Cemented Sandstone Research Articles

Characteristics of fluid inclusions in the sandstone‐hosted Qianjiadian uranium deposit, southwest Songliao Basin, northeastern China: Implications for the nature and evolution of ore‐forming fluids

AbstractThe Qianjiadian deposit is a typical sandstone‐hosted uranium deposit that is hosted mainly in sandstone and siltstone of the Lower Cretaceous Yaojia Formation, located within the transition between the Kailu Depression and Jiamatu Uplift in the Songliao Basin, northeastern China. We studied the geological characteristics of this deposit, and analysed the mineralized sandstone by scanning electron microscopy (SEM) and electron probe microanalysis (EPMA) to identify the host minerals of fluid inclusions associated with uranium mineralization and describe their petrographic characteristics. In addition, this research investigated the origin of ore‐forming fluids and the relationship between petroleum fluids and uranium mineralization, based on the following findings. (1) EPMA and SEM data show that uranium minerals are hosted mainly in hydrothermal quartz (HQ) within sandstone cement, which indicates that uranium minerals co‐precipitated with HQ. (2) The fluid inclusions in the HQ show low homogenization temperatures (102.5–169.5°C) and low salinities (1.7–6.1 wt%). In contrast, aqueous inclusions in quartz overgrowths show lower temperatures (60.5–117°C) and higher salinities (4.2–8.7 wt%). (3) Petroleum fluids trapped in HQ homogenize fluid and those along healed microfractures in quartz overgrowth or plagioclase overgrowths at ~70–80°C, those trapped in present‐day organic inclusions at ~85–95°C, and at the boundary of detrital mineral grains show higher homogenization temperatures (~135–145°C). These results indicate that the hydrothermal quartz cement is associated with uranium mineralization, and the ore‐forming fluid of Qianjiadian sandstone‐hosted uranium deposit are characterized by low‐temperature, low‐salinity hydrothermal fluids, and the changes of temperature and salinity of hydrothermal fluids may not be intimately associated with uranium mineralization.

Performance Analysis of Sulfate Pisha-Sandstone Cement Soil Based on the Grey Entropy Theory

The exploitation of Pisha sandstone is one of the most significant strategies for environmental protection and resource saving in the border areas of Shanxi, Shaanxi, and Inner Mongolia, where Pisha sandstone is widely distributed. The goal of this research was to develop a method by which to transform Pisha sandstone cement soil into a new type of material which could be applied in saline regions, thereby achieving a wider application. The micro-pore and structural characteristics of cement soil samples which had been soaked in 0.5 mol/L of sodium sulfate solution were analyzed using nuclear magnetic resonance (NMR), X-ray diffraction (XRD), and scanning electron microscopy methods (SEM). The results showed that the compressive strength of the sandstone cement soil decreased with the increase in the sulfate intrusion days. Under the erosion of sodium sulfate solution, the lower the cement content was, the more quickly the strength decreased. The main mineral composition diffraction peaks of sulfate sandstone, along with the diffraction peaks of sulfate corrosion products, tended to change during salt invasion. The mineral components in the sandstone can provide a material basis for the formation of the sulfate erosion products of cement soil. In addition, the grey entropy theory can be utilized to explore the correlation degree of the porosity, T2 spectrum area, bound fluid saturation, free fluid saturation, and unconfined compressive strength of the specimen. The results obtained in this research investigation provided a theoretic basis for the practical engineering applications of Pisha-sandstone cement soil in saline regions.

