Mudstone Layers Research Articles

Characterizing the influence of interlayers on the development and distribution of fractures in deep tight sandstones using finite element method

Understanding and interpreting the location, intensity, and aperture of tectonic fractures within deep geological structure are important to hydrocarbon exploration and development. Taken the Paleogene tight sandstones interbedded with mudstone interlayers in Tarim Basin as an example, we explored a novel finite-element-based geomechanical modeling to quantitatively predict fracture development and distribution, and effectively evaluate the vertical penetration of fractures through mudstone layers. This approach is based on the concept that fractures can be inferred from the redistribution of stress and strain with combination of certain failure criterion. We established several sets of sand-shale interbedded geological models and compared simulation results with observed core data. It was found that in tight sandstone reservoirs with few ductile interlayers, the elastic parameter is main factor affecting development of fractures. In contrast, the sand/mud thickness ratio is the most important factor controlling the development of fractures for sandstone interbedded with multiple mudstone layers. Furthermore, the lithology of interface/boundary was shown to strongly control the fracture penetration into mudstone interlayers. When the mudstone interlayer is thin enough, it is easily penetrated by fractures generated in sandstones, and fractures are more likely to develop around lithological interface. Such results were explained by the differences in rock mechanics properties and inconsistent deformation between layers and its derived tensile stress. The modeling results on fracture density and aperture showed good agreement with the core observation and micro imaging interpretation, and the average simulation accuracy of single wells has reached more than 80%.

The role of mechanical stratigraphy on the refraction of strike-slip faults

Abstract. Fault and fracture planes (FFPs) affecting multilayer sequences can be significantly refracted at layer–layer interfaces due to the different mechanical properties of the contiguous layers, such as shear strength, friction coefficient and grain size. Detailed studies of different but coexisting and broadly coeval failure modes (tensile, hybrid and shear) within multilayers deformed in extensional settings have led to infer relatively low confinement and differential stress as the boundary stress conditions at which FFP refraction occurs. Although indeed widely recognized and studied in extensional settings, the details of FFP nucleation, propagation and refraction through multilayers remain not completely understood, partly because of the common lack of geological structures documenting the incipient and intermediate stages of deformation. Here, we present a study on strongly refracted strike-slip FFPs within the mechanically layered turbidites of the Marnoso Arenacea Formation (MAF) of the Italian northern Apennines. The MAF is characterized by the alternation of sandstone (strong) and carbonate mudstone (weak) layers. The studied refracted FFPs formed at the front of the regional-scale NE-verging Palazzuolo anticline and post-date almost any other observed structure except for a set of late extensional faults. The studied faults document coexisting shear and hybrid (tensile–shear) failure modes and, at odds with existing models, we suggest that they initially nucleated as shear fractures (mode III) within the weak layers and, only at a later stage, propagated as dilatant fractures (modes I–II) within the strong layers. The tensile fractures within the strong layers invariably contain blocky calcite infills, which are, on the other hand, almost completely absent along the shear fracture planes deforming the weak layers. Paleostress analysis suggests that the refracted FFPs formed in a NNE–SSW compressional stress field and excludes the possibility that their present geometric attitude results from the rotation through time of faults with an initial different orientation. The relative slip and dilation potential of the observed structures was derived by slip and dilation tendency analysis. Mesoscopic analysis of preserved structures from the incipient and intermediate stages of development and evolution of the refracted FFPs allowed us to propose an evolutionary scheme wherein (a) nucleation of refracted FFPs occurs within weak layers; (b) refraction is primarily controlled by grain size and clay mineral content and variations thereof at layer–layer interfaces but also within individual layers; (c) propagation within strong layers occurs primarily by fluid-assisted development ahead of the FFP tip of a “process zone” defined by a network of hybrid and tensile fractures; (d) the process zone causes the progressive weakening and fragmentation of the affected rock volume to eventually allow the FFPs to propagate through the strong layers; (e) enhanced suitable conditions for the development of tensile and hybrid fractures can be also achieved thanks to the important role played by pressured fluids.

