Permian Sandstones Research Articles

STUDY ON CHARACTERISTICS AND ORIGIN OF NANO-PORE AND FISSURES FOR DEEP BURIED CRETACEOUS TIGHT GAS RESERVOIR USING NANOTECHNOLOGY IN KUQA DEPRESSION- TARIM BASIN, CHINA

Although buried over eight thousand meters, there is still developing high production from tight gas reservoir with low porosity and ultra-low permeability in the sandstones of Bashijiqike Formation of Cretaceous in Kelasu area of Kuqa depression. It is an interesting issue for the origin of the ultra-deep reservoirs. This paper aims to present the characteristics and origin of the nano-pore and nano fissures of that ultra-deep reservoirs using field emission scanning electron microscope, laser scanning confocal microscope and other high resolution measure experiments. There are developed two kinds of nano-pores with pore diameters between 40 nanometers to 1000 nanometers, and many nano fissures with width between 10 and 600 nanometers in this sandstone reservoir. Based on their petrophysical properties and microstructure characteristics, the nano pores are connected by those nano fissures that are also as valid pore throats. So, there is valid permeability in sandstones with even very low porosity. The effective combination between nano pores and pore throats is the key factors for high production in this tight gas sandstones. This point can help us to predict gas reservoirs with effective and recoverable reserve in this kind of ultra-deep gas field, also maybe in other similar gas field.

Experimental study on the effect of CO2 injection on the permeability characteristics of reservoir sandstone.

Using the Ordos Basin dry sandstone and sandstone saturated with different saline concentrations as research subjects, a self-developed constant temperature and pressure CO2 injection simulation device was employed to conduct permeability tests on sandstone under varying effective stresses and CO2 injection pressures. The test results indicated that during the CO2 injection process, the permeability of dry sandstone was two orders of magnitude higher than that of sandstones saturated with different saline concentrations. When the effective stress increases from 10 MPa to 28 MPa, the fissure compressibility of reservoir sandstone is influenced by the saturation of different saline concentrations, with the compressibility coefficients for 0%, 15%, and 30% saline-saturated sandstone being 0.00495 MPa⁻1, 0.00614 MPa⁻1, and 0.01879 MPa⁻1, respectively. The primary reasons for the reduced permeability of sandstone are as follows: supercritical CO2 lowers the mechanical properties of sandstone; high-concentration saline induces crystallization within the sandstone, resulting in a blockage effect. High-concentration saline increases the fissure compressibility of sandstone, decreases the permeability of the sandstone reservoir, and ultimately affects the injectability of the CO2 in the reservoir.

Quantitative classification and prediction of pore structure in low porosity and low permeability sandstone: A machine learning approach

Quantitative classification and prediction of pore structure in low porosity and low permeability sandstone: A machine learning approach

Heterogeneity and permeability estimation of pore-throat structure at different scales in deep tight sandstone reservoirs: A case study of Paleogene Hetaoyuan Formation in Anpeng area, Nanxiang Basin, China.

Clarifying the pore-throat size and pore size distribution of tight sandstone reservoirs, quantitatively characterizing the heterogeneity of pore-throat structures, is crucial for evaluating reservoir effectiveness and predicting productivity. Through a series of rock physics experiments including gas measurement of porosity and permeability, casting thin sections, scanning electron microscopy, and high-pressure mercury injection, the quality of reservoir properties and microscopic pore-throat structure characteristics were systematically studied. Combined with fractal geometry theory, the effects of different pore throat types, geometric shapes and scale sizes on the fractal characteristics and heterogeneity of sandstone pore throat structure are clarified. On this basis, the estimation model of tight sandstone permeability was established. The results indicate that the reservoir physical properties in the study area are poor, the pore types are mainly dissolved pores, and the pore size is mainly distributed in the nano to submicron range. The fractal dimension fitting curve obtained based on the non-wetting phase model has obvious turning points, indicating that the pore-throat structure has multi-scale characteristics. The turning point of fractal dimension divides the pore-throat structure of tight sandstone into large-scale pore-throats with good connectivity (reticular or beaded pore-throats) and small-scale pore-throats with poor connectivity (dendritic or capillary pore-throats), indicating that tight sandstone has binary pore structure characteristics. The geometry of large-scale pore-throat is complex, which is difficult to meet the self-similar characteristics, with the average fractal dimension is 3.72. The small-scale pore-throat morphology is close to the capillary and has obvious fractal characteristics, with the average fractal dimension is 2.22. There are many small pores and micropores in the reservoir, and the pore volume has a significant positive correlation with the total porosity of the rock, but the contribution to the permeability is low. The development degree of large-scale pore throat is an important factor affecting the physical properties of tight sandstone. The turning point radius of fractal curve and the comprehensive fractal dimension can be used as good indicators for permeability estimation.

