Calcite-cemented Sandstone Research Articles

Pore pressure effects on the anisotropic joint elastic-electrical properties of artificial porous sandstones with aligned fractures

Fractures widely exist in rocks and joint interpretation of elastic and electrical survey data can improve the characterization of fractured rocks. However, the cross-property relationship between the elastic and electrical rock properties, which is the key to the successful joint elastic-electrical interpretation, remains poorly understood in fractured rocks with varying pore pressure. This knowledge gap was by developing a method for making calcite-cemented sandstones with penny-shaped fractures and by simultaneously measuring the anisotropic elastic and electrical properties of the artificial sandstones with and without fractures as a function of pore pressure through the development of an anisotropic joint elastic-electrical measurement system. We find that elastic velocities and electrical resistivities of the intact and fractured samples reduce with increasing pore pressure, and the reduction in the physical properties of the fractured rock is less significant than that of the intact sample. We also find that elastic and electrical anisotropic parameters decrease in the sample without fractures but decrease first and then increase with increasing pore pressure in the sample with fractures. Most interestingly, the anisotropic elastic and electrical properties are found to exhibit a strong linear correlation in the intact and fractured samples with varying pore pressure and a difference exists in the slopes between the samples with and without fractures. The experimental data are explained and interpreted through the contrary contributions of the dilation of the random microcracks and the aligned fractures with increasing pore pressure to the anisotropic elastic and electrical properties. The results reveal the pore pressure effects on the anisotropic joint elastic-electrical properties of sandstones with fractures and provide a potential for the detection of overpressure in fractured rocks through integrated elastic and electrical surveys.

Quantifying amplitude-variation-with-offset uncertainties related to calcite-cemented beds using a Monte Carlo simulation

Calcite cement often occurs locally, forming thin layers of calcite-cemented sandstone characterized by high seismic velocities and densities. Because of their strong impedance contrast with the surrounding rock, calcite-cemented intervals produce detectable seismic reflection signals that may interfere with target reflections at the top of a reservoir. In this case, the amplitude-variation-with-offset (AVO) of the effective seismic signature will be altered and may even create a false hydrocarbon indication. From the Monte Carlo simulation, we find that the presence of thin calcite-cemented beds increases the uncertainty of the Bayesian pore-fluid classification based on the AVO attributes intercept and gradient. In the case example of a North Sea turbiditic oil and gas field, the probability of a false-positive hydrocarbon indication increases from 3%–5% to 18%–21% assuming an equal probability of the occurrence of brine, oil, and gas. The results confirm that calcite-cemented beds can create a pitfall in AVO analysis. Realistic estimates of the AVO uncertainty are crucial for the risk assessment of well placement decisions.

Elastic and electrical properties of calcite-cemented artificial sandstones based on a new manufacturing method

Calcite cement is widely existing in sandstones and can significantly affect the elastic and electrical properties of sandstones. Accordingly, understanding the effects of calcite cement on the elastic and electrical rock properties is of key importance for the accurate characterization of sandstone reservoirs through seismic and electromagnetic surveys. To obtain such knowledge, we design and implement dedicated laboratory experiments to investigate the effects of calcite cement and porosity on the P- and S-wave velocities and electrical resistivity of synthetic calcite-cemented sandstones made using a new recipe. The experimental results show that elastic velocities and electrical resistivity generally decrease with increasing porosity when the content of the calcite cement keeps constant, whereas the elastic and electrical rock properties exhibit complex variation with cement content in sandstones with approximately the same porosity. Analyses and interpretation of the experimental data indicate that the elastic and electrical rock properties are affected jointly by porosity, the content and distribution of the calcite cement, as well as the microstructure of the pore space resulting from the different axial stress employed for the consolidation of the rock samples. The results have revealed the mechanisms of how porosity and calcite cement affect the elastic and electrical properties of calcite-cemented sandstones, and provided a theoretical basis for the accurate characterization of sandstone reservoirs through seismic and electromagnetic surveys.