ГЕОЛОГО-ПРОМИСЛОВІ ОСОБЛИВОСТІ РОЗКРИТТЯ ТА ВИПРОБУВАННЯ СЛАБОПРОНИКНИХ ГАЗОНАСИЧЕНИХ ВІДКЛАДІВ

The relevance of research is attributed to the scientific justification of the features of disclosure and testing of low-permeable gas perspective deposits. Under the low-permeable gas perspective deposits in the Carpathian region of Western Ukraine, we consider the rocks of the Cretaceous system (based on previous works). Deposits of the Cretaceous system in the cross section of the Carpathian region occupy more than half the volume of the rock mass within the boundaries of the flysch strata, and the bulk in the depth interval 1.5–3.5 km corresponds to the sediments of the Stryiska suite of the upper section of the Cretaceous system. That is, the object of research is quite significant in its volume. In selecting the perforation intervals and methods of secondary action on the productive layer in wells drilled in regions where low-permeable thin-layered flysch formations (thin interlayering sandstones, siltstones and mudstones) are observed, we propose a method by which the choice of methods of secondary action on the layer, the intervals of mudstone is justified. In this case, the gas perspective deposits (due to the increase in the permeability of the mudstone layers during their perforation, and later also during the hydraulic fracturing of the layer) are combined into one significant in thickness gas-bearing exploitation object, which increases the flow of gas into the wells. Target disclosure of the gas horizons and increase of their gas output to optimal geological and economic parameters require the use of new technologies, similar to those used in the extraction of gas from shale rock. The simulation of the hydraulic fracturing process of the sand-mudstone pack of Upper Cretaceous deposits in the range 3260–3330 m in the well 2-Tarasivka with the Meer MFrac program proved an increase of the fixed crack opening (in this case, from 0.02 to 0.22 cm), which will contribute to significant increase in the conductivity of the crack from 668 to 6000 mD·m, and it is an important factor in the extraction of hydrocarbon gas from low-permeable deposits, which will increase the productivity of wells. And this will contribute to a significant increase in the fuel and energy supply of Ukraine by its own hydrocarbon raw material.

The Spatial Distribution of CBM Systems under the Control of Structure and Sedimentation: The Gujiao Block as an Example

A multilayer coal bed methane system with a complex superimposed relationship developed vertically in the Gujiao block. Taking the cumulative thickness of mudstone between neighbouring coal seams and open faults as object of research, the key strata for the division of different CBM systems was defined. The connection of an open fault for the fluid pressure system and the spatial distribution of the CBM system were analysed. The results showed that the porosity of compact mudstone is extremely low. When the mudstone thickness of the roof and floor increases, the gas content increases. Taking the Lijiashe and Wangzhimao faults as the boundary lines, the fluid pressure gradient between the Shanxi and Taiyuan formations is less than 0.08 MPa /100 m in the northern fracture zone, such as the Zhenchengdi, the Xiqu, the Tunlan, and the northern Dongqu well fields. The vertical gas units of different strata may belong to the same CBM system. The structure is simple in the southern Lijiashe fault. The vertical gas units in the southern Lijiashe fault belong to different CBM systems. Combined with the minimum thickness of the mudstone layer (2 m) and coal seam (0.5 m) standards, the spatial distribution of CBM systems was analysed.

Issues in Engineering Geological Classification of Weathered Sedimentary Rocks: Case Study of Kati Formation, Parit, Perak

Kati Formation consists of alternating layers of sandstone, siltstone and mudstone that results in non-uniform weathering. The presence of surficial features due to weathering such as iron bands, iron recementation, iron staining and lithobionts coating in an uneven distribution on the outcrop causing more complexity to the heterogeneous sedimentary rocks which are also tectonically deformed. The presence of iron recementation in highly weathered rocks causing the rock strength to be higher than the moderately weathered rocks. Thus, several laboratory tests are conducted in this study to characterise the weathered rocks which are uniaxial compressive strength (UCS), point load strength index, slake durability test, jar slake test and Schmidt hammer rebound test. Among the laboratory tests conducted in this study, slake durability index test, Id2 is the ideal test to characterise the rock samples of different weathering grades as it gives a large range of laboratory test results which is 0.3 % to 15.0 % for completely weathered, 22.0 % to 67.0 % for highly weathered, 68.0 % to 83.0 % for moderately weathered and 87.0 to 98.0 % for slightly weathered to fresh rocks. The values obtained are not affected by the uneven distribution of iron oxide present in the rock samples.