Diagenesis and Reservoir Evolution of Low Permeability Sandstones: A Case Study of the Second Member of the Jurassic Sangonghe Formation, Central Junggar Basin, China

ABSTRACTThe low‐permeability oil‐bearing tuffaceous sandstones of the second member of the lower Jurassic Sangonghe Formation (J1s2) in the Well Pen‐1 west sag of the central Junggar Basin occur in a deep burial setting (> 4000 m). They contain abundant oil and gas shows and hold promising exploration prospects. However, the reservoir heterogeneity is strong due to complex lithofacies and diagenesis, leading to significant productivity differences between adjacent wells. Therefore, identifying the lithofacies and diagenesis of the J1s2 sandstones and elucidating their influence on the evolution of relatively high‐quality reservoirs is of utmost importance for oil and gas exploration and development in this area. Samples from the Well Pen‐1 west sag, representing the J1s2 sandstones, were investigated utilising core and thin section observations, scanning electron microscopy, X‐ray diffraction, fluid inclusions and carbon and oxygen isotope analyses. The J1s2 sandstones are mainly medium‐ to fine‐grained and moderate‐ to‐well‐sorted feldspathic litharenites and litharenites. The tuffaceous contents range from 2.6% to 25% and the authigenic clay contents, produced by diagenesis, range from 0.6% to 12%, although carbonate cements are not abundant (av. 3.1%). Four sandstone lithofacies have been identified based on mineral compositions, leading to variations in diagenetic evolution and reservoir quality. Early diagenetic events included compaction, alteration of tuffaceous matrix and feldspar, and development of smectite, chlorite, kaolinite and early calcite. Mesogenic alteration included feldspar and tuffaceous matrix dissolution, alteration of kaolinite, chlorite and illite, and precipitation of quartz, anhydrite, late calcite and ferrocalcite. The alteration of the tuffaceous matrix resulted in a complex pore‐throat structure in the J1s2 sandstones. The pebbly sandstone and conglomerate (SC) and fine‐grained sandstone (Sm) lithofacies are generally characterised by high compaction resistance, low tuffaceous matrix and cement contents, and abundant secondary dissolution pores, and they exhibit better reservoir quality and great potential for oil and gas enrichment.

Evolution of the internal structure and physical properties of Tongxin sandstone under high temperature

The stability of the surrounding rock under high temperature is pivotal to the efficient and safe production of high-temperature fluidized mining engineering. To investigate the stability of rocks under high temperature, this paper takes the roof sandstone of Tongxin coal mine, examining changes in its physical parameters such as mass, dimensions, wave velocity, porosity, and permeability after treatment at various temperatures (20–700°C). The results showed that parameters like mass and wave velocity decreased with increasing temperature, while dimensions, porosity, fracture density, and permeability increased. The patterns of change in these physical properties with temperature exhibit a high degree of consistency. Additionally, composition analysis and thermal analysis were conducted to understand the physical and chemical changes occurred in sandstone. Scanning electron microscopy was used to observe microstructural changes in the sandstone. After analysis, the evolution of the internal structure of Tongxin sandstone with heat treatment is categorized into three stages. (1) stable change Stage (20–450°C, 650–700°C): Dominated by dehydration and thermal stress, where pore and fracture structures develop slowly; (2) rapid change stage (450–550°C, 600–650°C): Dominated by the kaolinite dehydroxylation, leading to increased porosity but decreased average pore size; (3) intense change stage (550–600°C): Dominated by the quartz phase transitions, where the thermal stress generated by quartz phase transitions causes dramatic alterations in the internal structure of the sandstone. Furthermore, a correlation model between wave velocity and permeability of sandstone at high temperatures was established based on the interrelationship of these physical properties, providing a new method for real-time monitoring of permeability under high-temperature conditions.

