Mudstone Samples Research Articles

Paleoclimatic variability in the southern Tethys, Egypt: Insights from the mineralogy and geochemistry of Upper Cretaceous lacustrine organic-rich deposits

The effect of paleoclimate during global warming periods on Upper Cretaceous lacustrine deposits is not fully understood. In this study, comprehensive sedimentological, mineralogical, and geochemical data were used to unravel the past climatic changes and their influence on the accumulation of Campanian lacustrine organic-rich deposits in Egypt. Three sedimentary lithofacies associations are recorded within the uppermost part of the Quseir Formation. The laminated clay-rich mudstone lithofacies has the highest organic matter (TOC up to 1.2 wt.%). Sedimentological observations indicate that the bulk lithologic assemblage was originated in a shallow lacustrine setting. The studied samples consist of clay minerals, quartz, pyrite, carbonate, fluorapatite, and halite. The siliciclastic content reaches up to 78wt.%, whilst the carbonate content ranges from 3 to 22 wt.%. Compared to Post-Archean Australian shales (PAAS), the studied samples are significantly rich in Mg, Ca, P, Al, V, Ga, and Cr contents. The mudstone samples are characterized by high ratios (C-value, CIA, CIW, Mg/Ca, Fe/Mn, Ga/Rb) and low values (K2O/Al2O3), reflecting relatively warm and humid conditions during deposition of these sediments. The prevalence of smectite, as well as the sedimentary features, support that the lacustrine organic-rich mudstones are mainly deposited under warm seasonal climate (arid/humid). Additionally, the presence of dolomite and halite along with the Sr/Ba ratios (1.30–2.13), strongly suggest a saline water setting. The changes between aridity and humidity periods are probably the main reason for the significant variation in water chemistry, especially salinity. Therefore, the paleoclimatic variations largely controlled the depositional process during the formation of the Upper Cretaceous lacustrine organic-rich mudstones in the southern Tethys.

Constitutive modeling of rock materials based on variable-order fractional theory

In this work, we develop a novel variable-order fractional constitutive model to describe the uniaxial compressive behavior of rock materials. The varying-order function of the variable-order fractional model is assumed as a piecewise function of strain. The evolutions of mechanical properties in the closure, elastic, and yield stages within the stress-strain curve are captured by employing the varying-order function changing from 1 to 0. The peak post stage, that is, the strain softening stage, is described by varying-order function with the order exceeding 1. Based on the varying-order function corresponding to each deformation stage, the mechanical mechanisms for the four deformation stages are clearly identified. To validate the proposed model, a series of uniaxial compression tests are conducted on coal, salt rock, and mudstone samples under constant strain rates. We demonstrate that the proposed model is in a good agreement with the experimental data. A comparison indicates that the proposed model has an improved accuracy with the use of fewer parameters compared with other models. Consequently, the proposed variable-order fractional model is effective in describing the stress-strain relationships of rock materials.

Analysis of Mudstone Fracture and Precursory Characteristics after Corrosion of Acidic Solution Based on Dissipative Strain Energy

The deformation and failure of rock materials are closely related to the strain energy characteristics during the loading process. These strain energy characteristics and rock properties are greatly affected when the rock is subjected to the acidic solution. To study the effects of chemical solutions with different pH on the mechanical properties and strain energy mechanism of mudstone, the chemical corrosion mudstone samples are subjected to a uniaxial loading testing machine (CN64 electro-hydraulic servo). The corrosive effects of the acidic solution on the porosity, strain energy characteristics, and failure mode of mudstone samples were thoroughly investigated. The findings of this research indicate that: (1) The rate of change in the porosity and chemical damage coefficient of rock samples after chemical corrosion decreases, which is closely linear with the increase of solution pH; (2) The total strain energy, elastic strain energy, and dissipative strain energy decrease with the increase of pH, and, as a result, it is proposed that the observed turning point of the proportion curve of dissipated strain energy from decline to rise is used as a precursor point of the rock failure; (3) The stress value of the failure precursor point increases and the strain value decreases with the increase in pH value. However, the ratio of the stress value of the failure precursor point to the peak stress hardly changes with pH value, and its value is about 0.883; and (4) Rock samples soaked in a weak acidic chemical solution (pH 7.3 and 5.3) are damaged by tensile crack, while rock samples soaked in a strong acidic chemical solution (pH 3.3 and 1.3) are mainly damaged by the combination of tensile and shear. The findings of this study can be used to provide an experimental and theoretical foundation for monitoring rock engineering disasters such as slope, tunnel, and coal mine failures.