Stable isotopes of oxygen and hydrogen in meteoric water during the Cryogenian Period

We measured δ18O and δ2H values of muscovite and carbonate mineral separates from metamorphosed carbonate-bearing mudstone layers in late Tonian to early Cryogenian strata, including Sturtian glacial deposits, which were deposited in a coastal setting at an approximate paleolatitude of 30–35°S and now crop out on Islay and the Garvellach Islands, Scotland. From these values, we calculated δ18O and δ2H values of meteoric water that equilibrated with clay at diagenetic conditions which we infer were reached shortly after deposition (i.e. before the end of the Cryogenian Period) because sediment accumulation was rapid due to fast subsidence at that time. This calculation required removal of the effects of exchange with reservoir rocks, metamorphic volatilization and mixing with metamorphic fluids on δ18O and δ2H values. The values we calculated for meteoric water fall within the 2σ ranges δ18O = −1 to −4‰ and δ2H = 0 to −23‰, respectively. These ranges are similar to present day values at equivalent latitudes. This finding is consistent with sediment accumulation in the Cryogenian Period having occurred in a climate similar to present day (Ice Age) conditions. This conclusion is not at odds with the Snowball Earth hypothesis because one of its predictions is that sediment accumulation occurred as the climate warmed at the end of panglaciation, a prediction supported by sedimentological evidence of multiple glacial advances and retreats in our study area and elsewhere.

Uncertainty analysis of hydraulic fracture height containment in a layered formation

The prediction of hydraulic fracture height growth is of extreme interest in various engineering practices. To fully understand the dependence of the fracturing height in complex underground geological structures, it is crucial to develop theoretical and numerical hydraulic fracturing models to identify the key variables and their interactions to hydraulic fracture height containment. In this work, we adopt a three-dimensional hydro-mechanical coupling cohesive zone finite element model supported by logging data from a practical engineering project. A novel response surface method with Box–Behnken design is introduced to evaluate the statistical significance of tested variables and to forecast the optimal variable combinations for hydraulic fracture height. The key controlling variables are sorted according to their statistical significance from the analysis of variance, and only the minimum lateral stress ratios in mudstone layer and conglomerate layer are highly statistically significant. The minimum lateral in situ stress contrast is the predominant influence of fracture height containment regarding to their significant negative correlation, while the Young’s modulus contrast is probably not an important parameter in terms of direct control of fracture height. Upon optimization, 41 optimized variable combinations were achieved and could be served as references in regulating this engineering practice or even similar events.

Variability of organic carbon content and the retention and release of trichloroethene in the rock matrix of a mudstone aquifer

Contaminants diffusing from fractures into the immobile porosity of the rock matrix are subject to prolonged residence times. Organic contaminants can adsorb onto organic carbonaceous materials in the matrix extending contaminant retention. An investigation of spatial variability of the fraction of organic carbon (foc) is conducted on samples of rock core from seven closely spaced boreholes in a mudstone aquifer contaminated with trichloroethene (TCE). A total of 378 samples were analyzed at depths between 14 and 36 m below land surface. Mudstone units associated with deep water deposition have the largest foc, with a maximum value of 0.0396, and units associated with shallow water deposition have the smallest foc. Even though foc correlates with depositional conditions, foc still varies over more than an order of magnitude in continuous mudstone layers between boreholes, and there is large variability in foc over short distances perpendicular to bedding. Simulations of diffusion and linear equilibrium adsorption of TCE using spatially variable foc in the rock matrix show order of magnitude variability in the adsorbed TCE over short distances in the matrix and residence times extending to hundreds of years following remediation in adjacent fractures. Simulations using average values of foc do not capture the range of TCE mass that can be retained in a rock matrix characterized by spatially variable foc. Bounds on TCE mass within the rock matrix can be obtained by simulations with spatially uniform values of foc equal to the maximum and minimum values of foc for a given mudstone unit.