Ferrous Iron (Fe+2) Released From Iron‐Rich Chlorite as a Reductant for Unconformity‐Related Uranium Mineralization: Insights From Reactive Fluid Flow Modeling

AbstractDebate continues over the reducing mechanisms for the formation of unconformity‐related uranium (URU) deposits. This paper evaluates, for the first time, the potential of iron‐rich chlorite as a reductant for uranium mineralization using reactive fluid flow modeling method. Our results confirm that Fe2+, released from the breakdown of iron‐rich chlorite, can reduce aqueous hexavalent uranium to precipitate economically significant URU deposits similar in size and grade to those formed with CH4 as the reducing agent. The resulting uranium mineralization tends to occur in the basement and below the downwelling parts of overlying basinal fluid circulation cells, where oxidizing basinal fluid percolates across the unconformity and reacts with upward flowing reducing basement brine. Therefore, the basinal fluid circulation pattern controlled by the permeability of the sandstone aquifer is critical in determining the formation and distribution of URU deposits. When the sandstone layer is more permeable, the simulated uranium deposits become larger in size, and vice versa. If the sandstone permeability is <5 × 10−14 m2, no obvious uranium deposits can be formed. In contrast, the permeability of fault zones does not have a significant effect on uranium mineralization, although it does affect fluid flow behaviors within the fault zone itself. We also demonstrate that fault zones do not appear to be a prerequisite for the formation of URU deposits when Fe+2 serves as a reductant, which highlights important exploration implications. Uranium exploration should, in addition to continuing to target graphitic fault zones, also consider areas where faults and/or graphite units do not exist.

Gas storage in low-permeability rocks

Low-permeability, dense sandstone formations are ideal for underground gas storage in compressed air energy storage. To examine the gas permeability of fine sandstone under high pressure, a self-developed triaxial high-pressure gas permeameter was used for experiments under varying confining and axial pressures. The study identified unsteady seepage variations and stress dependence in porous media represented by fine sandstone and developed a gas leakage model. Results indicate that the seepage process includes rapid, slow, and cessation stages. Fine sandstone shows excellent storage capacity, with a daily leakage rate of 0.0463%, below required thresholds. This study guides high-pressure gas permeability theories and modeling.

Geothermal resources of Hungary: a play-based review

In this paper we review the geothermal resources of Hungary using the geothermal play concept and its scale-dependent hierarchical categorization. The Pannonian Basin belongs to an extensional domain play type, with two main plays: the porous Neogene basin-fill sediments and the fractured–karstified Paleozoic and Mesozoic basement rocks. Both plays can be divided into hydrostatic and overpressured play segments. The most widely utilized play level in the hydrostatic segment of the porous Neogene sediment play comprises laterally extended, highly permeable sandstone layers storing low-salinity thermal water with average flow rate of c. 900 l min −1 and temperatures between 50 and 100°C in the depth range of 1–2 km. The other prosperous play level is represented by the fractured–karstified Mesozoic carbonates in the hydrostatic segment of the basement play having elevated temperatures (90–120°C) and several thousand l min −1 flow rates. The play level related to fractured Paleozoic rocks is less important due to its limited extent. The overpressured segment of the basement play may have high flow rates but it is still an untapped resource as exploitation is limited by high overpressure, high temperatures (170–200°C) and highly saline hot waters.

Pore-throat structure, fractal characteristics and permeability prediction of tight sandstone: the Yanchang Formation, Southeast Ordos Basin

