Mineral Composition Of Rocks Research Articles

Oil Mobility Evaluation of Fine-Grained Sedimentary Rocks with High Heterogeneity: A Case Study of the Lucaogou Formation in the Jimusar Sag, Junggar Basin, NW China

The oil mobility in unconventional tight/shale oil reservoirs is complex because of the high heterogeneity in the matrix pore structure and connectivity. Thus, it is difficult to evaluate hydrocarbon exploration potential precisely. In this study, multistep temperature pyrolysis (MTP) Rock-Eval and 1D (dimensional) as well as 2D nuclear magnetic resonance (NMR) techniques were employed to systematically characterize the oil mobility in lacustrine fine-grained sedimentary rocks from the Lucaogou Formation in the Jimusar Sag, Junggar Basin, NW China. The results show that the Lucaogou Formation fine-grained sedimentary rocks can be subdivided into six types of lithofacies. With the increase of organic matter richness, the content of adsorbed oil increases, whereas the content of movable oil increases first and tends to stabilize (when total organic carbon (TOC) > 4%). This is because when TOC is low, the fine-grained sedimentary rocks need to self-adsorb before oil production; when TOC increases further, the generated oil will break through the absorption limit and charge into the adjacent reservoirs; therefore, the movable oil ceases to increase. Permeability is found to have a greater impact on movable fluid saturation than porosity. Meanwhile, good throat radius and pore connectivity are conducive to oil flow as movable oil is more sensitive to throats rather than pores. Furthermore, a higher content of brittle minerals is not necessarily favorable for oil flow; alternatively, more clay minerals are easy to from cements causing pore blockage, which will essentially hinder oil mobility. Overall, the organic matter content, reservoir pore structure, and rock mineral composition are the main factors affecting tight/shale oil mobility. On the basis of the above research, a conceptual model of oil mobility in different lithofacies’ reservoirs is established. These results have a reference significance for evaluating oil recoverability in fine-grained sedimentary rocks.

Characteristics of thermophysical parameters in the Wugongshan area of South China and their insights for geothermal genesis

The Wugongshan area is rich in medium–low temperature convective geothermal resources, among which there are more than 10 geothermal fields in Wentang, Wanlongshan, Wenjia, Hongjiang, etc. There are few basic geothermal geological studies in the geothermal fields and their peripheral areas; thus far, no systematic research work into the thermophysical parameters has been carried out. In this paper, 85 rock samples were collected from the surface and boreholes covering the strata and magmatic rocks in the study area. The results show that the average radioheat generation rate, the average thermal conductivity, and the average specific heat are 0.24–5.49 (μW/m3), 1.995–4.390 (W/mK), and 1.318–4.829 (MJ/m3K), respectively. The average thermal diffusivity ranges from 1.115 to 1.611 × 10-6 m2/s. The highest radioheat generation rate is Jurassic granite, and the lowest is quartz vein. The largest thermal conductivity and specific heat is the siliceous quartzite, and the smallest is the quartz vein. The highest thermal diffusivity is Cambrian metamorphic mica schist, and the lowest is siliceous quartzite. The radioactive heat generation rate, thermal conductivity, specific heat, and thermal diffusivity are closely related to the chemical composition, mineral composition, rock fabric, porosity, water content, and temperature and pressure conditions of rocks in the whole area. There is a linear relationship between thermal conductivity (K) and thermal diffusivity (κ), and the correlation equation is K = −0.3144k + 3.2172. Combined with the characteristics of thermophysical parameters, the genetic theory of deep crust heat generation + structural heat accumulation + siliceous quartzite heat conduction + granite heat preservation is preliminarily proposed.

Rock mechanical properties of immature, organic-rich source rocks and their relationships to rock composition and lithofacies

