Quartz Cement Volumes Research Articles

Burial of a sand sea: Heterogeneous compaction and cementation of the jurassic Etjo Formation, Namibia and what it tells

Aeolian sandstones are capable of forming excellent reservoir rock due to their textural and compositional maturity. Often spanning across large areas, such formations may however experience spatial variability in burial and temperature exposure potentially leading to a heterogeneous diagenesis and subsequent reservoir quality. To assess such potential variability, this case study focusses on the diagenesis of the aeolian units of the Lower Jurassic Etjo Formation in Namibia, exposed at three key localities lying >100 km apart from each other. The Etjo Formation has experienced two major burial phases controlled by the emplacement of (I) Jurassic Karoo and (II) Lower Cretaceous Paraná-Etendeka volcanic rocks separated by an intervening exhumation phase. Petrographic analysis reveals very different rock properties between the localities with respect to intergranular volumes (IGV), intensity of quartz cementation, and preserved porosities. Average IGV and quartz cement values are 30.7 % and 24.3 % at Gamsberg, 23.7 % and 6.5 % at Waterberg and 19.7 % and 15.4 % at Mt. Etjo. Point-counted intergranular porosity values are on average 2.6 % (Gamsberg), 10.8 % (Waterberg), and 1.5 % (Mt. Etjo). Waterberg samples had an on average 72 % larger nucleation surface area available for quartz cement growth than samples from Mt. Etjo. The higher quartz cement volumes at Mt. Etjo therefore argue for a higher thermal exposure than at Waterberg. This is supported by kaolinite-to-illite transformation only occurring at Mt. Etjo. We attribute the higher temperature signature to the proximity of Mt. Etjo to the center of lower Cretaceous Paraná-Etendeka volcanism and related elevated thicknesses of the lava pile or heat flow increase. At Gamsberg, there is evidence for a quartz cement generation that formed during the Jurassic. This earlier cement stabilized the grain framework and preserved the IGV during the following Etendeka burial event. The Etjo Formation provides an exceptional example for the wide range of very low to very high degrees of compaction and cementation that can occur within an otherwise homogeneous, texturally and compositionally mature sandstone formation. This study underscores the importance of accounting spatial variations in burial depths and temperatures when predicting the diagenetic properties of reservoir rocks.

Improved reservoir quality assessment by evaluating illite grain coatings, quartz cementation, and compaction – Case study from the Buntsandstein, Upper Rhine Graben, Germany

Reservoir quality (RQ) in the Buntsandstein of the Upper Rhine Graben is in the center of attention as it is a possible target formation for geothermal energy production and hydrocarbon exploration surrounding existing fields. An understanding of properties affecting reservoir quality of the target lithology is still fairly poor and the success of accurately targeting high RQ intervals is limited. This is due to the fact, that the effect of compaction and the interplay between enhanced chemical compaction (i.e. pressure dissolution of quartz grains), illitic contact coatings and quartz cementation in the lithology has been underestimated. The understanding and quantification of controlling factors on reservoir qualities in fluvio-eolian sedimentary rocks, deposited in a (semi-)arid climate has been improved in recent years and this case study highlights the benefits of detailed petrographic analyses in understanding diagenetic and compactional processes in this lithology. This is especially relevant in the observation of grain coating clay minerals, whose effect depends on their specific location, i.e. either at grain contacts between quartz grains (GTG coating) or at the interface between detrital quartz grains and the intergranular volume (GTI coating).Illitic GTI coatings affect syntaxial quartz overgrowth precipitation, as precipitation sites are locally blocked. The negative correlation between the grain coating coverage and quartz cement volumes support these findings across multiple sample sets, and they may locally preserve intergranular porosity. Illitic GTG coatings on the other hand enhance chemical compaction (i.e. pressure dissolution) and will reduce the IGV. The negative correlation between these two properties again underlines the negative effect of this process on reservoir properties. Studied samples from a deep Buntsandstein well in the central URG show low reservoir quality due to either intense quartz cementation (0.7–31.7%) or a high degree of mechanical and chemical compaction (IGV: 2.3–38.0%). Higher illitic GTG and GTI coating coverages play a substantial role in controlling reservoir quality development, as demonstrated by comparing data from other fluvio-eolian lithologies (Triassic Buntsandstein and Permian Rotliegendes) to results of this study. In relation to their respective burial histories, higher illitic GTI coating coverages always correlate with smaller syntaxial quartz cement volumes. Similarly, higher illitic GTG coating coverages always correlate with lower IGV values in the three compared sample series.As both, the precipitation of quartz cements and compaction, are a function of the burial history, i.e. effective stresses and experienced temperatures, understanding the interaction of both these processes may enable the prediction of reservoir properties in undrilled areas.

