High-porosity Sandstone Research Articles

CO2 Storage Site Characterisation using Combined Regional and Detailed Seismic Data: Harvey, Western Australia

Some 115 km2 of regional 3D seismic data were acquired in the first quarter of 2014 near Harvey, Western Australia, for the needs of the South West CO2 Hub project. The survey proved to be of great importance for a regional characterisation of the reservoir, identification of the large structures and key geological interfaces. However, small to medium size structures of interest for the development of the static and dynamic models were poorly imaged in this survey as the recording geometry was adjusted for the greater depths, which was between 2km and 3km. To improve the imaging of the shallow structures, a high-resolution (nested) 3D survey centered at Harvey 4 well was undertaken in 2015 (Urosevic et al., 2015). This survey utilised a single geophone and single, 24 s long, broadband (6–150Hz) sweep combined with high data density to improve signal to noise ratio that was initially lowered by not employing high-power sources and geophone arrays. The results of this high-resolution 3D survey demonstrate that high-density surveys are important even at the characterisation stage and are crucial for development of a detailed static model. For that purpose, both post and pre-stack inversions of these data were utilised to model distribution of paleosols, lenses of high clay content, which are assumed to serve as baffles for CO2 upward migration. A good correlation was established between very low impedance values and increased percentage of paleosols and on the other end of the scale very high impedance values and low porosity sandstones. A pre-stack migrated high-resolution cube and the attribute derived from it, such as coherency and impedance, enabled improved structural and stratigraphic analysis around Harvey 4 well. The results shown were of a crucial importance for the containment studies, development of the dynamic model and establishment of the injection intervals.

Multiscale modeling and analysis of compaction bands in high-porosity sandstones

We present a multiscale investigation on the initiation and development of compaction bands in high-porosity sandstones based on an innovative hierarchical multiscale approach. This approach couples the finite element method and the discrete element method (DEM) to offer direct, rigorous linking of the microscopic origins and mechanisms with complex macroscopic phenomena observed in granular rocks such as strain localization and failure. To simulate compaction band in granular cementitious sandstone, we adopt a bonded contact model with normal and tangential interparticle cohesions in the DEM and propose a dual-porosity structure consisting of macro-pores and interstitial voids for the representative volume element to mimic the typical meso-structure of high-porosity sandstones. In the absence of particle crushing, our multiscale analyses identify debonding and pore collapses as two major contributors to the formation of compaction bands. The critical pressures predicted by our simulations, corresponding to surges of debonding and pore collapse events, agree well with the estimations from field data. The occurrence patterns of compaction band are found closely related to specimen heterogeneity, porosity and confining pressure. Other deformation band patterns, including shear-enhanced compaction bands and compactive shear bands, were also observed under relatively low confining pressure conditions with a rough threshold at $$0.55P^{*}$$ ( $$P^{*}$$ is the critical pressure) on the failure envelop. Key microscopic characteristics attributable to the occurrence of these various deformation patterns, including fabric anisotropy, particle rotation, debonding and pore collapse, are examined. Shear-enhanced compaction bands and pure compaction bands bear many similarities in terms of these microscopic characteristics, whereas both differ substantially from compactive shear bands.

Evolution of permeability and Biot coefficient at high mean stresses in high porosity sandstone

A series of constant mean stress (CMS) and constant shear stress (CSS) tests were performed to investigate the evolution of permeability and Biot coefficient at high mean stresses in a high porosity reservoir analog (Castlegate sandstone). Permeability decreases as expected with increasing mean stress, from about 20 Darcy at the beginning of the tests to between 1.5 and 0.3 Darcy at the end of the tests (mean stresses up to 275MPa). The application of shear stress causes permeability to drop below that of a hydrostatic test at the same mean stress. Results show a nearly constant rate decrease in the Biot coefficient as the mean stress increases during hydrostatic loading, and as the shear stress increases during CMS loading. CSS tests show a stabilization of the Biot coefficient after the application of shear stress.