Synthesis of alkali-activated uncalcined Pisha sandstone cement composites

This work reports the utilization of natural Pisha sandstone (PS) as a new precursor to manufacture a novel alkali-activated cement. The rheological properties, compressive strength, drying shrinkage, hydration properties, CO2 emission and energy consumption of the alkali-activated uncalcined Pisha sandstone cement composites (AUPC) were investigated. AUPC mortar containing 1.0 wt% of sodium hexametaphosphate and 0.2 wt% of ligneous calcium sulfonate as plasticizers showed similar rheological properties to those of ordinary Portland cement (OPC) mortar. The compressive strength of AUPC could reach 52.7 MPa after curing at 80 °C for 12 h. The increase of activator concentration (up to 4 mol/L) and the extending of curing time (up to 12 h) significantly increased the hydration of feldspar, montmorillonite and blended slag in AUPC. This resulted in the formation of more low Al/Si ratio aluminosilicate gels, calcium silicate hydrate (C-S-H) gels, and calcium alumina silicate hydrate (C-A-S-H) gels. When the concentration of NaOH and the curing time exceeded 4 mol/L and 12 h respectively, the porosity of AUPC increased, thereby lowering the compressive strength. The 90-day drying shrinkage of the AUPC with 4 mol/L NaOH activator increased from 1.68 to 3.04 mm/m when the water-to-binder ratio increased from 0.25 to 0.45. The total CO2 emissions and energy consumption for producing AUPC were estimated to be much lower than producing OPC.

In-situ major and rare earth elements constraint on the growth mechanism of calcite cement in shallow marine sandstone: An example from the Donghe sandstones, Tabei uplift, Tarim Basin, NW China

Understanding the genetic mechanism of calcite cement can facilitate the prediction of its distribution, which can improve the accuracy of reservoir quality prediction, however, the growth mechanisms of calcite cement still is not a totally resolved problem. This study reported the results of petrographic, cathodoluminescence, and in-situ major and rare earth elements found in a buried shallow marine sandstone (Donghe sandstones), located in Tarim Basin, NW China. Calcite cements were found as blocky and poililotopic and occluded pores, precipitated in an open marine environment, showed dominantly dull-luminescence with partially dark-red luminescence, during which they precipitated from the dissolution of bioclastic grains and bacterial sulfate reduction (BSR). These were accompanied by the dissolution of Mn- and Fe-oxides/oxyhydroxides, the reduced Fe2+ precipitating as pyrites, and the reduced Mn2+ being incorporated into the lattice of the precipitating calcite cements, which resulted in precipitated calcite cements showing dark-red luminescence under cathodoluminescence images. During shallow buried, BSR were occurred because of water washing of accumulated hydrocarbon reservoirs, which dissolve the early precipitated calcite cements, and subsequently precipitated at the location where the decrease inpCO2 was associated with increasing permeability. Meanwhile, the reduced Mn2+ and Fe2+ that originated from the dissolution of Mn- and Fe-oxides/oxyhydroxides were incorporated into the lattice of calcite cements and precipitated as pyrites, respectively, resulting in the calcite cements showing orange-red and orange-yellow luminescence with a low and high content of Mn and Fe, respectively. Calcite cements precipitated during deep burial, which were characterized by significantly high contents of Fe, Mn, K, Si, and replaced quartz grains, during which precipitation the messenger film water that covered the surface of clastic grains as the mediation for diagenesis reactions. Based on these results, the model of genetic mechanism and diagenetic evolution path is established for shallow marine sandstone, which provides insights into the burial diagenesis processes of calcite cements in marine sandstone.

Geochemically induced shear slip in artificially fractured dolomite- and clay-cemented sandstone