The importance of facies, grain size and clay content in controlling fluvial reservoir quality – an example from the Triassic Skagerrak Formation, Central North Sea, UK

Clay-coated grains play an important role in preserving reservoir quality in high-pressure, high-temperature (HPHT) sandstone reservoirs. Previous studies have shown that the completeness of coverage of clay coats effectively inhibits quartz cementation. However, the main factors controlling the extent of coverage remain controversial. This research sheds light on the influence of different depositional processes and hydrodynamics on clay-coat coverage and reservoir quality evolution. Detailed petrographic analysis of core samples from the Triassic fluvial Skagerrak Formation, Central North Sea, identified that channel facies offer the best reservoir quality; however, this varies as a function of depositional energy, grain size and clay content. Due to their coarser grain size and lower clay content, high-energy channel sandstones have higher permeabilities (100–1150 mD) than low-energy channel sandstones (<100 mD). Porosity is preserved due to grain-coating clays, with clay-coat coverage correlating with grain size, clay-coat volume and quartz cement. Higher coverage (70–98%) occurs in finer-grained, low-energy channel sandstones. In contrast, lower coverage (<50%) occurs in coarser-grained, high-energy channel sandstones. Quartz cement modelling showed a clear correlation between available quartz surface area and quartz cement volume. Although high-energy channel sandstones have better reservoir quality, they present moderate quartz overgrowths due to lesser coat coverage, and are thus prone to allowing further quartz cementation and porosity loss in ultra-deep HPHT settings. Conversely, low-energy channel sandstones containing moderate amounts of clay occurring as clay coats are more likely to preserve porosity in ultra-deep HPHT settings and form viable reservoirs for exploration. Supplementary material: of data and technique used in this study are available at https://doi.org/10.6084/m9.figshare.c.6438450

Impact of grain-coating illite in preventing quartz cementation: Example from permo-carboniferous sandstone, Central Saudi Arabia

The origin and role of clay coatings (e.g., chlorite) in inhibiting quartz cementation in sandstones have been an area of research during the last three decades. However, the role of grain-coating illite in porosity preservation has received little attention. An integrated petrographic-mineralogical study is conducted to investigate the mode of occurrence of illite coatings, their coverage, thickness, and role in inhibiting the development of quartz overgrowth in a Permo-Carboniferous sandstone reservoir in Central Saudi Arabia. Results of the study reveal that illite coatings occur in two forms: tangential and radial to grain surfaces, both of which are effective in arresting quartz cementation. Illite-coating coverage exhibits and inverse relationship with quartz cement volume, and the coating coverage has been found to be grain-size dependent. Coarser-grained sandstones have higher coating coverage due to smaller surface area and smaller amount of illite required to form complete coatings. The study has found that thick illite coating (up to 27 μm) and high illite-coating coverage (≥80%) effectively arrest quartz cementation. Total pore-filling cements threshold of >20% has a negative impact on porosity. The findings of this study can be applied to other reservoirs of similar depositional environments to improve their reservoir quality prediction.