The Distribution and Origin of Carbonate Cements in Deep-Buried Sandstones in the Central Junggar Basin, Northwest China

Extremely high porosities and permeabilities are commonly discovered in the sandstones of the Xishanyao Formation in the central Junggar Basin with the burial depth greater than 5500 m, from which hydrocarbons are currently being produced. High content of carbonate cements (up to 20%) is also observed in a similar depth range. Our study aimed to improve our understanding on the origin of carbonate cements in the Xishanyao Formation, in order to provide insights into the existence of high porosity sandstones at greater depths. Integrated analyses including petrographic analysis, isotopic analysis, fluid-inclusion, and core analysis were applied to investigate the distribution and origin of carbonate cements and the influence of high fluid pressure on reservoir quality. Textural evidences demonstrate that there are two generations of carbonate cements, precipitated at the temperature of 90°C and 120°C, respectively. The carbonate cements with low δCPDB13 ranging from −19.07 to -8.95‰ dominantly occurred near the overpressure surface and especially accumulated at approximately 100 m below the surface. Our interpretation is that high content of carbonate cements is significantly influenced by early carbonate cements dissolution and migration under overpressure. Dissolution of plagioclase resulted in the development of internal pores and porosities of as much as 10% at 6500 m depth presumably.

Textural control on the quadrature conductivity of porous media

Induced polarization (IP) has been broadly used for environmental and hydrogeological applications and in civil engineering. The IP response of a porous medium without metallic particles (described by its quadrature conductivity or its normalized chargeability) is controlled by the interfacial electrochemistry of the electrical double layer and the pore-space geometry. We use the specific surface per unit pore volume normalized by the formation factor (i.e., [Formula: see text]) as the controlling textural parameter for the quadrature conductivity. This relationship is obtained by averaging the surface conductance over the pore volume. A database that contains 76 samples (including porous borosilicate glass, sandstones, and clayey sediments) is used to check the new scaling. In addition to these data, we have conducted new IP measurements on 13 samples from the Middle Bakken Formation corresponding to low-porosity clayey materials. Comparison between the experimental data and our model confirms that the ratio [Formula: see text] is the dominant textural parameter describing the quadrature conductivity [Formula: see text] of a broad range of porous media. The database was also used to test whether the quadrature conductivity depended either on [Formula: see text], or the specific surface area [Formula: see text], or the ratio [Formula: see text] ([Formula: see text] being the connected porosity). Although the quadrature conductivity scales with [Formula: see text] and [Formula: see text] for high-porosity sandstones, these relationships are not appropriate for the low-porosity clayey materials presented in this study. However, experimental data support the dependence of the quadrature conductivity on [Formula: see text], a published relationship obtained through the volume averaging approach.

Relations between shallow cataclastic faulting and cementation in porous sandstones: First insight from a groundwater environmental context

The interplay between fault zone cataclasis and cementation is important since both processes can drastically reduce the permeability of faults in porous sandstones. Yet the prediction of fault cementation in high-porosity sandstone reservoirs remains elusive. Nevertheless, this process has rarely been investigated in shallowly buried faults (<2 km; T°<80 °C) where its sealing capacity could be acquired early in the geological history of a reservoir. In this paper, the macro- and microscopic analysis of a fault zone in the porous Cenomanian quartz arenite sands of Provence (France) shows that silica diagenesis occurs in the most intensely-deformed cataclastic parts of the fault zone. This fault zone shows 19–48% of its total thickness occupied by low-porosity quartz-cemented cataclastic shear bands whose porosities range from 0 – ca. 5%. The analysis of the weathering profile around the fault zone reveals the presence of groundwater silcretes in the form of tabular, tightly silicified concretions cross-cut by the fault. Detailed transmitted light, cold-cathodoluminescence and scanning electron microscopy analyses of the silica cements (from the fault and the silcrete) reveal that all the silica cements originate from groundwater diagenetic processes. This study therefore shows that silica cementation can occur specifically in fault zones and as groundwater silcrete in the shallow context of a groundwater system, generated at the vicinity of an erosional unconformity.