Acid rain: an unsuspected factor predisposing Panzhihua airport landslide, China.

Understanding the effect of acid rain to landslides is crucial for a better landslide risk assessment. This work aims to reveal the unsuspected but key role of acid rain in Panzhihua airport landslide, China. Firstly, we propose a hypothesis that acid rain may aggravate the slaking behavior of mudstone at weak interlayer and make it more fragmented, eventually further reducing its shear strength and predisposing the Panzhihua airport landslide. Subsequently, mudstone samples are subjected to slaking durability test, respectively, using water with a pH of 7 and two dilute hydrochloric acid solution with pH of 5 and 3. Slaking durability index (Idn) is adopted aiming to quantitatively evaluate the impact of acid rain on the slaking. Moreover, the mechanisms of acid rain affecting the slaking behavior of mudstone are revealed by (1) analyzing cation compositions changes in different pH slaking fluid and (2) observing micro-structure change of mudstone-chip before and after acid rain treatment. Finally, three works are conducted as evidences to prove that acid rain indeed plays a key role in the occurrence of Panzhihua airport landslide, including (1) analysis of the link between the slaking behavior of mudstone and its shear strength, (2) comparison of cations between spring water at the edge of the toe of landslide and acid rain, and (3) comparison of mineral contents of mudstone samples collected from different locations. These findings have implications for comprehensively analyzing the formation mechanism of landslide in acid rain area (such as Europe, North America, and China).

Experimental investigation on the nanoindentation viscoelastic constitutive model of quartz and kaolinite in mudstone

The creep behaviors in deep underground engineering structures, especially in soft rocks, have a remarkable impact on the long-term stability of the excavations, which finally leads to the high risk and failure of it. Accordingly, it is essential to recognize the time-dependent deformation through the investigation of this phenomenon. In this study, the creep behaviors of soft rocks were examined to help understand the underlying mechanism of the extended time-dependent deformation. Due to the limited results about the time-dependent properties of the constituents of the rock that reveal their heterogeneity, the targeting nanoindentation technique (TNIT), was adopted to investigate the viscoelastic characteristics of kaolinite and quartz in a two-constituent mudstone sample. The TNIT consists of identifications of mineralogical ingredients in mudstone and nanoindentation experiments on each identified constituent. After conducting experiments, the unloading stages of the typical indentation curves were analyzed to calculate the hardness and elastic modulus of both elements in mudstone. Additionally, the 180 s load-holding stages with the peak load of 50 mN were transformed into the typical creep strain–time curves for fitting analysis by using the Kelvin model, the standard viscoelastic model, and the extended viscoelastic model. Fitting results show that the standard viscoelastic model not only can perfectly express the nanoindentation creep behaviors of both kaolinite and quartz but also can produce suitable constants used to measure their creep parameters. The creep parameters of kaolinite are much smaller than that of quartz, which causes the considerable time-dependent deformation of the soft mudstone. Eventually, the standard viscoelastic model was also verified on the quartz in a sandstone sample.

Geochemistry and zircon U-Pb ages of the Oligocene sediments in the Baiyun Sag, Zhujiang River Mouth Basin