As a typical tight reservoir and an important site for unconventional hydrocarbon accumulation, the Chang 6 member of the Yanchang Formation is characterized by complex pore structures and strong heterogeneity. Analysing and characterizing the pore-throat structure is highly important for optimizing oil recovery processes. To clarify the nonhomogeneity and structural characteristics of the pore throats in the southeastern Ordos Basin, tight sandstone from the Chang 6 member was selected for analysis. Casting thin section (CTS), scanning electron microscopy (SEM), cathodoluminescence (CL), X-ray diffraction (XRD), and mercury intrusion capillary pressure (MICP) analyses were conducted. The results, revealed that intergranular pores, feldspar-dissolved pores, intergranular-dissolved pores, and microfractures were the predominant pore types found within the samples. By combining the results of MICP analysis with those of fractal theory, the pore–throat structure of each sample can be categorized into two types: large-scale and small-scale. Fractal theory was employed to characterize the complex and irregular pore–throat structure of the reservoir quantitatively. The average fractal dimension of large pores (D1) was 2.8094, whereas for small pores (D2), it was slightly lower than that of large pores, averaging 2.5325. These findings underscore that large-scale pore-throat structures are more complex and exhibit greater heterogeneity. Compared with those of large pores, the pore–throat structure parameters of small pores exhibit a more significant correlation with reservoir properties and fractal dimensions. Therefore, small pores are the primary contributors to the reservoir storage space and are key factors influencing the pore-throat structure of the Chang 6 tight sandstone. Based on the pore-throat radius and considering the influence of fractal characteristics on the pore structure, a nonlinear permeability prediction model was created using multiple regression analysis. Among these equations, the pore–throat radius corresponding to a mercury saturation of 40% (r40) emerged as the most effective predictor of permeability for tight sandstone. The investigation of micropore–throat structures in tight oil reservoirs holds practical significance for comprehending the distribution and accumulation of tight oil and guiding oil field development.

Orthogonal experimental investigation on permeability evolution of unconsolidated sandstones during geothermal fluid reinjection: A case study in the Minghuazhen Formation, Tianjin, China

Geothermal fluid reinjection (GFR) is vital for sustainable sandstone hydrothermal geothermal resource extraction. However, the permeability damage mechanisms of sandstone geothermal reservoirs due to reinjection remain unclear. This study aimed to investigate the permeability evolution of unconsolidated sandstone during GFR through an orthogonal experiment. The influence mechanism of the factors, including temperature, injection rate, confining pressure, geothermal fluid concentration (GFC), and depth, on sandstone permeability is also discussed. The results showed the sandstone samples suffered permeability damage in 34.62–72.73 %. Particle clogging emerged as the primary mechanism responsible for the permeability damage, serving as a crucial bridge in understanding the influences of the factors on sandstone permeability. Efficient GFR requires the optimization of reinjection conditions tailored to the characteristics of geothermal reservoirs. Sensitivity analysis showed that the order-sensitive characteristic of permeability damage to the factors was injection rate > GFC > temperature > depth > confining pressure. Through the optimization of reinjection parameters, the heat exploitation performance under the worst permeability damage case can be improved by 1.77–1.93 times. This study could offer valuable insights into the challenges and considerations associated with GFR in sandstone reservoirs.

A comparison study of pore structure and permeability of sandstone by BSE-SE images

The pore structure and permeability of geotechnical materials are critical parameters that guide underground engineering. With advancements in digital imaging technology, scanning electron microscopy (SEM) has emerged as a vital tool for examining the pore structure and permeability of these materials. SEM operates in two primary modes: backscattered electron imaging (BSE) and secondary electron imaging (SE), each of which emphasizes different aspects of the material's structure. However, few studies have been conducted to elucidate the influence of these two modes on the quantification of geotechnical structural characteristics. This study undertakes a more comprehensive quantitative analysis of the structure and permeability of sandstones by juxtaposing the two modes. The findings indicate that the BSE mode excels in analyzing the structure and composition, whereas the SE mode highlights the surface morphology. In terms of image segmentation, BSE mode images are more conducive to effective segmentation. Although SE images permit viable segmentation when preprocessed, they tend to represent a greater number of discrete tiny pores. Furthermore, there exists a discernible correlation between pore size and shape, wherein larger pores exhibit heightened roughness and deviate more from sphericity. Notably, these larger pores predominantly contribute to the material's permeability. Given that the BSE mode more readily captures continuous pore structures, the permeability values derived from BSE images are significantly higher than those obtained from SE images. These findings hold profound implications for enhancing our comprehension of geotechnical materials' pore structure and permeability, thereby informing the strategic use of BSE and SE modes in related studies.