Geologic carbon sequestration in deep saline aquifers results in a low pH brine plume that pushes into subsurface storage reservoirs and can access pre-existing or induced microfractures. This work investigates the effect of acidic brine on displacement of an artificial fracture under stress in the dolomite- and clay-cemented Bandera Gray sandstone. Samples were held under stress in a custom flow cell housed within an industrial CT scanner, and either acidic (pH 4) or reservoir-simulated (pH 8.3) brine was flowed through the artificial fracture for seven days. CT imaging shows that acidic brine resulted in greater shear slip than reservoir-simulated brine, with 0.379 ± 0.022 mm shear slip after pH 4 flow and only 0.213 ± 0.011 mm slip after pH 8.3 flow. Fracture surfaces exposed to acidic brine had rougher surfaces and lower fracture toughness, respectively, than those exposed to reservoir-simulated brine. SEM images of fracture surfaces indicate a loss by area of Fe-dolomite cementing crystals (6.05 ± 1.37% to 3.78 ± 0.73%) after exposure with the acidic brine, as well as a corresponding porosity increase (24.6 ± 1.1% to 26.1 ± 1.1%). These results indicate dissolution and weakening of the dolomite cements by acidic brine frees individual grains at the fracture surface to move, resulting in geochemically-induced stress release. Acidic brine created from geological carbon sequestration can dissolve sandstone cements and lead to increased shear slip at fracture interfaces, but further work at larger scales and with more realistic fracture conditions is needed.

Illite and chlorite cementation of siliciclastic sandstones influenced by clay grain cutans

The distribution and amount of clay rim cements are different for Permian Rotliegend and Lower Triassic Bunter sandstones in the Southern Permian and German Triassic Basins, respectively. Both are similar fluvial-aeolian deposits in hot-arid endorheic basins. In both Permian and Triassic sandstones clay grain cutans can be present, but clay rim cement is often lacking or rare in Bunter sandstones. At first sight it would appear that the presence of cutans is thus not relevant for the development of clay cementation. However, at closer inspection, it appears that authigenic clay is indeed present in both cases. In Rotliegend sandstones, the authigenic clay mainly developed as rims around the clay grain cutans, which are often thin and well microlaminated with few intra-micropores and composed of platy particles. The clay crystals in the rim decrease the pore interconnectivity and lower permeability significantly. In Bunter sandstones, most of the authigenic clay developed within the cutans. These cutans are less well microlaminated, have ample micropores, and are composed of more equant-shaped particles. The latter looser structure facilitated clay authigenesis within the cutans and their micropores. This often led to exfoliation of the cutan laminae and expansion of the entire cutan. These expanded cutans also lower pore connectivity. The presence and thickness of clay rim cement on top of cutans and grains is thus not correlated with the thickness of the cutans but with the texture of the cutans. The latter determined where authigenic clays precipitated and the thickness of cutans is partly the result of clay authigenesis within the cutans. This demonstrates that the composition and the texture of sedimentary components constrained and controlled burial diagenesis.

ВЛИЯНИЕ АГЛОМЕРАТОВ НАНОПОРОШКА КАРБИДА ВОЛЬФРАМА НА СВОЙСТВА ЦЕМЕНТА

The object of the research is cement samples modified by agglomerates of tungsten carbide nanopowders obtained from carbide waste. The paper considers the influence of tungsten carbide nanopowder on the change in the main cement characteristics (density, water demand, setting time, kinetics of strength gain, compressive strength, bending strength). The optimum additive content in cement materials is 3%. This parameter is estimated by the maximum growth of compressive and flexural strength at all stages of hardening. The effect of WC powder additive on the structure of the cement matrix has been shown. Due to the high dispersibility of WC nanopowder, its particles act as additional crystallization centers, fill in the micropores of the cement stone and create a denser and stronger structure. The results of studying the microstructure of the cement stone with additions of WC powder indirectly confirm the results of strength characteristics. The cement-sand samples with the modifier are found to have a denser crystallized cement-sand stone compared to the control sample (without WC powder addition). The conducted researches have shown perspectivity of application of WC nanoparticles agglomerates as modifying additives for cement materials, which can be used in production of special purpose concrete (hydraulic, radioprotective, etc.). The economic effect of the use of WC nanoadditives obtained from hard-alloy production wastes can be obtained due to a reduction in costs at the stages of construction and operation of structures, buildings, structures based on it.