In this study, element geochemistry and zircon chronology are used to analyze the Oligocene sediments in the Baiyun Sag, Zhujiang River Mouth Basin. The experimental results are discussed with respect to weathering conditions, parent rock lithologies, and provenances. The chemical index of alteration and the chemical index of weathering values of mudstone samples from the lower Oligocene Enping Formation indicate that clastic particles in the study area underwent moderate weathering. Mudstone samples exhibit relatively enriched light rare earth elements and depleted heavy rare earth elements, “V”-shaped negative Eu anomalies, and negligible Ce anomalies. The rare earth element distribution curves are obviously right-inclined, with shapes and contents similar to those of post-Archean Australian shale and upper continental crust, indicating that the samples originated from acid rocks in the upper crust. The Hf-La/Th and La/Sc-Co/Th diagrams show this same origin for the sediments in the study area. For the samples from the upper Enping deltas, the overall age spectrum shows four major age peaks ca. 59–68 Ma, 98–136 Ma, 153–168 Ma and 239–260 Ma. For the Zhuhai Formation samples, the overall age spectrum shows three major age peaks ca. 149 Ma, 252 Ma and 380 Ma. The detrital zircon shapes and U-Pb ages reveal that during Oligocene sedimentation, the sediments on the northwestern margin of the Baiyun Sag were supplied jointly from two provenances: Precambrian-Paleozoic metamorphic rocks in the extrabasinal South China fold zone and Mesozoic volcanic rocks in the intrabasinal Panyu Low Uplift, and the former supply became stronger through time. Thus, the provenance of the Oligocene deltas experienced a transition from an early proximal intrabasinal source to a late distal extrabasinal source.

Variations in ultrasonic wave velocities of Miocene carbonate and clastic sedimentary rocks under dry and fully water saturated conditions

This study investigates the variations in P and S wave velocities (Vp, Vs) under dry and saturated conditions and attempts to develop empirical equations estimating Vs based on Vp. In this context, 48 intact core specimens extracted from the Miocene sedimentary rocks were examined. A significant decrease was observed in Vp under saturated conditions (Vp-sat) for several specimens. Thus, selected samples were further examined by thin section, X-ray diffraction, and scanning electron microscopy equipped with an energy dispersive spectrometer. Correlation analyses were performed to reveal the causes of reduction in Vp-sat in terms of porosity, microcrack density, and mineralogical content. Vs was also observed to have a decreasing trend under saturated conditions. Strength and elasticity loss after saturation were not well correlated with the reduction in Vp-sat. The measured velocity values were also compared with the ones derived from Gassmann’s theory. Despite the discrepancy in the literature, a new parameter explaining the negative difference between the values of saturated and dry Vp was suggested. The mudstone texture and the abundant microporosity were firstly declared to be indicative of a negative velocity difference among the same rock group having similar mineralogy. Afterward, two empirical equations estimating Vs were derived from the ultrasonic wave velocities measured in this study and the data compiled from published literature. These equations were applied to a different dataset and compared with the ones suggested by various researchers. Proposed equations were determined to be more widely applicable and valid for limestone, mudstone, and sandstone samples according to several accuracy metrics.

Source rock characteristics and sedimentary environment analysis of Lucaogou Formation in the eastern Jimusaer Sag, Junggar Basin

In recent years, shale oil has gradually become the focus of oil exploration. The source rocks of Lucaogou Formation in the east of Jimusaer Sag of the Junggar basin have great shale oil potential. However, there is still much controversy regarding the sedimentary environment of the source rocks which restrict shale oil exploration. In order to address this problem, 45 mudstone samples of Jx-1 well are selected for testing and analysis. The results show that the Permian Lucaogou Formation in the Jimusaer Sag is developed with high quality source rocks, and the organic matter type is mainly type II, and the source rocks are in mature stage. Both higher terrestrial plants and aquatic algae are important sources of organic matter in source rocks, but the source of organic matter is still dominated by lower aquatic organisms. The source rocks of Lucaogou Formation are mainly deposited in reducing environment, and the water salinity is high, which is conducive to the preservation of organic matter.

Deformed displacive halite crystals: Diagenetic or tectonic origin?

ABSTRACT Displacive halite crystals in fine-grained sediments are a common feature of evaporitic sequences worldwide. The crystal habit of halite precipitated from pure aqueous solution under laboratory conditions is a cube: equal side lengths (a = b = c) and rectangular angles (ab = bc = ac = 90°). Crystals with deformed-looking shape appear in mudstone of Permian evaporites of the Eastern Alps. They expose platy, rod-like, parallelepiped-like or rhomb-like shapes. The aim of the study was to explain the origin of these shapes. Samples were taken in the Altaussee, Berchtesgaden, and Hallstatt mines. Representative mudstone samples were scanned by high-resolution X-ray CT, and halite shapes were restored by 3D software. Additionally, 327 euhedral halite crystals were dissected from mudstone and subjected to individual shape analysis. Their sizes range from 3 to 30 mm in thirteen specimens. Crystals are compact, but a slightly platy preference prevails in all samples. Approximately 80% of all objects expose angles > 1° deviating from the right angle, but only ∼ 15% contain angles > 20°. The maximum deviation from the right angle was ca. 33°. Most objects combine a platy shape (a ≈ b ≠ c) with only one angle deviating from the right angle (2× 90°). The acute angle is preferably oriented parallel to the layering. We interpret a primary rectangular crystal growth to platy shapes in the mudstone (reduced fluid circulation). The displacive halite crystals subsequently deformed during compaction of the mudstone. Halite in deformed shapes was replaced by datable polyhalite ca. 15 My after deposition, which rules out deformation during Alpine tectonics.

