Sandstone Layers Research Articles

Studies on the Source and Phase Characteristics of Oil and Gas: Evidence from Hydrocarbon Geochemistry in the Pingbei Area of Xihu Sag, the East China Sea Shelf Basin, China

The Pingbei area is the main accumulation area of oil and gas in the Xihu Sag. The phase characteristics of oil and gas in this area are complex, and the understanding of their genesis is still unclear. In this paper, based upon discussions of crude oil and natural gas geochemical data, integrated with local geological features, we discuss the sources, migration, and phase state characteristics of oil and gas in the Pingbei area of the Xihu Sag. The study results show that the crude oil and natural gas in the Pingbei area are coal-derived and the oil and gas produced by the humic organic matter during the mature stage. The oil and gas source correlation showed that crude oil and natural gas have good affinity with the Eocene Pinghu Formation (PH) coal-bearing source rocks. Crude oil has the characteristics of near-source accumulation and short-distance migration while natural gas is supplied from a dual source: the coal-derived hydrocarbon rocks of PH in the deep part of the study area, supplemented by the coal-derived hydrocarbon rocks of PH on the bottom of the slope, and adjacent hydrocarbon-bearing sub sag. The distribution characteristics and geochemical migration indices of hydrocarbon show that the oil generated from the hydrocarbon source rocks of PH in the deep Pingbei area mainly migrates vertically along the fault connecting the reservoir and the source rocks to the trap, where it accumulates, while the natural gas exhibits deep and large faults that mainly migrate vertically, supplemented by its lateral migration along the composite transport system composed of faults and sandstone layers in the slope zone. The whole area presents the phase characteristics of “upper oil and lower gas, west oil and east gas”. The mechanisms of produced-exhausted and geochromatographic effects (PGE), as well as evaporative fractionation (EF) and phase-controlled migration fractionation (PMF), result in the obvious discrepancy of hydrocarbon’s properties on the vertical profiles.

Quantitative Evaluation of Suspended Solid Runoff from Large-Scale Landslide Areas Presumed to Be the Source of Turbid Water

In the uppermost stream of the Mimikawa River, in northern Miyazaki Prefecture, the contribution to river turbidity of a huge, collapsed slope alternating sandstone and mudstone layers was qualitatively shown in our previous study. In this study, the water level and turbidity were continuously observed, to obtain a quantitative estimation of this contribution. The conversion equation from the water level to the flow rate is required, but field measurements during the flooding term in the mountainous site are difficult. In this study, a high-resolution survey was conducted, and the relationship was determined via a small-scale hydraulic model shaped using a 3D printer from the survey results, to determine the relationship between the water level and the flow rate. The flow rate time series was reproduced with the distributed runoff model that is verified with the flow rate converted from the water level. The flow rate and turbidity load time series were also estimated from the long-term rainfall. The area of the bare soil surface of each small basin was obtained via satellite image analysis, and the soil yield from each surface condition was calculated. Furthermore, the amount of turbidity produced upstream of Kamishiiba Dam was calculated for each small basin. It was estimated that 24% of the turbidity was generated from the small basin covering 5.7% of the total catchment area. This study showed that it is possible to verify the hydrological model by obtaining the water-level–discharge relationship, even in the mountains, where it is difficult to observe the discharge on-site, via small-scale hydraulic model experiments.

A case study of sandstone reservoir characterization of wells in the Orange Basin, offshore South Africa

This study used seismic and petrophysics data to identify potential subsurface facies that are viable hydrocarbon productive areas in the southern part of the Orange basin of South Africa. Two target areas were identified as target areas 1 and 2. In Target Area 1, three facies were identified: sand, silt and shale. The sand facies has the best reservoir quality (volume of shale between 20 to 30%; porosity from 9 to 17%; permeability from 5 to 15 mD, and water saturation from 40 to 60%), while the shale facies present the lowest reservoir quality rock (volume of shale greater than 40%; porosity from 0 to 5%; permeability from 0 to 0.5 mD, and water saturation from 80 to 100%). Target Area 1 mainly consisted of silty shale and shaly silt. However, Target Area 2 had the same facies as Target Area 1, except with additional basement facies comprising well-sorted fine to medium-grain sands. Target Area 2 had considerably thicker sand layers; thus, the petrophysical properties are slightly better, with the basement facies having a volume of shale less than 20%, porosity between 20 and 23%, permeability greater than 15mD, and water saturation between 30 and 45%. The overall reservoir rock quality in the triangulated area is generally poor because of a high concentration of clay in the matrix in sandstone layers. Regarding economic potential, Target Area 2 was identified as the most productive area with better reservoir qualities due to the basement facies. The recoverable gas in Target Area 1 is 95 057 cubic metres, while area 2 has a volume of 151 923 cubic metres. This study has shown for the first time that the southern part of the Orange Basin is not economically viable.