Pressure-dependent joint elastic-electrical properties of calcite-cemented artificial sandstones

Understanding the correlations between the elastic and electrical properties of various types of rocks is the key to the successful joint interpretation of seismic and electromagnetic survey data to provide petrophysical parameters to better assess the subsurface earth. However, the pressure-dependent joint elastic-electrical properties of calcite-cemented sandstones remain poorly understood, even though such rocks are widely distributed in nature and are all experiencing pressures. To obtain such knowledge, a new method has been developed for the manufacture of calcite-cemented artificial sandstones and investigated comprehensively the effects of porosity and cementation content on the confining pressure-dependent joint elastic-electrical properties of the synthetic samples made using the new recipe. Confining pressure is found to more significantly affect the P- and S-wave velocities and electrical resistivity in the samples with higher and lower porosity, respectively, when their cementation content remains the same. On the other hand, cementation content impacts the pressure-dependent elastic and electrical properties more complexly, and the effects of cementation content can be influenced by the minor fluctuation of porosity in the samples, especially at low confining pressures. More interestingly, P- and S-wave velocities are found to approximately linearly correlate with electrical resistivity as confining pressure varies, and the slopes of the linear joint correlations are demonstrated to vary distinctly with porosity and cementation content. The experimental data are interpreted in terms of the competing effects of porosity and cementation content on the microstructure of the samples. The results have helped to reveal the nature that porosity and cementation content find on affecting the joint elastic-electrical properties with varying pressure and have important practical implications for discriminating the porosity and cementation effects that will pave the way to a more successful interpretation of the joint seismic and electromagnetic survey data.

Small-Scale Diagenetic Heterogeneity Effects on Reservoir Quality of Deep Sandstones: A Case Study from the Lower Jurassic Ahe Formation, Eastern Kuqa Depression

The Lower Jurassic Ahe Formation is an important exploration target for deep clastic reservoirs in the eastern Kuqa Depression. The Ahe Formation sandstones show heterogeneous porosity and permeability petrophysical properties. These properties have been poorly understood, limiting forecast of petroleum accumulations and making it difficult to develop the reservoirs. Based on cores, thin sections, SEM, and fluid inclusions, this study examined sandstone composition and texture and diagenetic heterogeneity at the core scale. The aim was to understand the influence of variations in detrital composition and texture on diagenetic and reservoir quality evolution. The Ahe Formation sandstones are dominantly fine- to coarse-grained litharenites, with minor feldspathic litharenites. In fining-up sand beds, detrital grain size determines the degree of mechanical compaction and, consequently, the abundance of porosity through ductile grains and muddy matrix. Local complete calcite cementation is a noticeable exception to this general trend. Three sandstone petrofacies have been defined based on texture and framework composition, detrital matrix, diagenesis, and pore types: (1) ductile-lean sandstone, (2) ductile-rich sandstone, and (3) tightly calcite-cemented sandstone. Different petrofacies experienced contrasting diagenetic and porosity evolution pathways. Ductile-lean sandstones underwent lower degree of compaction relative to ductile-rich sandstones during the eodiagenesis stage, and extensive grain dissolution occurred. The petrofacies remained relatively porous and permeable before early oil arrival. With continued burial, the porosity and permeability in the sandstones were further reduced by cementation. The petrofacies still had moderate porosity and permeability and were substantially charged when late petroleum migrated into the reservoirs. Thus, ductile-lean sandstones constitute effective reservoir rocks in deep reservoirs. By translating petrofacies to signatures of well logs, the effective properties of the reservoir rocks can be forecasted at the well scale.