Parameter calibration for high-porosity sandstones deformed in the compaction banding regime

This paper discusses the parameter calibration procedure for an elastoplastic constitutive model for high-porosity rocks. The model selected for the study is formulated in the frame of the critical state theory, which is here used in a form able to accommodate non-associated plastic flow and softening effects due to volumetric and deviatoric plastic strains. The goal of this study is to generate a set of model constants able to capture both the stress-strain response and the compaction localization characteristics (e.g., stress and inclination at the onset of the deformation bands). For this purpose, data about the compaction localization properties of four extensively characterized sandstones have been considered. In particular, the strain localization theory has been used as a calibration tool, using explicitly information about the pressure-dependence of the localization mechanisms observed in experiments. The model constants have been defined by matching the constitutive response upon hydrostatic compression, as well as the stresses at the transition from high-angle shear bands to pure compaction bands, and from compaction bands to homogeneous cataclastic flow. It is shown that such procedure generates a set of model constants able to capture satisfactorily both the rheological response upon triaxial compression and the salient features of the compaction localization process.

The relationship between mineral content and acoustic velocity of sandstone reservoirs in Junggar basin

Sandstone reservoirs have generally high porosity in the Shawan formation of the Chunguang oil field, Junggar basin, because they developed in geological conditions of shallow and weak compaction. High porosity always links lower acoustic velocities in sandstone. However, when it is more than a certain value (approximately 27.5%), the porosity is not in accordance with acoustic velocities. In addition, cast thin sections illustrated incoherence between pore types and porosity. Fluids and mineral content are the two main factors changing acoustic velocities. This means that acoustic velocities of the high-porosity sandstone are mainly affected by the mineral content and fluid properties. Hence, data from litho-electric analysis are used to measure velocities of the compression shear waves, and thin sections are used to identify the mineral content. By the application of cross-plot maps, relations of acoustic velocities and mineral contents are proposed. Mineral contents include mainly quartz, feldspar, and tuff. In normal rock physical models, the shale content is calculated from well logs. The mineral grain is often regarded as pure quartz grain or average mineral composition. However, the application of the normal rock physics model will be inaccurate for high-porosity sandstone. Experience regression functions of the velocity model are established to estimate acoustic velocities. Also, mineral content logs could be predicted by using the P-wave acoustic log, and the rock physics model would be enhanced by using these logs of dynamic mineral contents. Shear wave velocity could also be estimated more accurately.

An experimental investigation of the role of microfracture surfaces in controlling quartz precipitation rate: Applications to fault zone diagenesis

We present the results of quartz growth experiments, which were designed to assess the role of microfracture surfaces in controlling quartz-precipitation rates during fault-zone diagenesis. Experiments were run in hydrothermal cold-seal vessels at 300–450 °C and 150 MPa confining pressure for up to 1344 h. Microfractures routinely form at grain contacts during these experiments. Microfracture kinematic-aperture distributions indicate that microfractures form within the first 48 h of each experiment. Regardless of experimental temperature or duration, microfracture-sealing cements account for approximately the same amount of new quartz cement in each experiment. With increasing experimental duration, sealed microfractures were progressively overgrown by grain-boundary overgrowth cements. Spatial and temporal trends in the distribution of overgrowth- and microfracture-sealing cements indicate that precipitation rates on newly formed microfractures greatly exceed those on detrital-grain boundaries. This effect persists regardless of natural iron-oxide grain coatings present in a subset of our experiments. While our results agree with previous research that demonstrated increased growth rates on fracture surfaces in faults in fully lithified rock, fundamental differences in the nature of deformation in our experiments provide insight into quartz cementation in cataclastic deformation bands in faults offsetting high-porosity sandstones.

