Mudstone Strata Research Articles

Comparison of Evaporite-Related Source Rocks and Implications for Petroleum Exploration: A Case Study of the Dongying Depression, Bohai Bay Basin, Eastern China

The Dongying Depression (Bohai Bay Basin, eastern China) was widely filled with evaporite (anhydrite and halite) layers during the Paleogene period, especially the middle of the fourth member of the Shahejie Formation (Es4). Most evaporite layers are interbedded with mudstone strata. The strata of Es4 are divided into three sections, referred to as the upper layers, evaporite layers, and lower layers, respectively. The analysis of elemental concentrations, elemental ratios, and Pr/Ph suggests that the lower layers were deposited in an intermittent saline lake environment within a relatively dry climate. The evaporite layers were formed in a highly saline lake environment, whereas the upper layers were formed in a brackish-saline to fresh-water environment. Organic matter (OM) abundance indices, including total organic carbon (TOC), chloroform extracts, total hydrocarbon content (HC), hydrocarbon generation potential (S1 + S2), and OM type, show that the source rock potential for petroleum generation in the upper layers is best, that in the evaporite layers is fair, and in the lower layers it is poor. Carbon isotopes (δ13C) of hydrocarbons in the evaporite and lower layers were heavier than those in the upper layers. Thermal maturity parameters show that the thermal evolution process of OM in the upper layers was faster where evaporite were present compared with evaporite-free areas, while the thermal evolution of OM in the lower layers was slower in these regions. The high thermal conductivity of evaporites may have accelerated the thermal evolution of source rocks in upper layers and allowed hydrocarbon generation at a shallower burial depth. This resulted in the earlier appearance of the petroleum generation window compared to in evaporite-free areas. Meanwhile, the thermal evolution of OM in the lower layers was restrained, and consequently the hydrocarbon generation window was widened, which implies the potential for petroleum exploration in deep strata under the evaporite sequence. This is a common phenomenon in evaporite-bearing basins, according to previous and present studies.

Key factors in hole-sealing and pressure-relief failure of hydraulic fracturing straddle packer in coal mine

Hydraulic fracturing technology is widely used in coal mines, and straddle packers are a key component of this technology. However, during the application of a straddle packer, potential problems occur, including hole-sealing failure and shooting rod events at the pressure-relief stage, which may result in serious harm to on-site personnel. Therefore, we here investigated the application of straddle packers in five coal mines. The failure type, rock layer in which failure occurred, and frequency of use of straddle packers were analyzed. The most common failure type was packer capsule rupture, and this mostly occurred in sandstone and mudstone strata. The frequency of use was directly related to the rock stratum structure, while hole-sealing failure occurred most easily near to rock stratum interfaces. Analysis of the mechanical behavior of straddle packer hole-sealing failure showed that soft rock strata and capsule rupture were the key factors leading to such failures. Hard and stable rock formations, appropriate expansion of the diameter of boreholes, and strict manufacturing quality control can play important roles in preventing hole-sealing failures of straddle packers. Residual high-pressure water in the packer section or upper part of the straddle packer was the key factor causing pressure-relief failure and shooting rod events; to prevent these issues, high-pressure water should be fully released, anti-channeling chains should be used, and existing straddle packer technology should be optimized.

Geochemistry and stable isotopic variations of water and dissolved sulfate from landslide areas in Niigata and Nagano Prefectures, Japan. A case study

The chemical and isotopic characteristics of waters from typical landslide areas in Niigata and Nagano Prefectures, Japan, are described in this article. The study areas are Mushigame, Nigorisawa, Yomogihira (Niigata Pref.) and Kanayamazawa (Nagano Pref.) where landslides have often occurred. We present the chemical composition and δD-δ18O values of surface and groundwaters from the areas in order to characterize a geochemical nature of the waters involved in the landslides. The waters were of meteoric origin as indicated by the δD-δ18O values. The Mushigame waters were enriched in NaCl, suggesting contribution of fossil seawater trapped in oil-producing mudstone strata in the area. The Nigorisawa and Yomogihira waters are less in Cl- and relatively more enriched in HCO3- and SO42-. The Na-K-Mg relationship of all the waters except the Kanayamazawa waters indicated close equilibration with some minerals such as K-feldspar, K-mica, chlorite and silica in the rocks, suggesting water-rock interaction at deeper depths, although the waters were greatly diluted by surface groundwater on the way to the surface as indicated by their δD-δ18O signatures. Dissolved sulfate is a major anion and was originally produced by oxidation of sulfides in volcanic rocks. Variable δ34S and δ18O values of sulfate suggest reduction of such sulfate to sulfide probably at shallow levels under different reducing conditions followed by re-oxidation, inducing the variability in the isotopic values. Thus, multiple oxidation-reduction processes may be common in the areas.

