Medium Sandstone Research Articles

Application of back-propagation neural networks in the distribution of oil sand reservoirs\u2014a case study from the western slope of the Songliao Basin, China

High-quality hydrocarbon source rocks are present in the upper Cretaceous layer in the western slope of the Songliao Basin. Oil and gas have accumulated in these rocks at the shallow edge of the basin, which has led to the formation of oil sand resources. This study uses the back-propagation (BP) neural network method to predict the distribution of oil sand reservoirs and is the first study of its kind in China. First, based on the basic data collected by core sample, well log and geochemical analyses, and the reasonable selection of samples, the cores are divided into mudstone, siltstone, fine sandstone, medium sandstone, and sand, according to lithology. Second, a three-layer BP neural network model is constructed with two hidden S-type layers and one linear output layer. Third, through a comparison of the effect of different numbers of training sessions of the sample data, we demonstrate that the accuracy of the model can be increased to 90% after training the network 100,000 times. Then, the log-derived data of rocks with unknown lithologies are input into the neural network to predict whether they contain oil sands. We show that the BP neural network method can predict the distribution of oil sand reservoirs in the target horizon of the study area, and the results are consistent with research results on the corresponding sand reservoirs and sedimentary facies. Thus, we conclude that it is feasible to use the BP neural network method to predict the distribution of oil sand reservoirs.

Mixed carbonate-siliciclastic sequence development influenced by astronomical forcing on a distal foreland, Miocene Dam Formation, eastern Saudi Arabia

Given increasing importance of tertiary reservoirs in the Middle East, the Miocene (Upper Aquitanian to mid-Langhian) Dam Formation in eastern Saudi Arabia was cored along a 40 km transect, to develop a high-resolution sequence framework, and examine the role played by astronomical forcing, moderate glacio-eustasy (20- to 40-m sea-level changes), and tectonic deformation in this distal foreland setting. The Dam Formation (up to 80 m thick) contains updip siliciclastics which interfinger downdip with marine carbonates. Siliciclastic facies include paleosol-capped, red mudrock (mudflat), green and gray mudrock (near-shore siliciclastic lagoon), rare mud clast–bearing sandstone (fluvial), and massive very fine to medium sandstone (terrestrial sand sheets, rare tidal channels). Carbonate facies include brecciated carbonates (paleosols), lime/clay-clast quartz wackestone (lag gravel), carbonate laminites and microbial heads and mounds (tidal flat and shallow subtidal), argillaceous quartzose marl (near-shore lagoon), peloid mudstone-wackestone (restricted lagoon), variably quartzose skeletal peloid mudstone to packstone (open lagoon), oolitic grainstone (hypersaline beach, tidal channel), mollusk packstone (hypersaline lagoon border), and foram-mollusk packstone-grainstone (seagrass meadows). The Dam Formation contains eight sequences, each of which contains 2 to 4 parasequence sets or individual parasequences. Sequences and individual parasequences are capped by paleosols and/or erosion surfaces, and microbial laminite caps are rare. Bases of parasequences locally are veneered with ravinement gravel lags or siliciclastics beneath shallowing-upward subtidal carbonates, and lack intercalated deeper water facies. Statistical analysis of core-gamma ray (API) vs depth (m) shows that accumulation rates were ~ 3 to 4 cm/kyr, considerably faster than long-term accommodation rates (~ 1.2 cm/kyr), indicating deposition occurred for only 40% of the time, the remainder of the time being non-depositional during many short-term lowered sea levels, evidenced by the coeval oceanic δ18O curve. Spectral analysis of the core-gamma ray logs show astronomical forcing within the eccentricity, obliquity, and precessional bands. Spectral analysis indicates that the eight Dam sequences (average 10 m thick, range < 5 to 18 m) are a mix of long-term obliquity (~ 1.2 Myr) and long-term eccentricity (400 k.y.) cycles. The 3- to 4.4-m sets/parasequences are short term (~ 100 kyr) cycles, and the 0.7- to 1.25-m parasequences are obliquity and precessional cycles. Given the updip position of the study area, repeated glacio-estutatic sea-level changes caused repeated shallow flooding (< 20 m) and emergence, which generated paleosol-bounded or erosionally capped sequences and parasequences. Although the distal foreland was undergoing synsedimentary deformation and slow but locally variable subsidence, which influenced variable thickness of units, astronomically driven glacio-eustasy was the dominant influence on timing of sequence and parasequence development.