Impact of Grain-Coating Clays on Porosity Preservation in Paleocene Turbidite Channel Sandstones: Nelson Oil Field, UK Central North Sea

The Forties Sandstone Member is an important deep-water reservoir in the Central North Sea. The role of depositional characteristics, grain-coating clays, and diagenesis in controlling the reservoir quality of the sandstones is poorly understood. The main aim of the study is to understand the role of depositional characteristics, grain-coating and pore-filling clays, and diagenesis in controlling the reservoir quality evolution of turbidite-channel sandstones. The study employed a multi-disciplinary technique involving thin section petrography and scanning electron microscopy (SEM) to investigate the impact of grain size, clay matrix content, mode of occurrence of grain-coating chlorite and illite, and their impact in arresting quartz cementation and overall reservoir quality in the sandstones. Results of our study reveal that porosity evolution in the sandstones has been influenced by both primary depositional characteristics and diagenesis. Sandstones with coarser grain size and lower pore-filling clay content have the best reservoir porosity (up to 28%) compared to those with finer grain size and higher pore-filling clay content. Quartz cement volume decreases with increasing clay-coating coverage. Clay coating coverage of >40% is effective in arresting quartz cementation. Total clay volume of as low as 10% could have a deleterious impact on reservoir quality. The Forties Sandstone Member could potentially be a suitable candidate for physical and mineralogical storage of CO2. However, because of its high proportion (>20%) of chemically unstable minerals (feldspar, carbonates, and clays), their dissolution due to CO2 injection and storage could potentially increase reservoir permeability by an order of magnitude, thereby affecting the geomechanical and tensile strength of the sandstones. Therefore, an experimental study investigating the amount of CO2 to be injected (and at what pressure) is required to maintain and preserve borehole integrity. The findings of our study can be applied in other reservoirs with similar depositional environments to improve their reservoir quality prediction.

Rock Acoustics of Diagenesis and Cementation

We simulate the effects of diagenesis, cementation and compaction on the elastic properties of shales and sandstones with four different petro-elastic theories and a basin-evolution model, based on constant heating and sedimentation rates. We consider shales composed of clay minerals, mainly smectite and illite, depending on the burial depth, and the pore space is assumed to be saturated with water at hydrostatic conditions. Diagenesis in shale (smectite/illite transformation here) as a function of depth is described by a fifth-order kinetic equation, based on an Arrhenius reaction rate. On the other hand, quartz cementation in sandstones is based on a model that estimates the volume of precipitated quartz cement and the resulting porosity loss from the temperature history, using an equation relating the precipitation rate to temperature. Effective pressure effects (additional compaction) are accounted for using the Athy equation and the Hertz–Mindlin model. The petro-elastic models yield similar seismic velocities, despite the different levels of complexity and physics approaches, with increasing density and seismic velocities as a function of depth. The methodology provides a simple procedure to obtain the velocity of shales and sandstones versus temperature and pressure due to the diagenesis-cementation-compaction process.

Diagenesis and its controls on reservoir quality of the Tambar oil field, Norwegian North Sea

A study of the diagenetic evolution of the Late Jurassic sandstones of the shallow marine facies in the Tambar oil field, Norwegian North Sea was carried out to understand its controls on reservoir quality. Core samples and a set of wireline logs obtained from three wells were used to carry out petrographic studies including light optics and scanning electron microscopy, well log analysis, fluid inclusions, X-ray diffraction (XRD) for whole rock samples, clay mineral extracts and stable isotopes. The Tambar reservoir sandstones range mainly from siltstone to fine-grained sandstones, exclusively arkoses cemented mainly by microcrystalline quartz, euhedral quartz overgrowth, dolomite cement, illite and chlorite. Early diagenetic/eodigenetic minerals include pyrite, calcite, and microcrystalline quartz and late diagenetic/mesodiagenetic minerals include quartz overgrowth, dolomite, illite and chlorite. The δ18O values of dolomite cement in the Tambar sandstone reservoir range from −11.77‰ to −3.57‰ PDB, while the δ13C values of the dolomite cement are from −5.07‰ to −1.12‰ PDB. Homogenization temperature for fluid inclusions trapped in authigenic quartz fall between 130 °C and 169 °C. The reservoir quality is controlled mainly by early formed grain-coating microcrystalline quartz that precipitated from the dissolution of sponge spicules which significantly inhibits quartz overgrowth precipitation. Oil emplacement do not show significant control on reservoir quality because considerable quartz cementation has taken place before the onset of major oil charge. Only the coarse-grained sands show a correlation between quartz cement volume and water saturation. However, the interpreted temperature for dolomite cementation is significantly higher for water leg-suggested effect of oil emplacement on the late dolomite. Understanding these controls on the reservoir quality will enhance the exploration strategy for the Tambar oil field.