Correlative conformity or subtle unconformity? The distal expression of a sequence boundary in the Upper Cretaceous Mancos Shale, Henry Mountains Region, Utah, U.S.A.

ABSTRACT In models of siliciclastic sequence stratigraphy, the sequence boundary in distal marine environments, where the strata are mudstone dominated, is usually considered a correlative conformity—the seaward extension of a subaerial unconformity. Despite its wide usage in the literature, objective recognition criteria of a correlative conformity remain lacking, largely due to the limited number of case studies directly examining the characteristics of sequence boundaries in offshore mudstone-dominated environments. This study focuses on the mudstone-dominated transitional interval between the Tununk Shale Member and the Ferron Sandstone Member of the Mancos Shale Formation exposed in south-central Utah to extend our understanding of the characteristics of a sequence boundary developed in the distal shelf environment of a ramp setting. An integrated sedimentologic, petrographic, and sequence stratigraphic analysis was conducted to characterize the sequence boundary that separates the Tununk from the Ferron depositional system (hereafter referred to as the T-F sequence boundary) and its lateral along-depositional-strike variability. Although manifest as a mudstone-on-mudstone contact, the T-F sequence boundary in all three measured sections is a subtle unconformity, characterized by erosional truncation below and onlap above, and marks a distinct basinward shift in facies association. The T-F sequence boundary also marks the change from the Tununk offshore mud-belt system to the Ferron Notom delta system, and therefore represents a surface that divides two genetically different depositional systems. Based on two distinct marker beds that bracket the T-F sequence boundary, the T-F sequence boundary can be traced across the study area with confidence. The lateral variability in the characteristics of the T-F sequence boundary along depositional strike indicates that it was produced by an allogenic base-level fall. Offshore shelfal mudstone strata may contain a significantly higher incidence of subtle unconformities analogous to the T-F sequence boundary than currently appreciated. Careful sedimentologic and petrographic analyses, combined with lateral correlations constrained by reliable chronostratigraphic marker beds, are essential for identifying subtle unconformities in shelf mudstone successions. The accurate recognition of subtle unconformities in mudstone strata is critical to apply the sequence stratigraphic approach appropriately to distal shelf environments, as well as to better constrain the timing and cause (allogenic vs. autogenic) of relative changes of sea level recorded in these rocks.

Genetic Mechanism of Geothermal Anomaly in the Gaoyang Uplift of the Jizhong Depression

The Gaoyang uplift is rich in geothermal resources, but there are few studies on the regional geothermal genetic mechanism. A large number of geothermal wells fail in position calculation because of a shortage of basis. By using the methods of P-wave velocity structure imaging and magnetotelluric sounding, it draws the following conclusions: 1) The crustal thickness in the Gaoyang uplift area is relatively thin, about 32 km; 2) The cutting depth of the Gaoyang East fault exceeds 28 km and has reached the bottom of the lower crust. It is a large tensile fault that provides a channel for mantle heat flow into the shallow crust; 3) The Gaoyang uplift and its surrounding depressions form a concave-convex base fluctuation mode, which is conducive to the accumulation of heat flow to the uplift; 4) The deep carbonate thermal reservoir in the Gaoyang uplift is overlaid with Cenozoic sand and mudstone strata, with a thickness of more than 3000 m and low thermal conductivity, which is conducive to the preservation of thermal storage heat. Therefore, the shortening of the heat conduction path caused by regional crustal thinning, convective heat conduction of large faults, concave-convex structure, and thick Cenozoic caprock are the reasons why the heat reservoir temperature in the Gaoyang uplift is higher than that around, which provides a theoretical basis for geothermal development and utilization.