Mechanical properties of layered rocks are critical in ensuring wellbore integrity and predicting natural fracture occurrence for successful reservoir development, particularly in unconventional reservoirs for which fractures provide the main pathway for hydrocarbon flow. We examine rock mechanical properties of exceptionally organic-rich, immature source rocks from Jordan, and understand their relationships with rock mineral composition and lithofacies variations. Four depositional microfacies were identified: organic-rich mudstone, organic-rich wackestone, silica-rich packstone and fine-grained organic-rich wackestone. The four types exhibit various mineralogical compositions, dominated by carbonates, biogenic quartz and apatite. Leeb hardness ranges between 288 and 654, with the highest average values occurring in silica-rich packstone and organic-rich mudstone. The highest uniaxial compressive strength (derived from the intrinsic specific energy measured using an Epslog Wombat scratch device), and compressional- and shear-wave velocities were measured in organic-rich mudstones (140 MPa, 3368 m s −1 and 1702 m s −1 , respectively). Porosity shows higher average values in organic-rich wackestones and fine-grained organic-rich wackestones (33–35%). Silica-rich packstone and organic-rich mudstone have brittle properties, while organic-rich wackestone and fine-grained organic-rich wackestone are ductile. High silica contents are correlated positively with brittleness. A strong hardness–brittleness correlation suggests that Leeb hardness is a useful proxy for brittleness. Our study allows a better understanding of the relationships between lithofacies, organic content and rock mechanical properties, with implications for fracking design to well completion and hydrocarbon production. Further work involving systematic sampling and a more rigorous study is still required to better understand the spatial distribution of target lithologies and their mechanical properties.

Case Study on the Effect of Acidizing on the Rock Properties of the Mahu Conglomerate Reservoir

The development of the Mahu tight reservoir has adopted horizontal wells with staged fracturing. In the fracturing, there is a problem of a high fracturing pressure. Acid treatment is often used to lower the fracturing pressure on site. At present, the impact of this acid treatment on the physical parameters of the rocks of the reservoir in the Mahu region has not been systematically studied. Aiming to solve this problem, this paper conducted an experimental study on how acid dissolution affects the physical properties of the Mahu conglomerate, including its porosity, permeability, triaxial rock mechanical parameters, tensile strength, and mineral composition. First, the experimental scheme was designed. Next, a series of experiments were conducted. Finally, the experiment results were analyzed comparatively before and after acidizing. The acid composition, concentration, and contact time were the main factors for the analysis, based on which the acid system and related parameters were recommended. This study showed that the Mahu conglomerate exhibited brittle plasticity characteristics under stress. The carbonate content in this region was low, while the feldspar content was high, so it was necessary to use mud acid to effectively dissolve feldspar, clay, and other silicates. After acidizing, the porosity was 200% of the original value. The permeability increased by up to 14 times. The tensile strength decreased significantly by up to 84%. The value of Young’s modulus of the rock decreased by up to 63.6%. The value of Poisson’s ratio was reduced by up to 40.7%. A combination of 6% HF + 15% HCl is recommended, with an effective acid treatment time of over 60 min for the Mahu conglomerate. Acidizing could significantly change the mechanical properties and permeability of the rock of the Mahu conglomerate reservoir, thus effectively reducing the formation fracturing pressure. This research provides technical support for Mahu acid dipping in horizontal well fracturing.

Effects of minerals (phyllosilicates and iron oxides) on the responses of aliphatic hydrocarbon containing kerogens (Type I and Type II) to analytical pyrolysis

Organic matter in sediments is dominated by kerogen, a high molecular weight geomacromolecule. Kerogen can be subdivided into Types I to IV that provide paleoenvironmental and petroleum potential information. Kerogen typing can be performed by several chemical methods including elemental analysis (H/C and O/C), FTIR and pyrolysis-gas chromatography techniques. However, kerogens occur naturally within mineral matrices and these can influence the chemical responses. We have examined the effects of a range of minerals (namely kaolinite, lizardite, ripidolite, illite, montmorillonite, haematite, goethite, limonite and magnetite) on the responses of kerogen to pyrolysis-gas chromatography-mass spectrometry. We used aliphatic hydrocarbon containing kerogen Types I and II from Carboniferous Midland Valley shales of Scotland and the Jurassic Oxford Clay of southern England, respectively, as well as a pure synthetic aliphatic polymer, polyethylene. We find that the aliphatic organic matter in Type I kerogens is transformed by interaction with minerals during pyrolysis to give a signal incorrectly suggesting more contributions from land plant-containing kerogens, such as a large number of aromatic molecules. Pyrolysis with goethite, limonite and magnetite leads to almost complete destruction of the organic matter. Hence, the mineral composition of sedimentary rocks during pyrolysis should be considered when assigning kerogen types. Failure to consider the effects of minerals can lead to incorrect assignment of kerogen type and, therefore, erroneous interpretations of paleoenvironments and petroleum potential.