The strength characteristics and progressive failure mechanism of soft rock-coal combination samples with consideration given to interface effects

The coal-measure rock masses of China's Ordos Basin are mainly composed of typical soft rock with low strength, which bear loads together with the coal seams during coal mining activities. Accurately determining the mechanical behaviors of combination soft rock and coal masses can provide a basis for underground coal engineering designs. First, uniaxial compression tests were conducted on medium sandstone samples, coal samples, and coal-rock combination samples. It was observed that the peak axial strain, uniaxial compressive strength, stress-strain curve shape, and elastic modulus of the combination samples were closer to those of the coal samples, indicating that the mechanical characteristics of soft rock-coal combination samples were mainly affected by the coal. Then, using the static equilibrium and deformation continuous conditions, along with the Griffith strength criterion, the ultimate compressive strength of materials at different positions in a combination model was analyzed. The results revealed that the interface constraint effects had reduced the strength of the sandstone near the interface and improved that of the coal near the interface. The interface constraint effects were verified by comparing the generalized Poisson's ratio of materials at different positions with those of the standard coal and sandstone samples. Finally, based on the final failure modes, and the temporal characteristics of the acoustic emission (AE) count rate and peak frequency, the progressive failure mechanism of the soft rock-coal combination samples was studied. This failure process was found to be characterized by the synergistic failure of the material and structure. Furthermore, the effect of coal-rock height ratio on the strength of materials at different positions was discussed, which interpreted the experimental results observed in the previous related studies, namely, with the increase in the rock-coal height ratio, the strength of the combination samples had increased, and the impact tendency had been significantly enhanced.

Numerical Simulation and Theoretical Study of the Effect of Fracturing Strata on Multi-Stage Fracturing in the Roof of an Outburst Coal Seam

ABSTRACT Multi-stage fracturing is one of the key techniques that use boreholes instead of tunnels and can effectively transform a coalbed methane (CBM) reservoir and increase its permeability. However, due to the lack of systematic and in-depth research on the use of multi-stage fracturing for gas drainage, the development of multi-stage fracturing is slow, and there are many problems, which to some extent restrict the application of multi-stage fracturing in low permeable outburst coal seams. In this study, the No. 21 coal seam in the Zhongmacun Coal Mine was taken as the research object, and the lithologies of the roof and floor of the No. 21 coal seam and the in-situ stress were tested. Then, three types of fracturing strata were numerically simulated to study the effect of fracturing strata on the fracture initiation, propagation and morphological characteristics induced by multi-stage fracturing in the roof of an outburst coal seam. The influence of the brittleness index of fractured strata on the formation of a fracture network was analyzed theoretically. The results indicate that the sequence in decreasing order of the fracturing range and failure mode is the medium sandstone model, the sandy mudstone model, and the mudstone model. It is useful to form a fracture network when the fractured strata in the roof of the outburst coal seam is a medium sandstone or siltstone. By comparing the brittleness index of the roof and floor lithologies of the No.21 coal seam in the Zhongmacun Coal Mine, the proper strata for the multi-stage fracturing in the roof of the outburst coal seam was selected. The results of this study provide a theoretical basis and data support for selecting proper strata for multi-stage fracturing in the roof of an outburst coal seam.