Quantification of the dependence of the emanation of radon from water on the pH and temperature of water

Uranium- and thorium-series isotopes are able to dissolve in hot spring waters and decay into radon, which could potentially cause health risk of the public. Besides, several environmental investigations can be carried out by measuring radon concentrations in hot spring waters. Accordingly, the measurement of radon in hot spring water is a significant issue.A practical method for measuring radon in hot spring water was developed by using a silicon semiconductor detector, RAD-7. The overall effect of detection efficiency and radon leakage of the measuring system were examined to calibrate the measured concentration of radon in hot spring waters. In addition, the effect of water temperature and pH on emanation of radon from standard 226Ra solutions as well as that of the measuring time on precision of the measurement were investigated. The correlation among temperature, pH and emanation of radon was then obtained. Further, the established method was applied to in-situ measurement to investigate the concentration of radon in hot spring waters from different geological areas in Taiwan.According to the percent error of the measurement, the measuring time for environmental samples was set as 50 min. The overall effect of radon leakage and detection efficiency of the measuring system was estimated to be 99.19 ± 7.47% with a standard rock source of 222Rn from NIST. Standard sources of 222Rn were prepared from 226Ra standard solutions with different pH values. The 226Ra standard solutions were sealed and stand for 38 days in a 250 mL vial specific for the RAD-7. After measured by the WAT250 mode of the RAD-7, the data were measured as (52.5 ± 8.9)%∼(83.6 ± 11.0)% of the prepared concentration of 222Rn (from 226Ra). The emanation rate of 222Rn from standard solution under pH 1.44–9.07 was (53.2 ± 6.9)% ∼(84.3 ± 11.1)% and decreased with pH to a minimum value at 7.00. Moreover, the effect of temperature increases with pH. High correlated trend curves (r2 = 0.9829–0.9999) for 222Rn emanation rate versus pH under different temperature were obtained. A smooth curved surface fitted by the MATLAB for emanation rate of 222Rn was accordingly derived. The 222Rn concentrations in waters of the Beitou Hot Spring (acid-digestion sulfate spring on sedimentary rock strata), Wulai Hot Spring (sodium bicarbonate spring on metamorphic rock strata), Choshuitan (sodium bicarbonate spring on mudstone strata) and Zhaori Hot Spring (acid-digestion sulfate spring on andesite strata) were measured with RAD-7 by WAT250 for 50 min and then calibrated by 222Rn emanation rate estimated above, which were 215 ± 37 (Bq/m3), 6797 ± 1073 (Bq/m3), 2124 ± 577 (Bq/m3) and 26644 ± 2768 (Bq/m3).

The reservoir property modeling of volcanic reservoir: An application to the Shahejie Formation, Zaoyuan oilfield, eastern China

As an increasing number of volcanic reservoirs has been discovered all over the world, the study of volcanic reservoirs has caught more attention. However, researchers mainly focus on the characterization of the external architecture of volcanic rock mass and the superimposed relationship between the rock mass, while paying little attention to the fine anatomy of the internal structure of the rock mass, making it difficult to effectively guide the development of the volcanic reservoir. Taking Zao35 block in Huanghua depression, Bohai Bay Basin, China, as an example, we find a workflow of establishing a high-resolution volcanic stratigraphic structure model and reservoir properties models by integrating well- and seismic-data analysis. The seismic data indicate that the volcanic activities consist of three stages: Stages I and III have positive seismic amplitude reflections, whereas stage II has a negative seismic reflection. The well data indicate that the lithology column of target formation in our study area consists of eight volcanic strata and seven mudstone strata. According to the lithology interpreted using well data, stages III, II, and I consist of five, one, and two volcanic eruption periods, respectively. We have established a structure and stratigraphy model with eight zones of volcanic rock and seven zones of mudstone barrier (15 zones in total) to effectively distinguish volcanic rocks (products of the eruption period) from sedimentary rocks (products of the volcanic dormant period). Based on the seismic attribute analysis, we have combined the sequential indicator simulation and local anisotropy from the seismic amplitude attribute. The model of volcanism can be reproduced by sequential indication simulation of local variogram with variable range directions. Based on the principle of “facies control” (perform lithology simulation in different regions according to lithofacies), we have used the sequential indication method to simulate volcanic rocks. The simulation results have a very good agreement with the shape of lava flows spreading around the crater. To describe the internal structure of the volcanic rock body, reveal internal lithology distribution characteristics, and reservoir physical properties, we provide an effective research method for the development of volcanic reservoir geologic research.