An Experimental Apparatus for Monitoring Radon during Compression of Coal/Rock Samples and Its Preliminary Application

Based on the radionuclide distributions in sedimentary coal-bearing strata, this study analyzed the microrelease mechanisms of radon in coal-bearing strata. It was found that the microrelease process includes three stages: emanation, migration, and exhalation. Based on this, an experimental apparatus was independently designed for monitoring radon during compression of coal/rock samples from coal-bearing strata, whose major components include an electrohydraulic servocontrolled rock mechanics testing system, an airtight container, coal/rock samples, radon output device, and a continuous emanometer. The developed apparatus was preliminarily utilized for uniaxial compression tests on mudstone samples taken from the #21105 coalface of the Fourth Coal Mine in Yili Coalfield, China. The test results show that before sample failure under the uniaxial compressive load (UCL), the radon concentration is negatively correlated with the applied UCL and the magnitude of imposed elastic deformation. Increasing the applied load shortens the period of stable deformation, gradually decreasing the porosity of the rock, and as a result of declining the concentration of radon emanation from the rock. Finally, suggestions for future research are proposed, including mathematical equations to express the correlations between different experimental parameters and fractal characteristics of radon release from porous media.

Provenance and Sedimentary Context of Clay Mineralogy in an Evolving Forearc Basin, Upper Cretaceous-Paleogene and Eocene Mudstones, San Joaquin Valley, California

Mudstone samples from the Moreno (Upper Cretaceous-Paleocene) and Kreyenhagen (Eocene) formations are analysed using X-ray diffraction (XRD) and X-ray fluorescence (XRF) to determine their mineralogy. Smectite (Reichweite R0) is the predominant phyllosilicate present, 48% to 71.7% bulk rock mineralogy (excluding carbonate cemented and highly bio siliceous samples) and 70% to 98% of the <2 μm clay fraction. Opal CT and less so cristobalite concentrations cause the main deviations from smectite dominance. Opal A is common only in the Upper Kreyenhagen. In the <2 μm fraction, the Moreno Fm is significantly more smectite-rich than the Kreyenhagen Fm. Smectite in the Moreno Fm was derived from the alteration of volcaniclastic debris from contemporaneous rhyolitic-dacitic magmatic arc volcanism. No tuff is preserved. Smectite in the Kreyenhagen Fm was derived from intense sub-tropical weathering of granitoid-dioritic terrane during the hypothermal period in the early to mid-Eocene; the derivation from local volcanism is unlikely. All samples had chemical indices of alteration (CIA) indicative of intense weathering of source terrane. Ferriferous enrichment and the occurrence of locally common kaolinite are contributory evidence for the intensity of weathering. Low concentration (max. 7.5%) of clinoptilolite in the Lower Kreyenhagen is possibly indicative of more open marine conditions than in the Upper Kreyenhagen. There is no evidence of volumetrically significant silicate diagenesis. The main diagenetic mineralisation is restricted to low-temperature silica phase transitions.

Geochemistry and detrital zircon geochronology of the Lower Jurassic clastic rocks of the northern Qiangtang Basin, northern Tibet: Implications for palaeoenvironment, provenance, and tectonic setting