The Origin of Quartz Cement in the Upper Triassic Second Member of the Xujiahe Formation Sandstones, Western Sichuan Basin, China

High-precision in situ δ18O values obtained using secondary ion mass spectrometry (SIMS) for μm-size quartz cement are applied to constrain the origin of the silica in the deep-buried Upper Triassic second member of Xujiahe Formation tight sandstones, western Sichuan Basin, China. Petrographic, cathodoluminescence (CL), and fluid inclusion data from the quartz cements in the Xu2 sandstones indicate three distinct, separate quartz precipitation phases (referred to as Q1, Q2, and Q3). The Q1 quartz cement was formed at temperatures of approximately 56–85 °C and attained the highest δ18O values (ranging from 18.3 to 19.05‰ Vienna Standard Mean Ocean Water (VSMOW)). The Q2 quartz cement was generated at temperatures of approximately 90–125 °C, accompanying the main phase of hydrocarbon fluid inclusions, with the highest Al2O3 content and high δ18O values (ranging from 15 to 17.99‰ VSMOW). The Q3 quartz cement was formed at temperatures of approximately 130–175 °C, with the lowest δ18O values (ranging from 12.79 to 15.47‰ VSMOW). A portion of the Q2 and Q3 quartz cement has a relatively high K2O content. The dissolution of feldspar and volcanic rock fragments was likely the most important source of silica for the Q1 quartz cement. The variations in δ18O(water) and trace element composition from the Q2 quartz cement to the Q3 quartz cement suggest that hydrocarbon emplacement and water-rock interactions greatly altered the chemistry of the pore fluid. Feldspar dissolution by organic acids, clay mineral reactions (illitization and chloritization of smectite), and pressure dissolution were the main sources of silica for the Q2 and Q3 quartz cements, while transformation of the clay minerals in the external shale unit was a limited silica source.

The origin of silica cements revealed by spatially resolved oxygen isotope microanalysis and electron-beam microscopy; Heidelberg Formation, Germany

The origin and growth mechanisms of microcrystalline quartz cement in sandstones and its relationship to quartz overgrowths remain puzzling even though it has an important effect on rock properties in deeply buried sandstones; microcrystalline quartz cement inhibits the growth of quartz cement and so leaves sandstones with anomalously high porosity deep in sedimentary basins. Here we have examined the relationships between quartz cement, and duplicate layers of chalcedony/amorphous silica and microcrystalline quartz from outcrops of the Upper Cretaceous Heidelberg Formation, Germany. We have used an array of techniques including: NanoSIMS for spatially-resolved stable oxygen isotope analysis, and high-resolution chemical analysis of the silica cements using electron microprobe analysis, as well as crystallographic studies using TEM and EBSD. Based on burial history modeling and the low degree of compaction of the Heidelberg Formation sandstones, we conclude that maximum temperature due to conductive heating was about 42 °C; quartz overgrowths with δ18O of 18.8 ‰ developed from meteoric-derived formation water with δ18O of about −12 ‰ V-SMOW. Based on published precedents and an awareness that silica solubility decreases with decreasing temperature, we assume that chalcedony/amorphous silica and then microcrystalline quartz developed during cooling of flowing formation water. Chalcedony with δ18O of 27.4 ‰ developed from meteoric-derived formation water with δ18O of between −1 and −3 ‰ V-SMOW depending on whether the initial temperature of the cooling formation water was 45 or 55 °C. The interpreted higher δ18O of the warmer water responsible for chalcedony and microcrystalline quartz growth is typical of basin-center waters that have undergone considerable water–rock interaction. It is likely that the fluid movement was caused by thrust tectonics in the Coniacian to Campanian. The duplicate layers of chalcedony/amorphous silica and then microcrystalline quartz suggests that there were two thrust-induced fluid expulsion events.