Horizontal Fracture Formation in the Cenozoic Mudstone of the Western Qaidam Basin

Fractures can provide valuable information on the tectonic evolution of the Earth’s crust. Our field observation data, rock core data, and thin section observations reveal several horizontal fractures in the Cenozoic mudstone strata in the western Qaidam Basin, Qinghai Province, China. These horizontal fractures have rough surfaces and are filled by gypsum. The fractures are mainly curved; however, some straight fractures are also found. There are many intersections between adjacent curved fractures, which often adjoin to form “T” shape fractures. Fracture apertures are mainly between 1 mm and 1 cm, with a maximum of 3 cm. Both the size and density of fractures decrease as strata denudation thickness decreases and depth of burial increases. Horizontal fractures are only found in a core area of anticlines with severe strata denudation. Accordingly, these fractures are related to vertical extension due to residual stress in the process of rapid uplift and denudation. The horizontal fractures cut earlier vertical tectonic fractures, indicating their formation after tectonic extrusion. Electron spin resonance (ESR) dating of fracture-filling materials revealed that horizontal fractures appeared in two stages. The first stage was approximately 1.8 Ma ago, while the second stage was approximately 0.3 Ma ago. The size and density of the later-stage horizontal fractures are markedly greater than the earlier-stage fractures. Based on the driving stress when fractures are formed, which is obtained from fracture parameters, the denudation rate in the later stage was approximately 2∼3-fold greater than that in the earlier stage. According to the evidence, during the Quaternary, the western Qaidam Basin experienced pulsating tectonic extrusion, producing uplift and denudation and an increase in tectonic activity. Such findings suggest a gradual increase in episodic tectonic activity in the entire Tibetan Plateau region during the Quaternary.

Pecopterids from the Permian of the Extreme South of the Paraná Basin, Brazil

In this paper we characterize pecopterids found in deposits of the Passa Dois Group (Upper Permian of the Paraná Basin) in a new outcrop, on the side of the BR-153 road, located at the municipality of Aceguá, Rio Grande do Sul State, Brazil. Pecopterids are common in the Permian paleophytogeogragic realms, signaling interglacial phases of the Late Paleozoic. Pecopteris are one of the floral elements of the Permian Glossopteris flora and become extinct in the Permo-Triassic boundary. Mudstone strata levels bearing phytofossils with different preservation qualities allowed the recognition of Pecopteris sp. and cf. Scolecopteris sp. floral elements. Taphonomic and depositional processes that conditioned the preservation of phytofossils are suggested accordingly the specimens described. Phytofossil specimen described support and highlight polymorphic nature of Gondwanan pecopterids. These new findings demonstrate potential for further paleobotanical studies in the upper portion of the Passa Dois Group and strength the notion of Permian strata for the Teresina Formation.

Analysis of Hydrogeochemical Characteristics of Tunnel Groundwater Based on Multivariate Statistical Technology

