Red Sandstone Beds Research Articles

Pliocene subsurface fluid flow driven by rapid erosional exhumation of the Colorado Plateau, southwestern USA

Abstract Erosion may modify the architecture of subsurface flow systems by removing confining units and changing topography to influence patterns of fluid circulation or by inducing gas exsolution from subsurface fluids, influencing compositional and buoyancy patterns in flow systems. Here, we examine the geologic record of subsurface flow in the sedimentary rocks of the Paradox Basin in the Colorado Plateau (southwestern USA), including the distribution and ages of Fe- and Mn-oxide deposits and bleached, former red-bed sandstones. We compare our results to those of previous geo- and thermochronology studies that documented as much as 2 km of erosional exhumation at ca. 3–4 Ma and Fe-and Mn-oxide precipitation at 3.6 Ma along fault zones in the region. We used (U-Th)/He and K-Ar dating to document two new records of subsurface flow of reduced fluids between 3 and 4 Ma. The first is precipitation of Mn-oxides along the Moab fault (Utah, USA) at 3.9 ± 0.2 Ma. The second is clay mineralization associated with laterally extensive bleaching in the Curtis Formation, which we dated using K-Ar illite age analysis to 3.60 ± 0.03 Ma. The coincidence of the timing of bleaching, Fe- and Mn-oxide formation in multiple locations, and erosional exhumation at 3–4 Ma raises the question of how surface erosion may have induced a phase of fluid flow in the subsurface. We suggest that recent erosion of the Colorado Plateau created steep topographic gradients that enhanced regional groundwater flow, whereby meteoric water circulation flushed reduced fluids toward discharge zones. Dissolved gases, transported from hydrocarbon reservoirs, also may have been exsolved by rapid depressurization.

Detrital Chromian Spinel Geochemistry from Miocene Red Bed Sandstones of Northeastern Iraq: Implication for Provenance and Tectonic Setting

The provenance and tectonic setting of clastic sediments present in the Miocene Red Bed Series located in northeastern Iraq were analyzed. The analysis is based on the composition of detrital chrome spinel and represents the first examination of this topic. The detrital chrome spinel exhibits Cr # (Cr/(Cr + Al)) values ranging from 0.54 to 0.77 and Mg # (Mg/(Mg + Fe2+)) values ranging from 0.37 to 0.69. This indicates a potential incorporation of ultramafic sources, possibly including peridotites (namely Harzburgite and Lherzolite precursors), within the supra-subduction zone. The analysis of modified Cr-spinels extracted from the Red Beds indicates that the source rocks underwent a process of metamorphism and that the Cr-spinel particles were surrounded by a magnetite rim. The Cr-spinel compositions of the examined samples are situated tectonically in the fore-arc setting of peridotite protoliths, except for a small number of samples that exhibit boninite affinity. The observed resemblance in the Cr-spinel number content between the examined specimens and the Zagros Ophiolite peridotites suggests that the Red Beds may have originated from the Cretaceous Mawat Ophiolite Complex. The deposition of Paleocene Walash-Naopurdan rocks indicates that the lower allochthon served as a source during this process. The tectonic context of the Red Bed Series suggests that the allochthonous terranes in the Mawat region were transported and obducted onto the northeastern Arabian passive margin in the early Miocene period. The deposition of the Red Bed Series occurred concurrently with the closure of the Neo-Tethys Ocean in the Miocene.

Evaluation Method for Determining Rock Brittleness in Consideration of Plastic Deformation in Pre-Peak and Failure Energy in Post-Peak

The assessment of rock brittleness holds significant importance for understanding and predicting the mechanical properties and engineering behavior of rocks. Due to the lack of a unified definition of rock brittleness, numerous evaluation methods for brittleness indexes have been proposed by scholars both domestically and internationally in recent decades, resulting in diverse evaluation outcomes. In this study, we first summarize the existing rock brittleness evaluation methods and highlight their respective advantages and disadvantages. Subsequently, considering the pre-peak plastic deformation of the rock mass, the pre-peak brittleness index factor is introduced. Furthermore, taking into account the total energy consumed by the rock mass for failure after the peak, the post-peak brittleness index factor is proposed. These two components of the brittleness index describe the characteristics of different stages of the stress-strain curve, leading to the development of a novel brittleness index. The proposed method is then applied to evaluate the brittleness of both red-bed sandstone and cyan sandstone, revealing the variation of rock brittleness under different working conditions. Finally, three existing evaluation methods are selected to validate the rationality of the proposed method. The results demonstrate that for red-bed sandstone, the proposed brittleness index exhibits maximum values under natural conditions at all confining pressures. The four brittleness indexes consistently characterize the brittleness of red-bed sandstone under natural conditions. Under saturated conditions, the brittleness indexes exhibit different patterns of variation. For cyan sandstone, the three brittleness indexes—B7, B9, and Bnew—exhibit a similar trend in characterizing the brittleness of cyan sandstone under natural conditions and freezing-thawing conditions, while the trend of B17 is essentially opposite to that of the previous three indexes. The research findings provide guidance for the assessment of sandstone brittleness.