Porosity evolution of two Upper Carboniferous tight-gas-fluvial sandstone reservoirs: Impact of fractures and total cement volumes on reservoir quality

Abstract Cyclic fluvial, coal-bearing sandstones from the Upper Carboniferous are important tight-gas reservoirs in the Lower Saxony Basin, NW Germany. Two wells from two fields located 10 km apart, both comprising of Westphalian C/D fluvial sandstones were studied. Well A was affected by approximately 1.2 km and well B of approximately 2.8 km of basin inversion after maximum burial depths larger than 5 km. Both fields display significant differences in recovered resources. An integrated core study combining sedimentological, petrographic and petrophysical data illustrates the impact of different diagenetic pathways. While permeability values appear in the same range of 0.01–1 mD for both fields, lithologies from well A of field A have average matrix porosities of 6% in a generally unfractured reservoir, whereas lithologies from well B of field B have matrix porosities of 10% in an overall fractured reservoir. Intense siderite/ankerite cementation in matrix and fractures is present in sandstones from both fields and can be related to fluid migration during basin inversion. In contrast, the strong alpine tectonic inversion affecting field B results in a lower thermal exposure since the late Cretaceous, resulting in smaller quartz cement volumes than in field A and larger matrix porosities in field B. Thus, matrix porosity is overall better due to smaller total cement volumes in field B. The study indicates a strong impact of fractures on tight sandstone reservoir quality and quartz cementation on hydrocarbon production performance from the sandstone matrix. Observations may be applicable to similar, strongly inverted tight gas sandstone fields and may directly affect exploration strategies.

Reservoir quality and burial model evaluation by kinetic quartz and illite cementation modeling: Case study of Rotliegendes, north Germany

Silicate reaction kinetics provide a complementary means to other established paleothermal indicators such as organic maturation for evaluating thermal reconstructions. In this study we combine the use of an organic maturation model with kinetic models for quartz and illite cementation to evaluate burial history scenarios for five subsalt wells in lithologically and structurally complex Rotliegendes reservoirs. Models for organic maturation are most sensitive to maximum temperature and provide no direct evidence for the time of peak temperature or the overall duration of high temperatures. By contrast, the kinetics of quartz cementation are much more strongly influenced by the duration of exposure to high temperatures compared with organic indicators. Kinetic models for fibrous illite formation similarly are sensitive to time and temperature and provide predictions for the time of illite formation that can be compared with radiometric dates. Used collectively, these organic and inorganic paleothermal indicators provide improved constraints on thermal evolution compared with conventional approaches. In this study we use these indicators to evaluate two alternative burial history scenarios. Scenario 1 incorporates a hypothesized Jurassic heat flow peak together with significant Late Jurassic deposition and subsequent erosion. Scenario 2 omits the Jurassic heat flow peak and omits the deposition and erosion of the Upper Jurassic. Although both of these scenarios are consistent with organic maturation data, scenario 2 leads to a far better match with quartz cement volumes and fibrous illite K-Ar dates.

Controls on reservoir quality in exhumed basins – an example from the Ordovician sandstone, Illizi Basin, Algeria

Petroleum exploration in many North African intracratonic basins targets Early Paleozoic sandstones as the primary reservoir objective. These sandstones are often characterized by highly variable reservoir quality (0.0001–1000 mD), and the ability to predict and selectively target areas of enhanced porosity and permeability is crucial to unlock the hydrocarbon potential. The objective of this study is to characterize the primary controls on reservoir quality in an Ordovician field in the Illizi Basin of Algeria through detailed core and petrographic analysis, and establish if variations in thermal history across the field have a material impact on reservoir quality. The best reservoir quality is observed in facies where primary intergranular porosity has been preserved in fine to coarse grained quartzarenites with less than 1% fibrous illite. These lithologies are most commonly found within the high-energy, tidally reworked, post-glacial facies sandstones of the uppermost Ordovician succession. Observed differences in quartz cement volume within compositionally and texturally similar samples from the southern and northern parts of the field are interpreted to reflect variations in thermal exposure due to deeper burial. This interpretation is supported by field-wide numerical modelling of sandstone diagenesis. This study indicates that subtle variations in thermal history can have a material impact on the spatial trends in reservoir permeability. Thermal history, therefore, is an important consideration in reservoir quality studies in exhumed basins where variations in present-day burial depth will be a poor guide for evaluating reservoir quality risk across a basin or play.