A Study on the Influence of Double Tunnel Excavations on the Settlement Deformation of Flood Control Dikes

This article combines numerical simulation and field monitoring methods to study the stability of the overlying Liuyang River embankment in the tunnel crossing between Huaqiao Station and Rice Museum Station of Changsha Metro’s Line 6. Using AutoCAD, 3Dmine, and COMSOL Multiphysics, a calculation model of the entire subway tunnel section crossing the flood control embankment under the coupling of fluids and solids was established. The process of tunnel-crossing the embankment and the variation in spatial displacement and plastic strain in different geological layers were analyzed from the perspective of time evolution and spatial distribution. The research results show that during the process of crossing the embankment, the deformation of the east bank is greater than that of the west bank, and crossing the west bank is the relatively riskier stage of the entire project. Moreover, during the process of crossing the embankment, the overlying soil layer will produce a plastic strain zone, and only a small amount of plastic strain is generated in the surrounding sandstone layer of the tunnel walls. In terms of the magnitude of plastic strain, the plastic strain area produced by the leading tunnel’s surrounding rocks is larger than that of the following tunnel. As the excavation progresses, a funnel-shaped settlement displacement gradually forms during the passage of the leading tunnel, and this settlement funnel gradually expands during the passage of the following tunnel, with the maximum settlement point transitioning from directly above the leading tunnel to the middle position between the two tunnels. Using the jitter filter algorithm and the adjacent average method to process the field monitoring data, the results show that the monitored deformation results well match the simulated settlement results.

Paleoecology and paleoenvironment of the Early Cretaceous theropod-dominated ichnoassemblage of the Los Corrales del Pelejón tracksite, Teruel Province, Spain

The earliest Cretaceous (mid-late Berriasian) tracksite Los Corrales del Pelejón is an important dinosaur trackway site in Teruel Province in Spain. The ichnoassemblage occurs in the Galve Formation (Galve sub-basin, Maestrazgo Basin) and comprises around 40 tracks assigned to theropods and ornithopods. In this paper, we undertake a paleoenvironmental analysis of the succession, which includes the overbank deposits of a fluvial environment, and discuss the implications of these findings for trackway preservation and orientation. The track-bearing unit is composed of fine- to very fine-grained, thinly bedded sandstone layers with wave ripples and traces of the Mermia ichnofacies, which were deposited as splay deposits within an ephemeral overbank pond. Two different theropod ichnotaxa (Megalosauripus cf. transjuranicus and Grallatoridae indet.) of three different size classes occur as small-, medium-, and large-sized tracks. The ornithopod tracks are classified as cf. Iguanodontipus isp. Five theropod trackways (M. cf. transjuranicus) show a bimodal orientation pattern with a similar orientation to NE-SW wave ripple crests, suggesting that these animals were walking parallel to the shoreline of the ephemeral overbank pond. Three of them walked in a similar subparallel orientation, but there is no evidence suggesting gregarious behavior as they have slightly different orientations and/or speed values. The Los Corrales del Pelejón site is an example of environmental influence on dinosaur behavior.

