Mixed Rock Research Articles

RTG-GNN: A novel rock topology-guided approach for permeability prediction using graph neural networks

Digital core permeability is a crucial factor in rock engineering, reservoir simulation, and underground applications, serving as the foundation for evaluating fluid flow underground. However, predicting digital core permeability using artificial intelligence presents challenges such as high model calculation complexity, strong resource dependence, and low efficiency. To overcome these challenges, this paper presents a novel hybrid enhanced rock topology-guided graph neural network (RTG-GNN) to accurately characterize the pore structure of digital cores and predict permeability. This model takes advantage of the physical attributes of pores and throats, utilizing them as features for nodes and edges. It improves information exchange through a gating mechanism and incorporates an attention mechanism for weighted aggregation of neighboring data, culminating in accurate permeability predictions. Furthermore, a scheme for constructing a comprehensive permeability distribution dataset (CPDD) is proposed for digital cores. This scheme effectively mitigates challenges like inadequate training data, subjective sampling bias, and data drift. Multiple experimental results demonstrate that RTG-GNN surpasses most methods in various performance indicators. Additionally, the RTG-GNN model boasts a lightweight design, occupying only 17% to 44% of the size of mainstream models and requiring just 15% to 38% of GPU resources for training. Remarkably, while maintaining high accuracy and low error rates, the computational load is reduced by a factor ranging from 19.06 to 636.41, the training speed is enhanced by 12.12 to 86.47 times, and the inference speed is increased by 6.76 to 16.14 times. The experiment further confirms that both single core and mixed rock datasets, acquired through the CPDD scheme, exhibit excellent generalization and reliability.

Creep level qualitative evaluating of crushed rock based on uncertainty measurement theory and hierarchical analysis

A large number of tectonically mixed rock belts and complex tectonic zones are distributed in the southwestern part of China. In these areas, high geostress and tectonic stresses have caused some underground rock layers to be crushed and broken, eventually forming crushed rock zones. Which may undergo creep deformation under long-term loads. The manuscript is based on a typical crushed rock in the southwestern China. Firstly, the factors affecting creep deformation were analysed, and the response law of each influencing factor to rock creep is demonstrated. Then, the theory of uncorroborated measures and hierarchical analysis were used to systematically correlate the factors influencing creep. Thereby, a creep level qualitative evaluating model of crushed rock is established. Finally, this model was used to qualitatively evaluate the creep level of the crushed rock in the study area. It is concluded that the creep level qualitative evaluating of this crushed rock is rated as Class II, which is characterised by a low creep level and small creep deformations (0–10 mm). The research results can provide a reference for the creep analysis of crushed rock and provide a basis for the safe construction of engineering slopes.

Geochemical characterisation of mangrove sediments in Tiko (SW, Cameroon, West Africa): Implication for provenance, paleoweathering and sediment maturity

Geochemistry was performed on clastic core sediments from the Tiko Mangroves estuary, South western region of Cameroon, to categorise the rock source composition, tectonic setting, past weathering intensity of the source area in relation to past climate and sediment maturity in relation to sedimentary cycle. Plots of La/Co, La/Sc, Cr/Th, discriminant function (DF1&2), TiO2vs Al2O3 and TiO2 vs Zr point to an acid (felsic) and mixed (intermediate) rock source composition for the Tiko sediments. The acid composition portrayed by the sediments is also confirmed by their LREE (Light rare earth elements) abundance, and a negative Europium anomaly on chondrite normalisation, while the intermediate (mix) composition reflects the multiple sources of the sediments (Douala Basin and basaltic debris from Mount Cameroon). Binary plots Discriminant function (A-P) M, and Discriminant function (A-P) MT signpost active tectonic domain for the studied sediments, that resulted from the tectonothermal of the Pan African orogenic history and eruptive activity of the mount Cameroon. The weathering indexes denoted as CIX (chemical index of weathering) and PIX (plagioclase index of weathering) for the Tiko sediments advocate an intense source area weathering in a humid hot climate. The PIX advocate a high-level plagioclase lixiviation. The low values of ICV (index compositional variation) less than 1 (<1) couple with correlation plots Zr/Sc vs Th/Sc, and Zr vs (La/Yb)N confirms that Tiko mangrove sediments are matured recycled sediments with compositional variations. This is the first comprehensive provenance study of mangrove ecosystem sediments in Cameroon.