The genesis of the Dahebian Zn-Pb deposit and associated barite mineralization: Implications for hydrothermal fluid venting events along the Nanhua Basin, South China

The Dahebian stratiform barite deposit, which is situated in the Nanhua Basin, is one of the world’s largest and most important barite deposits. This deposit was formed through sedimentary exhalative processes in the early Cambrian stage. Recently, beneath stratiform barite ore, stratabound Zn-Pb-(Fe-bearing) sulfide mineralization, which is hosted by the Doushantuo Formation, was discovered. Ores that represent Zn-Pb sulfide mineralization are mainly discordant but stratabound. The Doushantuo Formation consists of laminated interbedded layers of micritic dolomite and carbonaceous mudstone, associated with phosphorite and chert. The sulfide ores consist primarily of sphalerite, pyrite and galena. The associated gangue minerals are barite, quartz, minor calcite, as well as trace amounts of apatite and hyalophane. Hyalophane found in the stratiform barite ore is richer in barium but poorer in potassium than that associated with the sulfide ores. This outcome corresponds to ascending hydrothermal fluids that are richer in barium along with sulfide mineralization. Investigation of the ore fluid reveals relatively high temperature (142–368 °C) with scattered salinity (0.53–25.62 wt% NaCl eq.). This finding suggests that fluid cooling and/or mixing with seawater could be the primary mechanisms that prompted the ore formation. Sulfides selected from the zinc-lead sulfide ores and stratiform barite ore have similar sulfur isotopic compositions, with δ34S values ranging from 13.5‰ to 30.0‰; δ34S values of barite from these ores are analogous, with δ34S values ranging from 29.5‰ to 55.1‰. The sulfur isotopic fractionation between minerals (barite and sulfide) and the early Cambrian seawater is ΔBarite-Seawater = 7.8‰ and ΔSulfide-Seawater = −12.1‰, respectively, demonstrating that the sulfur in the Dahebian zinc-lead deposit and the stratiform barite deposit were derived from the early Cambrian stratified seawater column. Lead isotopic compositions of sulfide ores indicate that these ore metals are mainly sourced from rocks of the Doushantuo Formation and the underlying fold basement.Therefore, the Dahebian zinc-lead deposit and stratiform barite deposit probably arose from Selwyn-Type sedimentary exhalative hydrothermal fluids. Zn-Pb sulfides were mineralized in the feeder zone of these hydrothermal fluids, and the stratiform barite ore deposits represent the upper sedimentary-exhalative mineralization. This finding points to intensive Zn-Pb-Fe-Ba hydrothermal fluid venting into the Nanhua Basin and to relatively oxic deep ocean waters during the early Cambrian.

Early Triassic Griesbachian microbial mounds in the Upper Yangtze Region, southwest China: Implications for biotic recovery from the latest Permian mass extinction