Study on Local Brittleness of Rock Based on Multiple Linear Regression Method: Case Study of Shahejie Formation

Rock brittleness index is an important parameter that affects the volume fracturing effect of an unconventional reservoir. The higher the brittleness index, the larger the communication area of fracture network formed under volume fracturing. However, rock brittleness has strong heterogeneity, which seriously affects the propagation of volume fracture network. Most existing rock brittleness characterization methods start from macro brittleness and cannot analyze the influence of heterogeneity on reservoir brittleness. The mineral composition analysis method can characterize the heterogeneity of a reservoir by analyzing the influence of mineral composition on brittleness, but this is limited to qualitative analysis on different minerals, resulting in large errors. In this study, the relationship between macro mechanical properties and micro mineral composition of rocks was analyzed using multiple linear regression analysis and a large number of experimental core parameters on the Shahejie Formation to understand the quantitative characterization of rock brittleness distribution in a heterogeneous reservoir. The results showed that quartz, plagioclase, analcite, siderite, and pyrite were high brittle minerals while clay minerals were low brittle minerals. This method provides a new focus for the analysis of brittle distribution of heterogeneous reservoirs and an additional method to study rock fracture law.

Ages of the Ediacaran Volyn-Brest trap volcanism, glaciations, paleosols, Podillya Ediacaran soft-bodied organisms, and the Redkino-Kotlin boundary (East European Craton) constrained by zircon single grain U-Pb dating

Combined single zircon crystal CA-ID-TIMS U-Pb dating, micropaleontological study, and quantitative XRD analysis were used to correlate sections of the Podillya-Moldova Ediacaran succession with sections of the Volyn Large Igneous Province (Volyn LIP). Five U-Pb zircon ages were obtained from Volyn LIP rocks, and four from three distinct Podillya bentonite horizons. Biostratigraphic determinations were made for one section in Volyn, and two sections in Podillya. It was concluded that Volyn LIP activity had two phases. A first phase of mafic and felsic magmatism was characterized by eruption of numerous basalt lava flows, and lasted from c. 580 to 567 Ma; a second phase of pyroclastic felsic magmatism, with no lavas, lasted from 567 to c. 545 Ma. The two phases can be distinguished in the quantitative mineral composition of rocks from the Kobryn 1K borehole in western Belarus. In the central area of the Volyn LIP, near Brest, rocks of the felsic phase were removed by erosion before deposition of the Kotlin subgroup. The Podillya Ediacaran succession, from the top of the Hrushka formation to the top of the Kalyus beds, (i.e. the Mohyliv-Podilsky group), hosts soft-bodied organisms. This succession corresponds to the felsic phase of the Volyn LIP. The age of the Redkino-Kotlin subgroup biostratigraphic boundary, at the base of the Nagoryany formation, is constrained at 555–545 Ma, close to the end of the Volyn LIP activity. The newly discovered glaciation in Volyn is dated as c. 567 Ma. The period of development of kaolinitic paleosols on top of the Volyn LIP effusive rocks is dated at c. 570–550 Ma.

Tectogenesis as a factor influencing the stability of rock masses of the Northern iron ore district of the Kryvbas


 
 
 In the Northern iron-ore district of the Kryvbas basin, mining of deposits of poor magnetic ores is underway in two quarries – Pervomaiskyi and Hannivsky. Oftentimes, their mine contours contain low-valuable or unusable magnetite-silicaceous quarzites of containing ferrous and slate horizons. As a result, excavated ore mass contains ores and partly containing rocks of various mineral and chemical compositions and physical and technical properties. This caused diversity (15 mineral kinds) of the compositions of ores mined from the both deposits. Variability of mineral compositions of ores and rocks is due to manifesta- tions of numerous geological processes such as sedimentation, metamorphism, tectogenesis, metasomatism, supergene, etc. Various mineralogical, geochemical courses of the processes resulted in variability in physical, technical characteristics of ores and containing rocks, and this, in turn, caused various stability of rock masses during drilling-explosive works and mining. Stability of pit walls of the quarries of iron-ore deposits of the Kryvyi Rih basin is a sharp issue for mining enterprises. Solutions to those issues have been analyzed in various aspects. A number of factors take effects on the indicator of stability of rock masses within deposits, particularly mineralogical, geochemical, tectonic, etc. This paper looks at the influence of the tectonic factor specifically, which actively manifests within the Pervomayskyi and Hannivskyi deposits as fractures, breccias, mylonite of ores and rocks. During the surveys, there were designated tectonically altered kinds of ores and rocks within the both deposits, which take effects on seismic stability of pit walls of the both quarries, and also we put areas with various tectonic intensities on schematic maps. For this purpose, we used geological methods of study (selection of samples, mapping), mineralogical (determining quantitative proportions of minerals, texture, structure of ores and rocks). The study provides the data of mineralogical estimates of tectonically altered types of ores and rocks of productive layers of the Pervomaiskyi and Hannivskyi deposits of the Kryvyi Rih basin. We charted maps with areas of various manifestations of tectonic changes within productive layers of the both deposits.
 