The Lower Jurassic clastic rocks in the Qiangtang Basin, northern Tibet, are generally regarded as the early sediments of the new Mesozoic Qiangtang Basin (Late Triassic to Early Cretaceous). However, little attention has been paid to the palaeoenvironment, provenance, and tectonic setting of this unit. The QZ‐16 well, located in the east of the northern Qiangtang Basin, firstly recovered the Lower Jurassic strata (Quemo Co Formation) in the Qiangtang Basin. In this paper, the whole‐rock major‐trace elemental characteristics of 34 mudstone samples and detrital zircon geochronology of one sandstone, collected from the Quemo Co Formation of the QZ‐16 well, are reported in order to infer the palaeoenvironment, provenance, and tectonic setting. Quemo Co mudstones are dominated by SiO2 and Al2O3, while Fe2O3, CaO, K2O, and MgO are the second most abundant oxides. They are generally characterized by high REE concentrations, enrichment in light rare earth elements (LREEs), marked negative Eu and Ce anomalies. The REE contents of the mudstones are mainly controlled by terrigenous detrital minerals input. The Lower Jurassic mudstones have relatively high chemical index of alteration (79.1–83.6) values, indicating moderate to high degree of chemical weathering conditions of the source area. Sr/Ba and Rb/K ratios reflect that the marine transgression impact was limited and the mudstones were generally deposited in a brackish water environment. Major, trace, and REE contents indicate that the mudstones were mainly derived from intermediate igneous rocks with subordinate felsic igneous rocks. Prominent age peaks at ca. 200 ~ 400 Ma, 700 ~ 970 Ma, 1,700 ~ 1,900 Ma, and 2,100 ~ 2,700 Ma of the detrital zircon grains suggest that the Lower Jurassic clastic rocks were derived from the Central Qiangtang and Songpan–Ganzi complex. Moreover, tectonic discrimination diagrams based on major and trace elements show that the source of the Lower Jurassic sediments were most likely derived from a collision tectonic setting.

Experimental and Numerical Studies on Crack Initiation and Coalescence in Sandy Mudstone with Prefabricated Cross-Flaws Under Uniaxial Compression

The purpose of this paper is to study the crack initiation, propagation, and coalescence of the sandy mudstone sample with two sets of prefabricated cross-flaws under uniaxial compression. This study is different from previous studies on single or multiple parallel prefabricated flaws. The prefabricated cross-flaws are characterized by the dip of the rock bridge with the direction of the main flaw ( β ) and the angle between the direction of main and minor flaws ( γ ). The effects of these two parameters on crack initiation, propagation, coalescence, crack initiation stress, and coalescence stress are analyzed. Moreover, numerical simulation of the uniaxial compression experiments is performed using PFC2D with a flat-joint model, and the simulation results are in good agreement with those from the experiments. The results demonstrate that the dip angle of the rock bridge with the direction of the main flaw ( β ) has strong effects on the crack initiation and coalescence stresses. The larger the angle between the direction of main and minor flaws γ , the greater the crack initiation and coalescence stresses. The crack initiation stress is reduced for the case with cross-flaws compared with that with non-cross-flaws. Meanwhile, the connection type of main flaws and the width of the crack coalescence zone are difficult to observe through the experiments and are discovered from the numerical simulation.

Experimental investigation of the mechanical properties of mudstone in southwestern China

In this paper the mechanical properties of mudstone with different immersion durations (e.g., immersion in water for 0, 7 and 14 days), at different temperatures (20, 35 and 50 °C) and in different confining pressures (0.6 and 1.2 MPa) were investigated using uniaxial compression tests, conventional triaxial compression tests and scanning electron microscopy. The test results indicate that in the temperature range from 20 °C to 50 °C the water content can significantly reduce the mechanical parameters, including the strength and elastic modulus of the mudstone samples. In contrast, the Poisson ratio of the mudstone samples increases with the increase in the water content. In addition, there is little difference between the effects of an immersion time of 7 days and 14 days on the mudstone samples, indicating that the confining pressure plays a protective role on the mudstone samples.

Experimental Analysis on Creep Properties of Frozen Silty Mudstone Considering Conservation of Energy