Mineral dissolution and precipitation induced by hydraulic fracturing of a mudstone and a tight sandstone in the Powder River Basin, Wyoming, USA

Water-based hydraulic fracturing requires high-pressure injection of large quantities of water mixed with chemicals into the subsurface, where fluids contact mineral surfaces. Chemical imbalance between minerals and the injected fluids provides the potential for chemical reactions that may induce physical alterations to the rock. We investigate mineral reactivity during in situ water-rock interaction between unconventional reservoir rocks and hydraulic fracturing fluid. We characterized two unconventional reservoir rocks, a calcareous mudstone (B Bench of the Niobrara Formation) and a calcite-cemented sandstone (Wall Creek Member of the Frontier Formation) of the Powder River Basin, Wyoming, USA, and developed, analyzed, and quantified the aqueous geochemistry of a hydraulic fracturing fluid (HFF). These datasets informed the design of numerical simulations that predict mineral-HFF interactions at in situ conditions over a 31-day timeframe. The two unconventional reservoir rocks have similar mineral assemblages, albeit in different proportions, which leads to similar dissolution/precipitation reactions. Calcite, feldspar, and authigenic clay minerals begin to dissolve, and secondary calcite, anhydrite, and illite form. Calcite dissolution increases porosity of the B Bench from 2% to 2.9% and porosity of the Wall Creek from 5% to 5.8%. Mineral-fluid reactions manifest in flowback fluids as temporal geochemical trends consistent with limited field data.

Long term reactivity of CO2 in a low salinity reservoir-seal complex

An understanding of the long-term reactivity of different rock types to injected CO2 is needed for sequestration site assessment. Relative to saline aquifer studies, the long term reactivity of CO2 in low salinity aquifers has received little attention. Currently in Australia, the Surat Basin is being appraised for its large-scale CO2 storage potential within low salinity aquifers. Sixteen core samples from the Precipice Sandstone and Evergreen Formation – the notional target reservoir and seal complex – were characterized for mineral content; helium, mercury-injection and micro CT porosities; air permeability; and, imaged with SEM-EDS. Samples consisted of quartz rich reservoir sandstones, feldspar and clay rich or calcite cemented sandstones (secondary reservoir), mudstones (sealing complex), and oolitic ironstones (sealing complex) derived from braided river, fluvial-deltaic, and restricted marine shoal depositional environments, respectively. The reservoir sandstone samples characterized here had measured total porosity that ranged from 11 to 23% with pore throats mainly between 90 and 100 μm, and core air permeability from 558 to 3397 mD. In the Precipice Sandstone reservoir sample μCT plugs, 98% of the pore space was connected with calculated vertical permeability 145–4611 mD and horizontal 4291–8200 mD. Feldspar and clay rich sandstone and mudstone samples from the overlying Evergreen Formation had porosity that ranged between 0.2 and 22.9%, with a wide range of pore throat sizes from ~0.005 to 30 μm, and permeability from 0.2 to 28.1 mD, respectively. Ironstone and mudstone samples from the Westgrove Ironstone Member (Evergreen Formation) had porosity from 0.7 to 9.7% and a low permeability of 0.04 mD.Kinetic geochemical CO2 reactivity models made predictions over two time-scales: 30 or 1000 years. Selected models also accounted for the potential presence of 10 ppm SO2 gas. The Precipice Sandstone quartz-rich reservoir sandstones had consistently small amounts of reactive minerals and mineral trapping or scaling of the reservoir was not predicted over 30 years, with the pH approximately 4.5 after 30 years. Samples from the Evergreen Formation included feldspar and clay rich sandstones and mudstones, several contained variable amounts of carbonate cement. Their response to CO2 was more influenced by mineral content than rock type. Plagioclase feldspars and Fe-rich chlorite were the main silicate minerals that reacted to produce siderite and ankerite mineral trapping up to 2.57 kg/m3 CO2. In the very unlikely event that CO2 rich fluids migrated upwards as far as the Westgrove Ironstone Member, chlorite is predicted to alter to siderite. This study indicates that the Precipice Sandstone reservoir in the study region has a low likelihood of mineral scaling which is favorable to avoid CO2 injectivity issues. Mineral trapping as ankerite and siderite could be expected to trap CO2 in the chlorite and plagioclase rich Evergreen Formation seal lithologies. Further work is suggested on validating long term predictions with observation data from natural analogue studies.