 

ГЕОЛОГІЧНІ ПІДСТАВИ ДЛЯ РОЗРОБКИ 3D МОДЕЛІ СТІЙКОСТІ ГІРСЬКИХ МАСИВІВ РОДОВИЩ ПІВНІЧНОГО ЗАЛІЗОРУДНОГО РАЙОНУ КРИВОРІЗЬКОГО БАСЕЙНУ

The stability of rock massifs depends on a number of factors: geological, mineralogical, physical and technical, which affect it in different ways. The problem of stability of rock massifs is facing all iron ore enterprises, because it leads to accidents and violation of mining technology. Enterprises of the Kryvyi Rih basin are not an exception. Development of the specialised geoinformation system will solve actual problems of designing of seismically stable sides of quarries and prevention of landslides of rock massifs. This work is devoted to the main stages of software product development. It will take into account the influence of geological factors on the stability of rock massifs. Justification of expediency of taking into account mineralogical and geological factors was carried out on the example of the Hanivske deposit, which belongs to the Northern iron ore district of the Kryvyi Rih basin. The outlined proposals are based on the calculation of the stability reserve factor of rock massifs, taking into account the variability of the structure of the productive and host strata of the Hanivske deposit. In addition, no less important indicator is the influence of the geological factor, which takes into account the stratigraphic scheme of the deposit and its tectonics. The results of previous studies were taken into account when developing the recommendations. The basis for the correct functioning of a specialised geoinformation system are actual block geological models of the deposit, the database of which should contain information on mineral and chemical composition of ores and rocks, indicators of their physical and technical properties, manifestation of discontinuities, presence of aquifers, areas of metasomatic changes and weathering. The authors emphasised the importance of creating in the software product the function of selecting the method of calculating the stability coefficient of the sides. As a result, the software product should identify areas with stable, intermediate stability and unstable sides within the field. Automation of calculations of stability of rock masses and their visualisation will optimize monito

ГІС ДЛЯ МОНІТОРИНГУ СТІЙКОСТІ ГІРНИЧИХ МАСИВІВ ПІВНІЧНОГО ЗАЛІЗОРУДНОГО РАЙОНУ КРИВОРІЗЬКОГО БАСЕЙНУ

Pervomayske and Gannovske deposits of the Northern iron ore district are characterised by the variable structure of the productive and host strata, as well as the variability of their physical and technical parameters. This fact necessitates a detailed study of the mineral composition of rocks and other geological factors that affect the stability of rock massifs. The issue of variability of stability of rock massifs is acute for mining enterprises. The stability of rock massifs is conditioned by the manifestation of geological (syngenetic and epigenetic) processes. So the influence of mineralogical, physical and technical characteristics of ores and host rocks influences the variability of their strength indicators, stability of rock massifs in the faces of Pervomaysk and Gannovka quarries. The influence of physical and technical properties of ores and rocks was studied on the basis of research of their mineralogical, chemical, textural and structural features. The strength of rock massifs of iron ore deposits is also influenced by anthropogenic factors, the effect of which is conditioned by technical and technological parameters of mining activities. Mapping the areas of influence of various geological processes on the productive and host strata of Pervomayske and Gannovske deposits will increase the efficiency of drilling and blasting and mining operations. This work is devoted to the construction of 3D models, on the example of Pervomayskoye deposit. On the models different on stability areas of the deposit are allocated on the basis of mineralogical, stratigraphic, tectonic factors, and also on the basis of study of physical and technical parameters of ores and host rocks. A generalised model of the deposit is also given, taking into account the above-mentioned factors. The obtained models of stability of rock massifs of the deposit taking into account the influence of natural factors can be used for perspective planning of blasting, mining operations and for prevention of emergency situations connected with seismic activity of the sides of iron ore pits.