Analysis of the geothermal formation mechanisms in the Gonghe Basin based on thermal parameter inversion

Gonghe Basin is the most critical distribution area of hot dry rock (HDR) geothermal resources in China. Geophysical results (resistivity, velocity, density, etc.) provide an indirect way to image the spatial distribution to understand its genesis, which has a multisolution problem. However, heat flow and thermal conductivity are the most direct and reliable evidence to reveal the distribution of geothermal resources. The conventional thermal conductivity and heat flow obtained based on geologic surveys and rock samples are highly accurate, but it is challenging to characterize the regional distribution characteristics. In this paper, we develop the heat transfer adaptive finite-element equation parameter inversion workflow to invert the thermal parameters and analyze the geothermal formation mechanisms in the Gonghe Basin. First, the synthetic model test verifies that our inversion process has reliable accuracy and strong antinoise ability. For the Gonghe Basin HDR geothermal example, we use high-precision aeromagnetic data to estimate the Curie depth using the improved Parker-Oldenburg method. Then, the regional conductivity thermal and temperature field can be established according to the empirical formula as the initial input model for the inversion procedure. In addition, we collect the temperature data of six HDR geothermal wells across the Gonghe-Guide Basin as a priori constraint information. Combined with the inverted thermal parameters (thermal conductivity, geothermal gradient, and heat flow), geologic structure, and geophysical result, we infer that the high thermal conductivity layer at a depth of 5–10 km provides the main deep heat source channel and the shallow low thermal conductivity fine sandstone layer of Neogene provides good caprock protection for forming a high-temperature geothermal reservoir.

A nature-based solution to reduce soil water vertical leakage in arid sandy land

Quick water seepage in sandy soils results in a serious restriction for ecological restoration, becoming a worldwide urgent research issue in arid areas. Search for practical solutions are focused on reserving the limited rainfall into topsoil layer to meet plant water demand. Based on fieldwork, this study evaluates the blocking effects of Pisha sandstone on soil water leakage, by inserting sandstone layers in the different soil depths (10–15 cm, 20–25 cm, 30–35 cm), which act as isolation layers within the sandy soil. Results showed that the inserted Pisha sandstone layers significantly decreased the soil water infiltration rates at the different stages and cumulative infiltration. The inserted sandstone layer at the depth of 10–15 cm presented the highest permeability resistance, and its initial infiltration rate, stable infiltration rate and average infiltration rate decreased by 102.42 mm h−1, 144.30 mm h−1 and 132.79 mm h−1, respectively. Meanwhile, the soil water content in the 5–10, 20–25, and 30–35 cm soil layers increased by 2.17%, 2.20%, and 1.37%, respectively, due to the inserted Pisha sandstones. This experimental study proves that the insertion of Pisha sandstones effectively hinders the downward penetration of soil water in sandy soil. These findings provide a new alternative for an effective solution of the fast water leakage problem of sandy land.

Experimental investigation of drilling cutting transport using foam

This study investigates the stability of foam in drilling fluid, focusing on the impact of surfactant (sodium dodecyl sulfate) and hydroxyethyl cellulose polymer. The research analyzes various influential factors and examines three rock types (carbonate, limestone, and sandstone) commonly encountered in drilling operations. Additionally, the study explores the behavior of drilling cuttings from light, medium, and heavy reservoir oil types (Bangestan, Asmari, and Soroush) found in Iranian oil fields. The influence of salinity, specifically divalent (calcium) and monovalent salts (sodium), on foam performance is also investigated.Experimental findings reveal that higher concentrations of critical micelle concentration do not significantly affect foam stability, but are used in preparing the foaming solution. Furthermore, different polymer concentrations (ranging from 0% to 0.5%) enhance foam stability, except at a concentration of 0.5% where foam formation is impeded. Interestingly, the transfer of more stable foam and a greater volume of cuttings to the surface is facilitated in sandstone layers. The presence of heavy oil significantly impacts the rate of cuttings transfer compared to lighter oils.

Surface and Subsurface Engineering Geological Characterization for Empirical Design Excavation Method and Support System of Jragung Dam Diversion Tunnel, Indonesia

The diversion tunnel of Jragung Dam was initially designed using the New Austrian Tunnelling Method (NATM) based on assessment of limited drill cores. Due to the uncertainty in the use of single empirical tunneling method and limited data for the tunnel design, this research was conducted to better characterize the rock masses and provide design alternatives for the tunnel construction. In addition to drill core evaluation, surface engineering geological mapping was carried out to obtain complete parameters of Rock Mass Rating (RMR) and Q rock mass classifications for designs of the tunnel excavation method and support system. The results show that the tunnel area consists of highly weathered sandstone and claystone layers with extremely poor to fair rock mass quality. The rock masses have very favorable and fair discontinuity strike and dip orientations with respect to the tunneling direction. The recommended excavation methods are top heading and bench and multiple drifts. The tunnel mostly requires rock bolt and shotcrete support system and the tunnel sections with poor rock mass quality require additional installation of steel sets. This research highlights potential collapse of the extremely poor quality rock masses near the tunnel portals during excavation of the diversion tunnel.