Stability Analysis and Control Measures of Large-Span Open-Off Cut with Argillaceous Cemented Sandstone Layered Roof

This paper is based on the condition where layered argillaceous cemented sandstone as an engineering background is met by the No. 207 fully mechanized working face open-off cut (Wanli No. 1 Coal Mine). Through mechanical theory analysis and field practice, the engineering safety problem of the large-span argillaceous cemented sandstone layered open-off cut roof supporting structure was analyzed. The roof caving arch height of the open-off cut roadway in 207 working face was obtained based on the mechanical mechanism of instability and caving of the layered surrounding rock mass roof. The anchor cable suspension and bearing stability of the open-off cut roof were analyzed in terms of the layered beam structure model. Meanwhile, combining with conditions, reasonable and effective support countermeasures and key parameters are proposed for such open-off cut roadway and enhance the actual supporting engineering on-site. These research results could provide engineering reference for an open-off cut roadway with composite roof conditions featured to weak cementation and weak interlayer.

Coupled CFD–DEM numerical modelling of perforation damage and sand production in weak sandstone formation

A coupled microscopic simulation method was implemented to investigate the bond breakage behaviour during perforation and sand production in a weak cemented sandstone. A new contact bond model was implemented in the open-source program CFDEM to simulate a large cylindrical sample of one hundred thousand numerical particles using a high-performance computer (HPC) system. The coupled simulation of the particle–fluid phases and the information exchange between them required intensive computational power and highly efficient parallelization algorithms, yet it provided the detailed information on the distribution of bond and bond breakage, new contact formation, particle migration and the associated change in the internal porosity across the sample. The perforation process was found to create a damage zone that contained a compacted core of unbonded particles. The compacted core expanded quickly under fluid flow condition and released its particles to migrate towards the perforation tunnel. Small particles were produced first from the perforation damage zone, while fewer particles of larger sizes were produced later in an overall transient sand production phenomenon.

Influence of multiphase carbonate cementations on the Eocene delta sandstones of the Bohai Bay Basin, China

This study deciphers the origin and distribution of multiphase carbonate cementations and assesses their implications for reservoir quality of the Eocene delta hydrocarbon sandstones in the Bohai Bay Basin. Petrographic observation and stable isotopic analysis identified two phases of carbonate cementations: (1) eodiagenetic phase including non-ferroan calcite cement precipitated at 25–42 °C (δ13CVPDB: 1.0‰ to +5.7‰ and δ18OVPDB: 10.7‰ to −7.5‰); and (2) mesodiagenetic phase which includes ferroan calcite cement (δ13CVPDB: 11.8‰ to −5.7‰ and δ18OVPDB: 17.0‰ to −12.6‰) and ankerite cement (δ13CVPDB: 13.7‰ to −5.9‰ and δ18OVPDB: 17.1‰ to −12.4‰) precipitated at 90–134 °C. The eodiagenetic non-ferroan calcite cement that is mainly sourced from microbial methanogenesis of organic matter in adjoining mudrocks, extensively cemented at or near sandstone-mudstone contact. Mesodiagenetic ferroan carbonate cements are derived from predominantly thermal decarboxylation of organic acids and subordinate dissolution of eodiagenetic calcite cements in sandstones. These ferroan carbonates are preferentially concentrated at distances of 0.5–1.0 m from sandstone-mudstone contacts. Kinetic modelling reproduces evolution pathways of different diagenetic minerals in the Es4s sandstones (the upper part of the fourth member of the Eocene Shahejie Formation). More importantly, results of modelling reveal that the dissolution of non-ferroan calcite cements would not enhance total reservoir quality due to the precipitation of more stable ferroan calcite and ankerite cements. Therefore, the occurrence of multi-phase carbonate cementations at different sections of sandstones leads to significant destruction of porosity and reservoir heterogeneity during progressive diagenesis.

Characteristics of Tight Sandstone Cement in Panke Area

In order to improve the success rate of oil and gas exploration and further study the impact of tight sandstone cements on reservoirs, this paper uses a large number of rock thin sections, scanning electron microscopy, and physical property analysis to analyze the cements of the Chang 6 reservoir in the Panke area, southern Ordos Basin. Features have been studied in depth. The results show that the cements in the study area mainly include: authigenic cements such as illite and chlorite, carbonate cements such as iron calcite and iron dolomite, and siliceous cements. As the depth of the stratum increases and the temperature rises, the content of illite gradually increases; the main distribution area of chlorite cement is shallower than that of illite; with the increase of buried depth, the content of carbonate cement has a increase trend ; The content of siliceous cement in the deeper stratum is higher than that in the shallow layer.