Stability of Detrital Heavy Minerals on the Norwegian Continental Shelf as a Function of Depth and Temperature

Abstract Petrographic analysis of 1931 thin sections from sandstones of Pliocene to Devonian age on the Norwegian continental shelf indicate that the heavy minerals zircon, tourmaline, rutile, apatite, and spinel survive to depths exceeding 5000 m and to temperatures of around 200°C without showing signs of intrastratal dissolution. On the other hand, the maximum depth and temperature of occurrence for amphibole are 841 m and 40°C, around 1500 m and 55°C for sphene when 0.5 km of probable uplift is taken into account, 2231 m and 92°C for kyanite, 2559 m and 95°C for epidote, 2810 m and 109°C for staurolite, and 4589 m and 175°C for garnet. Initial signs of garnet dissolution occur at around 2000 m depth, and all garnets are partially dissolved at depths greater than 3500 m. Monazite is present down to 4858 m and 182°C and chloritoid to 3811 m and 149°C. It is, however, uncertain whether this is due to the low number of samples deeper than 4858 m and to the rare occurrence of chloritoid, or to intrastratal dissolution. The same sequence of heavy-mineral stability is found when volume of quartz cement is used as a proxy for time-temperature exposure. Quartz cement is not found in samples containing amphibole or detrital sphene. A maximum of a fraction of a percent quartz cement is found in samples containing kyanite, up to 1% together with epidote, up to 6% with staurolite, up to 24% with chloritoid, up to 25% with garnet, whereas maximum values of 25–35% are present together with zircon, tourmaline, rutile, patite, spinel, and monazite. The sequence of persistence of heavy minerals versus depth found in this study is very similar to results from several previous studies, although unstable heavy minerals survive to greater depths in very young and rapidly buried sandstones, and there are some differences regarding the stability of sphene and epidote. Heavy mineral dissolution has created secondary pores and possibly has contributed to precipitation of kaolinite, illite, quartz, and carbonates in the studied sandstones, but these reactions are of very limited volumetric importance.

Low-Porosity Haloes at Stylolites in the Feldspathic Upper Jurassic Ula Sandstone, Norwegian North Sea: An Integrated Petrographic and Chemical Mass-Balance Approach

Abstract Although stylolites in quartz-rich sandstones have been described and modeled, more feldspathic sandstones have not been similarly investigated. The Upper Jurassic Ula sandstone, Norwegian North Sea, a subarkose with 15 to 25% feldspar, displays stylolites with prominent low-porosity haloes up to 10 cm wide. Detailed mineralogical and chemical profiles across several stylolites and haloes are used to investigate the role of feldspar in mineral reactions at stylolites and to constrain the transport of chemical components. Quartz and feldspar are intensely dissolved at the margins of stylolites. The surrounding haloes are characterized by: dissolution of quartz and feldspar, intense compaction, anomalously high feldspar content, high concentrations of clay, and oxide cement volumes that increase away from the stylolite. Chemical mass-balance calculations demonstrate that silica and elements associated with feldspar (Al2O3, Na2O, K2O) were exported from the haloes. Grain-size data and evidence for aluminum loss from the halo indicate that primary concentrations of clay and mica were not higher near the stylolites and that the petrographic and geochemical patterns reflect the diagenetic concentration of clay. The analysis suggests that K-feldspar dissolution at and near the stylolite resulted in precipitation of authigenic illite near the site of K-feldspar dissolution, due to the low solubility of Al3+. This in turn catalyzed chemical dissolution of quartz in the low-porosity halo. Transport of CaO, MgO, and MnO into the haloes suggests that the haloes were zones of higher pH. The increased pH, perhaps due to the presence of clays, would have also increased quartz solubility, consistent with evidence for silica export from the halo. The dissolution of K-feldspar in the absence of kaolinite would also have raised pH around the site of K-feldspar dissolution and illite precipitation, favoring carbonate precipitation. This interpretation of stylolite diagenesis differs from conventional models based on quartz-rich sandstones, in which quartz cement volumes decrease away from stylolites. Models for compaction in arkosic sandstones need to account for the enhanced dissolution of quartz that is driven by K-feldspar dissolution and for catalysis by the resulting illite.

