Ordos Basin Of China Research Articles

Calculation of salt basin depth using fluid inclusions in halite from the Ordovician Ordos Basin in China

During the Middle Ordovician, the salt deposits of the Majiagou Formation of the Ordos Basin in North China formed, the halite including various genetic types of fluid inclusion. In this study, fit-testing of primary inclusions was performed to reconstruct the physical and chemical conditions during original halite sedimentation. During the post-sedimentation stage, salt was subjected to elevated temperatures (62–70°C) and pressures of tens of megapascals. From these measurements and mathematical calculations, the gas pressure was identified in the primary fluid inclusions, which allowed us to estimate that the brine column thickness in the salt basin was approximately 40 m

Impacts of diagenesis of tight sandstone gas reservoir on reservoir physical properties: A case study, Sulige gas field, Ordos Basin, China

Diagenesis is one of the most important factors impacting the performance of many reservoirs and is perhaps the most important factor impacting the performance of tight sandstone reservoirs, such as those of the Sulige gas field in the Ordos Basin of China. However, the relationship between diagenesis and related parameters determining reservoir physical properties remains unclear. Therefore, we have analyzed experimental data from high-pressure mercury intrusion porosimetry, scanning electron microscopy, and thin sections in addition to using a porosity recovery calculation model to investigate microscopic characteristics, diagenesis, and pore-evolution processes of the low-permeability tight gas reservoir of the He-8 unit of the Sulige gas field in the Ordos Basin. In addition, we have identified the impacts of diagenesis on reservoir characteristics and established the relationship between diagenesis and reservoir quality evolution. We also used the Beard primary porosity model to recover the primary porosity, and to built the reducing and enhancing calculation models for intergranular pore, dissolution pore, and intercrystalline pore during diagenesis. Based on the quantitative relationship between diagenesis processes and porosity evolution, we found that the results of simulation calculation and experimental works were in close agreement with minimal error.

Assessing bare-soil evaporation from different water-table depths using lysimeters and a numerical model in the Ordos Basin, China

In semiarid and arid regions, the evaporation from bare soil is highly sensitive to changes in the depth to the water table. This study quantifies the relation between water-table depth and the groundwater contribution to evaporation in the Ordos Basin in China. In-situ field experiments were combined with numerical simulations of heat, vapor and liquid water flow. Based on lysimeter experiments and a calibrated numerical model, a relation between depth to groundwater and evaporation rate was established for the lysimeter site. In addition, a sensitivity analysis considering the hydraulic conductivity and the inverse of the air-entry pressure (van Genuchten α) was established. For the field site, the results showed that for the water-table depths less than 52 cm below the ground, evaporation is independent of the water-table depth. For water-table depths exceeding 52 cm, an exponential relation between depth to groundwater and evaporation is observed. No phreatic evaporation occurs for water tables deeper than 105 cm, which is nearly two times the capillary fringe height. The sensitivity analysis showed that the extinction depth decreased with decreasing hydraulic conductivity and increased with α. The field-specific results and the sensitivity analysis provide valuable information to understand the dynamic processes of soil evaporation in the Ordos Basin. From a methodological point of view, the proposed modelling approach and the integration of lysimeter data proved to be a highly efficient combination to study evaporation dynamics in semi-arid and arid environments.

Experimental Investigation of Hydraulic Fracturing for Multi-type Unconventional Gas Co-exploitation in Ordos Basin

The accumulated gas production and production cycle would be significantly increased by exploiting the unconventional gas resources of coalbed methane and tight gas jointly in coal measure strata (tight gases–coal interbedding reservoir) in Ordos Basin in China. However, whether hydraulic fractures could connect multiple gas zones effectively in the vertical plane is a key problem. To study hydraulic fracture geometries and vertical propagation rules for coal measure strata, six groups of true triaxial experiments in natural outcrops collected from the eastern margin of Ordos Basin in China were carried out. Results show that unlike conventional symmetric fracture geometry in layered media, the principle of fracture vertical propagation in coal measure strata follows the least resistance and most preferential propagation path. The natural fractures and cleats in coal seam make the overall fracture morphology asymmetric. A moderate stress difference between vertical stress and maximum horizontal stress (4 MPa in this paper) is the key for connection of multiple layers as well as SRV enhancement. Developed natural fractures and bedding planes in interlayer could alter fracture extension path and impede hydraulic fracture crossing layer interface. In the process of hydraulic fracture propagating from interlayer to layer interface and then to the coal seam, the fluid pressure in hydraulic fractures is characterized by an apparent fluctuation of decreasing first and increasing later. Furthermore, the greater the elastic modulus of the interlayer, the greater the pump pressure fluctuations.