Geophysical benthic habitat mapping in Lake Tanganyika (Tanzania): Implications for spatial planning of small-scale coastal protected areas

Optimizing community-based fisheries management to enhance both food resource and biodiversity conservation in large lakes requires detailed knowledge of benthic habitats, which determines suitability for fish breeding sites. This information is unavailable for much of Lake Tanganyika, whose fisheries are threatened by a warming climate, destructive harvesting practices, and sediment pollution. Lake Tanganyika possesses a remarkably diverse fish population. Much of this is concentrated in areas with water depths less than 30 m and on rocky substrate. Here, geophysical tools were used to map benthic habitats in a 21 km2 co-management area of the lake in western Tanzania. Echosounding defined the position of the 30-m isobath, which varies with proximity to deltas and rift-related faults. Side-scan sonar discriminated among four unique substrates: crystalline bedrock, calcite-cemented sandstones, mixed siliciclastic sediments, and shell-rich sediments. Unlithified mixed silts and sands constitute over 91% of the study area. Rocky substrate composed of crystalline basement and calcite-cemented sandstone make up the less than 9% of the substrate in the study area. Crystalline bedrock was present from 0 to 30 m water depth, whereas the calcite-cemented sandstones were encountered in water less than 5 m deep. The spatial organization of rocky substrates is interpreted to be controlled by basin structure and lake level history; these habitats make ideal targets for establishing new small-scale protected areas. The techniques illustrated in this study are broadly applicable elsewhere in Lake Tanganyika, and to other large lakes where data needs for placing conservation reserves are lacking.

Mechanical characterization of thermally treated calcite-cemented sandstone using nanoindentation, scanning electron microscopy and automated mineralogy

In this study, we proposed a series of experimental and imaging techniques to investigate variations in the mechanical properties (reduced modulus, Er and hardness, H) of minerals and mineral-to-mineral contacts at a nanoscale in calcite-cemented sandstone subjected to high temperature treatment (500 °C) in a furnace. Automated mineralogy combined with environmental scanning electron microscope (ESEM) was used to determine minerals and mineral-to-mineral contacts in oven-dried and furnace-heated calcite-cemented sandstone. The crack density in oven-dried and furnace-heated calcite-cemented sandstone samples was measured using a MATLAB code written for ESEM image processing. The results showed significantly increased microcrack densities in furnace-heated samples compared with oven-dried samples. Additionally, nanoindentation tests were performed to determine the Er and H of the minerals and mineral-to-mineral contacts subjected to high temperature treatment. The Er and H of the minerals and mineral-to-mineral contacts significantly decreased in furnace-heated samples compared to oven-dried samples. Finally, an effort was made to understand the relationship between nanoscale mechanical properties and mesoscale properties of calcite-cemented sandstone. The decreased Er and H of the different minerals and mineral-to-mineral contacts due to high temperature treatment were likely to be caused by the decreased mesoscale mechanical properties of calcite-cemented sandstone.

Effects of a High Temperature (500 °C) on the Fracture Processes in Calcite-Cemented Sandstone Along Bedding-Plane Orientations

Effects of a High Temperature (500 °C) on the Fracture Processes in Calcite-Cemented Sandstone Along Bedding-Plane Orientations

A Statistical Approach for Analysis of Dissolution Rates Including Surface Morphology

Understanding mineral dissolution is relevant for natural and industrial processes that involve the interaction of crystalline solids and fluids. The dissolution of slow dissolving minerals is typically surface controlled as opposed to diffusion/transport controlled. At these conditions, the dissolution rate is no longer constant in time or space, an outcome observed in rate maps and correspondent rate spectra. The contribution and statistical prevalence of different dissolution mechanisms is not known. Aiming to contribute to close this gap, we present a statistical analysis of the variability of calcite dissolution rates at the nano- to micrometer scale. A calcite-cemented sandstone was used to perform flow experiments. Dissolution of the calcite-filled rock pores was measured using vertical scanning interferometry. The resultant types of surface morphologies influenced the outcome of dissolution. We provide a statistical description of these morphologies and show their temporal evolution as an alternative to the lack of rate spatial variability in rate constants. Crystal size impacts dissolution rates most probably due to the contribution of the crystal edges. We propose a new methodology to analyze the highest rates (tales of rate spectra) that represent the formation of deeper etch pits. These results have application to the parametrization and upscaling of geochemical kinetic models, the characterization of industrial solid materials and the fundamental understanding of crystal dissolution.