Numerical Simulation of CO2 Migration and Geochemical Reactions in Shihezi Formation Caprock, China

CO2 geological storage, which is an effective way to reduce CO2 emissions, is of great significance to mitigate the current greenhouse effect. In long-term CO2 storage, although the chemical reaction rate of CO2–brine–rock is slow, it can significantly change the mineral composition of rock and the structure and properties of pores and joints, and then change the transport process and distribution state of CO2 in porous media. Therefore, a simplified 2D geological model is established based on the geological data of the Shihezi Formation in the Ordos Basin, China. The mechanism of the CO2–brine–rock reaction and its effect on mineral transformation and pore permeability are studied. In the early stage of CO2 geological sequestration, the rate of CO2 intrusion into the caprock is fast, the CO2–brine–rock reaction in the early stage is mainly a dissolution reaction, and the porosity and permeability of the caprock show an increasing trend. During the period from 100 to 1000 years of CO2 sequestration, the vertical distance of CO2 intrusion into the caprock does not change much. During this period, the type of CO2–brine–rock reaction is mainly a precipitation reaction, which reduces the porosity and permeability of the caprock and increases the sealing ability of the caprock to a certain extent. Our results can not only provide theoretical support for the site selection and risk assessment of CO2 geological sequestration, but also provide a theoretical basis and practical guidance for large-scale commercial storage in the future.

Measurement of Rock Electrical Parameters and Analysis of Influencing Factors of Quaternary Mudstone Biogas Reservoirs in Qaidam Basin

As a new and unconventional reservoir, Quaternary mudstone biogas reservoirs in the Qaidam Basin are widely distributed and thick. They have great potential for biogas exploration. However, the mudstone biogas reservoirs in this area characterized by weak cementation, high clay content, and easy hydrolysis. Using existing methods, it is difficult to obtain the rock electrical parameters and evaluate the saturation. This greatly affects the evaluation of oil and gas reserves. This study selected core samples from the Quaternary mudstone biogas reservoir in Qaidam Basin, prepared the samples through core conformation pretreatment, and conducted rock electrical experiments using the self-absorption water augmentation method to obtain the rock electrical parameters of the mudstone biogas reservoirs for the first time. Then, the factors influencing the rock electrical parameters of the mudstone biogas reservoir were analyzed. The results showed that: (1) the porosity of the mudstone core samples was generally greater than 20%, and the permeability was in the range of 0.05–5 mD. The rock mineral composition was dominated by clay minerals, followed by carbonate minerals, quartz, and feldspar. The clay mineral composition was mainly illite, followed by montmorillonite and chlorite, with a small amount of kaolinite. (2) The use of wire-electrode cutting and high-temperature, heat-shrinkable tube to wrap the core sample played a supporting and protecting role in the core. Using the self-absorption water augmentation method, information was obtained showing that the distribution of cementation index m of the reservoir core samples ranged between 1.89–2.08, with an average value of 1.99, and the distribution of saturation index n ranged between 1.872–2.270, with an average value of 2.09. (3) Organic matter content had no obvious effect on rock electrical parameters. With the decrease of clay mineral content, the quartz content increased, the permeability increased, and the cementation index increased gradually. The saturation index increased with the increase in clay mineral content and the decrease of quartz content and permeability. The above results laid a petrophysical basis for the evaluation of the saturation of mudstone biogas reservoirs and could provide technical support for the comprehensive evaluation of the reservoir and the calculation of reserves.