Mapping CO2 in Real Time With Downhole Fluid Analysis in the East Irish Sea

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 166497, ’Mapping CO2 in Real Time in Hydrocarbon Reservoirs With Downhole Fluid Analysis: First Successful Experience in the East Irish Sea, UK Continental Shelf,’ by B. Quayle, S. James, and M. Quine, Centrica Energy, and I. De Santo, P. Jeffreys, and J.Y. Zuo, Schlumberger, prepared for the 2013 SPE Annual Technical Conference and Exhibition, New Orleans, 30 September-2 October. The paper has not been peer reviewed. This paper describes the first successful attempt on the continental shelf offshore UK to map carbon dioxide (CO2) in real time while logging during a drilling campaign in the East Irish Sea. Reservoirs in this sea’s basin contain varying proportions of CO2, nitrogen (N2), and hydrogen sulfide (H2S), in addition to oil and methane. Two of these wells develop the Rhyl gas field. Downhole-fluid-analysis (DFA) technologies were deployed with a wireline-formation-testing (WFT) tool to measure CO2 content accurately downhole. Introduction The Rhyl field was discovered in 2009 and received development approval in 2012. It is located 11 km north of the North Morecambe field. The North and South Morecambe fields were discovered in the 1970s, with some 7 Tcf of gas initially in place. Production from the Rhyl field extends the longevity of these assets. Vertical and horizontal variations in CO2 content in the Rhyl field were assessed across the Triassic Ormskirk sandstone, the upper member of the Sherwood sandstone group. The Ormskirk sandstone formation represents the principal reservoir target in the East Irish Sea, comprising high-porosity aeolian and fluvial sandstones with variable grain size and playa mudstones. Gas Composition in the Rhyl Field The composition of the gas found in Rhyl includes both hydrocarbon and nonhydrocarbon components such as N2 and CO2. The N2 is derived from late-stage hydrocarbon generation, while isotope data indicate that the CO2 has a magmatic origin. It is believed to have been exsolved from the magma of a series of Tertiary dolerite intrusions into the Ormskirk sandstones in close proximity to the Rhyl field. The samples collected during the drillstem test conducted on exploration Well 113/27b-6 provided several qualitative indications of a higher CO2 content than was originally expected from the correlation with the nearby North Morecambe field: a high gas density measured at the separator and difficulty in sustaining a flare.

Use of acoustic emissions to investigate localization in high-porosity sandstone subjected to true triaxial stresses

Acoustic emissions were used to investigate the evolution of damage and strain localization in Castlegate sandstone specimens subjected to true triaxial states of stress, where the intermediate principal stress ranged from equal to minimum compression to equal to maximum compression. Three failure modes were observed: shear band formation at low mean stresses, compaction localization at intermediate stresses, and no localization at high mean stresses. For shear bands, the onset of localization consistently occurred prior to peak stress, while compaction localization initiated at the beginning of a stress plateau. The band angle (defined as the angle between the band normal and the maximum compression direction) determined by fitting a plane through the localized acoustic emission events corresponded well with the shear band angle expressed on the specimen jacket. As expected, the band angle decreased with increasing mean stress. The theoretically predicted dependence of band angle on deviatoric stress state was not confirmed; however, data scatter due to natural variations in material could obscure such a trend. Each failure mode displayed a unique acoustic emission rate response; therefore, this rate response alone can be used to determine the failure mode and the onset of localization in Castlegate sandstone.