Pore Structure and Fractal Characteristics of Mixed Siliciclastic-Carbonate Rocks from the Yingxi Area, Southwest Qaidam Basin, China

Summary Evaluating reservoir properties at the pore scale is vital to better estimate hydrocarbon reserves and plan field development. The lacustrine mixed siliciclastic-carbonate deposits of the Upper Paleogene Xiaganchaigou Formation in the west Yingxiongling area form one of the most important hydrocarbon reservoirs in the southwestern Qaidam Basin (China). In this study, we analyzed well samples with X-ray diffraction (XRD), nuclear magnetic resonance (NMR), and mercury injection capillary pressure (MICP) data in integration with scanning electron microscopy (SEM) images to decipher the mineral composition and pore structure characteristics of the Xiaganchaigou Formation. We also calculate the fractal dimensions using MICP, NMR T2 spectrum, and SEM images based on fractal theory models. The results indicate that the mixed siliciclastic-carbonate samples of the upper section of the Xiaganchaigou Formation are mainly formed by dolomite and clay minerals with low siliceous and calcite content. Porosity is relatively low (2.01−9.83%) and positively correlated with dolomite content, thus indicating that the dolomite intercrystalline pores formed by infiltration and reflux dolomitization control the reservoir characteristics. The size of dolomite intercrystalline pores varies between several and hundreds of nanometers. The porosity has a poor correlation with permeability, which indicates that the pores are mostly primary, which lack the transformation of late dissolution. Three types of mixed siliciclastic-carbonate reservoirs are identified according to pore size distribution (&amp;lt;20 nm, 20−300 nm and multiple distribution), calculated using the NMR T2 spectrum. Fractal curves calculated by combining the MICP and NMR data are characterized by multisegments. The number of segments depends on the degree of heterogeneity of pore structure: two segment for high heterogeneity and three segment for low heterogeneity, also indicating a multifractal feature in mixed rock reservoirs. There is a negative correlation trend between porosity and fractal dimensions, and larger pores often have larger fractal dimensions. These results show that MICP-based fractal values are higher than those of NMR-based, which result from unconnected pores that the MICP is unable to reach. Fractal dimensions obtained from SEM have a small and narrow distribution range and are negatively correlated with the number of pores with smaller sizes. In essence, this study shows that the fractal dimension can be a concise index to evaluate the heterogeneity of lacustrine mixed siliciclastic-carbonate reservoirs, which can serve as an important reference for hydrocarbon development plans.

Crop productivity and soil inorganic carbon change mediated by enhanced rock weathering in farmland: A comparative field analysis of multi-agroclimatic regions in central China