Seepage Characteristics and Influencing Factors of Weakly Consolidated Rocks in Triaxial Compression Test under Mining-Induced Stress Path

The rock of weakly consolidated coal measure strata has the characteristics of low mechanical strength and strong water sensitivity. Under the stress and seepage disturbance caused by coal seam mining, the surrounding rock structure is prone to instability, which leads to mine safety accidents and water resources loss. In order to master the mechanical response and permeability evolution law of weakly consolidated rock under the disturbance of coal seam mining, the specimens of Jurassic mudstone, sandy mudstone, and sandstone in the Ili mining area of China were collected, and a triaxial compression seepage test was carried out. A comprehensive analysis was carried out on the mineral composition and microstructure characteristics of the rock. The results show the following: (1) Compared to the constant confining pressure condition, mining-induced stress promotes the fracture development rate of weakly consolidated rocks. The ratios of strain at the yield point of mudstone, sandy mudstone, and sandstone under mining-induced stress and constant confining pressure are 0.33, 0.43, and 0.79, respectively, and the ratios of strain at the failure point were 0.48, 0.52, and 0.72, respectively. (2) Under the condition of mining-induced stress, the permeability change range and the permeability recovery rate of the three types of rocks were different, which decreased in the order of mudstone, sandy mudstone, and sandstone. (3) In the process of the triaxial compression test, there was a strong hysteresis in the permeability change of the mudstone, and the permeability and hysteresis of the three types of rocks decreased with the increase in the clay mineral content. (4) Combined with the analysis of the rock mineral composition and microstructure characteristics, it is believed that the clay minerals in the rock after water mud and swelling are the main reasons for the hysteresis of the permeability change of weakly consolidated rock, and the content of clay minerals is the main factor affecting the permeability characteristics of the weakly consolidated rock.

The problem of the genesis of the Mesoarchean aluminosilicate rocks from the Karelian craton and their possible use as a quartz-feldspar raw material

The article presents original data obtained in the study of the chemical and mineral compositions of the Late Archean aluminosilicate rocks (formerly called silicites) from the Koikari and Elmus structures of the Vedlozero-Segozero greenstone belt of the Karelian craton (Central Karelia). A comprehensive study of these formations revealed their complex genesis as a result of the late imposition of hydrothermal and metamorphic alteration on sedimentary and volcanic-sedimentary rocks of feldspar-quartz composition. Due to the superimposed metasomatic (temperature?) impact on feldspar-quartz siltstones, Fe was removed from microinclusions in quartz and feldspar and its oxides were concentrated along the grain boundaries. Minerals such as monazite, parisite, allanite are also located either along the grain boundaries of quartz and feldspars, or together with calcite they fill microfractures, which makes it possible to get rid of them when preparing quartz-feldspar concentrates using various beneficiation technologies. According to most indicators limited by GOSTs, individual samples in their natural form meet the requirements for quartz-feldspar raw materials for use as part of batch in the production of diverse types of glass. Additional beneficiation of the feedstock (grinding, screening into narrow classes and further magnetic separation) leads to a decrease in Fe2O3 content to normalized values. The resulting quartz-feldspar concentrates with various particle sizes can be used in the production of building material and fine ceramics (sanitary and ceramic products, facing and finishing tiles, artistic, household porcelain and faience). The homogeneity of the mineral and chemical composition, the possibility of compact extraction and beneficiation (including in mobile small-sized installations) increase the prospects and competitiveness of this non-traditional feldspar raw material from Central Karelia.