2011 Drucker Medal Paper: Localized Compaction in Porous Sandstones

Compaction bands are narrow, roughly planar zones of localized deformation, in which the shear is less than or comparable to compaction. Although there are differences in their appearance in the field and in laboratory specimens, they have been observed in both for high-porosity (greater than about 15%) sandstones. Because the porosity in them is reduced and the tortuosity increased, they inhibit fluid flow perpendicular to their plane. Consequently, they can alter patterns of fluid movement in formations in which they occur and are relevant to applications involving fluid injection or withdrawal. Formation of compaction bands is predicted by a framework that treats localized deformation as a bifurcation from homogeneous deformation. This paper gives a brief overview of compaction localization but focuses on field and laboratory observations that constrain two parameters entering the bifurcation analysis: a friction coefficient μ and a dilatancy factor β. The inferred values suggest that normality (μ = β) is not satisfied, and compaction localization occurs on a transitional portion of the yield surface, where the local slope in a plot of Mises equivalent shear stress versus compressive mean normal stress changes from positive (μ &gt; 0) to negative (μ &lt; 0). These inferences are at odds with critical state and cap theories that typically assume normality and predict dilation on the portion of the surface where μ &gt; 0. In addition, the values suggest that the critical state (μ = 0) does not necessarily correspond to zero volume change.

A geometric model for the formation of deformation band clusters

Arrays of closely-spaced (approximately <70 mm) sub-parallel cataclastic deformation bands are common structures in deformed, high-porosity (∼10–35%) sandstones. The distribution of strain onto many small-displacement deformation bands is thought by some to result from strain-hardening of the cataclasite within individual bands. Examination of both normal and strike-slip faults with displacements ≤7 m from southeastern Utah, USA, and the North Island of New Zealand suggests, however, that clusters of deformation bands systematically develop at fault geometric irregularities (e.g., fault bends, steps, relays, intersections and zones of normal drag). The strain-hardening model does not account for clustering of deformation bands at fault geometric irregularities or the associated widespread coalescence of bands, and is not unequivocally demonstrated by post-peak macroscopic mechanical responses in laboratory rock deformation experiments. A geometric model is proposed in which individual bands within clusters develop sequentially due to migration of incremental shear strains at fault geometric irregularities as part of a slip localisation, asperity removal and strain weakening process. The geometric model, which does not require strain hardening of the fault rock, applies for the duration of faulting and a range of rock types in the brittle upper crust.

The propagation of compaction bands in porous rocks based on breakage mechanics

We analyze the propagation of compaction bands in high porosity sandstones using a constitutive model based on breakage mechanics theory. This analysis follows the work by Das et al. [] on the initiation of compaction bands employing the same theory. In both studies, the theory exploits the links between the stresses and strains, and the micromechanics of grain crushing and pore collapse, giving the derived constitutive models advantages over previous models. In the current post localization analysis, the bifurcation instability of the continuum model is suppressed by the use of a rate‐dependent regularization. This allows us to perform a series of finite element analyses of drained triaxial tests on porous sandstone specimens. The obtained numerical results compare well with experimental counterparts, in terms of both the initiation and propagation of compaction bands, besides the macroscopic stress‐strain responses. On this basis, a parametric study is carried out to explore the effects of loading rate, degree of structural imperfections, and confining pressure on the propagation of compaction bands.

Comparison of digital outcrop and conventional data collection approaches for the characterization of naturally fractured reservoir analogues

Abstract In this study, fracture systems developed within faulted, high-porosity sandstones in the decommissioned mines of Alderley Edge, Cheshire, UK are characterized using lidar (Light Detection And Ranging)-based analysis. The geometry of the mine workings prove to be conducive to the extraction of fracture attributes, whilst providing a degree of exposure of a notable Triassic-aged reservoir outcrop analogue (Helsby Sandstone Formation) not afforded at the surface. To test the fidelity of the approach, fracture statistics generated from lidar-derived digital outcrop models are compared to an equivalent dataset collected using conventional manual surveys, with digital outcrop and manually acquired fracture attributes used to populate discrete fracture network models. These are upscaled to provide equivalent porous medium properties, enabling the impact of uncertainties introduced into fracture modelling workflows by lidar-based techniques to be assessed. Whilst broadly comparable to fracture attributes obtained using manual surveys, the systematic underrepresentation of fracture properties is observed within lidar-derived dataset, resulting in the underestimation of fracture network flow capacity. The study results suggest that, whilst enhancing data acquisition rates and coverage of exposure surfaces, the use of digital discontinuity analysis may introduce additional biases into fracture datasets, increasing the level of uncertainty within resultant modelled networks.