Shale gas content evolution in the Ordos Basin

Gas content is a crucial parameter for shale reservoir evaluation and shale gas accumulation. Taking Shanxi and Taiyuan Formation shale samples from the Ordos Basin of China as an example, this research used thermal simulation, Rock-Eval pyrolysis, total organic carbon content (TOC) measurement, vitrinite reflectance (Ro) measurement, X-ray diffraction analysis, and methane isothermal adsorption based on formation conditions to study the evolution of shale gas content. A thermogenic gas content model was established through thermal simulation, Rock-Eval pyrolysis, and Ro measurement, while the influential factors of adsorbed gas content were analysed and a model was established through TOC and Ro measurements, X-ray diffraction analysis, methane isothermal adsorption, and reservoir conditions. Then, the research characterized the evolution of thermogenic gas, gas-in-place (GIP), adsorbed gas, and free gas through geologic time using a combination of thermogenic gas content, hydrocarbon expulsion and adsorbed gas content models.

Estimating carbon geosequestration capacity in shales based on multiple fractured horizontal well: A case study

Nowadays, shale reservoirs have been though as good candidates for CO2 geosequestration for reduction of carbon emission. When estimating the carbon geosequestration capacity, multiple fractured horizontal well (MFHW) is a prerequisite for consideration when shale reservoirs are included in candidate geological sites, as it is one of the critical factors for the successful development of shale reservoirs. In order to perfect our previous works, a new model is proposed to calculate the carbon geosequestration capacity of depleted shale reservoirs based on MFHW. The methodology calculates carbon geosequestration capacity of shale reservoirs by considering Knudsen diffusion, molecular diffusion, supercritical Langmuir adsorption, stimulated reservoir volume (SRV), and stress-sensitivity of the permeability. Utilizing the perturbation transformation technique, finite difference method, and Laplace transform technique, our method is more in accordance with the actual situations, allowing us to accurately estimate the carbon storage capacity by capturing the transient pressure response of the MFHW. A field case from the Ordos Basin in China presents that the CO2 geosequestration capacity is proportional to the dimensionless fracture half-length and conductivity. The properties of hydraulic fractures only affect the early period of the CO2 geosequestration process. For the adsorption coefficient and flow coefficient ratio, with the change of these parameters, the middle and late periods of the CO2 geosequestration process are mainly influenced.

Characterization of Pore Throat Size Distribution in Tight Sandstones with Nuclear Magnetic Resonance and High-Pressure Mercury Intrusion

Characterization of pore throat size distribution (PTSD) in tight sandstones is of substantial significance for tight sandstone reservoirs evaluation. High-pressure mercury intrusion (HPMI) and nuclear magnetic resonance (NMR) are the effective methods for characterizing PTSD of reservoirs. NMR T2 spectra is usually converted to mercury intrusion capillary pressure for PTSD characterization. However, the conversion is challenging in tight sandstones due to tiny pore throat sizes. In this paper, the linear conversion method and the nonlinear conversion method are investigated, and the error minimization method and the least square method are proposed to calculate the conversion coefficients of the linear conversion method and the nonlinear conversion method, respectively. Finally, the advantages and disadvantages of these two different conversion methods are discussed and compared with field case study. The research results show that the average linear conversion coefficients of the 20 tight sandstone core plugs collected from Yanchang Formation, Ordos Basin of China is 0.0133 μm/ms; the average nonlinear conversion coefficient is 0.0093 μm/ms and the average nonlinear conversion exponent is 0.725. Although PTSD converted from NMR spectra by the nonlinear conversion method is wider than that obtained from linear conversion method, the nonlinear conversion method can retain the characteristic of bi-modal distribution in PTSD.