Abstract The artificial ground freezing method is a dominant technique during the construction of massive coal mines and tunnels. The comprehensive understanding of creep properties, especially the accelerating creep process of frozen soft rock, can provide significant guidance regarding the excavation of the soft rock layer. In this article, a series of uniaxial compression tests and creep tests of silty mudstone samples was conducted. In conjunction with the experimental data, the creep properties of frozen silty mudstone at transient, steady, and accelerating creep stages are discussed in detail. Furthermore, based on the conservation of energy, a winding element is introduced to modify the classical Nishihara model, which reveals that the modified Nishihara model not only presents the creep-damage properties of frozen silty mudstone at the accelerating creep stage but also describes the primary and stationary creep stages accurately. Moreover, the fitted results using the modified Nishihara model are in good agreement with the test results. Ultimately, the results of sensitivity analyses corroborate that small changes in the stress level can cause large changes in the failure time. At the accelerating creep stage, the axial strain rate increases more rapidly when the coefficient winding element is small, causing a shorter time to approach failure. This modified creep model can present a theoretical basis for further revealing the objective law of rock creep and can provide an important reference for the safety assessment of coal mine and tunnel construction using the artificial ground freezing method.

Categories of Vendian catchments – sources of fine-grained aluminosiliciclastic materials for the Serebryanka and Sylvitsa group deposits (Middle Urals)

Research subject. The article discusses a number of geochemical features (distribution of lanthanides and Th, values of (La/Yb)N and Eu/Eu*) of fine-grained clastic/clay rocks of various Vendian regional stages of the western slope of the Middle Urals. The results were interpreted in the context of data on the composition of the catchment areas of a number of modern rivers and information on the distribution of the same characteristics in suspended matter and modern bottom sediments of the “downstream North Dvina River–White Sea”.Methods and materials. The content of La, Sm, Eu, Gd, Yb, and Th in as many as 200 samples of shales and mudstones of the Serebryanka and Sylvitsa groups served as a research material. The research method was an analysis of the localization of individual and average data points of clayey rocks of the Laplandian, Redkinian, Belomorian and Kotlinian regional stages on paired diagrams (La/Yb)N–Eu/Eu* and (La/Yb)N–Th developed on the basis of analytical data on the composition of bottom sediments of estuarine parts of different categories/classes of modern rivers. Results. It was shown that the sedimentary sequences of the western slope of the Middle Urals are mainly composed of fine-grained clastic material belonging to categories 1 (sediments of world major rivers) and 2 (sediments of rivers draining sedimentary/mixed sedimentary formations). This material is likely to have entered the sedimentation area from the east, from the growing Kadomian Varanger-Kanin-Timan folded-thrust belt/Timan orogen, or/and from the west (Riphean sedimentary sequences of Kama-Belsk aulacogen). In addition, the igneous and metamorphic rocks of the East European Platform basement composing the structures, which could be classified as “igneous/metamorphic terranes” under modern classification, were likely to have made a certain contribution. Conclusions. The obtained results agree well with the conclusions obtained earlier as a result of mineralogical and petrographic studies of the conglomerates and sandstones of the Serebryanka and Sylvitsa groups, as well as with the data from a previous geochemical analysis of clay rocks.

An integrated organic–inorganic geochemical characterization of Paleogene sediments in No.1 Structural Belt of the Nanpu Sag, Bohai Bay Basin, eastern China: implications for the origin of organic matter

Paleogene sediments, especially the third member of the Dongying Formation (Ed 3 ) and the first and third members of the Shahejie Formation (Es 1 and Es 3 ), have been regarded as the most important source rocks in the Nanpu Sag. Organic and inorganic analyses, including Rock-Eval pyrolysis, gas chromatography-mass spectrometry, and element geochemistry, in 91 mudstone samples, were used to reconstruct the palaeoenvironmental conditions, such as palaeoclimate, palaeo-salinity and palaeo-redox conditions, and to recognize the origin of organic matter. The results show that Es 3 has a higher TOC content than Es 1 and Ed 3 . Hydrocarbon genetic potential (S 1 + S 2 ) of the samples indicate fair to good hydrocarbon potential. The kerogen type of Ed 3 and Es 1 source rocks are Type II 1 –II 2 , while Es 3 source rocks are dominated by Type II 2 –III kerogens. Biomarkers and inorganic geochemical indicatives of source rocks, such as Pr/Ph, V/(V + Ni) and Cu/Zn, indicate a lacustrine environment with fresh to brackish water under suboxic to anoxic conditions during deposition. Ed 3 source rocks are characterized by low G/C 30 H (gamacerane/C 30 hopane) (<0.1), TT/C 30 H (tricyclic terpane/C 30 hopane) and S/H (serane/hopane), high Pr/Ph (pristane/phytane) and C 24 TeT/C 23 TT (C 24 tetracyclic terpane/C 23 tricyclic terpane), indicating mixed input of both algae and terrestrial higher plants, dominated by terrestrial higher plants. Es 1 source rocks display medium G/C 30 H, TT/C 30 H, S/H, Pr/Ph and C 24 TeT/C 23 TT, indicative of a mixed input of both algae and terrestrial higher plants. Es 3 source rocks are characterized by high G/C 30 H (>0.1), TT/C 30 H and S/H, low Pr/Ph and C 24 TeT/C 23 TT, typical of a mixed input of algae and terrestrial higher plants, with algal dominance. Ed 3 , Es 1 and Es 3 source rocks were mostly deposited in semi-arid to humid-warm climate conditions, with an average temperature higher than 15°C. This study suggests that suitable temperatures, a fresh to brackish lacustrine environment and suboxic to anoxic conditions could result in a high organic matter concentration and preservation, thus providing prerequisites for the formation of high-quality source rocks. Supplementary material : Tables S1–S3 are available at https://doi.org/10.6084/m9.figshare.c.5227684