Effects of water on rock fracture properties: Studies of mode I fracture toughness, crack propagation velocity, and consumed energy in calcite-cemented sandstone

Water-induced strength reduction is one of the most critical causes for rock deformation and failure. Understanding the effects of water on the strength, toughness and deformability of rocks are of a great importance in rock fracture mechanics and design of structures in rock. However, only a few studies have been conducted to understand the effects of water on fracture properties such as fracture toughness, crack propagation velocity, consumed energy, and microstructural damage. Thus, in this study, we focused on the understanding of how microscale damages induced by water saturation affect mesoscale mechanical and fracture properties compared with oven dried specimens along three notch orientations-divider, arrester, and short transverse. The mechanical properties of calcite-cemented sandstone were examined using standard uniaxial compressive strength (UCS) and Brazilian tensile strength (BTS) tests. In addition, fracture properties such as fracture toughness, consumed energy and crack propagation velocity were examined with cracked chevron notched Brazilian disk (CCNBD) tests. Digital Image Correlation (DIC), a non-contact optical measurement technique, was used for both strain and crack propagation velocity measurements along the bedding plane orientations. Finally, environmental scanning electron microscope (ESEM) was employed to investigate the microstructural damages produced in calcite-cemented sandstone specimens before and after CCNBD tests. As results, both mechanical and fracture properties reduced significantly when specimens were saturated. The effects of water on fracture properties (fracture toughness and consumed energy) were predominant in divider specimens when compared with arrester and short transverse specimens. Whereas crack propagation velocity was faster in short transverse and slower in arrester, and intermediate in divider specimens. Based on ESEM data, water in the calcite-cemented sandstone induced microstructural damages (microcracks and voids) and increased the strength disparity between cement/matrix and rock forming mineral grains, which in turn reduced the crack propagation resistance of the rock, leading to lower both consumed energy and fracture toughness (KIC).

Coupled experimental/numerical workflow for assessing quantitative diagenesis and dynamic porosity/permeability evolution in calcite-cemented sandstone reservoir rocks

Some of the world best hydrocarbon reservoirs (carbonates and siliciclastics) are also believed to be valuable for subsurface storage of CO2 and other fluids. Yet, these reservoirs are heterogeneous in terms of their mineralogy and flow properties, at varying spatial-temporal scales. Therefore, predicting the porosity and permeability (flow properties) evolution of carbonates and sandstones remains a tedious task. Diagenesis refers to the alteration of sedimentary rocks through geologic time, mainly due to rock-fluid interactions. It affects primarily the flow properties (porosity and permeability) of already heterogeneous reservoir rocks. In this project a new approach is proposed to calculate/quantify the influence of diagenetic phases (e.g. dissolution, cement plugging) on flow properties of typical sandstone reservoir rocks (Early Jurassic Luxembourg Formation). A series of laboratory experiments are performed in which diagenetic phases (e.g. pore blocking calcite cement in sandstone) are selectively leached from pre-studied samples, with the quantification of the petrophysical characteristics with and without cement to especially infer permeability evolution. Poorly and heavily calcite-cemented sandstone samples, as well as some intermediate cemented samples were used. The results show a distinctive dissolution pattern for different cementation grades and varying Representative Elementary Volumes (REVs). These conclusions have important consequences for upscaling diagenesis effects on reservoirs, and the interpretation of geochemical modelling results of diagenetic processes. The same approach can be applied on other type of cements and host-rocks, and could be improved by integrating other petrophysical analyses (e.g. petroacoustic, NMR).