CONTEXTFarmland enhanced rock weathering (ERW) is a negative emission technology that accelerates CO2 sequestration into the soil inorganic carbon (SIC) sink by applying calcium and magnesium-rich silicate rock to agricultural soils. Farmland ERW has great CO2 removal potential and is accompanied by various co-benefits, such as improving soil pH and providing mineral nutrients. However, the environment-driven mechanisms of ERW remain unclear, and the uncertainty on crop productivity also requires further evaluation in field trials. OBJECTIVEWe tried to explore the response of crop productivity and soil inorganic carbon to ERW in farmland and analyze the contribution and prioritization of environment drivers for the above change. METHODSA series of monitoring trials were established in a multi-agroclimatic region in central China from 2019 to 2021. Firstly, crop productivity and SIC change mediated by ERW were evaluated by applying 10 kg m−2 mixed rock powder in field trials. Secondly, the random forest and correlation network analysis were used to quantify the influence of climate, soil, and management drivers on yield and SIC change. Finally, the carbon footprint of silicate rock powder was calculated by life cycle analysis. RESULTS AND CONCLUSIONSFarmland ERW significantly improved the crop yield (7 ± 4.3%) and biomass (11 ± 4.6%) in the whole region. In the low water balance region, the net carbon sequestration during three years caused by ERW was 4.31 ± 0.82 t-CO2 ha−1, which increased the carbon capture of the agricultural system by 1.6–2.4 times. Contribution analysis showed that soil pH had the highest relative importance (18%) on yield change mediated by ERW. Low soil pH was conducive to yield and nutrient effectiveness increases, and the largest yield increase reached 31 ± 6.9%. Water balance (rainfall/evapotranspiration) was the dominant driver affecting (20%) inorganic carbon sequestration, the accumulation of SIC in low water balance reached 3.8 times that in high water balance regions. Moreover, fertilizer input also significantly correlated with ERW field effect, and N input significantly increased the SIC in the arid region. SIGNIFICANCEThis study provided detailed field data for large-scale potential analysis of ERW in farmland. These findings contribute to understanding the environmental influence of ERW and assisting decision-making for ERW layout. Moreover, our results could inspire soil improvement in regions with soil acidification and mineral nutrient shortages.

Silicate and carbonate mixed shelf formation and its controlling factors, a case study from the Cambrian Canglangpu formation in Sichuan basin, China

Abstract Mixed sedimentation is mainly a phenomenon by which siliciclastic and carbonate are mixed under a similar sedimentary environment. The study of the composition of mixed rock can reflect the sedimentary environment. The mixed sediments of the Canglangpu formation in the eastern Sichuan basin are widely developed and exhibit well exploration prospects. In this study, a classification diagram of mixed sediments suitable for the Canglangpu formation was proposed by combining lithologic characteristics with logging data. Based on the observations of typical outcrop profiles, the identification of thin sections, and drilling and geological logging data, a model was established to identify the electrical responses of the mixed sedimentary rocks in the study area. The results show that (1) the Canglangpu formation can be vertically divided into two complete transgression-regression cycles with the lower part mainly consisting of sedimentary carbonate diamictite and the upper part principally composed of clastic diamictite; (2) mixed sediments in the Canglangpu formation can be divided into six classes, namely carbonate sandstone, argillaceous sandstone, sandy carbonate rock, argillaceous carbonate rock, sandy mudstone, and Carbonate mudstone; (3) the sedimentary environment of the Canglangpu formation be divided into (a) mixed sediments in the shore, which include mixed lagoon and mixed tidal flat, (b) mixed sediments in the shallow marine shelf, which includes mixed beach bar, terrigenous detrital mixed shelf, and carbonate mixed shelf; (4) a mixed shore-mixed shelf sedimentary model has been established; it says that the mixed sediments show obvious characteristics of ring-like development around underwater paleo-heights. The formation mechanism of mixed sedimentation in the eastern Sichuan basin is mainly controlled by the change of sedimentary facies belt, and the development intensity is mainly influenced by early tectonic movement and sea-level change.

Mechanical behaviour of rock containing a persistent joint under uniaxial compression at different strain rates

Understanding the mechanical behaviour of jointed rock during seismic events is crucial for ensuring the stability of rock engineering. A series of uniaxial compression tests were conducted on granite specimens containing a persistent joint at strain rates (10−5–0.05/s). Acoustic emissions (AEs) were monitored to detect the rock fracturing, and the strain field on the specimen surface was measured by the digital image correlation technique. Three failure patterns of specimens were observed: rock splitting, joint slipping and mixed rock splitting–joint slipping. The shapes of typical strain–stress curves for the three patterns are different, but they are all characterised by multiple stress drops indicating significant rock fractures. When significant fractures occur, the frequency range of AEs expends and the dominant frequency of AEs becomes much larger. The specimen strength is affected by the strain rate, but this effect differs for different failure patterns of specimens. The joint inclination could influence the mode and profile of fractures near the joint, and the specimen strength and joint stiffness significantly decrease with increasing joint angle. This study could help better understand the behaviour of jointed rocks subjected to seismic loads.