Fault-controlled carbonate cementation: A case study from Eocene turbidite-delta sandstones (Dongying Depression) and implication for hydrocarbon migration

The subsurface distribution of carbonate cements in sandstones is cornerstone in understanding the fluid regime and origin of cements to shed the light on hydrocarbon migration. Petrography, cuttings, cores, well logs, and seismic data were integrated to better understand the distribution of carbonate cements in the Eocene turbidite-delta sandstones of the Dongying Depression. Petrographic examinations (including cathodoluminescence, CL) reveal that the carbonate cements are mainly calcite with minor ankerite that precipitated in deep burial settings. The significant abundance of carbonate cements near fault zones implies migration of fluids from deep settings through late fractures developed by compaction under deep burial conditions. The extent of carbonate-cemented zones may reach hundreds of meters away from the faults, while scatter carbonate cements are much more extensive. The relationship of pervasive and scattered calcites with oil-bearing sandstones suggests that the pervasive calcite cementation started as soon as the hydrocarbon and brine mixed fluid were displaced while the scatter calcite cements came after. The occurrence of abundant carbonate-cemented zones in the delta front oil-free sandstone intervals suggests possible hydrocarbon migration to the overlying sandstone units.

Carbonate cementation patterns and diagenetic reservoir facies of the triassic Yanchang Formation deep-water sandstone in the huangling area of the Ordos Basin, northwest China

The Chang 6 member sandstone is one of the main oil reservoirs of the Ordos basin, China, and consists of a lacustrine gravity flow deposited in Huangling area. Carbonate cement is volumetrically the most important diagenetic parameter controlling reservoir quality in the sandstones.Three generations of carbonate cements were recognized. The massive microcrystalline calcite, poikilotopic calcite and microcrystalline dolomite from the supersaturated alkaline pore water precipitate with basement cements at an early diagenesis stage. The ferroan calcite is mainly pore-filling and replacement cementation in the middle diagenesis stage, and ferroan dolomite is pore-filling cementation in the late diagenesis stage.The four types of reservoir facies and the distribution patterns were identified in the Chang 6 member sandstones. (1) Massive carbonate cemented facies with high contents of carbonate cements are distributed along the edges of the sandstone bodies, which are in the channels of the turbidite and sandy debris flow deposits. (2) Partial carbonate cemented facies occur along the margins of the massive carbonate cemented facies. (3) The porous facies with the lowest percentages of carbonate cements in sandstone are distributed in the middle of the sand-bodies of the sandy debris flow deposits. (4) High muddy debris facies that have high percentages of carbonate-cemented and high muddy debris sandstones occur essentially in the proximal overbank and lobe of the turbidite deposits.The reservoir quality of sandstones depended upon time of precipitation of carbonate cements on and diagenetic processes. Massive carbonate cements and the high muddy debris of sandstones undergo lots of carbonate cements and strong compaction in the early diagenesis stage, respectively. There was weak water rock reaction of dissolution in the later diagenesis process. Partial carbonate cemented facies and the porous facies of sandstones have sufficient water rock reaction and more dissolution pores.

Numerical simulations of cone penetration in cemented sandstone

In this study, three-dimensional simulations of cone penetration tests (CPT) in cemented sandstone have been investigated using the Discrete Element Method (DEM). We generated the realistic grain size distribution of sandstone in the numerical simulations using the Ustyurt-Buzachi Sedimentary Basin samples. Numbers of numerical CPT tests at different bond strength values were performed with the penetrometer vertically moved down at a constant rate. The results of the real-world particle size distribution show that the cone penetration resistance and side friction increase with increasing depths; and with increase in bond strength the cone resistance and side friction decrease, while the friction ratio increases. The result of numerical CPT tests in cemented sandstone was found to be in good agreement with the Soil Behaviour Type (SBT) classification system from CPT data.