Ancient Aeolian Reservoirs of the East Siberia Craton

Fine-grained strata deposited on the Eastern Siberian craton are predominantly considered to mainly consist of Neoproterozoic sandstones. Clastic rocks near the unconformity border of the Ediacaran and the Riphean are represented by sandstone and siltstone layers with thicknesses of several tens of meters, belonging to the Nepa, Tira, and Byuk horizons in the Nepa–Botuoba region. These Neoproterozoic sandstones have features characteristic of aeolianites formed under the action of high wind velocity in the Ediacaran period. Sandstone samples near the Riphean–Ediacaran boundary were collected from five deep wells and characterized for granulometry and mineral composition using optical microscopy, XRD, SEM, and ICP-MS techniques. These sandstones have a high proportion of quartz (60–98%) with minor amounts of feldspars, carbonate, and sulfate cements. Thin sections of the sandy rocks feature bimodal distributions of the grains throughout many sections, with large well-rounded quartz grains being several orders of magnitude greater than the silt matrix grains. The monomineralic quartz rocks have an overgrowth of quartz grains. These rocks can be petroleum reservoirs with good porosity and permeability, but in most of the studied intervals, a high content of anhydrite and dolomite interstitial cement significantly reduces both. The porosity of the rocks is low, while the permeability is very low, which may be associated with a significant amount of clay and cement material. Aeolianites normally contain large amounts of bimodal quartz (due to its high stability and resistance to weathering) and possess the presence of heavy minerals.

Seismicity and Rock Burst Hazard Assessment in Fault Zones: a Case Study

The coal exploitation in the Upper Silesia region (along the Vistula River) triggers the strata seismic activity, characterized by very high energy, which can create mining damage of the surface objects, without any noticeable damages in the underground mining structures. It is assumed that the appearance of the high energy seismic events is the result of faults’ activation in the vicinity of the mining excavation. This paper presents the analysis of a case study of one coal mine, where during exploitation of the longwall panel no. 729, the high energy seismic events occurred in the faulty neighborhood. The authors had analyzed the cause of the presented seismic events, described the methods of energy decreasing and applied methods of prevention in the selected mining region. The analysis concluded that the cause of the high energy seismic events, during the exploitation of the longwall panel no. 729 was the rapid displacements on the fault surface. The fault’s movements arose in the overburden, about 250 m above the excavated longwall panel, and they were strictly connected to the cracking of the thick sandstone layer.

Characterization of the Carnian Pluvial Episode in the Argana Basin (Western High Atlas, Morocco): An approach based on sedimentology, clay mineralogy and paleosols

We report on a multi-proxy study of the Carnian Pluvial Episode in the continental Argana Basin (Morocco) based on mineralogy, geochemistry, paleosols and sedimentary features. The study focus on two sections of the Irohalene Member (unit t5) of the Timezgadiouine Formation dated as late Julian-early Tuvalian through clam shrimp (conchostracan) associations. The sedimentary study of the Irohalene Member defines three parts: lower, middle and upper (t5.1, t5.2, t5.3 respectively). The base of the lower part comprises channels with gravel and sandy bars related to proximal areas, and fluvial meandering systems, overbank and floodplain deposits developing upwards. Extensive floodplains showing the development of siliciclastic and carbonate lakes mostly represent the t5.2 and t5.3 parts, although the sedimentary characteristics of the former reflect more humid and permanent water conditions. These more humid characteristics are also indicated by the drastic change from calcareous paleosols in t5.1 to smectitic Vertisols in t5.2, which occur in relation to thin layers of lacustrine carbonates, greenish lutites, and hybrid oolitic sandstones of palustrine environments. Stable carbon and oxygen isotope data are consistent with a general tendency towards the arid conditions observed in the Tadart Ouadou t6 Member of the Bigoudine Formation, of fluvial-eolian origin and younger in age. This general tendency is interrupted by at least two humid yet hot intervals characterized by pedogenic carbonates featuring lighter isotope compositions and lacustrine carbonates precipitated under conditions of high temperatures and evaporation rates.