Permeability and flow impact of faults and deformation bands in high-porosity sand reservoirs: Southeast Basin, France, analog

Outcrops of the Cretaceous high-porosity sandstone of the Southeast Basin, France, show two main types of deformation structures: a large number of small-offset, shear-enhanced cataclastic deformation bands (DBs); and a small number of large (meters to decameters)-offset ultracataclastic fault zones. Microstructural analyses of the cataclastic DBs show that fragmentation produces strands of cataclastic fragment-supported matrix, separated by weakly fractured host rock, which cluster to form the DBs. The ultracataclastic fault zones, however, are composed of a matrix-supported ultracataclasite material. Permeability data show that the DBs reduce host-rock permeability by 0.5 to 2 orders of magnitude, whereas the ultracataclasites reduce permeability by approximately 4 orders. Simple calculations considering the structural frequency, thickness, and permeability of these faults suggest that, although the DBs may have an impact on single-phase flow, it is most likely to be less than a 50% reduction in flow rate in extensional contexts, but it may be more severe in the most extreme cases of structural density in tectonic shortening contexts. The larger ultracataclastic faults, however, despite their much lower frequency, will have a more significant reduction in flow rate, probably of approximately 90 to 95%. Hence, although they are commonly at or below the limit of seismic resolution, the detection and/or prediction of such ultracataclastic faults is likely to be more important for single-phase flow problems than DBs (although important two-phase questions remain). The study also suggests that it is inappropriate to use the petrophysical properties of core-scale DB structures as analogs to larger seismic-scale faults.

Mechanical behavior and localized failure modes in a porous basalt from the Azores

Basaltic rocks are the main component of the oceanic upper crust, thus of potential interest for water and geothermal resources, storage of CO2and volcanic edifice stability. In this work, we investigated experimentally the mechanical behavior and the failure modes of a porous basalt, with an initial connected porosity of 18%. Results were acquired under triaxial compression experiments at confining pressure in the range of 25–200 MPa on water saturated samples. In addition, a purely hydrostatic test was also performed to reach the pore collapse critical pressure P*. During hydrostatic loading, our results show that the permeability is highly pressure dependent, which suggests that the permeability is mainly controlled by pre‐existing cracks. When the sample is deformed at pressure higher than the pore collapse pressure P*, some very small dilatancy develops due to microcracking, and an increase in permeability is observed. Under triaxial loading, two modes of deformation can be highlighted. At low confining pressure (Pc &lt; 50 MPa), the samples are brittle and shear localization occurs. For confining pressure &gt; 50 MPa, the stress‐strain curves are characterized by strain hardening and volumetric compaction. Stress drops are also observed, suggesting that compaction may be localized. The presence of compaction bands is confirmed by our microstructure analysis. In addition, the mechanical data allows us to plot the full yield surface for this porous basalt, which follows an elliptic cap as previously observed in high porosity sandstones and limestones.

Hydrocarbon accumulation mechanism of low permeable tight lithologic oil fields in the Yanchang Formation, Ordos Basin, China