Early Triassic microbialites are distributed widely in the shallow marine facies of the Tethys Region, especially in the carbonate platform where they were deposited immediately after the latest Permian mass extinction (LPME). Ten Griesbachian domed microbial mounds were found in an outcrop of the uppermost first member of the Feixianguan (FXG) Formation at Baimiaozi, which is located in Beibei in the Upper Yangtze Region of southwest China. Field investigations and thin-sections analyses indicated that oolitic limestone, bioclastic limestone, microbialite, marl, and mudstone deposits are present in the first and second members of the FXG Formation, among which the thickness of the microbial mound above the massive oolitic limestone at the carbonate platform was approximately 3–4 m. Three facies were identified at the microbial mounds, namely, a mound base, mound body, and mound cap. Irregular laminae were found in the brown-colored microbialite of the mound base. The main mound body, which is composed of gray microbialite, is 1.0–1.5 m high and 2.0–3.0 m in diameter at the base. Dark gray grainstone found in the mound cap is covered by a thin layer of shelly limestone containing intact fossils of bivalves and gastropods, which are indicative of a simple ecosystem consisting of microbes and primary consumers. Brown-colored mudstone and marl layers of the second member of the FXG Formation overlie the microbialite, and this indicates that growth of the microbial mounds was halted by a sudden increase of terrestrial inputs and rapid transgression. Early Griesbachian conodonts of Hindeodus parvus? were identified from the mound limestone and the overlying strata of the second member of the FXG Formation, which is suggestive of the presence of a microbialite-dominated ocean in the Upper Yangtze Region during a certain interval after the LPME.

Rare earth element and yttrium content of coal in the Banko coalfield, South Sumatra Basin, Indonesia: Contributions from tonstein layers

The South Sumatra Basin is one of the most prolific coal basins in Indonesia. Most of the coal from this area is used as thermal coal. Seredin and Dai (2012) explained the importance and significance of coal deposits as potential sources for rare earth elements and yttrium (REY); the concentrations of these elements in coal are similar to their abundances in conventional REY deposits. Production of the REY in coal deposit is mainly from the coal's fly/bottom ash. Examination of coal with high REY concentrations is becoming a very important issue. The study area is located in the Banko coalfield, Sumatra, Indonesia. This area is one of the best candidates for studying REY in coal because there are tuffaceous mudstone layers (tonsteins) reported by previous researchers that may have enriched the coal in REY. For this study, coal and non-coal beds from the same stratigraphic interval were sampled and analyzed using thin and polished section, X-ray diffraction (XRD), inductively coupled plasma atomic emission spectroscopy (ICP-AES), and inductively coupled plasma mass spectrometry (ICP-MS). Special attention was given to coals near the tonstein layers. Based on the data collected, it was concluded that all the tonsteins studied are of volcanic origin and are silicic and alkali tonsteins. The elements Al and Si are higher in coal samples collected just beneath the tonstein layers and the highest REY concentration found was in a sample beneath a silicic tonstein layer. These data suggest that there is an opportunity to extract REY from coal's fly/bottom ash because the REY content of the ash is as much as 10 times higher than the REY content of the coal feed to power plants.

Engineering geological study of the Mai Khola Hyroelectric Project, Ilam, eastern Nepal

The Mai Khola Hydroelectric Project, a run-of-river scheme, has a capacity 15.6 MW. It has design discharge of 16 m3/s, design net head of 112.71 m and includes 2192 m long inverted-D shaped headrace tunnel with 4.3 m diameter, concrete dam of 10.6 m height and semi-surface powerhouse. The project area consists of rocks of the Middle Siwalik Subgroup, comprising of sandstone, siltstone and mudstone, inter bedded frequently. Sandstone is predominant in head works area, headrace tunnel and is completely absent in a surge tank, and penstock alignment. Siltstone alternating with thin layer of mudstone is predominant in powerhouse area. The headrace tunnel outlet portal and surge shaft lie on the hill slope characterized by colluvial deposits. The penstock alignment passes through highly weathered siltstone and mudstone. The semi-surface powerhouse and the tailrace canal lie on the lower alluvial terrace. The Main Boundary Thrust (MBT) is the major structure observed at about 90 m upstream from the weir axis. The average Q-value of rock mass along the headrace tunnel surface mapping was 0.062–1.33 and after excavation the value was 0.004–0.23. An extremely poor to poor relation was observed between the rock mass class on surface mapping and exceptionally poor to very poor on excavation. Analysing the results of the surface and underground study of the rock mass, the excess support is required during construction.