Garnet: The archive of prograde metamorphic records of granulite-facies terranes

<p indent="0mm">Whether granulite terranes can preserve prograde metamorphic records is a perturbing topic in metamorphic petrology, and related research is rare. This is mainly because the metamorphic records of the granulite terranes have information asymmetry. Generally, the mineral assemblages and compositions of granulite-facies rocks are considered to reflect peak and retrograde metamorphism, rather than prograde metamorphism. However, this assumption must be made with caution. Case studies of granulites from the junction area of Shanxi and Hebei province in the North China Craton suggest that garnet could serve as “temperature and pressure resistant containers” under granulite-facies conditions, and retain the evidence of pre-peak metamorphism. For pelitic granulites, their protoliths are supracrustal rocks, and must have undergone prograde metamorphic processes from the Earth’s surface to the deep crust. Therefore, a confirmatory study seeking prograde metamorphic records for pelitic granulites from the Huai’an-Datong area was conducted. In the studied pelitic granulites, inclusions in garnet display well-preserved microstructural zoning, with radical zonal distribution patterns: Crowded quartz grains in the core, a few K-feldspar-kyanite-bearing assemblages in the mantle, and some sillimanite flakes in the rim. The major element zoning (here, major endmember proportions X_Sps, X_Alm, X_Pyr, and X_Grs are used) of garnet is relatively homogeneous. However, the high rare earth elements (HREEs) exhibit bell-shaped profiles decreasing from the core to the rim. This suggests that even if the major elements of garnet are homogenous due to high-temperature diffusion, microstructural zoning and some trace elements with slow diffusion rates could be protected from peak- and retro-grade high-temperature modification. For garnet mafic granulites, their protoliths are considered as gabbro dykes, and there is no robust evidence of prograde metamorphism from the upper to lower crust; therefore, their tectonic significance is unclear. Here, an exploratory study of prograde metamorphism for garnet mafic granulites from the Huai’an-Hengshan area was conducted based on the study experience of pelitic garnet. In the investigated mafic granulites, garnet also displays a zoned distribution of various mineral assemblages from its center to rim, including quartz- and titanite/ilmenite-bearing assemblages in the core, rutile-bearing assemblages in the outer mantle to the inner rim, and a resorbed rim. Fortunately, garnet of the mafic granulites displays well-preserved compositional growth zoning patterns, with bell-shaped X_Sps, increasing X_Pyr from core to mantle, and a resorbed rim (X_Sps). The HREEs (for example, Yb) show bell-shaped patterns similar to those of the X_Sps. Both textural and compositional zoning patterns in the studied mafic granulites documented multiple generations of garnet growth history from the prograde to peak metamorphic stages. These findings provide robust metamorphic evidence for the Paleoproterozoic subduction and assembly of the Jin-Yu Mobile Belt in the North China Craton. The above two case studies suggest that high-temperature granulite-facies metamorphism may not completely reset the multi-generation growth information retained in the garnet. In contrast, minerals in the matrix only record the history of intensive re-equilibration at the peak or rehydration retrogression metamorphism. Thus, garnet is an important archive of prograde metamorphic information. This study further suggests that caution should be exercised when determining the peak<italic> P-T</italic> conditions of granulites using garnet core compositions.

Study of the structure and development of oil deposits in carbonate reservoirs using field data and X-ray microtomography

At present, a large number of scientific works devoted to the study of the features of the geological structure and the development of oil deposits in complex carbonate reservoirs are based on the use of any one research method. This article shows the advantages of the integrated use of modern methods of core research, including X-ray microtomography and electron microscopy, as well as data from hydrodynamic and field studies of wells. The advantage of the approach used is the ability to study the deposit at three levels: core-well-development object, it is reasonable to transfer micro-survey data to the regularities of the implementation of technological processes of oil production. The objects of research in this article are the Famennian oil deposits of two neighboring fields, which, at first glance, are analogues in terms of the similarity of the enlarged geological and physical characteristics. Comprehensive studies of core samples from these deposits made it possible to establish differences both in the mineral composition of rocks and in the structure of their void space, especially in the size and distribution of pore channels. So, with approximately equal porosity for one of the deposits, the presence of two types of voids and a twofold prevalence of the size of the largest of them were established. According to the complex of laboratory methods for studying the core, the reservoir is classified as a porous type (no cracks were found). The performed interpretation of the hydrodynamic studies confirmed this fact, made it possible to establish the presence of a dependence of the reservoir permeability on the formation pressure (deformation of the void space of the reservoir), and also to build the corresponding individual dependence for each well. Comparison of the equations approximating the dependence data showed a more pronounced deformation of the reservoir, which is characterized by the presence of large pores and caverns. That is, a complex of laboratory and hydrodynamic studies made it possible to establish the probability of deformation of the void space of a carbonate reservoir even in the absence of cracks in it. In turn, the reservoir deformation is singled out as the most probable cause explaining the different rates of decline in well flow rates of the fields under consideration.

Reservoir characteristics and factors influencing shahejie marl in the shulu sag, bohai bay basin, eastern China