Following tunnel excavation, which is influenced by hydraulic fracturing and geological structure, a series of hydrochemical reactions occur in the karst aquifer, which has a significant impact on groundwater hydrology and the earth process. Based on five sets of 38 samples collected in the Tongzi Tunnel in 2020 and 2021, the main geochemical processes and water quality conditions of the karst aquifer system during tunnel construction were revealed by multivariate statistical analysis and graphical methods. The results showed that water-rock action is the main mechanism controlling groundwater chemistry in the study area; HCO3-, Ca2+, and Mg2+ are associated with the widely distributed carbonate rocks in the study area. SO42- is derived from gypsum and sulfate rocks and special strata, which are another important source of Ca2+. Sodium-containing silicates and reverse cation exchange as the causal mechanisms of Na+ whereas F- is derived from fluorite. According to the mineral saturation index calculations, the dissolution and precipitation of minerals such as alum, gypsum, calcite, dolomite, and salt rock have an important influence on the main chemical components in water. The 38 samples were subjected to cluster analysis, and the results could be classified into seven categories. The representative clusters 1, 3, and 5 were selected for principal component analysis. Clusters 1 and 5 of groundwater represent weathering, dissolution, and ion exchange of carbonate and sulfate rocks and are closely related to the lithologic limestone, limestone intercalated with carbonaceous mudstone, carbonaceous mudstone, and coal-measure strata in the aquifer. Cluster 3 is dominated by upper surface river water and characterizes the geochemistry in natural water bodies dominated by the dissolution of carbonate, sulfate, and salt rocks. Finally, groundwater quality is mostly found in Class IV, with NO3- and F- being the main contaminants in the water.

Genesis and petroleum geological significance of calcareous mudstone in Beier Sag

Beier sag is one of the most oil-rich sags in the south of Hailaer Basin in China. The purpose of the study is to clarify the identification characteristics and geological origin of calcareous mudstone strata in Nantun Formation of Beier sag, so as to provide new ideas for the identification and evaluation of high-quality source rocks, and to provide effective support for efficient exploration of oil and gas in the basin. The comparative analysis of logging curves and seismic reflection characteristics shows that the calcareous mudstone layer has the “three high and one low” logging response with high GR (Natural Gamma Ray), high LLD (Deep Investigate Induction Log), high AC (Acoustic) and low DEN (Rock Density), showing continuous and strong seismic reflection characteristics. Through the comparative analysis of experimental data of rock thin slice and microscopic image and organic geochemistry, the layer lithology is mainly gray-black mudstone, locally mixed with oil shale and marl, rich in calcium and algae. The layer has the high abundance of organic matter, good organic matter type ( II1-I ), high hydrocarbon-generating potential and high hydrocarbon-exhausting efficiency. It is the highest quality source rock found in Beier sag, and is the material base to form the large scale oil and gas accumulation. The formation of calcareous mudstone formed in the brackish water and wide lake basin by a wide range, represents the anoxic event early in Cretaceous period and is the significant stratigraphic correlation marker bed of the sag. In addition, this layer and the fan-delta front sand widely distributed in the lower part of Nantun Formation form the most favorable reservoir combination, which controls the plane distribution of oil and gas reservoirs. Above and below the calcareous mudstone strata, oil and gas are very rich.

Architectural characteristics of the normal-dextral fault in mudstone strata: Anatomy based on the cores from the Bohai Bay basin

Clay smearing formed by source layer intrusion and/or fault grinding is an important type of fault sealing, especially the continuous clay smearing zone in the mudstone section, which is a typical feature of fault architecture. However, an observation of the cores taken from the faults arcoss mudstone strata in the Bohai Bay rift basin shows that no significant clay drag and clay smearing are formed in the fault zone, but a complex fault zone occurs in the mudstone strata, and its fault cores and damage zones obviously differ from fault architecture in other lithologies in the following aspects: (1) asymmetrical broken cleavages develop on the flank of the main sliding surface in the fault core; (2) the breccia belt is typical of the structural style of the damage zone, most of which are not obvious sliding surfaces, but the directional alignment of breccia indicates the sliding direction of the breccia belt; (3) the width of the damage zones and the number of breccia belts within the damage zone are asymmetrical, and the thickness of breccia belts of the footwall damage zone is approximately twice that of the hanging-wall damage zone; (4) the breccia belts are separated by stable undeformed strata, and the strain of the breccia belts in the footwall damage zone increases gradually close to the fault core. The main factor that affects the asymmetrical width of breccia belts in the hanging-wall and the footwall damage zones could be the lithologic variables and the drag deformation width produced by the uplift of the salt-gypsum roller in the footwall of the fault. The complex structure of the fault in mudstone strata has important guiding significance for the later evaluation of longitudinal fluid migration along the fault and fault sealing in petroliferous basins.