Mixed Carbonate-Siliciclastic Deposits in a Channel Complex in the Northern South China Sea

New high-resolution 3D seismic data image a submarine channel complex in the northern slope of the South China Sea. The channel complex stretches hundreds of kilometers across the slope and flows into the deepsea from the siliciclastic shelf margin, linking neritic environment to the pelagic plain. The evolution of the channel complex developed two sedimentary stages, stage I (19.1-18.5 Ma) and stage II (18.5-17.5 Ma), separated by erosional surfaces. In the first stage, the complex was filled with pure siliciclastic sediments, forming thick-massive sandstone intercalated by thin layers of mudstone. During the stage II, the channel complex was deposited five carbonate-siliciclastic cycles. The unexpected channel-fill carbonate deposits present allochthonous characteristics, suggesting the siliciclastic channel was surprisingly used to transport carbonate sediment from the adjacent neritic carbonate platform. By analyzing the periodical carbonate sedimentary process in the siliciclastic channel complex, we infer that it was related to the in situ carbonate production of the neritic carbonate platform and was most likely to be controlled by the relative sea-level changes. Unlike line-source carbonate slope aprons or small-sized carbonate channels, the large-sized siliciclastic channel complex links directly neritic carbonate platform to deepwater basin and can transport large volumes of neritic carbonates to the pelagic environment in a short period. The new findings help to estimate the contributions of neritic siliciclastic shelf and carbonate platform to deepwater slope more accurately. This study suggests that channel systems are more complex than expected and have significant implications on the conceptual models describing the deepwater sedimentary theory.

Experimental and DEM investigations of temperature effect on pure and interbedded rock salt

Rock salt is considered as a favorable host rock material for natural gas storage due to its low permeability, fast self-healing ability and unique creep behavior. Characterizing engineering properties of rock salt especially interbedded salt under complicated geological storage conditions associated with temperature variations remains challenging. This research integrates physical and numerical experiments to investigate the temperature effect on the engineering behavior of pure and interbedded salt. Thermo-mechanical coupled triaxial compression tests are carried out on both pure (Ash-grey Rock Salt and Charcoal-grey Rock Salt) and impure (Rock Salt with Mudstone interbedded) rock salt specimens. To reveal the underlying mechanism of thermal-induced deformation and cracking of salt at grain scale, we configure a discrete element model and incorporate smooth-joint contacts to capture the interfacial behavior in the bedded salt. Experimental results indicate that the compressive strength and failure mode of rock salt is highly susceptible to temperature. The increasing temperature enhances the ductility of salt whereas lowers its peak compressive strength. Numerical results provide a micromechanical explanation that the attenuated compressive strength of salt under high temperature is attributed to the evolution of micro-cracks. The presence of the mudstone layer increases the overall material strength, confines the crack propagation orientations, and therefore limits the transverse deformation of the bedded salt. This study is expected to bring new insights into the micro-mechanical study of bedded salt and improve the long-term assessment of geological storage facilities.

Monitoring the Vertical Distribution of Rainfall‐Induced Strain Changes in a Landslide Measured by Distributed Fiber Optic Sensing With Rayleigh Backscattering

AbstractDistributed fiber optic sensing with Rayleigh backscattering, which has been recognized as a novel technique for measuring differences in temperature or strain, was adopted in a borehole to a depth of 16 m in an actual landslide to detect a vertical profile of strain changes. Strain changes were measured every 6 hr from 19 June 2017 to 18 October 2017 with a spatial resolution of 10 cm and strain resolution of 1.87 με. The measurements provided a clear‐cut vertical profile of the strain changes caused by rainfalls that cannot be detected by conventional methods. The results show that there are two types of deformation in the landslide mass: (1) sliding at the boundary between tuff and mudstone and (2) creep in mudstone layers. Activation of deeper sections of the landslide by heavy rainfalls has also been detected.

The Upper Oligocene of Montgat (Catalan Coastal Ranges, Spain) : new age constrains to the western Mediterranean Basin opening.