Supercritical CO2 injection and residence time in fluid-saturated rocks: Evidence for calcite dissolution and effects on rock integrity

CO2 injection and storage in geological reservoirs is an attractive prospect for mitigating the anthropogenic production of greenhouse gases and global warming. The technology could lead to mineral precipitation and therefore stable storage over geological time scales. This contribution investigates the evolution of three calcite-rich reservoir rock analogues during injection of and exposure to supercritical CO2 (scCO2), i.e., two limestones (Tuffeau and Savonnieres) and a synthetic calcite-cemented sandstone (CIPS). Three types of exposure protocols have been conducted: (i) scCO2 injection and a four-hour residence time in an initially dry rock; (ii) scCO2 injection and a two-hour residence time in an initially brine-saturated rock; and (iii) scCO2 injection and a four-hour residence time in an initially brine-saturated rock. Two aspects are monitored during these experiments: (i) the evolution of the pore fluid chemical composition; and (ii) the evolution of the rocks’ physical properties (i.e. porosity, permeability, P-wave velocity and electrical resistivity). Additionally, some scCO2 injection and exposure experiments in the brine-saturated rocks have been conducted using X-ray tomographic monitoring.X-ray tomographic monitoring suggests that scCO2 first displaces the water, leading to an average water saturation of about 70–90%. Then, scCO2 dissolves in the pore brine, leading to a homogeneous decrease by about 3% in water saturation of the sample. As a result, the pore brine acidifies even after 2h of exposure only, which leads to calcite dissolution and a significant increase in the brine’s concentration in calcium cations. For the samples and most exposure experiments, evidence of calcite dissolution is inferred from the measured physical properties. For the brine-saturated Tuffeau limestone and CIPS sandstone, calcite dissolution leads to significant mechanical weakening. For the brine-saturated Savonnieres limestone, the sample subject to two-hour residence time shows evidence of calcite dissolution, whereas the sample after four-hour residence time does not. Calcite re-precipitation could be the cause of this unexpected response after four hours.

Origin of diagenetic calcite cements in the continental Qaidam Basin, NW China: Implication for fluid flow and hydrocarbon migration

Diagenetic carbonate cements have been widely investigated to reveal fluid information in marine basins. However, they have not been well studied in continental basins. This paper addresses this issue using a case study in the northern Qaidam Basin, northwest China. The calcite cements in the reservoir sandstones were investigated by an integrated analysis including petrology, mineralogy, carbon and oxygen isotopes and fluid inclusion. Calcite is the dominant cement and main cause of poor porosity in the sandstones. The δ13CPDB and δ18OPDB values of calcite cements display a wide isotopic range, varying from −13.5 to 2.5‰ and −16.7 to −4.7‰, respectively. However, the δ13CPDB and δ18OPDB values from the central basin are within a limited range (−4.2 to −2.0‰ and −11.2 to −9.4‰, respectively). Dull- and bright-luminescence zones and two populations of fluid inclusion indicate two generations of calcite (Ca 1 and Ca 2) in N1 well. Ca 1 and Ca 2 have similar δ13CPDB values (−8.7 to −7.0‰), whereas the δ18OPDB values (−13.7 to −13.1‰) of Ca 2 are more negative than those (−10.8 to −9.8‰) of Ca 1.Three origins of calcite cement were clearly identified: (i) hydrocarbon source rocks in thermal degradation stage, (ii) lacustrine carbonates in deep strata, and (iii) paleo-lakewater. Two episodes of organic fluids derived from the Jurassic source rocks flowed towards the basin margin, resulting in two rounds of acidic→hydrocarbon→alkaline fluid evolution identified in the reservoir sandstones. Calcite (Ca 1) precipitation generated highly calcite-cemented sandstones, which severely downgraded the reservoir quality. However, the second episode of acidic fluid created new porous zones for subsequent hydrocarbon accumulation. Although the cementation history in the central basin did not occlude all porosity, the presence of fluids derived from buried lacustrine carbonates suggests a poor prospect for hydrocarbon migration. The good spatial correlation between the occurrence of the 13C-depleted calcite cement and the presence of hydrocarbons suggests that the 13C-depleted calcite (relative to lacustrine carbonates) may be an indicator of hydrocarbon migration in the northern Qaidam Basin.