Energy Dissipation and Fracture Mechanism of Layered Sandstones under Coupled Hydro-Mechanical Unloading

Rock burst is easy to occur in the water-rich roadway of coal mines, which is closely related to the energy dissipation and fracture mechanism of rocks under coupled hydro-mechanical (H-M) unloading. Therefore, in combination with the triaxial loading and unloading process and H-M coupling effect, the mechanical test of layered sandstones under coupled hydro-mechanical unloading (TLUTP) was conducted. The energy dissipation and fracture mechanism were revealed. The results show that: (1) The influence of layered angles on the peak volumetric strain is more sensitive than that of confining pressure under conventional triaxial loading with H-M coupling (CTLTP). On the contrary, the influence of confining pressure on the peak volumetric strain is more sensitive than that of layered angles under TLUTP. (2) With increasing layered angles, the peak elastic energy density under CTLTP shows the “W” shaped evolution characteristic, while that of under TLUTP shows the “N” shaped evolution characteristic. (3) The “Energy Flow” chain is proposed. Meanwhile, combined with the domino effect and the structural evolution theory, the energy dissipation and fracture mechanism of layered sandstones under coupled hydro-mechanical unloading are both revealed. The conclusions obtained can provide certain fundamental theoretical references for the effective prevention of rock burst in a layered water-rich roadway.

Numerical Tracking of Natural Gas Migration in Underground Gas Storage with Multilayered Sandstone and Fault-Bearing Caprocks

The structure of caprocks is often greatly altered by different scales of faults or fissures in long-term geological tectonic evolution, and the sealing performance may be deteriorated. In this paper, a simplified geological model characterized as multilayered sandstone and fault-bearing caprocks extracted from the Shuang 6 underground gas storage located in the Liaohe oilfield was established. Different fault geometry (e.g., fault length, fault dip angle, and fault type) and seepage attributes (porosity and permeability) were considered to illustrate their impacts on natural gas migration during the cyclic high rate of injection and production of natural gas. The results showed that the seepage anisotropy and the natural gas front are strongly affected by the formation properties and, especially, are hindered by the low permeability sandstone layers. The difference in the lateral migration distance of natural gas in different layers can reach 110 m at the end of the injection period, with an annual injection volume of 108 m3. The migration of natural gas along the fault zone is mainly controlled by the permeability of faults, followed by fault scale, fault dip angle, and fault type. The sealing failure of caprocks in the fault zone does not occur based on the simulated gas migration distribution, showing that a very limited amount of natural gas migrates into the caprocks.

In situ estimation of effective rock elastic moduli by seismic ambient vibrations

In this study, we performed a non-invasive ambient noise investigation of unsaturated rock structures in the Bohemian Paradise (Bohemian Cretaceous Basin, Czech Republic). Our study focused on two key topics: 1) An in situ elastic moduli estimate of competent, horizontally deposited sandstone layers using ambient noise array measurements. Recordings were processed using an f-k array analysis, from which frequency-dependent Love and Rayleigh wave dispersion curves, as well as Rayleigh wave ellipticity, were retrieved. Data were inverted for the P- and S-wave velocity profiles, from which Young's and shear moduli were successfully estimated. 2) A study of the local response of the Kapelník rock tower. We analysed a dataset of ambient noise recordings obtained from the top of the tower and its foot. Information regarding tower oscillation frequencies and directions, together with amplification ratios, were retrieved from a particle motion polarisation analysis and from site-to-reference spectral ratios. Euler-Bernoulli beam theory was also employed for interpreting measured data using elastic moduli estimated from noise array measurements.