Analysis on the Difference of Micropore Structure Characteristics and Physical Properties of Mixed Rock Reservoir

The accumulation of multiple source components leads to the superposition of multiple pore throat systems and similar pore throat distribution may correspond to different seepage capacity in the mixed rock reservoir. Taking the mixed rock reservoir of III+IV oil formation in Nanyishan Oilfield, Chaidamu Basin, as an example, the development characteristics of pore throat system and its controlling factors in the mixed rock were summarized by means of thin slice, scanning electron microscopy, X-ray diffraction and high pressure mercury injection, and the reservoir physical property differences and their genetic controlling mechanism were analyzed. The results show that the reservoir space of the mixed reservoir mainly consists of intergranular pore fracture, micropore fracture, and micropore-dissolution pore. The diverse and complex pore types make the reservoir space combination type, capillary pressure curve shape and reservoir physical properties are not completely corresponding. The higher the content of brittle minerals, the higher the reservoir permeability. However, the development of iron dolomite mainly destroys the reservoir. The permeability of reservoir is mainly controlled by the development degree of macropores and the pore size which is the main contribution to permeability, and the proportion of macropores increases with the increase of permeability.

Nuclear magnetic resonance response characteristics and quantitative evaluation method of fluid saturation of lacustrine shale oil

The quantitative evaluation of fluid saturation is important for formation evaluation of shale oil. However, there is currently no effective method to identify the fluid occurrence state and quantitative evaluate the fluid saturation of lacustrine shale oil because of the complexity of diagenetic minerals and pore types. In this paper, a method is proposed for the quantitative evaluation of fluid saturation based on nuclear magnetic resonance (NMR), X-ray diffraction (XRD) and scanning electron microscopy (SEM) measurements for shale samples of Fengcheng Formation, Mahu sag in Junggar Basin, China. These studies revealed that the shale oil rocks mainly contain quartz, feldspar, dolomite, calcite and clay minerals, both develop organic and inorganic pores. The fluids mainly occur in the form of bitumen, clay bound water, bound water, bound oil and movable oil in this study area. Based on the findings from these experiments, a mixed rock index (MI) and mud index (SI) were proposed to divide the shale oil formations into three types, including sand shale, dolomitic shale and mudstone. A T1−T2 2D NMR fluid occurrence state characterization map was established to identify the different fluids by the MI, SI and NMR characteristics. Furthermore, a method was proposed to quantitatively calculate the coefficient distribution of bound and movable fluids for shale oil in different formations. Finally, the proposed method was successfully applied into the lacustrine shale oil in the Fengcheng Formation to identify fluid occurrence state and evaluate fluid saturation quantitatively.

Identifying Emeishan basalt by supervised learning with Landsat-5 and ASTER data

Multispectral-sensor images are advantageous in terms of discriminating major lithologies due to their high spatial resolution and intermediate spectral resolution, in addition to their low cost and high accessibility in comparison to hyperspectral images. In this study, Landsat-5 Thematic Mapper ™ and the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) data—which are the most widely used multispectral data for the discrimination of the mixed rock units—are utilized to identify basalts in our study area. Further, prior knowledge regarding basalt-distribution areas in our study region is obtained from the geological-survey results conducted by the Sichuan Geological Survey at 2005, which is used as the reference of correction to assess our identified results. Small portions of this prior area of basalt distribution were verified through field checks, which were then determined as sites for use as training data for remote-sensing imagery. Three supervised-classification algorithms within ENVI 5.3—k-nearest neighbors (KNN), maximum likelihood classification (MLC), and support vertical machine (SVM)—were utilized for model identification. As a result, six models were constructed, including the KNN prediction of basalts by ASTER images, SVM prediction by ASTER, MLC prediction by ASTER, KNN prediction by Landsat-5 images, SVM prediction by Landsat-5, and MLC prediction by Landsat-5. The performances of the six models, in terms of precision and accuracy, show that the optimum model is Landsat-5 by SVM, with a precision of 70.92% and accuracy of 99.97%, followed by the ASTER by SVM model, with a precision of 67.72% and accuracy of 99.89% and the Landsat-5 by KNN model, with a precision of 57.23% and accuracy of 99.85%.