Diagenesis and controls on reservoir quality of Lower Triassic red bed sandstones (Buntsandstein) from a marginal basin facies, southwest Germany

After decades of hydrocarbon exploration and production in the Lower Triassic Buntsandstein from the Central European Basin center, the marginal southern German Buntsandstein has gained interest as a potential lithology for geothermal applications and lithium brine extractions. For this purpose and in order to better understand reservoir quality, porosity and permeability in combination with point-counted petrographic data of sandstones from SW Germany covering the Lower, Middle, and Upper Buntsandstein are studied. Textural, mineralogical, and petrophysical data show that the eolian and fluvial sandstones represent a heterogeneous reservoir system. Permeabilities vary over six orders of magnitude from 0.001 to 515 mD, while porosities are in the range of 7.9–21.5%. Different controls on reservoir quality exist. Large grain sizes ≥0.31 mm together with total quartz contents are the major controls on reservoir quality. Additional controls on reservoir quality are exerted by authigenic phases. Since porosity loss is dominated by compaction, the influence of authigenic clay phases on reservoir quality is minor. However, illite coatings on detrital quartz efficiently inhibited syntaxial quartz overgrowth. Where illite coatings lack, syntaxial quartz overgrowths locally stabilized the granular framework and, thus, preserved primary porosity. These authigenic phases, however, represent a local control, which cannot be applied on a regional scale. Similarly, enhanced secondary porosity due to feldspar dissolution is locally restricted and minor. The comparison with other studies on the Buntsandstein shows that its reservoir system is highly heterogeneous over small and large scales and within the same stratigraphic levels. In this study, quartz-rich and authigenic clay-poor sandstones with relative large grain sizes interpreted as eolian deposits reveal the best reservoir qualities.

Geochemistry of the Middle Eocene Gercus Sandstone, Shaqlawa Area, Northern Iraq: Implications for Provenance, Tectonic Setting and Paleoweathering

The study of the geochemical characterization of sandstone rocks contributes to understand the nature of the source rocks, their tectonic setting and the effects of paleoclimate and paleo-weathering. Accordingly, twenty sandstone samples from the red sandstone units of the Gercus Formation were collected from the northeastern limb of Safin Anticline at Shaqlawa Area – Northern Iraq. The studied sandstone samples show depletion in major, trace elements contents, except for CaO, MgO and Ni, Co and Cr respectively, and general depletion in the rare earth elements (REE) contents. Major element geochemistry indicates that the red bed sandstones of the Gercus Formation are of an oceanic island arc (OIA) tectonic setting. On the other hand, the results revealed that the paleo-weathering of the source area of the Gercus Formation experienced intensive chemical weathering under humid and/or warm climate conditions. The maturity and paleoclimate conclusions of the studied samples suggest a low feldspar content and low values of K2O / Al2O3 ratio, which in turn resulted from the dominance of low grade metamorphic and sedimentary rock types in the source area.

Reservoir characteristics and controlling factors of the Lower paleogene sandstones in the southeast part of Jiyang Sag, BohaiBay Basin, China

With the continuous improvement of exploration degree in Bohai Bay Basin, the red beds in Jiyang Sag has become a hot exploration spot in recent years, the red beds is buried deep, and the distribution of favorable reservoirs has become a key factor restricting oil and gas exploration. This paper combines logging, core and seismic data, mainly introduces the sandstone reservoir characteristics of red beds in the southeast of Jiyang depression, and analyzes the influence of sedimentary environment and diagenesis on its reservoir characteristics respectively. The results showed that the red bed sandstone is mainly composed of arkose, quartz-feldspar and feldspar-quartz graywackes, with good porosity and permeability. Under the control of the original sedimentary environment, three types of reservoir sand bodies are mainly developed in the study area: proluvial fan, fan delta and beach bar deposits, among them, fan delta deposits have better storage performance. The diagenetic environment was early alkaline and late acidic, compaction and cementation greatly reduced primary pores and a large number of secondary pores are produced due to minerals dissolution. Under the combined influence of sedimentary environment and diagenesis, the red-bed reservoirs have various types of reservoir spaces, with obvious differences in porosity and permeability.