Attenuation characteristics of ground penetrating radar electromagnetic wave in aeration zone

Amplitude, an important electromagnetic wave attribute, is considered as a useful parameter to describe the energy attenuation of electromagnetic wave. In this study, SIR-3000 Radar Equipment was employed to investigate the amplitude of electromagnetic wave in medium sandstone with different water content and depth. Results suggest that, with same water content, the decreasing of electromagnetic wave energy with increasing depth can be described by maximum peak amplitude, while, the decreasing of electromagnetic wave energy with increasing water content at certain depth can be described by average amplitude. In addition, energy of electromagnetic wave is negatively correlated to the dielectric constant because of the positive correlation between water content and dielectric constant. This study can provide significant information to the water measurement in aeration zone.

A grain size auto-classification of Baikouquan Formation, Mahu Depression, Junggar Basin, China

Abstract The borehole electrical imaging log offers general visual borehole wall, leaving no doubt that the conductivity contrast is sufficient to obtain a qualitative gain-size distribution of rocks. In this study, an automatic grain-size classification method is proposed using gray values of borehole electrical images from Baikouquan Formation in Mahu Depression. The first stage is comparing electrical images with cores. Gravels, sands, silts and clays are all discovered in the cores. The gravels are “mottled” in electrical images, and the bigger the spots, the coarser the gravels. The images of sands are homogeneous bright colored, and the coarser the sandy grains, the brighter the images. The electrical images of silts and clays are homogeneous brown and dark-brown colored. The second stage is auto-discriminating four categories of grain sizes roughly using averages and variances of gray values. The variances of gray values of gravels are high, whereas those of sands are medium. The gray averages of silts are between 160 and 220, whereas those of clays are larger than 200. The third stage is auto-classifying three kinds of gravels or sands finely using frequency distribution of gray values. The gray values of frequency peaks of cobbles are less than 50 and frequencies are larger than 15%, whereas those of pebbles are less than 50 or larger than 200 and frequencies are between 10% and 20%; almost gray frequencies of granules are less than 10%. The dominated gray values of coarse sandstones, medium sandstones and fine sandstones are less than 50, between 50 and 160 and ranged from 160 to 240, respectively. The proposed method is demonstrated to be useful and fast to auto-classify grain size of various rocks in conglomeratic environments.

Impact of rock type on the pore structures and physical properties within a tight sandstone reservoir in the Ordos Basin, NW China

The pore–throat systems and physical properties of tight sandstone reservoirs are complex, and deposition is thought to be a fundamental control for them. In this study, the impacts of the full ranges of rock types (from pebbly coarse sandstone to fine sandstone) on the pore structures and physical properties of the Permian tight sandstone reservoir in the eastern Ordos Basin were investigated comprehensively through a series of experiments including conventional physical testing, thin-section analysis, scanning electron microscopy, nuclear magnetic resonance analysis and high-pressure mercury injection tests. The results showed that the coarser-grained sandstones tend to have higher feldspar content and lower percentage of cements, leading to strong dissolution, weak cementation and improved porosity and permeability. The medium sandstone has the highest level of quartz and the lowest average content of feldspar, resulting in strong heterogeneity of physical properties. Only those medium sandstone reservoirs with relatively high content of feldspars have better physical properties. Additionally, the coarser-grained sandstones contain relatively large dissolution pores (nearly 200 μm), whereas the finer-grained sandstones have more intercrystalline pores with a relatively more homogeneous pore structure. The pebbly coarse sandstone and coarse sandstone reservoirs are favorable targets with best physical properties.

Recognition of turbidite stages in the Massapê oil field, Recôncavo Basin - Brazil, using well logs

The integration of rock and well logs data is fundamental for the interpretation of depositional systems and sequences, a fundamental support for E&P activities. This approach was applied on the low permeability turbidite sandstones and slurries of the Maracangalha Formation, in the Massapê Oil/Gas Field, Recôncavo Basin. Well-log interpretation allowed the recognition of 23 turbidite stages, each of which records a specific period of growth of a fan system. These stages are composed of four main facies, all observed in each one of these stages: (a) fine to medium sandstones, with porosities greater than 9%, being the best reservoirs; (b) silty to very fine sandstone slurry facies, that corresponds to very muddy reservoirs with porosities less than 9%, forming important permeability barriers; (c) siltstones and (d) shales. Turbidite stages appear in the Gamma Rays logs (GR) with funnel, block and serrated log motif patterns, corresponding to dominantly thinning and fining upward signatures, recognized in several wells in the Massapê Field. The detailed correlation of these well-logs showed that these stages are present in the whole field. The mapping of high resolution turbiditic stages, considering the correlation of their four facies, will allow a better location of wells, resulting in a more efficient exploration and production development, capable of increasing the recovery of important oil reserves that still exist in the area, optimizing costs and giving greater robustness to E&P processes.