Spatial distribution prediction of pre-Sinian volcanic and metamorphic rocks in Xinhenan-Sandaoqiao area, Northern Tarim Basin

The main type of oil and gas reservoir in the Xinhenan-Sandaoqiao area is buried hills. The distribution pattern and scale of reservoirs in this area are obviously controlled by pre-Sinian basement strata. However, the lithologic combination and spatial distribution pattern of the pre-Sinian basement in this area are still unclear. We have used the method of multifactor comprehensive analysis to study the spatial distribution of pre-Sinian basement volcanic and metamorphic rocks. First, we determine the lithology and lithologic association of volcanic and metamorphic rocks according to cores and thin sections. Guided by the seismic reflection characteristics of different lithologic combinations, different lithologic combinations are identified on the profile by combining the seismic reflection characteristics of single well and multiwell. Second, using cluster analysis technology, we select three seismic attributes that are sensitive to lithology from 10 attributes, make crossplots of the three seismic attribute values, and ensure the distribution range of attribute values corresponding to different lithologic combinations for plane lithologic identification. Finally, we combine the recognition results of the plane and the profile to summarize the plane lithology distribution of the pre-Sinian basement surface. Six distribution types were identified deep metamorphic bedrock area in Kuqu depression, dynamic mixed metamorphic rock and intermediate-acidic intrusive rock area, metamorphic bedrock in thrust napple slopes area, thermal contact metamorphic rock area, intermediate-acidic intrusive rock area, and dynamic metamorphic rock area and gneiss area in faulted uplift core.

Movements and Its In‐Process Control of Ground and Built Structures due to Tunnelling in Urban Areas

Tunnelling‐induced environmental responses including ground movements and deformation of the environmental structures are the two prominent issues needed to be carefully disposed in urban areas. This paper tries to develop a theoretical framework to evaluate and control the environmental deformation to avoid two disasters including ground collapse and structural failure. According to the mechanical responses of the ground due to tunnelling, the urban ground in China can be roughly divided into three types including composite multilayered ground, highly water‐bearing ground, and mixed rock ground. The displacements of ground and environmental structures due to tunnelling in multilayered ground are emphatically discussed in view of its largest proportion distribution. The cumulative and mutational characteristics of the displacements are considered, which can be the main basis for distinguishing the occurrence of two types of disasters. Based on the development of the displacements, the three‐stage evolution model of disaster is established. Combined with the existing displacement and degeneration of the ground and structures and the safety factor, the displacement control standards can be eventually determined. The theoretical framework of in‐process control is established which is based on the prediction, measurement, and control standards of the displacements. In the framework, the prediction model can be modified in time, and the tunnelling parameters can be adjusted synchronously. The current research is successfully applied in the Qinghuayuan Tunnel Project.

Fractal model for permeability estimation in low-permeable porous media with variable pore sizes and unevenly adsorbed water lay

Permeability can effectively represent the ability of fluids to flow in porous media, and is a crucial physical parameter for the exploration of unconventional oil reservoirs. In low-permeable porous media, such as tight rocks, not all pores function as fluid flow channels; meanwhile, the pore surface tends to form unevenly adsorbed water lay. To date, the two phenomena above have not been considered in previous studies on permeability estimation models. In this study, a new fractal permeability model with variable pore sizes was proposed resulting in two advancements: (1) unevenly adsorbed water lay attached to pore surfaces was considered; and (2) instead of the total pore space, the movable fluid space obtained from nuclear magnetic resonance (NMR) experiments was used as the fluid flow channels in the porous media. Helium permeability data of 26 mixed rock samples from tight oil reservoirs were used to establish the accuracy of the proposed permeability model. The results of a sensitivity analysis show that high porosity and large pore diameters play a positive role in generating high permeability in porous media. The permeability decreased with an increase in the fractal dimension of the total pore space and the tortuosity fractal dimension of the total pore space and movable fluid space, whereas it first decreased and then increased as the fractal dimension of the movable fluid space increased. Furthermore, the proposed model indicates that the movable fluid space and unevenly adsorbed water lay greatly influence the fluid flow, specifically at lower permeabilities (K＜0.1 mD), while the influence of the unevenness of the adsorbed water lay on fluid flow is negligible at high permeabilities. Overall, the accuracy of the permeability estimation of the proposed model is reliable both in low-permeable and conventional porous media, and can be effectively applied to unconventional oil reservoirs.