Techno-Economic Comparison of an Integrated CO2 Capture, Geological Utilization and Storage (CCUS) Projects Between Greater Green River Basin in U.S. And Ordos Basin in China

Techno-Economic Comparison of an Integrated CO2 Capture, Geological Utilization and Storage (CCUS) Projects Between Greater Green River Basin in U.S. And Ordos Basin in China

In-situ analyses of organic matter maturation heterogeneity of uranium-bearing carbonaceous debris within sandstones: A case study from the Ordos Basin in China

Carbonaceous debris (CD) within uranium reservoir sandstones of the Zhiluo Formation has been studied in the Shuanglong uranium deposit of the southern Ordos Basin, northern China. Organic matter maturation (OMM) heterogeneity of uranium-bearing CD was analyzed by means of in-situ methods with optical microscope, laser Raman microprobe and scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS). The results show that the contents of vitrinite, inertinite and minerals are 70.49%, 6.54%, and 22.97%, respectively, and the content of exinite is 0, indicating that vitrinite is the major maceral. Vitrinite reflectance (VR) of uranium-bearing CD ranges from 0.65 %Ro to 2.45 %Ro with the maximum difference of 1.80 %Ro and the mean value of 1.85 %Ro, and the distributions of VR frequencies are discrete, indicating that OMM of uranium-bearing CD might be heterogeneous. Uranium minerals in uranium-bearing CD are mostly coffinite and pitchblende (2–10 µm in diameter). For uranium-bearing vitrinite matrix in CD, VR, G peak shifts and uranium contents range from 1.14 %Ro to 1.84 %Ro, from 1586.5 cm−1 to 1600.1 cm−1 and from 0 to 6.0%, respectively, with the corresponding maximum difference of 0.71 %Ro, 13.6 cm−1 and 6.0%, respectively, indicating that uranium-bearing vitrinite matrix may be of heterogeneity. Shift differences of G peak (△G) and full width at half maxima of G peak (GFWHM) are unevenly distributed in uranium-bearing vitrinite matrix, and △G and GFWHM decrease with increased VR, which shows that OMM of uranium-bearing vitrinite matrix is heterogeneous, and its variation is associated with uranium enrichment. The uneven distributions of uranium minerals cause compositional contrast variations of vitrinite matrix. And compositional contrast is divided into three hierarchies by using backscattered electron images of SEM combined with the distributions of uranium contents, i.e., I (free uranium), II (uranium content less than 2%) and III (uranium content more than 2%). With uranium content increasing, VR shows an obvious rise while △G displays a significant decline. Moreover, when close to the uranium-rich area, VR exponentially increases, which implies that OMM increases with the increased difference in compositional contrast. And compared with VR in I hierarchy of compositional contrast, VR in II hierarchy and III hierarchy at least increases by 0.1 %Ro and 0.2 %Ro, respectively, which demonstrates that compositional contrast variation of organic matrix is relevant to OMM heterogeneity, and the increased difference in compositional contrast corresponds to higher OMM zone. Such findings are instructive in variation prospecting of OMM, and then OMM abnormality could act as a guide for uranium deposits prospecting.

Fractal Characterization of Nanopore Structure in Shale, Tight Sandstone and Mudstone from the Ordos Basin of China Using Nitrogen Adsorption

The characteristics of the nanopore structure in shale, tight sandstone and mudstone from the Ordos Basin of China were investigated by X-ray diffraction (XRD) analysis, porosity and permeability tests and low-pressure nitrogen adsorption experiments. Fractal dimensions D1 and D2 were determined from the low relative pressure range (0 < P/P0 < 0.4) and the high relative pressure range (0.4 < P/P0 < 1) of nitrogen adsorption data, respectively, using the Frenkel–Halsey–Hill (FHH) model. Relationships between pore structure parameters, mineral compositions and fractal dimensions were investigated. According to the International Union of Pure and Applied Chemistry (IUPAC) isotherm classification standard, the morphologies of the nitrogen adsorption curves of these 14 samples belong to the H2 and H3 types. Relationships among average pore diameter, Brunner-Emmet-Teller (BET) specific surface area, pore volume, porosity and permeability have been discussed. The heterogeneities of shale nanopore structures were verified, and nanopore size mainly concentrates under 30 nm. The average fractal dimension D1 of all the samples is 2.1187, varying from 1.1755 to 2.6122, and the average fractal dimension D2 is 2.4645, with the range from 2.2144 to 2.7362. Compared with D1, D2 has stronger relationships with pore structure parameters, and can be used for analyzing pore structure characteristics.