Detrital zircon U-Pb geochronology of the Moatize and N'Condédzi coalfields, Zambezi Karoo Basin of Mozambique: Implications for provenance, sediment dispersal and basin evolution

Detrital zircon U-Pb geochronology of the Moatize and N'Condédzi coalfields in the Zambezi Karoo Basin of Mozambique provides key insights into the regional provenance, sediment dispersal pathways and basin evolution. Borehole cores from the two coalfields reveal a stratigraphy spanning the early Roadian (middle Permian) to the Carnian (Upper Triassic). The Permian sandstones of the Moatize Coalfield (MC) yield three detrital zircon populations, with ages ranging from 1150 to 950 Ma, 900–780 Ma and 650–490 Ma. In contrast, the Permian sandstones of the N'Condédzi Coalfield (NC) have only one population, which ranges from 1150 to 950 Ma. During the Permian, the provenance area for the NC was the Tete-Chipata Terrane and Malawi Complex (1150–950 Ma) to the north-northeast. In the MC, the detrital zircon populations of the early lake delta depositional setting (Roadian to Wordian) indicate a main provenance in the Zambezi Belt (900–780 Ma) located to the south of the MC, with minor sourcing from the Nampula Block (1150–950 Ma and 650–490 Ma) to the east. The transition from a lake delta to an alluvial depositional setting is attributed to a major tectonic event in the MC, which involved the formation of a braided channel belt. Sandstones from this braided channel belt yield three detrital zircons populations (650–490 Ma, 900–780 Ma and 1150–950 Ma), indicating provenance from both the Zambezi Belt and the Nampula Block. The overlying sandstones in the MC show only a minor population from the Zambezi Belt (900–780 Ma), implying a shift in provenance to the Nampula Block that was likely induced by tectonics. The absence of detrital zircon populations of 900–780 Ma and 650–490 in the Permian sandstones of NC implies that the two coalfields were not connected during the mid to late Permian. It is likely that an intra-rift horst (the Mesoproterozoic Gabbro-Anorthosite Tete Suite) separated the two coalfields. The Lower Triassic sandstones of the NC yield a main detrital zircon population indicating provenance from the Tete-Chipata Terrane and Malawi Complex (1150–950 Ma). A minor population at 650–490 Ma is linked to increased aridity at the Permian – Triassic boundary, which caused expansion of the watershed across the Tete-Chipata Terrane and the Malawi Complex. The Upper Triassic sandstones in the NC yield a 1150–950 Ma detrital zircon population, indicating provenance from the Tete-Chipata Terrane and Malawi Complex and a return to the source-to-sink conditions seen in the mid to late Permian.

Insights into the wetting mechanisms in low-permeability sandstone reservoirs and its evolution processes: The shahejie formation in the Dongying Depression, Bohai Bay basin

During the accumulation and development processes of low-permeability oil, wettability plays a crucial role in the percolation capacity of fluids. In particular, the evolution of low-permeability sandstones involves complex changes in formation fluid properties and mineral types, leading to a deficient understanding of wetting mechanisms in low-permeability sandstones. This gap significantly impedes research on the accumulation mechanisms of low-permeability oil. Focusing on the low-permeability sandstones in Shahejie formation of Dongying Depression, Bohai Bay Basin, tests like Casting thin section, scanning electron microscope, contact angle measurement, molecular dynamics simulation and the Amott method based on nuclear magnetic resonance technology were performed to comprehensively investigate sandstone wettability and its evolutionary process. The results indicate that the adsorption capacity of hydroxyl groups or monovalent cations (K+and Na+) for water molecules causes residual intergranular pores, feldspar dissolution pores, quartz dissolution pores, clay mineral intercrystalline pores and micro-cracks to exhibit hydrophilicity; while the adsorption capacity of divalent cations (Ca2+and Mg2+) for oil molecules causes the surface of dissolution pores of carbonate minerals to exhibit neutrality or lipophilicity. Additionally, changes in formation conditions (temperature, pressure, ion types, and ion concentration) significantly control the wettability of pore surfaces, further determining the overall wettability of low-permeability sandstone reservoirs. Throughout the evolutionary process of low-permeability sandstones, the Amott-Harvey index of low permeability sandstone is greater than zero, and the wettability exhibited hydrophilic or neutral. From A-stage eodiagenesis to B-stage mesodiagenesis, the Amott-Harvey index is 0.82, 0.12, 0.37, 0.03, and 0.49, and the wettability of reservoirs transitions through phases of strong hydrophilicity, weak hydrophilicity, hydrophilicity, neutrality, and hydrophilicity, respectively. Finally, an evolutionary model of the wettability of low-permeability sandstone reservoirs was established, which is of great significance for predicting the quality of low-permeability sandstone reservoirs.