Origin and Evolution of Geofluids in the Eocene Red-Bed Sandstones of the Dongying Depression, China

Fluid inclusion and petrographic study focused on authigenic quartz, annealed microfractures in quartz grains (AMF) and carbonate cement, was performed in red-bed reservoir sandstones from the first member of the Kongdian Formation and the lower fourth member of the Shahejie Formation (Shengli oilfield, East China). Both hydrocarbon and aqueous inclusions are present. Microthermometry and Laser Raman spectroscopes of liquid-rich two-phase inclusions showed that the pressure-correction value of aqueous inclusions is < 15°C. Differences in homogenization temperature and salinity occur between inclusions in quartz and carbonate cements. The liquid-rich two-phase aqueous inclusions were classified into four groups based on the salinity-homogenization temperature plots. The liquid-rich two-phase hydrocarbon inclusions were classified into two groups. The timing of the first-stage diagenetic fluid was before 31.3Ma. Fluids were from syn-depositional water and diagenetic fluids of clay minerals in interbedded mudstones. The timing of the second-stage diagenetic fluid was between 31.3 and 26.4Ma. Fluids were rich in organic acids and were accompanied by filling of small amounts of low maturity oil. The timing of the third-stage diagenetic fluid was between 26.4 and 21.4Ma. Fluids were mainly influenced by dehydration of gypsum-salt rocks and decarboxylation of organic acids. The timing of the fourth-stage diagenetic fluid was after 21.4 Ma. Fluids were influenced by organic acids and carbon acids and were accompanied by filling of mature oil after 10Ma. During the burial history, the red-bed reservoirs experienced the evolution process of early alkaline diagenetic fluids, early acid diagenetic fluids, late alkaline diagenetic fluids and late acid diagenetic fluids, which caused the alternate evolution of multiple alkaline and acid diagenetic environments.

Eocene fault-controlled fluid flow and mineralization in the Paradox Basin, United States

Abstract Sedimentary rocks of the Paradox Basin of the Colorado Plateau (southwestern USA) record widespread manifestations of paleo–fluid flow and fluid-rock reactions including Cu, U-V, and Fe-Mn mineral deposits, Si and Ca metasomatism, hydrocarbon accumulations, and bleached sandstones. Many of these are spatially associated with faults. Here we show evidence for a widespread phase of fault-related fluid migration and mineralization at 41–48 Ma in the Paradox Basin. We measured K-Ar dates of multiple size fractions of clay-rich fault gouge, yielding statistically overlapping dates of authigenic (1Md) illite for the Salt Valley (47.0 ± 3.0 Ma), Kane Springs (47.7 ± 3.8 Ma), Cliffdweller (43.4 ± 4.6 Ma), Courthouse (41.9 ± 2.3 Ma), Lisbon Valley (45.3 ± 0.9 Ma), and GTO (48.1 ± 2.6 Ma) faults. The latter two have an illite Rb-Sr isochron age of 50.9 ± 3.5 Ma, and fault-adjacent bornite has a Re-Os isochron age of 47.5 ± 1.5 Ma. Authigenic illite from a paleo–oil reservoir near the Courthouse fault formed from the interaction of reduced fluids with oxidized red-bed sandstones at 41.1 ± 2.5 Ma. The Moab and Keystone faults have older authigenic illite ages of 59.1 ± 5.7 Ma and 65.2 ± 1.0 Ma, respectively. Our results show a close temporal relationship between fault gouge formation, red-bed bleaching, and Cu mineralization during an enigmatic time interval, raising questions about drivers of Eocene fluid flow.