Effect of burial history on diagenetic and reservoir-forming process of the Oligocene sandstone in Xihu sag, East China Sea Basin

Abstract Porosity evolution history, hydrocarbon charging periods, diagenetic and reservoir-forming characteristics of low-permeability and tight sandstone are presented in this study. The interpretation is based on information including thin section, scanning electron microscope, homogenization temperature of fluid inclusion, bulk rock X-ray diffraction analysis, clay X-ray diffraction analysis and stable isotopic composition. These sandstones are located in the Oligocene E3h Formation in the Xihu sag, East China Sea Basin, East China. Five diagenetic events in the H3, H4 and H5 members were determined, including connate alkaline pore water of the Oligocene sandstone, meteoric water leaching in late Miocene, organic acids and CO2 acidic fluids in early-middle Miocene, reservoir temperature decrease induced by tectonic uplift, and alkaline diagenetic environment from late Miocene to present day. Porosity evolution history of different grain-size lithology were evaluated by basin modelling and test data. The medium sandstone and coarse sandstone of the H3 member and medium sandstone of the H4 member not reached the tight state in the geologic time. The fine sandstone of the H4 member entered the tight state at 12 Ma, suffered acidic fluids to improve the tight state in late stage, and entered the tight state again since 6.5 Ma. The medium sandstone, fine sandstone and siltstone in the H5 member entered the tight state at 13.5Ma, 16Ma and 19Ma, respectively. Three hydrocarbon charging periods were determined, including 15.5 Ma-10.4 Ma in the middle Miocene, 9.6 Ma-7.0 Ma in the late Miocene and 6.2 Ma-0.0 Ma from the late Miocene to present day. Three diagenetic and reservoir-forming patterns were proposed, including accumulation without tightness, accumulation after tightness-improving reservoir quality with acidic fluid-accumulation while tightness, accumulation after tightness. The burial history cannot be ignored in the diagenetic events, porosity evolution history and hydrocarbon charging periods of similar low-permeability and tight sandstone in sedimentary basins.

Carboniferous glaciotectonized sediments in the southernmost Paraná Basin, Brazil: Ice marginal dynamics and paleoclimate indicators

Carboniferous glaciogenic strata (Itararé Gp.) in the southernmost Paraná Basin, Brazil exhibit soft-sediment deformation features previously interpreted as glaciotectonism. These sediments were studied in detail to confirm that they were deformed by ice and to assess the nature of the glaciation, depositional environments, and paleoclimate in this region during the Carboniferous. Five outcrops were described along a railroad transect that contains a conglomerate and diamictite facies with striated and faceted clasts, a medium sandstone facies, a fine grained silt/clay rhythmite and mudstone facies with dropstones and diamictite pellets, a sandy clinoform facies, and a folded sandstone with interbedded mudstone facies. The depositional environment for these sediments is interpreted as an outwash fan and fan delta from an ice-proximal, transitional terrestrial-to-estuarine setting. Rb/K values from the rhythmites reflect a transition from a freshwater to brackish environment and the Chemical Index of Alteration (CIA) of the rhythmites ranges from 65 to 73, reflecting a relative increase in the degree of chemical weathering through time. Deformation features include widespread folding, thrust faults, hydrofractures, décollement surfaces, and piggyback fold-thrust complexes. The deformation is interpreted as evidence of a push-moraine complex formed by at least two complete ice advance/retreat cycles. The occurrence of décollement surfaces, plastically deformed proglacial sediments, and hydrofractures indicate a dynamic, warm-based or polythermal glacier. Abundant outwash sediments, rhythmites with dropstones, and the shift to average CIA values all support a temperate paleoclimate. Deformation structures indicate a NW direction of ice shove that is in agreement with the regional-scale hypothesis that a NNW flowing lobe extended out of Uruguay during the Carboniferous and terminated in the southernmost Paraná Basin. This study demonstrates that late Paleozoic glaciation in this region was more dynamic than previously understood, with high frequency fluctuations in ice marginal positions.