Kaolin-Coating of Stylolites, Effect on Quartz Cementation and General Implications for Dissolution at Mineral Interfaces

Abstract The deeply buried (4 km, 160°C) quartz-rich sandstones of the Middle Jurassic Garn Formation in well 6507/7-1 offshore mid-Norway typically contain 20–30% quartz cement and only 3–9% porosity. The exception is a less than 1.5 m-thick interval at the top of the formation where quartz cement volumes typically drop to 8–16% and porosities are in the range 7–19%. The uppermost part of the Garn Formation is not isolated from the rest of the formation by any type of impermeable barrier; the sandstones are water-filled indicating that the change in cementation does not correspond to a hydrocarbon contact; and, there are no significant changes in grain size, grain coating abundance, grain mineralogy or other factors that could give a lower quartz surface area for development of quartz overgrowths within the uppermost 1.5 m of the sandstones. However, in the samples from the less cemented interval, illitization of authigenic kaolin is either not detectable or very restricted compared to deeper down in the cores where illitization of kaolin is usually complete. We propose that the cause of reduced quartz cementation in the top of the Garn Formation may be the coating of stylolites developed from primary illitic clay-rich and micaceous laminae with authigenic kaolin as quartz cementation proceeds and the authigenic kaolin-rich rock adjacent to the stylolites is dissolved. Although we do not have evidence indicating that this may be a common phenomenon of major importance for preservation of reservoir quality, our observations may help to unravel the mechanisms of quartz dissolution at stylolites. Specifically, we suggest that the lack of significant surface charge on kaolin compared to illite and mica may lead to a reduced thermodynamic drive for quartz dissolution at kaolin-quartz contacts. Similarly, lack of dissolution at clean quartz-quartz surfaces may be due to smaller and equal surface charges setting up oppositely directed electric fields that cancel each other, whereas the positive surface charge of carbonate minerals may actually enhance the electric fields from negatively charged illitic clay and mica, which may explain the extensive dissolution often seen at stylolites in limestones.

The effect of hydrocarbons on quartz cementation: diagenesis in the Upper Jurassic sandstones of the Miller Field, North Sea, revisited

Variable quartz cementation and porosity distribution in the Upper Jurassic Brae Formation deep-water sandstone reservoir of the Miller Field (UK, North Sea) is mainly controlled by the amount of coatings on the quartz grains. Oil was possibly present in the pore space for the last 40 Ma, but had no significant effect on preserving porosity in the oil leg relative to the water leg. Samples with anomalously high porosities, which commonly occur in the shallowest sandstone intervals, are microquartz coated, and it is a misinterpretation to explain these high porosities as due to hydrocarbons inhibiting quartz cementation. Porosity preservation due to microquartz coating is quite common in Upper Jurassic sandstones in the North Sea. Kinetic modelling of quartz cementation with the Exemplar program correctly predicts observed quartz cement volumes in the Brae Formation outside the microquartz-coated zones, when high quality petrographic data, including abundance of clay coatings, are provided as input. Significant overprediction of quartz cement volumes occurs only for samples where scanning electron microscope analysis shows microquartz coatings to be present. Failure to recognize grain coating as one of the major parameters controlling quartz cementation may lead to incorrect geological models of reservoir quality and selection of inappropriate exploration or production strategies.