An energy-based damage model for red-bed sandstones subjected to drying–wetting cycles

A series of experiments is performed to determine the damage behaviour and energy evolution of red-bed sandstones subjected to drying–wetting (D–W) cycles. The experimental results reveal that accompanied by the strain increases, the input energy density gradually increases, the elastic energy density increases slowly first, then increases linearly, and reaches the maximum value at the peak strength, then gradually decreases. The dissipated energy density continuously increases and then maintains a steady-state level before it finally increases rapidly. Furthermore, with the increase in the number of D–W cycles, all three energy densities of specimens present a linear downward trend. According to the damage variable defined by the energy principle, the damage evolution models of the uniaxial compression strength (UCS) and peak strain of the D–W specimen are established, which can well characterise the evolution of the UCS and peak strain of rocks after different numbers of D–W cycles.

Sedimentary environment constraints on the diagenetic evolution of clastic reservoirs: Examples from the Eocene “red-bed” and “gray-bed” in the Dongying Depression, China

The diagenetic characteristics and differences between the Eocene red-bed sandstone deposited in an arid environment and gray-bed sandstone deposited in a relatively humid environment were investigated systematically by an integrated analysis of petrography, X-ray diffraction, carbon and oxygen stable isotopes and fluid inclusion microthermometry. Strong eodiagenetic calcite and gypsum/anhydrite cementation occurred at the boundary of the Eocene red-bed sandstone, while the centre of the red-bed sandstone was characterized by relatively strong quartz dissolution and weak mesodiagenetic feldspar dissolution and ferro-carbonate cementation. Eodiagenetic calcite cementation at the boundary of the gray-bed sandstone and quartz dissolution were relatively weak, while the carbonate cemented zone at the boundary of the gray-bed sandstone was strengthened by mesodiagenetic ferro-carbonate cement. Relatively strong feldspar dissolution and ferro-carbonate cementation occurred in the centre of gray-bed sandstone beds. By comparing the diagenetic differences between the red-bed sandstones and gray-bed sandstones, it is shown that sedimentary environment had an obvious influence on reservoir diagenetic evolution and reservoir properties through influencing the chemical properties of connate water, the transformation rate and degree of clay minerals in interbedded mudstone, and the properties of diagenetic fluids during burial process.

Compaction control on diagenesis and reservoir quality development in red bed sandstones: a case study of Permian Rotliegend sandstones

Authigenic minerals formed during diagenesis in conjunction with compaction by burial have long been known to lead to porosity-loss of sandstones, and a subsequent deterioration in reservoir quality. The diagenetic impact on reservoir quality and permeability heterogeneity measured horizontal and vertical to bedding was characterized in three fluvio-eolian Lower Permian Rotliegend outcrops from the Flechtingen High, the northern Hesse Basin (both Germany) and the Vale of Eden (UK) using point-counting, polarized light-microscopy, helium pycnometry and permeability measurements. Results show significant porosity (10 to 35%) and permeability (0.01 to 10,000 mD) ranges largely independent of depositional environment. The major control on reservoir quality in Cornberg Sandstones are dolomite and siderite cementation in conjunction with illitization and illite and kaolinite cementation, leading together with quartz cementation to a mostly cemented IGV and poorest reservoir quality (avg. horizontal permeability: 0.96 mD). Flechtingen Sandstones are most intensely compacted due to the lack of significant early diagenetic cement phases and continuous illitic grain-to-grain coatings, which inhibited intense quartz cementation but enhanced chemical compaction at quartz grain contacts, resulting in intermediate reservoir quality (avg. horizontal permeability: 34.9 mD). Penrith Sandstones lack significant authigenic phases besides quartz due to carbonate dissolution during uplift. They show the least amount of detrital feldspars and clay minerals, leading to no major reservoir quality reduction by burial diagenetic clay mineral alterations, resulting in the highest reservoir quality (avg. horizontal permeability: 5900 mD). Additional results highlight higher horizontal to vertical permeability ratios kh/kv in less homogeneous sandstones of < 10 mD of 10, and in more homogenous, higher permeable sandstones > 1000 mD of 1. Although detrital and authigenic sample compositions vary throughout the studied areas, the general effect of grain coatings coverages on syntaxial cement inhibition and chemical compaction can be delineated. This study increases the understanding of porosity reduction in sandstones, as it confirms the necessity to differentiate between the illitic grain-to-grain coatings and illitic grain-to-IGV coatings. As a result, the enhancing effect of illite on chemical compaction on quartz grain-grain boundaries can be better constrained, as well as the effect of grain coatings on quartz cementation. This is relevant for reservoir quality and risk assessment in hydrocarbon and geothermal plays as well as in storage.