Characteristics of growth fault architecture and its evolution in mudstone strata: Evidence from the core of Bohai Bay basin

Mudstone strata are important components of extensional basins and play an important role in both hydrocarbon migration and preservation. However, studies on the architecture and evolution of faults across mudstone strata have not been conducted because of the influence of traditional views on mudstone sealing. Based on the core samples of the fault zone, we studied the architecture and evolution of growth faults across the mudstone strata using core sample scanning, petrology microscopes, and X-ray diffraction. During the plasticity–semi-plasticity, semi-consolidated, and consolidation stages, soft-sediment deformation structures (fossil body distortion, dike deformation, rearrangement of ooids), fracture and breccia successively formed. In the hanging wall damage zone and the fault core, fractures with a set of similar strikes are filled with calcites or dolomites, and they are then filled with clays or sands in the footwall damage zone with two groups of different strikes. In the fault core and the damage zone, breccias have different spatial distribution, mineral composition, and relative clay mineral content because of different causes. The number of breccia belts, which formed in the late stage, in the hanging wall damage zone is considerably less than that in the footwall damage zone primarily because of the uplift of salt gypsum in the footwall of the fault. As the displacement of growth faults gradually increases, the fault width rapidly increases in the early-middle stages, and it shrinks in the late stage, which may be that the activity intensity of the contemporaneous fault decreases rapidly after the Ed. The growth fault architecture that directly affects the permeability of mudstone strata emphasizes the need to re-evaluate fluid migration and sealing along faults in extensional basins.

Quantitative identification of relatively high-level radioactive sandstone in the Upper Triassic Yanchang Formation of the Ordos Basin, China

Most relatively high-level radioactive sandstone (HRSS) reservoir has considerable oil (or gas) resource potential. HRSS is often wrongly identified due to its similar logging response characteristics as mudstone, which leads to the omission of effective reservoirs. In this paper, a quantitative identification method for HRSS is proposed after the analyzing of the response characteristics and relationship between spontaneous potential log and natural gamma-ray log in conventional sandstone and mudstone strata. Take the Upper Triassic Yanchang Formation in Ordos Basin as an example: the responses of spontaneous potential log and the responses of natural gamma-ray log are synchronized and positively correlated in conventional sandstone and mudstone strata, but they are not synchronized in HRSS. Quantitative identification of HRSS was realized based on this synchronization feature, and a “virtual compensation” of natural gamma-ray log was performed. At the same time, logging evaluation method about HRSS has been discussed. The final results shows that this identification method work effectively, and can reduce the misjudgment and omission of effective reservoirs.

Application of sequence stratigraphic concepts to the Upper Cretaceous Tununk Shale Member of the Mancos Shale Formation, south‐central Utah: Parasequence styles in shelfal mudstone strata

AbstractAlthough sequence stratigraphic concepts have been applied extensively to coarse‐grained siliciclastic deposits in nearshore environments, high‐resolution sequence stratigraphic analysis has not been widely applied to mudstone‐dominated sedimentary successions deposited in more distal hemipelagic to pelagic settings. To examine how sequence stratigraphic frameworks can be derived from the facies variability of mudstone‐dominated successions, the Tununk Shale Member of the Mancos Shale Formation in south‐central Utah (USA) was examined in detail through a combination of sedimentological, stratigraphic and petrographic methods. The Tununk Shale accumulated on a storm‐dominated shelf during the second‐order Greenhorn sea‐level cycle. During this eustatic event, the depositional environment of the Tununk Shale shifted laterally from distal middle shelf to outer shelf, then from an outer shelf to an inner shelf environment. At least 49 parasequences can be identified within the Tununk Shale. Each parasequence shows a coarsening‐upward trend via upward increases in silt and sand content, thickness and lateral continuity of laminae/beds, and abundance of storm‐generated sedimentary structures. Variations in bioturbation styles within parasequences are complex, although abrupt changes in bioturbation intensity or diversity commonly occur across parasequence boundaries (i.e. flooding surfaces). Due to changes in depositional environments, dominant sediment supply and bioturbation characteristics, parasequence styles in the Tununk Shale show considerable variability. Based on parasequence stacking patterns, eleven system tracts, four depositional sequences and key sequence stratigraphic surfaces can be identified. The high‐resolution sequence stratigraphic framework of the Tununk Shale reveals a hierarchy of stratal cyclicity. Application of sequence stratigraphic concepts to this thick mudstone‐dominated succession provides important insights into the underlying causes of heterogeneity in these rocks over multiple thickness scales (millimetre‐scale to metre‐scale). The detailed sedimentological characterization of parasequences, system tracts and depositional sequences in the Tununk Shale provides conceptual approaches that can aid the development of high‐resolution sequence stratigraphic frameworks in other ancient shelf mudstone successions.