Diagenesis and Origination of Carbonate Cements in Deeply Buried Sandstones of the Eocene Es3 Member, Raoyang Sag, Bohai Bay Basin, China

Diagenesis is one of the most predominant factors controlling reservoir quality in the deeply buried siliciclastic sandstones of the third member in the Eocene Shahejie Formation (Es3), in the Raoyang Sag, the Bohai Bay Basin. In this study, thin section, cathodoluminescence (CL), scanning electron microscope (SEM), X-ray diffraction (XRD), Raman spectrum, carbon and oxygen isotopes, and fluid inclusion analyses are used to restructure paragenetic sequences and detect origins of carbonate cements recorded in this deeply buried member. Based on petrographic analyses, the Es3 sandstones are identified as lithic arkoses and feldspathic litharenites at present, but derived from original arkoses and lithic arkoses, respectively. Geohistorically, the Es3 sandstones have undergone two diagenetic episodes of eogenesis and mesogenesis. Events observed during eogenesis include chemical compaction, leaching of feldspar, development of chlorite coating and kaolinite, precipitation of the first generation of quartz overgrowth (QogI), dissolution of feldspar, and precipitation of calcite and nonferroan dolomite cement. Mesogenetic alterations include chemical compaction, precipitation of kaolinite aggregate and the second generation of quartz overgrowth (QogII), precipitation of ankerite, development of I/S and illite, and formation of pyrite. Carbon and oxygen isotopic data show that calcite cements are characterized by 13C ( δ 13 C PDB ranging from -0.7‰ to 1.0‰ with an average of 0.1‰) and 18O ( δ 18 O SMOW varying from 12.3‰ to 19.0‰ with an average of 16.2‰); these stable isotopic data combined with Z value (from 114.69 to 122.18) indicate skeletal debris ( δ 13 C PDB ranging from -1.2‰ to -1.1‰ with an average of -1.15‰; δ 18 O SMOW varying from 23.0‰ to 23.2‰ with an average of 23.1‰) and ooids in adjacent carbonate beds involved in meteoric water and seawater from outside jointly served as the carbon sources. For nonferroan dolomite, the δ 13 C PDB value of -4.1‰ is a little bit negative than the calcite, and the δ 18 O SMOW of 14.3‰ is coincident with the calcite, which suggest the nonferroan dolomites come from the diagenetic fluids with a similar oxygen isotopic composition to that of the calcite but modified by the external acidic δ 13C-depleted water. However, the ankerites are actually rich in 12C ( δ 13 C PDB ranging from -10.0‰ to -1.2‰, mean = − 4.3 ‰ ) and 16O ( δ 18 O SMOW varying from 10.1‰ to 19.4‰, mean = 14.9 ‰ ), when combined with the distribution of cutting down along the direction pointing to sand-body center from the margin and microthermometric temperature (Th’s) data mainly varying between 115.2°C and 135.5°C with an average of 96.0°C, indicating the main origination from the Es3 source rocks with effective feldspar buffer action for the acidic fluids in the margins of the Es3 sandstones. In addition, the necessary elements for ankerite such as Fe2+, Ca2+, and Mg2+ ions also come from organic matter and clay minerals during thermal maturation of the Es3 source rocks. The study provides insights into diagenetic processes and origination of carbonate cements in the Es3 sandstones; it will facilitate the cognition of predictive models of deeply buried sandstone reservoirs to some extent, which can reduce the risks involved in oil and gas exploration and development.