Diagenetic Heterogeneity of Deep Sandstones and Its Relationship to Oil Emplacement: A Case Study from the Middle Jurassic Toutunhe Formation in the Fukang Sag, Central Junggar Basin (NW China)

The Middle Jurassic Toutunhe Formation at depths of approximately 4000–6000 m has increasingly come into focus as a current deep reservoir target in the central Junggar Basin (NW China). Based on petrography, SEM, stable isotopes, and fluid inclusion analyses, the goals of this study were to investigate the effect of depositional lithofacies on sandstone diagenetic heterogeneity and to examine the relationship between diagenetic evolution and oil charge within a heterogeneous reservoir. Grain size controls the overall abundance of cement and porosity and reservoir properties through its effect on ductile lithic sand grains and hence on mechanical compaction. Early diagenetic calcite cement is an exception to this trend. Ductile lithic-rich, very fine-grained sandstones featured compaction of easily deformed, clay-rich grains, resulting in a very rapid loss of porosity during burial. In contrast, dissolution and cementation occurred as well as ductile compaction in the fine-grained sandstones. Two episodes of oil charge occurred in the relatively coarser-grained sandstone lithofacies. Diagenesis progressed alternately with oil emplacement, and some diagenetic alterations and oil charge occurred simultaneously. Ductile lithic-rich, highly compacted sandstones and tightly calcite-cemented sandstones can create permeability barriers embedded in permeable reservoir sandstones, probably resulting in heterogeneous flow.

Seismic prediction of sandstone diagenetic facies: Applied to Cretaceous Qingshankou Formation in Qijia Depression, Songliao Basin, East China

Abstract Seismic diagenetic facies is an important control on reservoir quality. This study investigated the feasibility of predicting sandstone diagenetic facies using conventional 3D seismic data by seismic sedimentology and calibrated by laboratory core-analysis data in the Qijia area of the Songliao Basin. Three core issues are related to seismic characterization of diagenetic facies, including how to correlate stratigraphic and diagenetic units from seismic data, how to evaluate the relationship between core-based diagenetic facies and seismic attributes, and how to find an effective way of mapping seismic diagenetic facies. Well- and seismic-based, high-resolution sequence analysis and seismic stratal slicing provide reservoir-scale (20 m) seismic representations of diagenetic units. Core-based analyses of sandstone diagenetic processes and diagenetic sequences reveal the kind of diagenesis that most influences reservoir quality. An investigation of reservoir parameters and acoustic rock properties further reveals the link between diagenetic facies and impedance, leading to a recognition of calcite cementation as the process that can be detected by seismic data. A seismic-based lithology cube (e.g., 90°-phased seismic volume) provides the amplitude (impedance) signal for detection of diagenetic facies. Stratal slices made from the seismic-based lithology cube are then used to interpret depositional facies and systems. Eventually a seismic diagenetic-facies map is generated through analysis of the relationships between depositional facies, impedance, and diagenetic facies. A case study of clay- and calcite-cemented sandstone in the Qijia area shows that although still in its infancy, seismic detection of sandstone diagenetic facies using conventional seismic data is definitely feasible.