Aquifer Area Investigation Using Resistivity Method in Cikopomayak, West Java, Indonesia

Administratively, the research area is located in around Cikopomayak Village, Jasinga, Bogor, West Java,Indonesia. Based on the Hydrogeological and Groundwater Basin Map, this research area was classied to non-potential groundwater basin. In addition, when the summer comes, the lack of water will be happened around the area. Consequently, the water reserve wasn't enough to fulll the necessity of freshwater. Thus, the research about identifying aquifer location was needed. This research aims to determine the depth and thickness of aquifer for the alternative ways to overcome the problem, particularly, for the Cikopomayak Village. Theresearch used the geophysical method by modeling the several data such as Geoelectric 1-D to know the aquifer location based on the section of medium resistivity value. Then, the geoelectric data was correlated to the borehole drilling data that is Gamma Ray (GR) Log, Spontaneous Potential (SP) Log, Resistivity Log, and the cutting data. Commonly, the most suitable lithology for an aquifer is sandstone because of its good characteristic of permeability and porosity. Logging data result will show the small value of GR log, SP Log lean-ing to the right, and low-value resistivity for indicating the aquifer zone. Based on geoelectric data, the research zone has 3 groups of resistivity value and lithology that are shaly clay-stone and siltstone with low-value resistivity (0-50 ohm.m), sandy claystone, medium sandstone, and coarse sandstone with medium value resistivity (50-150 ohm.m), fine sand-stone, sandy claystone, and andesite with high-value resistivity (&gt; 150ohm.m). The aquifer system in the research area was classied into 3 groups that are aquifer, aquitard, andaquiqlud.

A new integrated workflow for improving permeability estimation in a highly heterogeneous reservoir of Sawan Gas Field from well logs data

The Sawan Gas Field is one of the most promising gas fields in Pakistan with a cumulative production of 850 BCF. The repetition of coarse sandstone, medium sandstone, and sandstone-shale intercalation in the production zone cause extreme heterogeneity. Consequently permeability varies enormously (from 0.01 mD to more than 1000 mD). Nevertheless, verifiable and accurate estimation of permeability in the production zone with no previous laboratory-derived data is considered a challenging task. In this study, we explore a methodology for improving permeability estimation based on the combination of neural network (NN), multiple variable regression, and classification of data mining using conventional well logs (GR, LLD, RHOB, DT, and NPHI). The approach works in two-steps. First, we compute permeability using empirical, statistical, and virtual techniques on a fully cored well in order to select the specialized regression model that will be responsible for building data partitioning and classification of data mining task. To improve the efficiency of the classifier model, we combine the NN with multiple variable regression for predicting accurate permeability values. In step-2, the proposed regression model was employed to determine the final permeability values from data partitioning and classification of data mining. The final result of this study revealed that the proposed approach which combines NN, multiple variable regression, and classification of data mining provide more uniform, accurate, and qualitative estimation of permeability compared with stand-alone generic or global regression model. Also electrofacies (EFs) classification was conducted over the model to validate the proposed approach.