Abstract By analyzing the hydrocarbon accumulation conditions under the guideline of sequence stratigraphy, the hydrocarbon accumulation mechanism in the low permeability, tight reservoirs of the Yanchang Formation in the Ordos Basin is studied from the perspective of pore evolution, and the hydrocarbon accumulation evolution models are concluded. The low permeability, tight reservoirs of the Yanchang Formation are distributed within the range of good hydrocarbon source rocks. The reservoir sand-bodies are dominated by lacustrine gravity flow deposits and delta front deposits. In the early period when hydrocarbons are accumulated in the Yanchang Formation, the reservoirs are characterized by high porosity and high permeability. The oil and gas generated in the period are accumulated normally under the effect of buoyancy. In the middle-late period, the reservoirs have experienced continuous compression. The petroleum generated in the late period, the major hydrocarbon generation period, migrates and accumulates by infiltration along the previously remained paths under the effect of buoyancy and pressure difference between the source rocks and the reservoirs. When the reservoirs become tight, oil and gas stop migration. From the Late Jurassic to the middle period of the Early Cretaceous, oil and gas widely charge the high porosity sandstones near the source rocks when they migrate from the center of the lacustrine basin to the borders. In the late period of the Early Cretaceous, the diagenesis of sandstone reservoirs is enhanced, making the reservoirs tight. After the Late Cretaceous, the present low permeability tight lithologic reservoirs are formed due to the compressing and uplifing movement of the structure which dips down in the west and tilts up in the east.

Characterization of deformation bands associated with normal and reverse stress states in the Navajo Sandstone, Utah: Discussion

In a recent article about deformation-band networks in Utah, Solum et al. (2010) present the thesis that deformation bands forming in the contractional regime are more areally extensive and not associated with discrete faults, whereas those forming in the extensional regime are generally limited to damage zones around faults. This is an attractive thesis and, insofar as it holds true, of great value for predicting deformation-band distributions in high-porosity sandstone reservoirs. We believe that Solum et al. (2010) touch on an important relationship between tectonic regime and strain distribution and, as geologists, we are constantly looking for simple relationships like this one. However, the authors base their suggestion on very limited data (two scanlines of 16 and 60 m [52 and 197 ft]), comparing localities that differ not only in tectonic regime, but also in tectonic style or boundary conditions as well as petrophysical properties. To add to the reflections of Solum et al. (2010) on how tectonic regime may control deformation-band distribution, we find it appropriate to discuss factors other than tectonic regime that may be at least as important for the characteristics and distribution of deformation bands in deformed porous sandstone reservoirs. Furthermore, their permeability considerations could be improved considering the anisotropic and architectural characteristics of deformation bands and deformation-band clusters. The development of millimeter-thick deformation bands in highly porous sandstone is a fine-scale strain localization phenomenon that closely relates to the local state of stress and material properties at the time of deformation (e.g., Wong et al., 1997; Aydin et al., 2006; Schultz et al., 2010). This is a useful approach, particularly when dealing with deformation bands at the scale of laboratory samples and outcrops. However, the distribution of deformation bands on the hectometer to kilometer scale within a sandstone unit may, in many cases, …

Diagenetic controls on reservoir quality in Middle to Upper Jurassic sandstones in the South Viking Graben, North Sea

The deeply buried synrift play of the South Viking Graben is characterized by highly variable reservoir quality. An integrated approach incorporating petrophysics, petrography, and one-dimensional basin modeling methods was applied to investigate these variations. Analysis shows that average porosities below 4000 m (13,123 ft) (vertical depth below sea floor) range from approximately 5% to as much as about 25% in comparable quartz arenitic sandstones. From porosity-depth trends, three porosity categories can be recognized (normal-, low-, and high-porosity sandstones). Normal-porosity sandstones fall along the regional average porosity-depth trend. Low-porosity sandstones have been subject to extensive quartz cementation as a consequence of a higher degree of thermal maturity and plot below the regional porosity-depth trend. High-porosity sandstones plot above the regional porosity-depth trend. Here, quartz cementation has been inhibited by grain-coating microquartz, and thus porosity has been preserved. Hydrocarbon emplacement has previously been thought to have inhibited quartz cementation in the study area, but this study concludes that the reservoirs are mainly water-wet, allowing for continued quartz cementation despite the presence of hydrocarbon pore fluids. Predicting the distribution of microquartz-coated sandstones and the degree of thermal maturity is therefore fundamental for successful exploration in the deeply buried parts of the synrift play. This study presents a regional and stratigraphic framework for such predictions that may be incorporated into play models in the area.