Controlling factors on the morphology and spatial distribution of methane-related tubular concretions – Case study of an Early Eocene seep system

Up to 10 m in length and >1 m in diameter tubular, calcite-cemented sandstone concretions are hosted by the faulted Dikilitash unconsolidated sands and sandstones. These structures document shallow subsurface pathways of Early Eocene methane seepage in the Balkan Mountains foreland (NE Bulgaria). Their exceptional exposure allowed a unique study of the factors governing the morphology and spatial distribution of such fossilized fluid conduits. The large dimensions and subvertical, cylindrical shape of the most common tube type primarily reflects the buoyancy-driven, vertical path of an ascending gas-bearing fluid through permeable, mainly unconsolidated sandy host sediments. Tube morphology was also influenced by local stratigraphic anisotropies and might as well document differences in former seepage conditions. Mapping of >800 tubular concretions showed the NNW–NNE elongation and alignment of tube clusters and massive cemented sandstone structures. This suggests that Paleogene fault systems played a major role in directing the movement of fluids. However, within a single tube cluster, tubes are preferentially aligned, over distances up to 50 m along directions at an angle between 10° and 36° with respect to the inferred NNW–NNE, cluster parallel fault traces. In addition, cylindrical tubes of analogue dimensions are aligned over distances >100 m along N15° to N25°-oriented directions. It is hypothesized that this spatial geometry of tubular concretions reflects the complex geometry of deformations structures in fault damage zones along which fluids were preferentially channelled.

Calcite cement in Permian deep-water sandstones, Delaware Basin, west Texas: Origin, distribution, and effect on reservoir properties

Calcite cement is the dominant control on reservoir quality in turbidite sandstones of the Upper Permian Bell Canyon Formation, Delaware Basin. These well-sorted, very fine-grained arkoses were deposited in a basin-floor setting by channel-levee systems terminating in broad lobes. Calcite cement distribution in the East Ford and Geraldine Ford fields was mapped using core, log, and thin-section data. Calcite is concentrated in tightly cemented zones that are mostly less than 1 ft (0.3 m) thick. Areas that have high percentages of calcite-cemented sandstone (20%) occur along the margins of the sandstone bodies, in overbank and lobe deposits, where sandstone pinches out into siltstone. Areas that have the lowest percentage of calcite-cemented sandstone (10%) occur where the sandstone is thickest, in the channel facies. Isotopic composition of the calcite (13C = 1.8 to 3.0 [relative to the Peedee belemnite, PDB], 18O = 4.6 to 6.3 [PDB]) is consistent with the source of calcium carbonate being from dissolution of detrital carbonate rock fragments and marine skeletal debris. Because internal sources of calcite were apparently insufficient to account for the cement volume, cement components are interpreted as having been transported into the sandstones from organic-rich basinal siltstones and limestones. Feldspars buffered acidic formation waters near where they entered the sandstone, resulting in calcite concentrated near the sandstone margins. The calcite formed near maximum burial depths of 4800 ft (1.5 km) and temperatures of 104F (40C) from marine pore waters with 18O of approximately 0 (relative to standard mean ocean water). Most of the calcite-cemented zones are interpreted as being concretions that extend no more than a few meters laterally. Production data and geophysical log correlations suggest that some cemented zones are laterally continuous at least 1000 ft (300 m) and cause vertical reservoir compartmentalization. Laterally extensive calcite layers may be associated with the base of turbidite deposits.

Chemical signature of two Permian volcanic ash deposits within a bentonite bed from Melo, Uruguay

A Permian bentonite deposit at Melo, Uruguay is composed of a calcite-cemented sandstone containing clay pseudomorphs of glass shards (0-0.50 m) overlying a pink massive clay deposit (0.50-2.10 m). The massive bed is composed of two layers containing quartz and smectite or pure smectite respectively. The smectite is remarkably homogeneous throughout the profile: it is a complex mixed layer composed of three layer types whose expandability with ethylene glycol (2EG 1EG or 0EG sheets in the interlayer zone which correspond to low-, medium- and high-charge layers respectively) varies with the cation saturating the interlayer zone. The smectite homogeneity through the profile is the signature of an early alteration process in a lagoonal water which was over saturated with respect to calcite. Compaction during burial has made the bentonite bed a K-depleted closed system in which diagenetic illitization was inhibited. Variations in major, REE and minor element abundances throughout the massive clay deposit suggest that it originated from two successive ash falls. The incompatible element abundances are consistent with that of a volcanic glass fractionated from a rhyolite magma formed in a subduction/collision geological context.