Shahejie marl in the Shulu Sag is a crucial resource for unconventional hydrocarbon exploration in China. Although breakthroughs have been made in tight oil exploration in this area, the mechanisms underlying the formation of this marl reservoir and factors controlling its ‘sweet spots’ have not been thoroughly studied. To understand the pore structure characteristics and factors influencing the marl reservoir, we analyzed core samples from Wells ST1 and ST3. A series of experiments was conducted on the samples, such as X-ray diffraction, focused ion beam scanning electron microscopy, micro-CT, and total organic carbon test. Additionally, the physical properties of different marl rock fabrics were studied with auxiliary tests, such as mercury intrusion capillary pressure analyses, nuclear magnetic resonance, porosity and permeability tests, and thin-section observation. The results revealed that the marl reservoir is characterized by low porosity (1.61%) and low permeability (2.56mD). The porosity and permeability (1.61% and 3.26mD) of laminated marl were better than those (0.92% and 1.68mD) of massive marl. Clay minerals and quartz content in laminated (11.8 and 8.2%) was less than in massive marl (16.2 and 13.3%). The marl pores include intercrystalline pores, dissolution pores, and microfractures. Additionally, the laminated marl pores were primarily distributed along the dark lamina, with good connectivity. A few isolated and uniform holes were observed in the massive marl. Influenced by rock fabric and mineral composition, layered fractures were mainly developed in the laminated marl, while structural fractures were the main type of microfractures in the massive marl. The primary sedimentary mechanism was the main geological action underlying the differences in marl rock fabric; this mechanism affects the physical properties of the marl reservoir, which are key factors to be considered when searching for the marl reservoir ‘sweet spots’. Particular attention should be paid to these factors during tight oil exploration and development in similar sedimentary basins.

Experimental and mechanistic study of mudstone volumetric swelling at the bottom of salt cavern gas storage

Salt cavern gas storage is one of the vital strategic natural gas reserves and emergency peak shaving facilities all over the world. However, rock salt in China is primarily bedded salt, usually composed of many thin salt layers and interlayers (e.g., anhydrite, mudstone, and glauberite). During the water solution mining process of the cavern, the insoluble mudstones fall to the bottom and account for 1/3 up to 2/3 of the storage capacity. The bulk volume of the insoluble mudstones is almost twice its in-suit volume. It is of great urgency to investigate the swelling mechanisms of the bottom insoluble mudstones. Given this, we first analyzed the mineral composition of salt rock and insoluble mudstones by using XRD and SEM methods. Then, experimental studies were carried out considering both clay swelling and physical packing. At last, the zeta potential tests were conducted to reveal the swelling mechanisms of the bottom mudstones. Results show that the volumetric expansion of mudstones is made up of three parts: clay swelling, particle surface bound water volume, and pore space free water volume increase. Because the content of expansive clay in the bottom mudstones is less than 2%, and the high salinity brine in the cavern has excellent clay stability performance, clay swelling is not the main contributor to the volumetric expansion of the bottom mudstones. Measurement results show that the surface of the mudstones is negatively charged after hydration. Electrostatic repulsion can increase the spacing between small rock particles and creates approximately 47.6% of the pore space, which is the main factor in the volumetric expansion of mudstones. This study provides a theoretical basis for the mining solution and capacity enlargement during the construction of bedded salt cavern gas storage in China.

Correlation between the mineral composition and dissolution rate constant of sulfate rocks at different pressures and water flow velocities

Correlation between the mineral composition and dissolution rate constant of sulfate rocks at different pressures and water flow velocities

Research on formation mechanism of the mining-induced horizontal fractures in rock strata separation in the Jurassic coalfield, Huang long, Shaanxi, China

To analyze the mining-induced mechanism of the horizontal fractures due to bed separation in Jurassic coalfield, China, the mineral composition identification test, triaxial compression tests, and overburden response were performed in this study. The mineral composition of rocks is composed of terrigenous sand (cuttings, quartz, and feldspar), terrigenous silt, clay, and a few interstitial materials in adjacent rock strata. The differences of mineral compositions are proved to be the essential factors through triaxial compression tests on rock samples, which found that the strength, Young’s modulus, Poisson’s ratio, and plasticity of adjacent have great differences in rock samples. For predicting the variation of the horizontal fractures, the overburden is supposed to be a thin cube plate. And the range of sizes is corresponding to the scope of the extracted coal seam. Then, based on the elasticity thin plate theory, w(x, y)=C0(1+cos(xπ/a)) (1+cos(yπ/a)) is proposed as the deflection function and the stress and deformation of rock strata are obtained by the variational method and minimum potential energy principle. Hence, a mathematical model suitable is proposed to analyze the variation of the horizontal fractures. The Yuanzigou coal mine is taken as an example, the theoretical results are verified by an exploration borehole video imager. In conclusion, the distribution characteristics of rock strata and the differences in these physical parameters are the essential factors causing horizontal fracture between adjacent rock strata. Besides, the relationship between horizontal fractures and roof water inrush was also discussed in this study.