Mechanisms for overpressure generated by the undercompaction of paleogene strata in the Baxian Depression of Bohai Bay Basin, China

Abstract Undercompaction is an important mechanism for developing fluid overpressure in the mudstone strata. The main factors that affect undercompaction are the deposition rate (DR) and mudstone content (MC); however, the manner in which the DR and the MC influence the development of undercompaction is still unclear. To clarify the pressurization mechanism of undercompaction and the influence of DR and MC, the acoustic and density logging as well as the sonic-velocity–effective-stress schematic are used to identify the undercompaction overpressure. Furthermore, the PetroMod software is used to simulate the development of overpressure and to analyze the influence of the DR and MC on the undercompaction of Paleogene strata in the Baxian Depression of Jizhong Subbasin. The Baxian Depression causes the development of overpressure in the Dongying Formation, the first member of the Shahejie Formation, the third member of the Shahejie Formation, the fourth member of the Shahejie Formation, and the Kongdian Formation, whereas the undercompaction overpressure is observed to mainly develop in the Dongying Formation and in the first member of the Shahejie Formation. A prerequisite for performing the simulation is that the total thickness and deposition time of the formation should remain constant. Further, the effects of the varying DR and MC on the development of undercompaction are simulated. The results exhibit that the DR and MC are prerequisites for developing undercompaction overpressure. However, the DR only affects the undercompaction development period and exhibits little effect on the final overpressure amplitude. Meanwhile, the amount of MC not only increases the amplitude but also results in an early development of the undercompaction overpressure.

Thermal image and spectral characterization of roadway failure process in geologically 45° inclined rocks

Large-scale geomechanical model test was conducted in order to investigate stability of an un-supported tunnel with rectangular cross section embedded in 45° inclined alternating strata of sandstone, mudstone and coal seam. The loading path consists of two groups: cases A–G with overburden depths from 296 to 948m and small loading rate, and cases H–M with overburden depths from 1126 to 2047m and fast loading rate. Infrared thermography, incorporated with image processing and Fourier transform, was employed to characterize the rock responses. Averaged temperature field, 〈IRT〉, represents energy release rate, oscillating at stick–slip pattern with different periods and amplitudes. Overburden depth and loading speed have a significant impact on 〈IRT〉 curve, i.e. small overburden depth and loading speed corresponds to long period and small amplitude; whereas, great overburden depth and fast loading speed to short period and high amplitude. The processed thermal image best represents rock behavior by two major IRT distribution modes. For loading cases A–G, the coal strata were over stressed indicated by high IRT while the mudstone strata were less stressed represented by low IRT, corresponding to the static interlayer friction. For loading cases H–M, the mudstone strata were over stressed indicated by high IRT while the coal strata were less stressed indicated by low IRT, corresponding to the dynamic friction. Fourier spectra and spatial frequency were employed to characterize the infrared sequence. Ultra-high spatial frequency component is a precursor for predicting the imminent dynamic event. Low spatial frequency component may be served as an indicator of the tunnel-wide sphere of influence that the stress redistribution extends.

Application of computerized tomographic scanning to the study of water-induced weakening of mudstone

Water generally plays a harmful role in the stability of mudstone and has induced many geological disasters in mudrock strata. To visually examine water-induced deterioration of mudstone, X-ray computerized tomographic scanning was used to detect the real-time interaction between mudstone and water. This facilitated real-time visual observations of the evolving characteristics of water-induced cracks within the samples. According to these experimental findings, assisted with scanning electron microscopy and X-ray diffraction, the water-weakening mechanisms of mudstone can be divided into the following steps: (1) micro-sized discontinuities provide an initial path for water to invade the rock; (2) the incursive water induces volumetric swelling of clay minerals and dissolution of carbonate; and (3) these effects drive the propagation and connection of cracks. This understanding of the processes of degradation provides a visual evidence for the assessment of mudrock performance with respect to the stability of landslides, debris flows, and susceptibility of subsidence in mudstone strata.