From cradle to grave: how burial history controls the rock‐physics properties of quartzose sandstones

ABSTRACTRock‐physics properties of sands and sandstones are strongly affected by geological processes of the past, including deposition, compaction and exhumation. By honouring these geological processes, the rock‐physics modelling will be more predictive in areas with limited well control. This study performs rock‐physics modelling constrained by a given geological history, starting from deposition to mechanical and chemical compaction. Different geological factors, including sorting, grain size and clay coating, will affect the quartz cementation and rock stiffening of quartzose reservoir sandstones. By combining compaction models with rock‐physics contact theory, we can model the rock‐physics properties of sands/sandstones as a function of geologic time. We have demonstrated the approach on well log data from three selected wells on the Norwegian Continental Shelf, where the burial histories of the target reservoir sandstones are significantly different. We conclude that rock‐physics modelling constrained by burial history can be used to predict elastic properties quite accurately in these wells. The integrated approach presented in this study allows for more realistic rock‐physics depth trends in areas with complex burial history that can be used in AVO studies or to estimate net erosion associated with tectonic uplift.

Digital rock physics approach to simulate hydraulic effects of anhydrite cement in Bentheim sandstone

Abstract. Cementation of potential reservoir rocks is a geological risk, which may strongly reduce the productivity and injectivity of a reservoir, and hence prevent utilisation of the geologic subsurface, as it was the case for the geothermal well of Allermöhe, Germany. Several field, laboratory and numerical studies examined the observed anhydrite cementation to understand the underlying processes and permeability evolution of the sandstone. In the present study, a digital rock physics approach is used to calculate the permeability variation of a highly resolved three-dimensional model of a Bentheim sandstone. Porosity-permeability relations are determined for reaction- and transport-controlled precipitation regimes, whereby the experimentally observed strong decrease in permeability can be approximated by the transport-limited precipitation assuming mineral growth in regions of high flow velocities. It is characterised by a predominant clogging of pore throats, resulting in a drastic reduction in connectivity of the pore network and can be quantified by a power law with an exponent above ten. Since the location of precipitation within the pore space is crucial for the hydraulic rock properties at the macro scale, the determined porosity-permeability relations should be accounted for in large-scale numerical simulation models to improve their predictive capabilities.

DISTRIBUTION OF CARBONATE CEMENTS IN THE SANDSTONES OF THE SOUTHEAST OF THE DNIEPER-DONETSK DEPRESSION IN THE ASPECT OF FORMING SECONDARY POROSITY IN THEM AT HIGH DEPTHS

The main problem when searching for oil and gas at great depths is the lack of an economic reservoir. Since great depths correspond to high degrees of catagenetic transformations, the primary porosity of most rocks is reduced and all hopes are associated either with the relict primary porosity, the value of which progressively decreases with depth, or with secondary porosity. There are different views on the formation of secondary porosity in sandstones - the main reservoirs of the Dnieper-Donetsk Depression (DDD). Some of these views are briefly discussed in this paper. In all cases, carbonate cement plays an important role as its dissolution under the influence of various factors forms secondary porosity. The question arises as to how often sandstones with carbonate cement are found among the differences with a different cementing material, at what depths, in the sediments of what ages, are there many sandstones in which the formation of secondary porosity is possible by one or another mechanism. The aim of this work is to quantitatively assess the prevalence of carbonate sandstones among the sandy rocks of the southeastern part of the DDD, clarify the distribution of carbonate sandstones in depth and over the main stratigraphic horizons, and preliminary estimate the content of various carbonates in the cement of sandstones according to field and standard laboratory studies of core material. In the course of the work, data from more than 10 thousand samples were used, taken at various fields of the northern side and the northern near-zone zone of the southeastern part of the Far East region, approximately from the Belskoye field in the north-west to Kruzhilovskoye in the southeast inclusive. The sampling depth intervals for the samples used vary from 1 to 6 km, the age of the sediments they characterize is from Triassic to Devonian. The results obtained show that the content of carbonate in sandstone cement at great depths is high enough for its movement to cause the formation of secondary porosity on an industrially significant scale, which, in turn, provides good prospects for prospecting hydrocarbon deposits at great depths.