The Oligocene deposits of Montgat are integrated in a small outcrop made up of Cenozoic and Mesozoic rocks located in the Garraf-Montnegre horst, close to the major Barcelona fault. The Oligocene of Montgat consists of detrital sediments of continental origin mainly deposited in alluvial fan environments; these deposits are folded and affected by thrusts and strike-slip faults. They can be divided in two lithostratigraphic units separated by a minor southwest-directed thrust: (i) the Turo de Montgat Unit composed of litharenites and lithorudites with high contents of quartz, feldspar, plutonic and limestone rock fragments; and (ii) the Pla de la Concordia Unit composed of calcilitharenites and calcilithorudites with high contents of dolosparite and dolomicrite rock fragments. The petrological composition of both units indicates that sediments were derived from the erosion of Triassic (Buntsandstein, Muschelkalk and Keuper facies), Jurassic and Lower Cretaceous rocks (Barremian to Aptian in age). Stratigraphic and petrological data suggest that these units correspond to two coalescent alluvial fans with a source area located northwestwards in the adjoining Collserola and Montnegre inner areas. Micromammal fossils (Archaeomys sp.) found in a mudstone layer of the Pla de la Concordia Unit assign a Chattian age (Late Oligocene) to the studied materials. Thus, the Montgat deposits are the youngest dated deposits affected by the contractional deformation that led to the development of the Catalan Intraplate Chain. Taking into account that the oldest syn-rift deposits in the Catalan Coastal Ranges are Aquitanian in age, this allows to precise that the change from a compressive to an extensional regime in this area took place during latest Oligocene-earliest Aquitanian times. This age indicates that the onset of crustal extension related to the opening of the western Mediterranean Basin started in southern France during latest Eocene-early Oligocene and propagated southwestward, affecting the Catalan Coastal Ranges and the northeastern part of the Valencia trough during the latest Chattian-earliest Aquitanian times.

Evolution of formation pressure and accumulation of natural gas in the Upper Palaeozoic, eastern‐central Ordos Basin, Central China

The exploration achievements in the eastern‐central Ordos Basin have attracted tremendous attention in recent years. However, only a few studies were carried out on the relationship between formation pressure and distribution of natural gas in this area. The formation pressure of 293 wells, logging data of 154 wells, and fluid inclusion samples of 53 wells were analysed in this paper, and the results show that the mudstones from Permian Upper Shihezi Formation to Permian Taiyuan Formation are mostly undercompacted. Four gas‐bearing layers with significant differences in pressure profile can be further divided into Permian negative pressure system and Carboniferous normal pressure system. The distribution of current formation pressure is high in the east and north areas but low in the west and south areas. The sedimentary facies and distribution of formation pressure in Permian have a good consistency. The vertical development (zonation) of gas reservoirs is controlled by the undercompacted mudstone layers, and the reservoirs can be found in the “low pressure” areas of negative pressure strata in the Permian and “high pressure” regions of normal pressure strata in the Carboniferous. The pressure evolution and accumulation process can be divided into 3 stages: hydrocarbon‐generation supercharging accumulation during Late Jurassic—end of Early Cretaceous, sandstone resilience discharging accumulation during end of Early Cretaceous—Paleogene, and adjusting accumulation during Paleogene—Quaternary. The discovery of the relationship between formation pressure and gas reservoirs is significant to the future natural gas exploration in this area.

A study on a instability slope in Taiwan subjected to rainfalls

After the long-term monitoring on the Chaishan area in Taiwan from 2005 to 2012 by Kaohsiung City Government, the obtained results showed that annual lateral displacements in the region are about 7-8cm to the Taiwan Strait. The geological surface profiles of Chaishan area are in sequence weathered limestone, clay layer, limestone and mudstone layer, respectively. Thus the frictional resistance between weathered soils and rock layer could decrease after infiltration of rainwater due to impervious to water of the lowest mudstone layer. Typhoon invades often Taiwan each year, resulting in rainfall infiltration and rising groundwater level, as well as increased pore water pressure within the soil mass, causing the earth movements in some parts of Chaishan, especially in the Temple A (Shan Hai Temple) accompanied with cracking phenomenon. In this paper, limit equilibrium (LE) and finite element method (FEM) are used for slope analysis, in which the slope is considered as unsaturated soil. Results showed groundwater amounts are easy to accumulate and increasing pore water pressure give resulting in decreased safety factor. Both of groundwater level and rain durations were also considered in this study.