Experimental Investigation on Tensile Strength of Jurassic Red-Bed Sandstone under the Conditions of Water Pressures and Wet-Dry Cycles

Jurassic Red-Bed sandstone is one of the main types of rock composing unstable slopes on both banks of the Three Gorges Reservoir (TGR) in China. It is vulnerable to tensile strength degradation when exposed to cyclic wet-dry conditions under reservoir operation. Therefore, the law behind this needs to be clarified in order to accurately evaluate the stability of a sandstone slope located in the drawdown area of the TGR. In this paper, Jurassic Red-Bed sandstone specimens from the Majiagou landslide in the TGR area were collected for experiments with cyclic wet-dry procedures. After each wet-dry cycle, efforts were made to implement splitting tests, measure clay mineral contents and ion contents in the soak solution, and observe changes in microstructures. The tensile strength is experimentally demonstrated to be a process of damage accumulation under the integrated actions of water pressure, calcite dissolution, and clay mineral hydration, among which hydration of illite contributes the most to the damage. The loss of the cement material and the cyclic expansion and shrinkage of clay minerals weaken the cementation between grains, leading to the tensile strength degradation.

Diagenetic alterations related to sedimentary architecture of deltaic distributary channels in red beds of the Cretaceous Yaojia Formation, Songliao Basin

Fine-scale diagenetic alterations linked to architectural patterns of the sandstones were studied in meter-scale deltaic distributary channels of the Upper Cretaceous Yaojia Formation, Songliao Basin. The deltaic reservoirs of the Yaojia Formation are mostly developed in three types of distributary channels: major distributary channels, crevasse channels, and terminal distributary channels. The three types of distributary channels have different sedimentary architecture and lithofacies. The distribution of reservoir intervals varies greatly in the upward fining distributary channel depositional cycles (meter-scale). From the diagenetic alterations in the distributary channels successions, compaction, calcareous cementation, grain-coating clays with hematite in the red bed sandstones, and dissolution of feldspar or rock fragments are main control factors of high-quality reservoirs. More porosity loss is generated by strong compaction near the top and bottom part of the distributary channel succession. This could be caused by the high clay content and mud clasts which contribute much to the porosity loss in the sandstones. Calcareous cementation is mainly developed near the clay-rich intercalations. Dissolution pores in the sandstones without hematite cement and grain-coating clays are well developed in the middle position of the successions. The cementation is restricted in sandstones with unique hematite cement and grain-coating clays in the reddish crevasse distributary channels. The distribution pattern of high-quality reservoirs within the three different types of distributary channels in red beds is proposed by linking the diagenetic processes to depositional environments and architecture. High-quality hydrocarbon reservoirs are mostly preserved in the massive and cross-bedded sandstones from the middle part of the major distributary channel successions.

Diagenetic alteration and geochemical evolution during sandstones bleaching of deep red-bed induced by methane migration in petroliferous basins

Bleaching of red-bed sandstones is an important process related to the organic-inorganic interactions and diagenetic alteration, playing an important role in reservoir heterogeneity and carbon cycle in petroliferous basins. Distinguishing from the aeolian red-bed in the outcrop, a case study of deep red-bed of fluvial facies was conducted on the Permian Xiashihezi Formation, NE Ordos Basin, China. Four diagenetic facies can be identified as red, brown, green and white sandstones (RS, BS, GS, WS) with typical diagenetic alteration and geochemical characteristics. Thereinto, diagenetic alterations are dominated by clay minerals and carbonate cements including calcite and siderite. Carbon and oxygen isotopes showed a synergistic decreasing trend from RS, BS to WS with the increase of altered degree. The unaltered RS retained abundant early hematite, smectite and calcite, while strongly bleached WS had lowest feldspar but extensive kaolinite and illite, and manganese-rich calcite. Medium-altered BS and GS are characterized by late magnesium-rich siderite and chlorite coating respectively. The isotopic data, fluid inclusions and modeling indicated that CH4 is the important causal fluid for sandstones bleaching. Diagenetic alteration and geochemical evolution in deep red-bed of fluvial facies was influenced by upward-fining sedimentary sequence, open/closed diagenetic system and natural gas migration. The geochemical behaviors of Fe are responsible for the diversity of diagenetic facies. Fe2+ from redox between Fe3+ and CH4, integrated Mg2+ released from the transformation of smectite to illite into magnesium-rich siderite, or into chlorite in coarser sandstones in relatively open system. Extensive CH4-rich reducing fluids were oxidized to acidic fluids in the more open diagenetic system with coarser grained sandstones, resulting in the intense dissolution and the high-quality reservoirs distributed in the lower sequence of fluvial facies. Furthermore, CH4 oxidation by Fe(III) in deep red-bed can be extremely important CH4 sink and expected to potential sites of geological carbon sequestration.