The São Jerônimo da Serra Site, Rio do Rasto Formation (Middle/Upper Permian), Paraná Basin, Brazil: faciological and taphonomic context

ABSTRACT: In this contribution, we present an analysis of the main faciological features of the Sao Jeronimo da Serra Site, related to the Morro Pelado Member of Rio do Rasto Formation (Middle/Upper Permian), from Parana State, Brazil, integrated to its taphonomic context. The outcrop, rich in vertebrates and bivalves remains, presents a succession of red siltstones bedded with fine to medium sandstones in which three architectural elements were observed: SF (sheetflood deposits), FF (fine of flood plain) and CS (crevasse). These data, allied to the taphonomic signatures of fossils assigned to eight levels, allowed the identification of three taphofacies. The taphofacies I is characterized by preserved bones in floodplain facies, with different articulation grades and absence of any sign of transport. The taphofacies II is composed by small isolated or fragmented bones and fish scales deposited in terminal splay facies. The taphofacies III is characterized by sandy layers with bivalve shells and fish scales, preserved in crevasse-splay deposits. The obtained data shows that the studied sedimentary succession was generated on a dry trend environment, with sporadic water supply, which conditioned the fossil preservation.

A comprehensive analysis of the sedimentology, petrography, diagenesis and reservoir quality of sandstones from the Oligocene Xiaganchaigou (E3) Formation in the Lengdong area, Qaidam Basin, China

The sandstone from the Oligocene Xiaganchaigou Formation (E3) in the Lengdong area, Qaidam Basin, China, is extensively distributed and acts as an effective regional reservoir for hydrocarbon accumulation. Based on thin sections, scanning electron microscopy (SEM), X-ray diffraction (XRD) image analysis and cathodoluminescence (CL), a comprehensive analysis of the sedimentology, petrography, diagenesis and reservoir quality of the Oligocene sandstone reservoirs of the Xiaganchaigou (E3) Formation in the Lengdong area, Qaidam Basin, are studied. The sandstones in the Lengdong area are classified as argillaceous sandstone, silty sandstone, fine sandstone, medium sandstone, pebbly sandstone, conglomeratic sandstone and conglomerate based on their grain sizes, degrees of sorting, and matrix contents. Five sedimentary facies were identified in the Xiaganchaigou Formation (E3) of the Lengdong area, Qaidam Basin: distributary channel, interdistributary bay, mouth bar, central bar and distal sand sheet facies. Their major diagenetic processes include compaction, carbonate cementation and feldspar and calcite cement dissolution. The porosity and permeability values of the fine sandstone, conglomerate, and conglomeratic sandstone are the highest, followed by those of the medium sandstone and silty sandstone, and the argillaceous sandstone and pebbly sandstone have the lowest porosity and permeability values. The diagenetic sequence in the study area ranges from the eodiagenesis B stage to the mesodiagenetic A stage. Compaction has significantly reduced the primary porosity. Carbonate cements, mainly calcite cements, occlude pores by precipitating in intergranular and intragranular spaces. Dissolution largely contributes to increasing the secondary porosity. The well-sorted and relatively coarser-grained sandstones that formed in higher-energy sedimentary facies (distributary channel, mouth bar and central bar) have a better reservoir quality than the poorly sorted, clay-rich sandstones that formed in low-energy environments (interdistributary bay and distal sand sheet). Based on the comprehensive analysis of petrographic data, diagenesis and sedimentary facies, the reservoir in the Lengdong area is classified into three types: type A, type B and type C. Type A, located in the center of the study area, is the most favorable reservoir, while type C sandstone is the last to consider when looking for a good reservoir. This study can guide hydrocarbon exploration in the Lengdong area, as well as in northwestern China and similar areas throughout the world.

Characterization of braided river-delta facies in the Tarim Basin Lower Cretaceous: Application of borehole image logs with comparative outcrops and cores

