eastern margin. An additional 5km strata were added to current basin stratigraphy to account for observed paleogeothermal data. Variation studies were undertaken to account for uncertainties in the lithologies of the eroded strata, and the wide ranges of possible permeabilities of different lithologies and kinetic parameters of oil and gas generation. It is found that, if moderate permeabilities are used in the modeling for each lithology (known as the base model), oil and gas generation processes contribute little to the development of fluid overpressure, and fluid pressure in the basin is close to hydrostatic regardless of whether or not hydrocarbon generation in the Douglas Formation is included in the modeling. However, if permeabilities are assigned values one order of magnitude lower than in the base model, significant fluid overpressures are developed in the eroded strata in the upper part of the model. In the base model, oil generated in the Douglas Formation may migrate downward, driven by an overpressure zone situated above the Douglas Formation, but gas migrates upward. In the low-permeability model, however, the overpressures developed above the Douglas Formation are so high that both oil and gas generated in the Douglas Formation migrate downward. The numerical modeling results thus indicate that it is hydrodynamically possible for oil and gas generated in the Douglas Formation to migrate to the base of the basin and reach the sites of the unconformity-related uranium deposits.

Natural Gas Exploration Using Carbon Isotopic Fractionation Effect: A Case Study of Shanxi Formation, Upper Palaeozoic Group in the Center of Ordos Basin, China

Abstrac t—Carbon isotopic fractionation would occur during the process of natural gas generation and migration. As for 'self-generating and self-preserving' tight sandstone gas reservoir, carbon isotope better keeps the current maturity and migration results. In this paper, three local hydrocarbon generation centers and two relatively lower permeability zones were determined from the study area and it is suggested that natural gas migrated roughly from south to north direction through the analysis of carbon isotope isoline. Thus, we predict three favorable exploration areas using natural gas storage index combined with above isotopic results. It is a common geochemistry phenomenon that carbon isotopic fractionation would occur during the process of natural gas generation and migration. Carbon isotope becomes heavier as organic matter of the gas source rock becomes more mature; meanwhile, carbon isotope appears lighter along the direction of natural gas migration. Therefore natural gas carbon isotope records parent material source, and information of geology and geochemistry action that parent material and its products experience. Researchers in China and other countries have found and confirmed fractionation effect of natural gas carbon isotope, and built quantitative relations of δ13C1 and Ro that suggested specific mathematical relationship for the different type of source rocks. It is verified by indoor research that carbon isotopic fractionation range is wide under low temperature, and hydrocarbons that organic matter discharges contain lighter carbon isotope; as the temperature rises, fractionation effect is reduced, and carbon isotope of generated compounds become heavier(1),(2). It is also concluded by indoor experiments that natural gas always displays carbon isotopic fractionation effect to a different extent when it is injected into and diffuses through several types of lower permeability rocks respectively, in which montmorillonitic clay is the most obvious, kaolinitic clay takes the second and limestone fractionation effect is the least(3)-(6). Moreover, gas isotopic fractionation is influenced by migration dimension and channel. When natural gas flows in bulk through reservoir rocks, carbon isotopic fractionation is not obvious as the gas-rock medium contract area is small and the dwelling time is short; whereas the gas is seeping through the rocks with a smaller scale and slower flow rate, the fractionation is strong because of the long dwelling time and the adequate gas-rock contract, that leads carbon isotope along front of migration direction becomes light. As a result, an application of carbon isotope information is to infer the direction of natural gas migration and the possible enrichment zone. According to the geochemistry analysis and some other geologic evidence of gas reservoir in the eastern-central part of the Ordos basin, this paper just discusses how to predict natural gas migration and favourable exploration areas using carbon isotopic fractionation information.

Mathematical model for predicting the state of health of transformers and service methodology for enhancing their life

This paper presents the analysis and modeling of the mathematical characteristics of the dissolved gases in the transformer oil for the purpose of working out a schedule for oil filtration, which in turn regulates the quality of oil. A restriction, on the total volume of dissolve gases in the oil, improves the performance and enhances the life of a transformer. The stochastic characteristic of the gas generation inside the oil, has been monitored and studied over a long duration, through different times of the week and different months of the year. Looking at the characteristics of gas generation and routine process of gas removal (by oil filtration), a Markov model has been developed, to predict the state of health of a transformer and suggest a schedule for regular gas filtration, for the purpose of extending the life of the transformer. However, filtration itself doesn’t remove the source of the fault. Important parameters like, the capacity of oil tank, rate of gas generation and various other key parameters for restricting the volume of total gas, inside the transformer have been derived. The mathematical model can also optimize some of the design parameters of a transformer, depending on the service frequency of various users and their choice of life expectancy of the transformer.

The upper limit of maturity of natural gas generation and its implication for the Yacheng formation in the Qiongdongnan Basin, China

Vitrinite reflectance (VR, Ro%) measurements from residual kerogen of pyrolysis experiments were performed on immature Maoming Oil Shale substituted the samples for the gas-prone source rocks of Yacheng formation of the Qiongdongnan Basin in the South China Sea. The work was focused on determination an upper limit of maturity for gas generation (ULMGG) or “the deadline of natural gas generation”. Ro values at given temperatures increase with increasing temperature and prolonged heating time, but ΔRo-value, given a definition of the difference of all values for VR related to higher temperature and adjacent lower temperature in open-system non-isothermal experiment at the heating rate of 20°C/min, is better than VR. And representative examples are presented in this paper. It indicates that the range of natural gas generation for Ro in the main gas generation period is from 0.96% to 2.74%, in which ΔRo is in concordance with the stage for the onset and end of the main gas generation period corresponding to 0.02% up to 0.30% and from 0.30% up to 0.80%, respectively. After the main gas generation period of 0.96–2.74%, the evolution of VR approach to the ULMGG of the whole rock for type II kerogen. It is equal to 4.38% of VR, where the gas generation rates change little with the increase of maturation, ΔRo is the maximum of 0.83% corresponding to VR of 4.38%Ro, and the source rock does not nearly occur in the end process of hydrocarbon gas generation while Ro is over 4.38%. It shows that it is the same the ULMGG from the whole rock for type II kerogen as the method with both comparison and kinetics. By comparing to both the conclusions of pyrolysis experiments and the data of VR from the source rock of Yacheng formation on a series of selected eight wells in the shallow-water continental shelf area, it indicate that the most hydrocarbon source rock is still far from reaching ULMGG from the whole rock for type II kerogen. The source rock of Yacheng formation in the local areas of the deepwater continental slope basin have still preferable natural gas generative potential, especially in the local along the central depression belt (namely the Ledong, Lingshui, Songnan and Baodao sags from southwest to northeast) from the depocenter to both the margin and its adjacent areas. It help to evaluate the resource potential for oil and gas of the hydrocarbon source rock in the deepwater continental slope of the Qiongdongnan Basin or other basins with lower exploration in the northern of the South China Sea and to reduce the risk in exploration.