Application of Acoustic Emission to Estimation of Strata Denudation: A Case Study from the Ordos Basin, China

This paper presents a method for reconstructing denudation in sedimentary basins. The principle of this method is to use the Kaiser effect involving the acoustic emission (AE) of rock samples under a uniaxial load test. Based on the proposed method, the denudation thickness since the Late Cretaceous period in the southeastern margin of the Ordos Basin in China was estimated. The results showed that the denudation increased gradually from the northwest (1600 m a.s.l) to the southeast (1800 m a.s.l) of the study area. A comparison of the denudation results with those obtained from other methods illustrated that the AE method was reliable and easy to use. Finally, using computer simulation, we analyzed the influence of strata denudation and ground topography on the current in situ stress. The results suggested that due to the denudation, the horizontal stress played a dominant role in the distribution of the in situ stress of the study area, but the in situ stress could be reduced by the uplifted movement of the ground. The analysis could facilitate future applications of the AE method and improve understanding of the basin stress field.

Study of the method portrayed by a river by calculating the tooth rate of the logging curve

In order to accurately describe the reservoir distribution of the oil and gas field, the precision of the portrayal of the channel microfacies is getting higher and higher. Aiming at the existing problems and difficulties in sedimentary microfacies, a method of determining the location of river channels and describing the river channels with high accuracy is proposed by calculating the tooth rate of logging curves. The tooth rate is characterized by the amplitude of the tooth, the density of the tooth and the frequency of the tooth distribution. The general idea is to calculate the three parameters of logging first, and then give each parameter a certain weight coefficient, and get the tooth rate of the curve section of the log. According to the size of the tooth rate, the specific location of the log in the channel is analyzed, and the channel is finely depicted. Compared with the traditional method of sedimentary facies research, the method has improved the accuracy of boundary characterization and achieved semi-quantitative or quantitative accuracy. In the application of sedimentary research of the Hangjinqi area in the Ordos Basin of China, the position of the river is determined by the tooth rate, and the accuracy of the boundary is obviously improved. The research results can be applied directly to the production and guide the optimization of the favorable area and the deployment of the well position.

Enhancement of organic matter maturation because of radiogenic heat from uranium: A case study from the Ordos Basin in China

Carbonaceous debris (CD) within uranium-bearing strata has been studied in the Daying uranium deposit of the northern Ordos Basin, northern China. The influence of radiogenic heat from uranium on organic matter maturation was investigated through a series of tests including measurements of vitrinite reflectance (Ro), fission-track (FT) analysis in quartz grains, and the calculation of the radiogenic heat production rate of the samples. The results show that Ro in uranium-bearing strata generally increases as the burial depth increases, indicating that CD experienced normal burial coalification. However, Ro values of the samples rich in uranium are 0.062% Ro higher than those without uranium mineralization. Vitrinite reflectance bears a positive relationship with uranium content, and an inverse relationship with distance to the closest sandstone rich in uranium, indicating that uranium enrichment enhances organic matter maturation. The production of uranium decay makes FT observable in quartz grains, and the intensity of decay increases with proximity to the uranium ore body. The calculated radioactive heat production rate from the uranium ore body is 6.857 × 10−5 W/m3. During the long-term stable decay, as the uranium ore body theoretically results in an abnormal increase in temperature of 52°C without consideration of the loss of heat conduction, heat convection, and thermal radiation, this would yield a theoretical Ro increase of 0.209% Ro, reasonably greater than the observed. Therefore, the long-term stable radiogenic heat produced by uranium ore body can slightly enhance organic matter maturation, which is instructive in uranium prospecting.