Abstract Borehole electrical image log with core-like presentation is an efficient way to conduct sedimentary description in the subsurface. The key is to set up a series of geological diagnostic criteria. The subsurface Lower Cretaceous Bashijiqike Formation in the Kelasu belt, Kuqa Depression, described as a sedimentary facies of braided river-delta front, has measured a mass of borehole image logs and open-hole logs at depths more than 6000 m. Combined with comparative outcrops, limited cores and thin sections, high-quality borehole images were fine calibrated to reveal their sedimentary details, with the aim of subtle sedimentary interpretation in the uncored intervals. The image elements all have corresponding geological indications. The lithologies are interpreted including mudstones, siltstones, fine- and medium sandstones and conglomerates, and the sedimentary structures are interpreted in terms of various bedding types, scoured surface and stacking modes. Thus, interdistributary bay, sandy or conglomerate subaqueous distributary channel and channel mouth bar are interpreted based on the lithologies, sedimentary structures and textures, basal and top contacts and succession variations. Furthermore, determination of paleo-current direction using borehole image logs is proposed for some time, but is far from mature and need to consider tectonic deformation settings of studied stratigraphy. Based on this, the paleo-current in the Kelasu belt is interpreted to flow southward from the Southern Tianshan to the depression during the Early Cretaceous which is consistent with the results derived from the heavy mineral analysis. However, the subaqueous distributary channels could frequently migrate locally and the paleo-current direction changed at localities with an offset angle varied between 30° and 150°. Regionally, the scale of single sand-body decreases lakeward and the sand-body may stretch at a maximum distance of 685.1 m along the flow direction. The diagnostic criteria and the workflow for borehole image interpretation in this study will have potential application to subsurface sedimentary description and guide sedimentary architecture where cores are not available.

Thermal and mechanical coupling effects on permeability of weakly cemented sandstone

The weakly cemented sandstone is the host rock in the Cretaceous and Jurassic strata with low intensity, high water-bearing capacity and sensitivity to disturbance. In order to investigate the influence of temperature and confining pressure on the permeability of weakly cemented sandstone, scanning electron microscope were used to characterize microstructure of the particles, a series of triaxial creep tests were performed on coarse sandstone, medium sandstone and fine sandstone separately, and the effect of particle size on the permeability of sandstone was also analyzed. The results indicate that the increase of confining pressure led to the plastic deformation between the rock particles, that permeability was irreversible as the confining pressure changed, and that the permeability reduction rate of coarse sandstone was lower than that of fine sandstone. The sensitivity of weakly cemented sandstone to temperature and confining pressure increased as the sizes of the particles decreased, and the effect on permeability was more obvious.

Presence of charcoal as evidence of paleofires in the Claromecó Basin, Permian of Gondwana, Argentina: Diagenetic and paleoenvironment analysis based on coal petrography studies

The PANG0001 well, situated in the Claromecó Basin, Argentina, involves rocks that belong to the Tunas Formation [29], Permian of Gondwana. It is composed of fine to medium sandstones intercalating with black and green mudrocks and three coal seams up to 3 m thick. In the coals, a petrographic analysis was carried out to analyze the depositional environment and the diagenesis level reached by the Tunas Formation. The coals are composed of mono-maceral bands of Collotelinite and Gelinite, from the Vitrinite Group, or Fusinite, from the Inertinite Group. They were deposited in a wet swampy forest with rises and decreases of the phreatic level. The presence of inertinite or charcoal evidence the occurrence of palaeo-fires during the Permian. The vitrinite reflectance values are between 1.3% and 2.38%. The coals classify as semiantracitic to low volatile bituminous, with 10 to 25% of volatile matter. The vitrinite reflectance values indicate temperatures between 140 °C and 190 °C. The Tunas Formation reaches the oil to methane gas window, into a catagenesis to metagenesis range related to the organic matter diagenesis. Coal petrography is a good method for the environment and diagenesis analyses applied to the Tunas Formation.