Study on the pore structure and permeability evolution of tight sandstone under liquid nitrogen freezing‐thawing cycles based on NMR technology

To further raise the gas extraction efficiency of the tight sandstone, the liquid nitrogen (LN2) freezing-thawing cycles method can be employed to improve the permeability of the low-permeability reservoirs. Permeability is generally regarded as a macroscopic description of the pore structure and usually has functional relationship to pore structure properties. The permeability of the rock is closely related to the change of microscopic pore structure. The permeability of rock depends on how the subzero temperatures changed the microscopic pore structure of rock, but it has not yet been confirmed obviously. In this study, the nuclear magnetic resonance (NMR) technique was adopted to investigate the pore structure evolution law and permeability of the tight sandstone with different LN2 freezing-thawing cycles. The results demonstrate that the LN2 freezing-thawing cycles promotes pore development and micro-fracture connection, and enhances the pore connectivity. The proportion of meso-pores, macro-pores and micro-fractures in the sandstone samples increases significantly, which provides a channel for the sandstone gas flow and extraction. Total porosity and effective porosity of the samples present a trend of rapid increase as the number of LN2 freezing-thawing cycles increasing, while the residual porosity decreases as the number of LN2 freezing-thawing cycles increasing. Coates model, SDR model (mean T2 model) and PP model were used to calculate and evaluate the permeability of the samples subjected to different LN2 freezing-thawing cycles. Furthermore, PP model can provide a better permeability estimate than the classical Coates and SDR model.

Hydrothermal activity near the Permian–Triassic transition in the south‐western Ordos Basin, China: Evidence from carbonate cementation in Upper Permian sandstones

ABSTRACTCarbonate cementation in the Upper Permian sandstones informs the timing and temperature of hydrothermal activity in the south‐western Ordos Basin. This study presents a detailed examination of these hydrothermally influenced carbonate cements, constraining their age, carbonate diagenesis and relationship with regional geodynamic evolution. Sedimentological analyses demonstrate the development of deltaic plain and front sand bodies in the study area, which resulted in interbeds of volcanic matrix‐rich sandstones with matrix‐free sandstones. Petrography and electron microprobe analysis reveal four carbonate mineral growth phases of matrix‐free sandstones in the following sequence; scarce pure siderite, scarce Mg‐rich siderite, abundant blocky calcite and moderately abundant grain‐replacing calcite. The fluid inclusion data show anomalies of homogenization temperature of blocky carbonate cements during early diagenesis, over a wide range of ca 148 to 228°C. In addition, blocky carbonate cements show low δ13C (−5.9 to −13.1‰ Vienna PeeDee Belemnite) and δ18O (clustered tightly from −12.4 to −14.6‰ Vienna PeeDee Belemnite) values, interpreted to result from elevated temperatures during cementation, associated with activation of basement faults and concomitant hydrothermal fluid intrusion triggered by oceanic crust subduction in the south‐west margin of the Ordos Basin. Using in situ calcite U–Pb geochronology, the timing of hydrothermal activity was constrained to ca 247.0 ± 11 to 248.2 ± 4.7 Ma. This work provides a case study for applying intergranular calcite U–Pb dating to determine the absolute timing of fluid flow in sedimentary basins, offering tremendous potential to capture snapshots of various diagenetic evolution stages in sediments. The proposed diagenetic model can also provide new insights and understanding regarding hydrothermally influenced sediments. More importantly, hydrothermal activity may have commenced earlier than previously thought. The North Qinling Orogen uplift and associated Mianlue oceanic crust subduction may have begun at the Permian–Triassic transition with a protracted hydrothermal event in the south‐western Ordos Basin.

Prediction of permeability of various geotechnical materials under different temperatures based on physical characteristics and machine learning

Accurately and efficiently predicting permeability at different temperatures is crucial in the fuel science, as it enables optimization of production efficiency, and ensures safety and stability during storage and transportation. There remains a deficiency in predictive methods using comprehensive and reasonable training datasets and validating their applicability across different materials, particularly when considering stress and temperature. This study presents a machine learning-based method to predict permeability, incorporating physical characteristics such as microscopic image features and damage factors. The image sample data augmentation method was improved for a sufficient and meaningful training dataset, and gas permeability of sandstone under different pressure conditions after various temperature treatments were obtained. After summarizing the variation patterns of each characteristic with temperature, the necessity of using selected predictive features was validated, and sample sets were constructed. We trained an extreme learning machine neural network for sandstone’s permeability under different conditions, and the relative error was compared with other representative machine learning methods. Subsequently, the same data acquisition and prediction process were applied to granite and bentonite to validate the proposed method's applicability. The findings indicate that the proposed predictive method can stably provide accurate gas permeability predictions for various geomaterials, with relatively small prediction error rates (sandstone-4.1782%, granite-4.1782%, bentonite-3.2479%). Moreover, the stability of the relative error is steadily better than other representative machine learning methods. This approach achieves an effective prediction of gas permeability under various conditions, and can potentially simulate the evolution of gas permeability for fuel sector.