Micro-fracture Development Impact Factors Analysis and its Effects on Well Production of Hua-qing Area Ordos Basin in China

Micro-fractures, which are characterised by complex-non-structural geneses, are critical in tight sand reservoir production. Taking the Chang 6 reservoir of the Huaqing area, the Ordos basin, as an example, the influential factors of the micro-fracture development are analysed from a novel perspective. Studies show that the influences of above factors on micro-fracture are listed as follows: brittle mineral content > median grain size > stratum thickness > overburden pressure. In same lithology, enrichment position of rigid compositions is a key factor to the micro-fracture generation. However, sandstones are rigid and prone to forming micro-fracture. The effect of diagenesis on micro-fracture development is quite complex. Micro-fractures generate at the contact position between soft and hard materials. And then, the effects of micro-fractures on the well production have been analysed, which hence offers scientific guidance for further high-efficiency development of the Chang 6 tight sandstone oil reservoir in this area. [Received: December 28, 2016; Accepted: March 28, 2018]

Micro-fracture development impact factors analysis and its effects on well production of Hua-qing area Ordos Basin in China

Micro-fractures, which are characterised by complex-non-structural geneses, are critical in tight sand reservoir production. Taking the Chang 6 reservoir of the Huaqing area, the Ordos basin, as an example, the influential factors of the micro-fracture development are analysed from a novel perspective. Studies show that the influences of above factors on micro-fracture are listed as follows: brittle mineral content > median grain size > stratum thickness > overburden pressure. In same lithology, enrichment position of rigid compositions is a key factor to the micro-fracture generation. However, sandstones are rigid and prone to forming micro-fracture. The effect of diagenesis on micro-fracture development is quite complex. Micro-fractures generate at the contact position between soft and hard materials. And then, the effects of micro-fractures on the well production have been analysed, which hence offers scientific guidance for further high-efficiency development of the Chang 6 tight sandstone oil reservoir in this area. [Received: December 28, 2016; Accepted: March 28, 2018]

Organic geochemical characteristics of the upper Triassic source rocks in the Eastern Ordos Basin, China

In this paper, the organic geochemical characteristics of the upper Triassic Yanchang Formation source rocks in the Panlong oilfield in the eastern Ordos Basin of China were systematically studied. The results showed that the organic matter abundance of the dark mudstone source rocks of the Chang 7 segment is high and reaches the standard of good or best source rocks. The organic matter type of the Chang 7 segment source rocks is mainly Type II1 with strong oil generation ability. The biomarker compounds studies showed that the Chang 7 segment source rock has reached the mature stage, reflecting its good organic matter type and high evolution degree. The salinity of the original sedimentary water of this source rock is low, reflecting a terrestrial sedimentary environment. The oil source comparison confirmed that the crude oil Yanchang Formation in the Panlong oilfield was contributed by the Chang 7 source rocks.

Depositional and diagenetic controls on the reservoir quality of Upper Triassic Chang-7 tight oil sandstones, southwestern Ordos basin, China

The seventh oil layer of the Upper Triassic Yanchang Formation (Chang-7) tight oil sandstone reservoirs is a major exploration target. A significant amount of hydrocarbons has been discovered in these reservoirs in the southwestern Ordos basin in China. The Chang-7 tight sandstones are characterised as tight with low porosity, low permeability, and strong heterogeneity. This study investigates the sedimentary facies, diagenesis, and their impact on the reservoir quality of the Chang-7 tight oil sandstones. The sandstones were deposited in a deltaic-lacustrine depositional system. Three major depositional facies are identified consisting of delta front fed by braided rivers and meandering rivers, and slump turbidite fans. The depositional environment exerts a key control on reservoir quality. The distinct low-energy sedimentary environment produced fine to very fine-grained sandstones with high matrix and mica contents, characterised by low initial porosity and permeability. Diagenesis mainly comprised mechanical compaction and cementation by quartz, carbonate minerals and various clay minerals. The reservoir properties of the Chang-7 sandstones are generally poor, with porosity of 1.4–20.7% (average porosity 8.6%) and permeability of 0.001–116.7 mD (average 0.2 mD), which are attributed to significant compaction and cementation. Mechanical compaction was more important than cementation for reducing porosity, whereas secondary dissolution porosity was significant for the Chang-7 tight oil sandstones due to closer proximity to the underlying Chang-73 source rocks.