An experimental investigation of geomechanical properties of deep tight gas reservoirs

An ultra-deep well (-3700 m–7,000 m) has high formation temperature (100–180 °C) and interlaced distributed lithology in longitude which has significant influence on wellbore stability and hydraulic fracture propagation. At present, few findings are available for a systematic understanding of different lithologies' deformation properties and fracture-cave structures under high-temperature. In this paper, a series of rock mechanical tests are conducted to study the rock mechanical properties of different lithologies under different temperatures. The results show that the influences of temperature on strength and failure mode for different lithologies are not the same. Under high temperature, the strength of fine sandstone and argillaceous fine sandstone increases slightly while the strength of medium sandstone and mudstone decreases seriously. As to failure mode, mudstone suffers from tensile failure and sand stones suffer from shear failure under the experiment condition in this study. For carbonate without crack, strength decreases as temperature increases when the temperature is higher than 80 °C; for carbonate with macrocrack developed, if the rock failure zone is parallel to the crack trend, the fracture strength will rapidly decline as the temperature increases. However, if the rock failure zone is vertical to the crack trend, the influence of temperature will be small. For shale, the strength and elasticity modulus are lower and the Poisson's ratio is higher under the condition of high temperature and low confining pressure. As to failure mode, shale suffers from tensile failure at 25 °C, and suffers from shear failure or tensile-shear combined mode at 110 °C. Based on the experiment results, a modified Duncan model for mudstone considering temperature influence is established, which could be used in the study of wellbore stability and hydraulic fracturing.

Análise de Biofácies dos Depósitos do Oligoceno Superior (Formação Qom) na Zona Urumleh Dokhtar, Irã

In order to determine the accurate paleoenvironmental conditions of Qom Formation, one stratigraphic section was studied in the Urumieh-Dokhtar Zone (West of Ashtian). 60 thin sections of the Qom Formation, 71 m thick, were prepared and the distribution of benthic foraminifera was analyzed. On the basis of the recognized foraminifera, the age of Qom Formation in the studied section is assigned to the Late Oligocene (Chattian). In this study, sex different biofacies and two lithofacies types have been recognized, that can be grouped into two depositional environments, back ramp (alluvium channel, lagoon and shoal) and the ramp (inner, middle and outer ramp). The alluvium channel is characterized by Gms (gravel with massive layering and abundant matrix). The shoal is represented by fine to medium sandstone: calcite cemented submature litharenite. The lagoon is dominated by the presence of bioclastic bryozoan corallinacea wackestone- packstone and bioclastic foraminifera corallinacean wackstone to packstone. The inner ramp is characterized by bioclastic benthic foraminifera (perforate and imperforate) wackestone-packstone and the middle ramp represented by bioclastic perforate foraminifera wackstone-packstone and bioclastic Operculina packstone. Outer ramp is characterized by bioclast benthic and planktonic foraminifera mudstone.

Determining the relationship of thermal conductivity and compressional wave velocity of common rock types as a basis for reservoir characterization

A comprehensive dataset detailing thermal conductivity and acoustic (compressional) wave velocity of 1430 oven-dry rock samples from clastic sedimentary (sandstone, arkose, greywacke), carbonatic (limestone, marl, dolomite, marble, coquina), plutonic (gabbro, gabbrodiorite, diorite, granodiorite, granite) and volcanic (basalt, andesite, rhyolite) rock types is presented. Correlation of thermal conductivity, compressional wave velocity and porosity are discussed in detail for each tested rock type. The study confirms that thermal conductivity of dry rocks can be predicted from acoustic velocity for porous rock types such as volcanites and sandstones, while non- and low-porous rocks show no to minor trends. With a prediction accuracy ±0.5Wm−1K−1 and a confidence of >80% for sediments and mafic volcanites the calculated data is far more comprehensive than data collected from literature, and is likely accurate enough for most first exploration approaches or geoscientific models before detailed site-scale investigation or modelling is conducted.To investigate the effect of water saturation on thermal conductivity and compressional wave velocity 118 sedimentary samples (arkose and fine-, medium- and coarse sandstones) were saturated in de-aired water and the heat conduction and acoustic velocity were remeasured. The obtained data shows that both thermal conductivity and compressional wave velocity of saturated samples markedly increase in contrast to dry samples. The extent of the thermal conductivity and compressional wave velocity gain is mainly controlled by porosity. Thermal conductivity of saturated samples increases twice as much for higher porous samples than for low porous fine and medium sandstones. In contrast, the gain of compressional wave velocity of saturated sandstones decreases with increasing porosity.