Cretaceous anoxic–oxic changes in the Moldavids (Carpathians, Romania)

This study focused on the Cretaceous black shale successions, followed by red shales that crop out at the outer regions of the Romanian Carpathians, in the Moldavids. The oldest parts of the black shale units deposited in an abyssal plain during Late Valanginian–Late Barremian time; they are mainly characterized by hemipelagic and pelagic muddy siliciclastic rocks and carbonates, commonly intercalated with fine-grained turbidites. During the sedimentation of the middle part of the black shale units in the Late Barremian–Early Albian interval, the depth of the basin increased, as the carbonate hemipelagic sedimentation was replaced by a mainly siliceous one. Only a few thin turbidite intercalations are present. The youngest part (Albian pro parte) of the black shale units is characterized by a turbiditic sedimentation, with mainly sandy sequences of middle and lower deep-water fans. We may assume that the depth of the basin continuously decreased. The presence of authigenic glauconite in the Albian sandstones suggests a palaeoenvironmental change, linked to the occurrence of oxygenated turbidity current circulation. A significant shift in the sedimentation regime in the Eastern Carpathian Moldavids took place in the Late Albian, when Cretaceous Oceanic Red Beds (CORB) occurred. This type of sedimentation lasted up to the Coniacian. The lower part of the CORBs that contains radiolarites intercalated with variegated shales, pyroclastic tuffs and thin sandstones is interpreted as a hemipelagic and pelagic sedimentation in the abyssal plain environment, where rarely turbidites occurred. Upwards, there are mainly burrowed variegated red and green shales. The youngest parts of CORBs are characterized by increased thickness and frequency of the turbidites. While the main part of the CORB is carbonate free or has very low carbonate content, the upper part of these strata becomes rich in marl and mudstone strata, indicating a decrease of the basin-depth. The accumulation of black shales in the Eastern Carpathians during the Late Valanginian–Late Albian interval is linked to the existence of a small, silled basin of the Moldavian Trough, in which restricted circulation led to the density stratification of the water column, resulting in the deposition of anoxic Lower Cretaceous sediments (i.e., the black shales). Because of the tectonic deformation that took place within the Lower–Upper Cretaceous boundary interval, the restricted circulation had changed to an open circulation regime in the Moldavian Trough. Hence, the anoxic regime was progressively replaced by an oxic one, across the Albian–Cenomanian boundary interval. The beginning and the end of the CORBs in the Moldavid units depend thus on various palaeogeographic and palaeoenvironmental settings, and it was controlled by the regional tectonic activity.

A toppled structure with sliding in the Siwalik Hills, midwestern Nepal

Not only the high mountainous regions but also the southern hilly regions of Nepal frequently suffer from landslides and debris flows. An interesting toppled structure with sliding was distinguished on an excavated slope facing the major highway in one such region, the Siwalik Hills. These hills are mostly composed of alternating beds of sandstones and mudstones of the Siwalik Group. A distinct contrast in the direction of dip of the strata was recognized on the excavated slope; the strata in the upper portion of the slope dipped southward, whereas those of the foot slope dipped northward. This indicated that toppling of strata had occurred. The northward direction of toppling and the angle and axis of rotation of the block were determined by examining the distribution of the attitudes of the strata. Steeply dipping strata influenced by southward thrusting along the major tectonic line, contrasting rigidity between sandstone and mudstone, and rapid dissection by a river were the major causes of the toppling. The bottom surface of the toppled part dipped gently not only northward but also partially westward, showing that the toppled block slid westward after toppling. Evidence for tension cracking and scarplets on the top slope indicated that the block was still actively sliding. Because steeply dipping sandstone and mudstone strata are widely distributed in the Siwalik Hills, similar structures are inferred to exist in many places. Therefore, the investigation of such slope structures is important, and their instability must be evaluated before road construction and excavation projects are carried out in Nepal.