A method to determine nuclear magnetic resonance (NMR) T2cutoff based on normal distribution simulation in tight sandstone reservoirs

The T2cutoff is an important input parameter in nuclear magnetic resonance (NMR) applications. The accuracy of the T2cutoff affects the prediction reliability of parameters associated with the identification and evaluation of formations. Current methods are based on regional statistics; they have limited applications and the predicted values are not always reliable. In this study, a total of 36 core samples, drilled from Triassic tight sandstone reservoirs of the southwest Ordos Basin in China, were used for laboratory NMR measurements under fully brine saturated and irreducible water conditions. Based on the morphological character analysis of experimental NMR T2 spectra, we demonstrate that NMR T2 spectra can be fully simulated using the normal distribution function. In addition, a new method was proposed, which can predict various T2cutoffs based on the morphological differences of NMR T2 spectra, and the irreducible water saturation (Swirr) was calculated, which represents the ratio of the sum volume of clay bound water and capillary bound water to total pore volume. The reliability of this method was verified by comparing the predicted T2cutoff and Swirr values with those of core NMR experimental results. Finally, we extended this method into field applications in several tight sandstone reservoirs in China. The results show that total T2 distributions of the formation were fully simulated by the normal distribution function, and various T2cutoff and Swirr values were precisely predicted. Meanwhile, tight sands permeability curves, predicted based on the Timur-Coates model, were also accurately estimated. The results of our study may be applied to improve tight sandstone reservoir identification and evaluation using NMR logs.

FRACTAL CHARACTERIZATION OF TIGHT OIL RESERVOIR PORE STRUCTURE USING NUCLEAR MAGNETIC RESONANCE AND MERCURY INTRUSION POROSIMETRY

Tight oil sandstones have the characteristics of narrow pore throats, complex pore structures and strong heterogeneities. Using nuclear magnetic resonance (NMR) and mercury intrusion porosimetry (MIP), this paper presents an advanced fractal analysis of the pore structures and petrophysical properties of the tight oil sandstones from Yanchang Formation, Ordos Basin of China. Firstly, nine typical tight oil sandstone core samples were selected to conduct NMR and MIP test for pore structure characterization. Next, with the pore size distribution derived from MIP, it was found that the relationships between NMR transverse relaxation time [Formula: see text] and pore size are more accordant with the power function relations, which were applied to derive pore size distribution from NMR rather than the linear relation. Moreover, fractal dimensions of micropores, mesopores and macropores were calculated from NMR [Formula: see text] spectrum. Finally, the relationships between the fractal dimensions of different size pores calculated from NMR [Formula: see text] spectrum and petrophysical properties of tight oil sandstones were analyzed. These studies demonstrate that the combination of NMR and MIP can improve the accuracy of pore structure characterization and fractal dimensions calculated from NMR [Formula: see text] spectrum are effective for petrophysical properties analysis.

Impact of inner reservoir faults on migration and storage of injected CO2

CO2 geological storage (CGS) is an effective way to mitigate greenhouse gas emissions, and geological security is one of the most important issues in CGS. The faults distributed in geological formations make multi-layered reservoirs interconnected systems. A three-dimensional (3D) numerical model was established to evaluate the effects of inner reservoir faults on the CO2 migration and storage capacity of an actual CGS demonstration project in the Ordos Basin of China. The results show that the faults in the layered reservoir system could significantly affect the migration of injected CO2. The cross-layer faults at the bottom of the faulted reservoir could act as preferential passages between the upper and lower geological formations, causing the CO2 in the reservoir formation to move upward to adjacent layers rather than to lateral migration. CO2 migration along the inner-layer faults widely occurred at the top of the reservoir formation, decreasing the pressure accumulation and CO2 saturation around the injection well. Based on the simulation, CO2 will have migrated into the Heshanggou Formation after 300 years, and most of the CO2 will be trapped in the bottom sub-layers, with no CO2 intruding into the upper caprock. The spatial and temporal evolution of the injected CO2 was well presented for the faulted reservoir system, suggesting that the faults inside the multi-layered reservoir are beneficial to CGS.