Mixed siliciclastic-carbonate deposition and cyclical evolution of the Upper Shahejie Formation and its impact on reservoir development in the Eocene Huanghekou Sag, Bohai Bay Basin, East China

We have systematically investigated a typical lacustrine mixed siliciclastic-carbonate sedimentary system in this study, which primarily deposited in the Eocene Upper Shahejie Formation (Es1 formation) of the Huanghekou Sag, Bohai Bay Basin. Using synthetic lithologic data sets, we summarized eight lithofacies from mixed siliciclastic-carbonate rocks and classified them into siliciclastic- and carbonate-dominated groups, indicating the variation of relative intensity between fluvial/deltaic process and carbonate productivity. Combining petrological and mineral component analysis, most lithofacies are compositionally replaced by micrite dolomite and show vertically different scales of cyclicity. The high-level cyclicity suggests the alternation between siliciclastic- and carbonate-dominated mixed rocks, whereas the lower level indicates interbedding of different dolomite-dominated lithofacies. The controlling factors on both levels are different. The higher level is probably controlled by climate changes, which finally resulted in the relative variations of siliciclastic supply, although the lower level of cyclicity changes more frequently and is considered to have been controlled by fluctuating hydrodynamic conditions. Typical mixed rock depositing patterns are summarized as the “sandwich type,” which are mainly distributed at siliciclastic-dominated sections and the “interbedded type” at carbonate-dominated sections. Besides, reservoir physical properties among lithofacies display significant differences as well, and the grain-supported framework of carbonate-dominated lithofacies suggests most constructive in keeping intergranular pores. The investigated lacustrine cyclical sedimentation in mixed siliciclastic-carbonate rocks has been less concerned in previous studies and may serve as a useful reference for lacustrine mixed rocks analysis in other rifted basins.

Reservoir characterization and production history matching of Lower Cretaceous, Muddy Formation in Ranch Creek area, Bell Creek oil field, Southeastern Montana, USA

A detailed reservoir description of the Lower Cretaceous, Muddy Formation is an essential step for developing an enhanced oil recovery project in the Bell Creek field. The applied workflow highlighted the role of key uncertainties that were not well addressed during early field development. Several challenges including: stratigraphic subdivision of the reservoir units, rock type complexity due to diagenesis, petrophysical data inconsistencies and fluid contacts have historically led to hurdles in reservoir development. The study outlines a workflow used to refine some of these uncertainties and generate better inputs for the geologic model. The Ranch Creek area at Bell Creek field in southeastern Montana is characterized by a clastic sequence of mixed rock types from argillaceous mudstone, bioturbated tightly cemented to clean sandstone lithofacies. Delta front to fluvial-dominated depositional settings as well as diagenesis played an important role in controlling reservoir quality and geometry. The facies complexity is reflected in rock properties controlling fluid migration through the reservoir intervals. The lithofacies types are clustered into three groups, each with characteristic porosity and permeability cutoffs based on core plugs measurements. The rock types are linked to depositional facies obtained from the petrographic description and calibrated to the clusters derived from the capillary pressure measurements. Porosity distribution within the reservoir is biased to the diagenetic complexity and rock type distribution. Three different transforms were used to calculate the corresponding permeability to the modeled porosity parameter. Modeling water saturation for the three different capillary pressure groups using the J-function approach improves the spatial distribution of water saturation and leads to a better match of historical oil and water production rates.Reservoir simulation is crucial to improve our understanding of rock and fluid properties. Integrating dynamic data such as pressure, production and injection rates helps to reduce the uncertainty of the model parameters. The history match process highlights the need to tune the output of the static model. Iterations on the static model, focused on changing the ranges of key parameters such as facies volume fractions and water saturation that contain the most uncertainties. Applying these changes results in an improved geological model that provides a better data to the simulation model. The tuned static model was able to reproduce the observed results. Matching the pressure data supports a valid oil-in-place which is consistence with historical volumetric calculation.