MINERALOGICAL, BIB‐SEM AND PETROPHYSICAL DATA IN SEAL ROCK ANALYSIS: A CASE STUDY FROM THE VIENNA BASIN, AUSTRIA

The Vienna Basin is a major hydrocarbon province with a long exploration history. Within the basin, secondary migration from Upper Jurassic source rocks into stacked Middle Miocene (Badenian) sandstone reservoirs was formerly considered to have occurred almost entirely along major fault zones. However recent exploration data has suggested that in areas where no major faults are present, oil may have migrated vertically through the sandy mudstone intervals separating individual reservoir units, which are therefore imperfectly sealed. In order to investigate possible secondary migration through the semi‐permeable mudstones, this study links variations in gross depositional environment (GDE) to variations in mudstone properties (e.g. mineralogy and pore size distribution). The study focussed on the mudstones which seal reservoir sandstones referred to locally as the “8.TH” and “16.TH” units. The bulk mineralogical composition of 56 mudstone and sandy mudstone (and minor intercalated muddy sandstone) samples from seal layers in 22 wells was studied by X‐ray diffraction analysis, broad ion beam – scanning electron microscopy (BIB‐SEM), mercury intrusion porosimetry (MICP) and N2 adsorption. These data are interpreted in the context of GDE maps of the Vienna Basin which were previously established using seismic and well log data.Results indicate that the gross depositional environment strongly controlled the pore space characteristics of the mudstones. The sandy mudstones in the NW part of the study area were influenced by a complex eastward‐prograding deltaic system which deposited coarse detritus into a major palaeo depression (“Zistersdorf Depression”) located in the centre of the basin. Higher overall porosity and a dominance of larger pore size classes, probably resulting in reduced seal quality, were observed for sandy mudstones from well locations within feeder channels and also from within the Zistersdorf Depression. Similarly, sandy mudstones from locations associated with the long‐term input of coarser sediments in shoreline, coastal and proximal offshore settings in the NW and central parts of the study area are considered to be of lower sealing quality compared to fine‐grained mudstones deposited in distal, open‐marine settings which prevailed in the SE part of the study area throughout the Middle Miocene.In general, pore geometries were influenced by mineralogical composition; quartz‐ and detrital carbonate‐rich samples show equidimensional pores, while more elongated pores (with a higher average aspect ratio) characterize clay‐rich samples. Furthermore, matrix mesopores (2‐50 nm) determined by N2 sorption are more abundant in clay‐rich versus quartz‐rich samples, and show a pronounced positive trend with increasing percentage of illite‐smectite mixed‐layer clay minerals.This study shows that regional‐scale mudstone seals in the Vienna Basin have been influenced by variations in sedimentation associated with lateral variations in gross depositional environment during the Middle Miocene. The observed pore characteristics will serve as input data for future models of secondary migration.