ANALISIS POTENSI AIR TANAH BERDASARKAN PENGUKURAN GEOLISTRIK 2D DI DAERAH KAPUAS, PROVINSI KALIMANTAN TENGAH

Groundwater is water that occurs naturally in the subsurface, and is contained in the aquifer. Groundwater is the water source of the most widely used by the public for purposes of clean water. To determine the condition of the aquifer configuration one of the methods that can be used is the 2D electrical resistivity. Exploration drilling is one effective way to determine directly the presence of groundwater, but to determine the location of the drilling necessary geophysical approach to describe subsurface conditions prior to drilling. 2D electrical resistivity method is a geophysical method that utilizes dynamic electrical properties of the media path. 2D electrical resistivity measurements have been carried out in Kapuas, Central Kalimantan using Wenner configuration, the distance between the electrodes 10 meters, penetration depth reaches 80 meters, and and measuring some 21 track. Water potential zones were identified based on having resistivity range 15-25 ohm.m, contained in the sandstone layer, which is covered in a layer of impermeable mudstone layers. Location that has a high potential for groundwater recommended drilling done on the track P-07 and P-13, with an estimated depth of the aquifer between 25 meters and 30 meters. The highest groundwater potential lies between the drilling data PH-23 and PH-24, with the resistivity at that point reaches 300 ohm.m. Keywords : Groundwater, aquifer, sandstone, 2D electrical resistivity, resistivity, Kapuas

A taphofacies model for coquina sedimentation in lakes (Lower Cretaceous, Morro do Chaves Formation, NE Brazil)

The Barremian–Aptian Morro do Chaves Formation, NE Brazil, was deposited in lacustrine environments during the rifting between the African and South American plates. This formation contains shell beds (coquinas) interbedded with mudstone and sandstone deposits. Shell concentrations may be used to identify paleontological and paleoenvironmental data that allow to interpret the depositional energy, paleoenvironmental conditions and the accumulation history of the formation. The following features of the shell beds were analyzed: (i) taphonomic aspects (sorting, fragmentation, abrasion, rounding and orientation of the shells), (ii) sedimentological aspects (grain packing, abundance of shells, type of matrix and sedimentary structures) and (iii) stratigraphic aspects (bed dimension and shape, bounding surfaces). Two siliciclastic lithofacies (clay mudstones with mud cracks and laminated sandy mudstones) and four carbonate lithofacies (wackestones, packstones, grainstones and rudstones) are described based on the textural features. The carbonate facies were successively divided into six taphofacies (T1 to T6) with the help of an artificial neural Self-Organizing Map (SOM) and evaluated by non-metric multidimensional scaling (NMDS), based on taphonomic features of the carbonate lithofacies. Taphofacies and lithofacies formed at lacustrine margins. Clay mudstone layers with mud cracks and without shells fragments are interpreted as distal parts of alluvial fans or fluvial distributary systems at the edges of the lake system. Rudstones and packstones (taphofacies T1, T2, and T3) with fragmented and oriented shells, wave ripples and horizontal lamination correspond to upper shoreface deposits. Rudstones, packstones and wackestones (taphofacies T4, T5 and T6) that display shells with low signs of fragmentation and abrasion and laminated clay mudstone are interpreted as lower shoreface deposits. A depositional model of the coquinas of the Morro do Chaves Formation is proposed based on the characteristics and distribution of the taphofacies. Moreover, the use of an embedded Markov Chain enabled the relation of the different upper and lower shorefaces in a preferential sequence of deposits of shallowing- and deepening-upward cycles.