Energy Catastrophe of Jointed Rock Slope Considering Spatiotemporal Variability of Strength

The spatiotemporal variability of rock mass strength for jointed rock mass is crucial for the stability of open-pit mine slopes. The Dagushan Open-pit Iron Mine was taken as the background to systematically study the unloading damage and energy evolution of high slope jointed rock mass. Through a field survey of mixed rock slope and fuzzy C-means clustering optimization analysis, the random distribution parameters of two dominant joint sets of rock mass were determined, and a three-dimensional joint network of rock mass was reconstructed. Additionally, a numerical analysis method is proposed for the energy evolution of jointed rock mass considering the spatiotemporal variation of strength, and the variation trends of gravity potential energy, elastic strain energy, and dissipation energy of jointed rock slope were analyzed. Moreover, based on the cusp catastrophe theory, an energy criterion was established for jointed rock mass slope instability. Furthermore, the energy evolution mechanism of deformation and failure of jointed rock slope under mining was elucidated. The slope safety factor obtained from the energy cusp catastrophe dissipation energy cusp catastrophe is the same as that obtained from the limit equilibrium Janbu method.

Petrophysical analysis of Upper Qishn Clastic Member Reservoir, Tawila Oilfield, Yemen

A comprehensive petrophysical study of the Upper Qishn Clastic Member reservoir was conducted at Tawila oilfield in Yemen. An integrated well-logging dataset was analyzed to detect the reservoir properties and hydrocarbon potential. Lithologic data, shale/matrix models, elastic, seismo-facies, and fluid analytical analyses and crossplots were generated to characterize the reservoir. The clastic reservoir was subdivided into four petrophysical zones (S1A, S1B, S2, and S3) having different reservoir properties and hydrocarbon content. Crossplots of dia-porosity, apparent-matrix density, and elastic rock properties revealed a reservoir with mixed rock types primarily composed of quartz, calcite, and feldspars, with rare kaolinite and heavy minerals. A laminated-shale model was found to apply to the S2 and S3 reservoir zones, while dispersed- and structural-shale models best describe the S1A and S1B zones. Pickett and Buckles saturation crossplots indicated that the reservoir is not at irreducible water saturation. The Qishn reservoir exhibits regular and uniform facies of similar thickness throughout the study area with a slight dip toward the northwestern part of the oilfield. Estimated petrophysical parameters for the reservoir are represented by vertical petrophysical analogs and property distribution maps. The vertical petrophysical analogs show that the S2 and S3 zones attained clearly defined reservoir properties compared to S1A and S1B zones. The S2 zone has an effective porosity of 11–19%, permeability of 2–670 mD, and hydrocarbon saturation < 57%. The S3 zone offers an effective porosity of 12–21%, permeability of 4–2000 mD, and hydrocarbon saturation < 61%. Both S1A and S1B zones show weak petrophysical parameters and low hydrocarbon content. Hydrocarbon-distribution maps reveal significant hydrocarbon saturation north of the main east–west-trending fault reaching a maximum associated with low shale volume and excellent effective porosity and permeability values. It was concluded that the S2 and S3 zones are the most hydrocarbon-rich zones in the reservoir and should be further studied and mapped across the study area and the nearby oilfields.