Distinguishing different classes of mass-transport deposits in LPIA strata exposed in eastern Paraná Basin, Brazil

Ice age mass-transport deposits (MTD) occur in multiple stratigraphic intervals within the Permocarboniferous Itararé Group, in the Paraná Basin, and are exposed in several disconnected quarries and road cuts around the region of Mafra, southern Brazil. The most typical deposits are diamictites with muddy matrix, but silty-sandy diamictites and metric intercalations of deformed layers of sandstones and siltstones occur subordinately. Similar MTD facies have been described from better exposed and time equivalent successions in western Argentina. There, because of the km-scale exposures, MTDs can be mapped in their plenitude, giving better insights about their emplacement. In this paper, we describe the Brazilian MTDs and infer their modes of transport and deposition supported by comparisons with published Argentinean examples. Based on matrix constitution, presence of coherent blocks and rafts, and sedimentation phases, at least four types of MTDs can be identified: 1) Mud-rich MTD and isolated sandstone blocks; 2) Resedimented MTD represented by diamictites with folded blocks of diamictite, possibly corresponding to two phases of remobilization; 3) Sand/silt-rich MTD (evolved stage), and formed by deformed bands of sandstones and siltstones (mature stage); and 4) MTD with rafts of heteroliths and sandstone blocks. Usually MTDs rest on and scour into a fine-grained substrate such as thin-bedded turbidites and shale. The regional geological setting at Mafra points to an open embayment. The collapse of inclined depositional slopes related to high sediment supply associated with deglaciation was the major control on MTD development. However, the inferred proximity of the ice margin during slope failure was variable, suggesting MTD formation during different stages of a glacial cycle.

Groundwater exploration using 2D resistivity method in water supply for prevention of peatland fire at Kahayan Kuala area, Central Kalimantan

Resistivity data acquisition using the two-dimensional (2D) resistivity method has been carried out at 8 locations in the Kahayan Kuala area, Pulang Pisau District, Central Kalimantan. The aim of the study was to identify the distribution type and depth of aquifer layers and other geological materials in subsurface based on their resistivity characteristics. The study results showed that resistivity values at the northern part of the study area ranging from 13.28–282 and 2.09–88.37 ohm-meter, while at the southern part, the resistivity values ranging from 0.32–25.75 ohm-meter. At the northern part of the study area, the resistivity values range of 41.77–88.37 to 67.7–187.79 ohm-meter are peat and mud layers, range of 25.35–88.37 to 55.29–282.28 ohm-meter are sandstone layers, and range of 2.09–34.31 to 13.28–67.78 ohm-meter were clay and sandy clay layers. At the southern part, the range of resistivity values from 5.97–25.75 ohm-meter are peat and mud layers, range of 4.46 - 25.75 ohm-meter are sandstone layers, and range of 0.32–5.97 ohm-meter are clay and sandy clay layers. Sandstone layers are interpreted as confined aquifers, peat and mud layers as unconfined aquifers, and clay and sandy clay layers as non-aquifer or sealing layers. At the northern part of the survey area, the depth of aquifer layers ranging from 10–40 meter and at the southern part the depth of aquifer layers ranging from 20–50 meter.

Jurassic system stratigraphy at the Kamianka village. Part 1. The Kozhulinska formation