An Analysis of the Roles of Stress, Temperature, and pH in Chemical Compaction of Sandstones: Discussion

In a recent article on chemical compaction of sandstones, Sheldon et al. (2003) claim that models of quartz cementation based upon the diffusional transport of dissolved silica originating from the clay-induced dissolution (CID) of quartz at stylolites are untenable because no chemical driving force is available to drive silica diffusion in the aqueous phase. As examples of such untenable models, they cite a number of our papers (Oelkers et al. 1992, 1996; Walderhaug 1994; Bjorkum 1996; Bjorkum and Nadeau 1998; Bjorkum et al. 1998; Walderhaug et al. 2001). In the following discussion of their paper we will demonstrate that there is indeed a driving force for aqueous silica diffusion within the CID quartz cementation model and point out several flaws in the Sheldon et al. (2003) treatment of quartz dissolution at stylolites. Contrary to the claims of Sheldon et al. (2003), the quantitative models of quartz cementation presented in Oelkers et al. (1992, 1996), Walderhaug (1994, 1996), and Bjorkum et al. (1998) do not claim to present, nor do they depend upon, a detailed understanding of the mechanisms of quartz dissolution at stylolites. These models are based upon an overwhelming number of petrographic observations indicating that (1) quartz dissolution in quartzose sandstones occurs predominately at quartz-clay and quartz-mica interfaces (clay-induced dissolution; CID) and (2) this enhanced quartz dissolution requires negligible stress (see Bjorkum 1996). The actual mechanism which increases quartz solubility at these interfaces is unclear; it may result from an electrostatic interaction between quartz and clay mineral/mica surfaces, but this has yet to be demonstrated unambiguously. Nevertheless, just as Newton's law of gravitation enables useful calculations without providing insight into the underlying causes of gravity, these quartz cementation models provide accurate predictions of quartz cement volumes in quartzose …

The Effect of Stylolite Spacing on Quartz Cementation in the Lower Jurassic Sto Formation, Southern Barents Sea

ABSTRACT The shallow marine Lower Jurassic quartz arenites of the Sto Formation in the southern Barents Sea comprise (1) intervals where dispersed detrital clay is absent, and where the spacing between clay-rich laminae that evolved into stylolites upon burial is exceptionally large, up to several meters, and (2) intervals where minor detrital clay matrix occurs, clay laminae are very common, and stylolite spacing is typically less than a centimeter. Point counting of thin sections and cathodoluminescence micrographs shows that quartz cement contents are far lower in the intervals where stylolite spacing is exceptionally large, 4-11%, versus 10-20% outside these intervals. There is also a correlation between distance to nearest stylolite and volume of quartz cement. Samples located a centimeter or less from a stylolite contain 10-20% quartz overgrowths, for distances of 3-20 cm quartz cement content is 4-10%, and only 3-8% when the closest stylolite is more than 20 cm distant. Modeling of quartz cementation with the ExemplarTM diagenetic modeling program indicates that the observed trend of decreasing quartz cement abundance outwards from stylolites is not caused by variations in grain size, degree of grain coating, or content of quartz grains, i.e., the trend is not due to more quartz surface area being available for overgrowth formation close to stylolites. On the contrary, the modeling suggests that the samples situated more than 20 cm from stylolites contain 5-8% less quartz cement than what would have been the case given a more normal stylolite abundance. This study indicates that sandstones with exceptionally few clay-rich or micaceous laminae and without clay or mica at individual grain contacts will be significantly less quartz cemented and more porous than other sandstones with similar temperature histories. However, such sandstones seem to be highly unusual on the Norwegian continental shelf. This suggests that exceptionally low abundance of stylolite precursors may be of only local importance for preserving reservoir quality at elevated temperatures, and that it is normally not necessary to include stylolite spacing and distance to the nearest stylolite as variables in quantitative models of quartz cementation.