Seismic recognition and quantitative characterization of siliciclastic–carbonate mixed sedimentary rocks in the Bohaiwan Basin, Northeast of China

Mixed sediments often occur in continental lake basins due to the coupling effects of the climate, hydrodynamic conditions, tectonic processes, and material abundances of provenances, and the mixed sedimentary rock formed in this process has dual characteristics. The response of mixed sedimentary rock to seismic waves is different from that of conventional sandstones or carbonate rocks and easily causes errors in identification and characterization by conventional research methods. In this paper, taking the mixed sedimentary rock in the southern part of the Laizhouwan Sag in the Bohaiwan Basin as an example, the petrological characteristics of the mixed sedimentary rock are studied and its sedimentary model is established. On this basis, a comparative study of the seismic waveform characteristics is carried out. Compared with sandstone, the waveform continuity of mixed sedimentary rock is poor, the amplitudes are weak and the frequencies are low, but opposite results are found compared with the conglomerate formed by gravity flow at the bottom. Contrast analysis of the acoustic impedance shows that the value of the acoustic impedance of mixed sedimentary rocks is higher than that of sandstone and is generally greater than 6.2 × 106 kg/m3 m/s. Therefore, the threshold value between sandstone and mixed rock is set to 6 × 106 kg/m3 m/s in inversions to delineate the boundary between mixed sedimentary rock and sandstone. Finally, using seismic attribute analysis and inversion technology, the scales of the horizontal and vertical distributions of the mixed sedimentary rock platform are predicted. The range of the platform is 7100 m in length, 3100 m in width, and 15.6 km2 in area. Longitudinally, the mixed sedimentary rock layer spans the third layer of the third member of Shahejie Formation (ES33).

Provenance and tectonic setting of the Eocene Nanka Sandstone, Anambra Basin, Nigeria

The Anambra Basin is a Cretaceous intracratonic basin, trending NE–SW in the southern part of Nigeria. The basin is believed to have been formed during the Santonian tectonic episode, which affected the entire Benue Trough. Inorganic geochemical data of the Nanka Sandstone was applied in studying its tectonic setting and provenance. The sandstones have fractionated chondrite-normalised rare earth element (REE) patterns, enriched light rare earth elements (LREE) and nearly horizontal REE with negative europium (Eu) anomalies. These patterns as well as a wide range of LREE/HREE (heavy rare earth elements) ratios and negative Eu anomalies suggest a felsic and/or mixed rock type source. A provenance discriminant function diagram utilising major oxides indicates a quartzose recycled sedimentary source. A plot of La/Th vs. Hf suggests a mixed felsic/basic source; a plot of Cr/Th and Th/Sc indicates contributions from felsic and basic end members, suggesting a mixed-source provenance. A Th/Co vs. La/Sc diagram, V-Ni-Th*10 ternary plot and Th/Co vs. La/Co suggest mixed source rock. Provenance investigations allude to sediment derivation from felsic, intermediate and mafic components and probably inputs from recycled older sediments. Tectonic discrimination based on major and trace elements indicates a passive margin setting.

Geochemical investigations in bulk and clay size fractions from lower Krishna river sediments, southern India: implications of elemental fractionation during weathering, transportation and deposition

Major and trace elements, including rare earth elements (REEs) geochemistry of bed sediments from the lower reaches of Krishna river suggest their origin mainly from weathering of Deccan basalts and Precambrian rocks of the southern India. The elemental compositions indicate evidence of sedimentary sorting during weathering, transportation and deposition. Highly depleted Na2O and high values of Chemical Index of Alteration (CIA) are suggestive of moderate to strong chemical weathering environment in lower reaches of Krishna river. Major, trace and REE concentrations of bulk sediments suggest marginal variations in relative contribution from mafic and felsic minerals. The bed sediments contain significantly low concentrations of trace elements like Ba, Sr, La, Ce and Nd and moderately low concentrations of V, Zr, Rb, Zn, Ni and Cr relative to upper continental crust (UCC) composition. Most of the other trace elements and REEs are almost similar to UCC indicating their origin from weathering of mixed rock types in catchment areas of Krishna river. Strong relationships of ΣREEs with Ti and Zr in bed sediments suggest that REEs are mainly controlled by heavy minerals such as titanite and zircon. A marked increase in REEs in clay fractions of sediments indicates their adsorption and/or substitution on clay lattice.