Sandstone-Hosted Uranium Deposits as a Possible Source for Critical Elements: The Eureka Mine Case, Castell-Estaó, Catalonia

The Eureka deposit in Castell-estaó in the Catalan Pyrenees is a Cu–U–V deposit, hosted by Triassic red-bed sandstones, and classified here as a low-temperature, sandstone-hosted stratabound metamorphite U deposit. The main mineralisation is stratabound, related to coal-bearing units and produced during the Alpine deformation by migration of hydrothermal fluids. In this stage, the original sedimentary and diagenetic components (quartz and calcite, micas, hematite and locally apatite) were replaced by a complex sequence of roscoelite, fine-grained REE phosphates, sulphides and Ni–Co arsenides and sulpharsenides, Ag–Pb selenides, bismuth phases, sulphosalts and uraninite. The black shales of the Silurian sediments underlying the deposit and the nearby Carboniferous volcanoclastic rocks are interpreted as the source of the redox-sensitive elements concentrated in Eureka. The sulphur source is related to leaching of the evaporitic Keuper facies. The REE transport would be facilitated by SO4-rich solutions. The reduction of these solutions by interaction with organic matter resulted in the widespread precipitation of REE and redox-sensitive elements, including many critical metals (V, Bi, Sb, Co), whereas barite precipitated in the oxidized domains. The occurrence of similar enrichments in critical elements can be expected in other similar large uranium deposits, which could be a source of these elements as by-products.

Dolomitization of shallow-water, mixed silicilastic-carbonate sequences: The Lower Triassic ramp succession of the Transdanubian Range, Hungary

The Lower Triassic succession of the Transdanubian Range in Hungary comprises limestones, dolomite, marl, sandstones and siltstones, deposited in tidal flat, lagoon and ooid shoal environments on the marginal ramp of the western Neotethys. Seven cores were chosen for petrographic and stable isotope investigations aiming to reconstruct the paragenetic sequence with special regard to the dolomitization and hydrothermal events. Five lithotypes were differentiated: (i) dolomite, (ii) sandy, silty, clayey dolomite, and dolomarl, (iii), dolomitic siltstone and sandstone, (iv) dolomitic limestone, and (v) limestone. In these lithotypes, three types of dolomites are present: non-ferroan replacive, ferroan replacive, and ferroan cement. Fabric retentive and fabric destructive non-ferroan replacive dolomitization are interpreted to have occurred by seepage reflux. Supporting evidence includes the presence gypsum and anhydrite in the Lower Triassic beds. Stable isotope values of the ferroan dolomite (δ18O of −10.7 to −4.2‰ and δ13C of −4.8 to 4.7‰) suggest dolomitization by fluids of relatively high temperature. The similar stable isotope values (δ18O of −9.3 to −5.8‰ and δ13C of −1.9 to 2.5‰) of the non-ferroan dolomite phase suggest that the reflux dolomite was overprinted by this second dolomitization event. Traces of exotic minerals, such as barite, chalcopyrite, galena and sulphosalts were found as fillings of vugs and fractures in the dolomite-cemented sandstone. The metals could have been sourced from the underlying Permian red sandstone beds. The heterogeneous sediment composition had profound impact on the diagenesis of these sedimentary successions.

Effects of Different Temperatures on the Softening of Red-Bed Sandstone in Turbulent Flow

The rates of chemical reactions are highly dependent on temperature, meaning that the actual geological rock mass is affected by different temperatures. Only when the temperature effect is considered can the mechanism of the influence of temperature on the interaction between water and rock be further understood. It was found that the condition of turbulent flow is more likely to promote the softening of red-bed sandstone than the conditions of laminar flow and static water in an experimental study on the softening effects of different flow patterns on red-bed sandstone. Therefore, based on a multi-functional self-circulating open channel hydraulic test system, this paper designs and completes equal volume saturated tests of red-bed sandstone at low temperature (1 °C), medium temperature (23 °C), and high temperature (45 °C) under the turbulent conditions of three equal temperature gradients. The chemical action of the circulating solution in water flow at different temperatures, the propagation of micro-cracks in rock and the changes in mechanical indexes are discussed. The influence laws and mechanisms of the different temperatures on the softening of red-bed sandstone in turbulent flow are revealed. The results show that low-temperature flow can inhibit the softening of red-bed sandstone in the range of 1–45 °C. With the increase in water flow temperature, the development degrees of micro-structures and the mechanical damage of the corresponding rock become more notable. That is, temperature affects the physical and chemical water-rock interactions and then changes the internal structure of rock, thus affecting the softening and failure processes of red-bed sandstone. The study provides a theoretical basis for the further investigation of the softening laws and mechanisms of other red layered soft rocks by temperature under turbulent conditions.