Marine vs. terrestrial environments during Early Triassic deposition on the northeastern margin of the Central European Basin – a multidisciplinary study on the Middle Buntsandstein of the Bartoszyce IG 1 borehole, NE Poland

The Middle Buntsandstein Subgroup of the Lower Triassic of north-eastern Poland has been investigated in the context of possible marine ingressions into the Central European Basin. To better constrain these, palynofacies analysis and mineralogical analysis of mudstones were undertaken on samples taken from the Lidzbark and Malbork formations penetrated by the Bartoszyce IG 1 borehole, serving as stratotype section of both lithostratigraphic units. Microfacies and geochemical analyses, including C and O isotope analysis, were conducted additionally on oolitic limestones of the basal Lidzbark Formation (the lowermost Middle Buntsandstein), and the boron content was measured on all mudstone samples. Seven palynofacies types are distinguished: types 1 to 3 within the Malbork Formation and types 4 to 7 within the Lidzbark Formation. Types 1 and 2 originated on a floodplain, type 3 probably in a deltaic or barrier setting, and types 4 and 5 in a brackish lagoon or a more open basin, possibly of marine origin. Palynofacies type 6 reflects long transport and reworking, whereas palynofacies type 7 is interpreted as formed due to pedogenic processes. Clay minerals and quartz, accompanied by feldspars, calcite and dolomite are the main components of the mudstones investigated. The clay mineral association consists of illite or a mixture of illite and smectite, and chlorite. The Lidzbark Formation and the lowermost part of the Malbork Formation show less variability in mineralogical composition than the upper part of the Malbork Formation. Smectite admixtures were detected only in the upper part of the Malbork Formation (the uppermost Middle Buntsandstein). The boron content, achieved after aqua regia digestion, ranges from 70 to 121 mg/kg (96 mg/kg at average), oscillating generally around 90 mg/kg. A higher boron content, bound in silicate structure, is associated with the upper part of the Malbork Formation. All major mineralogical and geochemical changes coincide more or less with the transition from the supposed marine to the terrestrial environmental realm, interpreted from lithological and sedimentological observations within the lowermost part of the Malbork Formation. However, diagenetic alteration of the clay minerals, and of the boron content, could not be ruled out. The oolitic limestones, mainly grainstones, contain admixtures of quartz grains and rare bioclasts, the ooids nuclei being peloids or unrecognizable. The carbonates are almost exclusively composed of low-Mg calcite. A high content of Mg and the presence of small amounts of dolomite suggest that the ooids were primarily composed of high-Mg calcite and are comparable with similar Early Triassic deposits in Tethyan settings. The δ 13 C values range from –2 to +1‰ VPDB, fitting well with the known ranges of Lower Triassic marine carbonates. An observed δ 13 C depletion towards the top of the oolite-bearing part of the section may reflect a local shallowing trend that led to overall salinity decrease. A possible connection with the one of the global oceanic geochemical episodes has to be tested further. The results obtained suggest a marine origin of the lower Middle Buntsandstein deposits studied and document a terrestrial origin for the upper Malbork Formation.

Geochemical studies on high-efficiency hydrocarbon expulsion of Triassic lacustrine shale in the Ordos Basin, China: implications for tight and shale oil accumulation

The organic-rich sediments developed at the bottom of Chang7 Member of the Upper Triassic Yanchang formation, which can be divided into oil shale and black mudstones, are dominate source rocks for low permeability and tight oil reservoirs in Mesozoic petroleum system of Ordos Basin. The hydrocarbon expulsion characteristics of the organic-rich sediments have been investigated by geology and geochemistry data. By comparison with the geochemical parameters of crude oil samples and mudstone extracts, Chang7 shale has many geochemical evidences on high rates of hydrocarbon expulsion. Lower conversion rate of chloroform extracted bitumen “A”, especially for the samples with TOC higher than 10%; lower productivity indexes and lower hydrogen indexes reducing with TOC increasing; lower ratios of saturates/aromatics but higher contents of polar components for residual bitumen “A”; heavier stable carbon isotopes of residual hydrocarbons. It should be noted that the source rock evaluation and oil-source correlation would be misunderstood based on these abnormal geochemical data caused by high-efficiency hydrocarbon expulsion. Calculation for rock samples have shown that the expelled rates of the shale samples averaging 70% are significantly higher than those of the mudstone samples. Chang 7 shale with large amounts of hydrocarbons generated and expelled is critical for hydrocarbon accumulation in low-permeability and tight oil reservoirs. Also, Chang7 organic-rich rocks is an important exploration field of shale oil in the future.