Construction of an intergranular volume compaction curve for evaluating and predicting compaction and porosity loss in rigid-grain sandstone reservoirs

To evaluate compaction as a factor in porosity evolution, a plot of intergranular volume vs. depth was constructed using data from relatively uncemented reservoir sandstones from a variety of depths, ages, and geographic locations. The resulting intergranular-volume-decline curve reveals that sands compact mechanically and intergranular volume declines rapidly, from about 40 to 42% at the surface, to about 28% at 1500 m. Between about 1500 and 2500 m, intergranular volume continues to decline slowly, until the framework stabilizes at around 26% (maximum potential porosity in the absence of cement or matrix). No further significant decrease in intergranular volume is observed to the depth limits of the data set at 6700 m. Comparison of intergranular volume and volume of quartz cement for different formations reveals no obvious balance between intergranular pressure solution (as monitored by intergranular volume) and quartz cementation. This indicates that grain-to-grain pressure solution and quartz cement precipitation do not proceed concomitantly on the thin-section scale. Moreover, grain compaction is limited (to about 26% intergranular volume) in rigid-grain sandstones, which suggests that the occurrence and distribution of deep porosity is a function of the volume of cement available to fill the intergranular pores. Therefore, deep, porous sandstones are relatively uncemented rather than undercompacted.

Quartz cementation inhibited by crestal oil charge: Miller deep water sandstone, UK North Sea

AbstractIn the Miller Field, diagenetic quartz abundance, isotopic compositions and salinities of quartz-cementing fluids display a distinct pattern which is related to the structural depth of the reservoir sandstones. Quartz cement volumes increase from the crest of the field (average 6.0±1.5%) towards the flanks of the field (average 13.2±2.1%) and directly reduce reservoir porosity. By integrating petrographic observations with results of fluid inclusion measurements and O isotope analyses of diagenetic quartz, the pattern of quartz cementation is seen to be related to the reservoir filling history. Oil filled the crest of the reservoir first and prevented extensive quartz cementation. At greater depth in the reservoir oil zone, quartz overgrowths continued to precipitate until inhibited by the developing oil column. Oxygen isotope compositions of diagenetic quartz imply that quartz cement continued to precipitate in the water zone of the reservoir up to the present day.

Modeling Quartz Cementation and Porosity in Middle Jurassic Brent Group Sandstones of the Kvitebjørn Field, Northern North Sea

Petrographic study of deeply buried Middle Jurassic Brent Group sandstones from the Kvitebjorn gas field in the Norwegian sector of the North Sea shows that quartz cement volumes range from less than 1% to almost 30% over short distances, and porosity ranges from 5 to 30%. A clear correlation between quartz surface area and quartz cement volume indicates that this variation is due to differences in quartz surface area available for quartz overgrowth formation, which, in turn, is a function of grain size, abundance of grain coatings, and quartz clast abundance. The correlation between quartz surface area and quartz cement volume also suggests that the quartz cementation process is a strongly precipitation rate-controlled process, and that quartz cementation can indeed be modeled quantitatively by modeling the precipitation step in the quartz cementation process. Using temperature history, detrital mineralogy, grain size, and grain coating abundance as input, quartz cement volumes were for 90% of the samples modeled to within 4% or less of observed values with the EXEMPLAR® diagenetic modeling program. Modeled porosities deviate from measured values by less than 3% for 75% of the samples, and the difference between measured and modeled porosities exceeds 5% for only two of the 40 samples. Deviations between modeled and measured quartz cement volumes do not correlate with distance to nearest stylolite, but a tendency for underestimating quartz cement in samples with low quartz surface areas may possibly be present. Comparison with results from modeling of quartz cementation in other sandstones shows that optimal fit between measured and modeled quartz cement volumes is not always obtained with the same values for the kinetic parameters controlling quartz precipitation rate per unit surface area as a function of temperature. The variation in optimal kinetics between data sets is probably partly due to inaccurate temperature histories, but improving the quartz surface area function may also reduce the range of optimal values for the kinetic parameters. Olav Walderhaug is employed in Statoil's exploration technology division in Stavanger, Norway, where diagenesis and reservoir quality prediction are his main areas of responsibility. Olav holds an M.Sc. degree in petroleum geology from the University of Bergen and a D.Sc. degree in sandstone diagenesis from the University of Oslo. His research interests are mainly within sandstone diagenesis and related topics, including the development of quantitative models of cementation, porosity evolution, and basin subsidence.

Abstract: Quantification of Quartz Cement and Pressure Solution Volumes, North Sea Jurassic sandstone

Abstract: Quantification of Quartz Cement and Pressure Solution Volumes, North Sea Jurassic sandstone