Can carbonate cementation be inhibited in continental red bed sandstone?

The Lower Cretaceous Bashijiqike Formation in Kuqa depression belongs to a continental red bed sandstone with poor reservoir quality, and is deeply buried to 5.5–7.0 km but some successions can host porosity >10% and permeability larger than 10 mD. Carbonate cements are important pore-filling constituents, and they control the reservoir quality and heterogeneity of Bashijiqike sandstones. Red hematite-bearing grain-coating clay minerals inhibit the pore occluding quartz cements and preserve intergranular pores. Here we investigate the ability of grain-coating clays to inhibit carbonate cements. Thin sections, scanning electron microscopy (SEM), X-ray diffraction (XRD), and cathode luminescence (CL) analysis were performed to investigate the petrography, pore systems, diagenesis and diagenetic minerals of the red bed sandstones. The results show the hematite-bearing grain coatings give the ‘‘red bed’’ colors of the sandstones. The sandstones are heavily mechanically compacted, and the predominant pore-filling cements include carbonates and clay minerals, while quartz cements are volumetrically not important in reducing porosity. The grain-coating clays are mainly hematite-bearing mixed-layer illite and smectite. The paragenetic sequences of various types and generations of diagenetic events were reconstructed. Sandstones containing effective mixed-layer illite and smectite coatings, in which the intergranular pores can be preserved, have high porosities. Carbonate cement is never found in sandstones with effective grain coating clays, and carbonate cement precipitates where there are no sufficiently thick and continuous grain coats. The clay coatings retard or inhibit eogenetic calcite cements and progressively restrict late-stage carbonate (dolomite) cementation during burial. The hematite-bearing grain-coating clays help the red bed sandstones retain good reservoir quality at great depths.

Examination of Rainfall-Induced Landslide Failure Mechanisms via a Centrifuge Physical Simulation Test

Rainfall is one of the most important factors contributing to landslides, and gentle bedding incline, high-rainfall induced landslides are common throughout the world. Field observations and theoretical analyses have been used to assess slope instability caused by permeability variation. In this study, the influence of rainfall infiltration on gentle bedding incline slope behaviour was investigated using a centrifuge physical simulation test. The magnitude, pattern and development of pore water and earth pressure at the interface; the shear failure surface features; and the corresponding deformation and failure processes were considered. A model with interbedded sand and mud was created, and a centrifuge was used to simulate both natural and rainfall conditions. The weak intercalation was composed of single-material silty clay, and the landslide mass was composed of red-bed sandstone. A combination of photography, pore water pressure measurements and earth pressure measurements were used to examine the relationship between the pore water pressure, earth pressure and failure modes. When the slope experiences overall instability, the curves of the earth pressure and pore water pressure dramatically decrease. The results reveal that the failure shear surface largely depends on the differential creep caused by the properties of the rock mass and the rainfall infiltration.

Creep tests of red-bed sandstone after high temperature

Temperature is an important factor affecting rock deformation and stability. The influence of temperature on the creep characteristics of red-bed sandstone is revealed through the creep test of red-bed sandstone after different high temperatures, and a creep damage model considering the temperature effect based on the Burgers model is established to analyze the influence rule of temperature on model parameters. The research results indicate that 510~550 °C is the nodal temperature range for the change in mechanical properties of red-bed sandstone. Below this temperature range, with the temperature increases, the peak strength of specimen increase, ductility increases, and the long-term strength decreases. Above this temperature range, with the temperature increases, the peak strength and peak strain of specimen decrease, and the long-term strength increases. The instantaneous strain increases linearly with the increasing temperature. Under the same stress conditions, with the temperature increases, the instantaneous strain of specimen, the steady-state creep rate and the time to enter the stable creep stage increase, the time to enter the accelerated creep stage decreases, and the acceleration creep curve becomes steeper.