Mineralisation controls for the diverse Cape manganese occurrences, South Africa

Abstract The Cape Fold Belt, comprising folded sedimentary sequences from predominantly the Palaeozoic Cape Supergroup, hosts a multitude of manganese occurrences and mineral deposits, many of which were subject to historical mining activity. Although size, grade and quality issues negate their modern-day exploitation for the steel-making process, the mechanisms by which Mn has enriched at these sites holds scientific value for our understanding of low-temperature Mn (bio-)geochemical cycling. Deposits located within the Cape Fold Belt comprise structure-hosted Mn deposits and a little described class of Mn mound deposits associated with chalybeate thermal springs (temperature = 41 to 48°C). Although the relationships between the two remain tenuous, detailed study of both classes provides insight into the conditions that favour Mn accumulation in the near-surface and sub-aerial environments. Comparisons between the physicochemistry of manganiferous- and non-manganiferous thermal springs suggest that manganese solubility is favoured by warm, acidic and slightly reducing fluids with elevated salinity. Transport, and associated fluid focusing typically within highly permeable sandstone units, serves to locate Mn mineralisation in near-surface structurally-complex trap sites that provide both accommodation space and conditions that are sufficiently oxidising. Where manganiferous spring waters spill out at surface, oxidation is caused by contact with atmospheric O2 to form Mn mound deposits, and ambient Mn-oxidising microbiota (e.g., genus Ramlibacter, and members of Burkholderiales, Rhodocyclaceae, and Oxalobacteraceae) are likely to play a role in enhancing the kinetics of this process. Relative to the structure-hosted deposits, these Mn mound deposits are typically lower grade (higher iron content) and relatively friable (high porosity and fine grain sizes). Supergene and diagenetic processes can lead to localised upgrade of the Mn mound material, although typically not to ore grade.

Prediction of deep low permeability sandstone seismic reservoir based on CBAM-CNN

The prediction of sand body is a key focus in evaluating reservoir distribution, playing a crucial role in oil and gas exploration and development. To clarify the development characteristics of the sand body in the Huangyan structural belt of the Xihu Sag and effectively identify the hidden channel, a new approach is proposed. This approach incorporates a Convolutional Block Attention Module (CBAM) into a Convolutional Neural Network (CNN). Well logging and seismic data were used to predict the distribution of sand body in the lower section of the Huagang Formation (H12). Firstly, based on known sand-stratum ratio data and well-side seismic data, the seismic attributes that are sensitive to the reservoir response are preferred through correlation analysis, which are used to construct a Polynomial Linear Regression (PLR) model to obtain the preliminary sand distribution prediction results. Secondly, the grid is divided according to this result. Then, three sample selection methods, namely proportional, fixed-range, and random, are used to construct three sample sets in conjunction with the geologic model guide. Finally, CBAM-CNN, CNN, Random Forest (RF), Support Vector Machines (SVM), and Backpropagation Neural Network (BPNN) are utilized for sand body prediction. The results indicate that when using the same model, the sample set selected by the fixed-range selection method yielded the best prediction outcome. When using the same sample set, the CBAM-CNN model outperformed the others. Among various strategies, training the CBAM-CNN model with the sample set derived from the fixed-range selection method led to the highest test set R2 of 0.913. The results of the sand body distribution indicate that fine river channels are more continuous, and reservoir boundaries are clearer, significantly outperforming other schemes. This outcome can serve as a guide for further reservoir delineation.

Estimation of Vertical Permeability of Hugin Sandstone from Petrophysical Well Logs Using Ensemble Methods: An Enhanced Machine Learning Approach

Estimation of Vertical Permeability of Hugin Sandstone from Petrophysical Well Logs Using Ensemble Methods: An Enhanced Machine Learning Approach