Introduction. Despite the active research of Jurassic system deposits of Donbas during the last two centuries the issue of their stratigraphy principles is still debatable. The authors` analysis of the available scientific material revealed a conflict between the normatively established requirement to rely on lithostratigraphy during the sediment research, and the fact that the existing common system relies on biostratigraphy. This contradiction could be solved by the study of lithostratigraphic units of the Donetsk Jurassic deposits. Therefore, the purpose of the article was to research the Kozhulinska formation, establish its composition and describe its main features. The Lower Jurassic deposits from the outcrops near the village of Kamyanka (Izyum district) were studied, since the studyable stratotypes of most of the Jurassic strata, which are well exposed in the studied area, are available in this area. Material and methods. The scientific search was based on the analysis of literature, results of previous drilling and mapping works, as well as own field, laboratory and analytical studies. Clay samples were taken from different stratigraphic levels of the formation and from different lithological varieties. Laboratory research was carried out in two directions: using an electron microscope and a spectrometer to detect the material composition of clays. A palynological analysis of these deposits was also carried out, namely, stratified deposits with the help of microphytofossils, including, for the first time, according to the orthostratigraphic group – dinocysts. Results and discussion. The results showed that the Kozhulinska formation was mainly represented by shaly clays (gray, dark gray, sometimes blue-gray) that contained relatively thin layers of sandstones. The main mass of clays consisted of hydromicas, with an insignificant content of montmorillonite, and an even smaller amount of kaolin. The layers of quartz sandstones cemented with clay material were separately traced along the strike. Paleontological remains in the clays of Kozhulinska formation were rare. Palynological groups are mainly represented by pollen and spores, as well as remains of dinophytes, green algae and acritarchs. According to paleoalgological analysis, the sediments of the Kozhulinska formation are dated for the first time. Stratigraphy. Kozhulinska formation is represented by a fairly uniform layer of montmorillonite clays of hydromica composition. The clays are mainly siltstone, but there are, especially in the lower part, washed clean varieties. It is impossible to divide the formation into sub formation based on lithological features. According to biostratigraphic data, deposits are divided into Plinsbachian, Toarian and Aalenian. Paleogeography. At the end of the Early Jurassic and the beginning of the Middle Jurassic era, the territory of the Kamianska area was a low-lying part of the seabed that was drained during low tides. The land that surrounded it was also low-lying and almost did not supply detrital material, however, there is a regular removal of plant detritus. The source of the terrigenous material was the sedimentary complexes of the Donetsk Ridge. Features of the chemical composition of the world are a significant amount of iron (siderite, ocher, chamosite) and the absence of calcium and magnesium carbonates. At the end of the early Aalene, this unstable basin finally regresses, which leads to the accumulation of a powerful subaerial pack of sandstones, which weigh and border the cores of anticlines - elevated areas of the bottom or small islands.

Synopsis of Upper Tukau Deposits along the Sungai Rait Road, Miri District, Sarawak, Malaysia

The Upper Tukau Formation is exposed along the Sungai Rait Road and can be described as follows: the lowermost member is formed by soft clay and silt, the “Brick Shale”, and contains meandering sandy channels. The clay is thoroughly excavated in the area and used as raw material for bricks. The second and hilltop-forming layer is built by relatively hard amalgamated, and partly channelized sandstone layers, often lined with diagenetically precipitated iron minerals (i.e., pyrite and siderite concretions). The youngest member is characterized by intertidal channels and mud flats. Geochemical studies point to a polymictic source of already recycled sediments, deposited in predominantly oxic, partly suboxic environments, with rapid transitions observed from one milieu to the other. In broad terms, the entire sequence appears to be a shallowing-upwards cycle, leading from subtidal to intertidal realms. The surprising sharp boundaries between clay and sand packages are discussed and may be related to climate cycles.

Characterizations of Middle–Lower Triassic Reservoir Quality and Gas Accumulations in the Northeastern Margin of the Ordos Basin, China

New Middle–Lower Triassic gas reservoirs were found in the eastern margin of the Ordos Basin, which provides new possibilities for gas exploration in the Ordos Basin. However, there are few relevant studies on these gas reservoirs. In this study, we use porosity–permeability measurements, optical light microscopy, scanning electron microscopy, mercury intrusion capillary pressure experiments, core-scale low-field nuclear magnetic resonance experiments, fluid inclusion analysis, burial history reconstruction, organic geochemistry data, and wireline log data to systematically investigate the reservoir quality and accumulation pattern of Middle–Lower Triassic gas reservoirs. The Middle–Lower Triassic sandstones are mostly lithic arkose, with a small amount of feldspathic litharenite. In comparison to the upper Paleozoic tight sandstone, the Middle–Lower Triassic sandstones exhibit a superior pore structure and increased porosity and permeability. A significant portion of the Middle–Lower Triassic sandstones can be referred to as “conventional sandstones” (porosities >12% and permeabilities >1 mD). The source rocks of the Middle–Lower Triassic gas reservoirs are most likely the upper Paleozoic coal and organic-rich mudstone, and the accumulation of natural gas occurred in the Early Jurassic to Late Cretaceous. The gas accumulation in the sandstones of the Middle–Lower Triassic reservoirs was mainly driven by buoyancy. The initiation and extension of vertical faults with small displacement in the northeast margin of the Ordos Basin enabled gas to move through the faults from the deep upper Paleozoic source rocks to the Middle–Lower Triassic reservoirs. Since gas accumulation relies on buoyancy, conventional sandstone layers with good reservoir properties are more likely to reach high gas saturation and are the primary targets for exploration and development.