Per Arne Bjorkum

How Important is Pressure in Causing Dissolution of Quartz in Sandstones

ABSTRACT Textural observation of quartz dissolution at mica-quartz interfaces in sandstones from the Norwegian continental shelf show that mica grains have penetrated into quartz grains without being significantly deformed. Theoretical calculations of the mechanical properties of the mica suggest that quartz dissolution takes place at pressures less than 10 bars, i.e., at pressures only a fraction of the overburden load. Cathodoluminescence and element mapping show that K-and Al-bearing material (probably illitic and/or micaceous clay) is present at all interfaces where quartz-to-quartz dissolution and interpenetration is observed. Because illitic and micaceous clay is likely to have an effect upon quartz dissolution similar to that of mica, this suggests that what has been considered to be a ressure-solution process may instead be a clay-induced dissolution process. Because quartz-to-quartz dissolution is not observed in the absence of illitic or micaceous clay, this suggests that quartz dissolution is not mainly controlled by pressure. In a quartz-mica or quartz-illite system the rate of quartz precipitation, which is strongly controlled by the temperature, may therefore be the main control on quartz cementation (Oelkers et al. 1992, and in press).

Porosity Prediction in Quartzose Sandstones as a Function of Time, Temperature, Depth, Stylolite Frequency, and Hydrocarbon Saturation

The variation of porosity in quartzose sandstones is calculated as a function of depth, temperature gradient, burial rate, stylolite frequency, and hydrocarbon saturation. Calculations were performed by considering the effects of both mechanical compaction and chemical compaction/cementation. This latter process dominates at temperatures greater than approximately 90°C and is due to quartz redistribution within the sandstone. Quartz redistribution stems from clay-induced quartz dissolution at stylolite interfaces, coupled with diffusional transport of aqueous silica into the interstylolite sandstone and precipitation on quartz surfaces as cement. Many model parameters are obtained from theoretical calculations or laboratory measurements, and few basin-dependent parameters are required to make porosity predictions. A set of porosity predictions is presented in porosity/depth figures. Close correspondence between computed results and measured porosities in cores from a variety of sedimentary basins demonstrates the accuracy of the predictions.

The Effect of Grain-Coating Microquartz on Preservation of Reservoir Porosity

Clay coatings have been widely accepted by many workers as an explanation for preserving high porosity in deeply buried sandstones, but few workers have realized that similar effects can be produced by microcrystalline quartz coatings. This phenomenon can be expected only under special circumstances, but in such cases it can have profound consequences for exploration. In the Central Graben area of the southern North Sea, unusually high porosity (20-27%) and permeability (100-1000 md) are found in certain zones in Upper Jurassic sandstones at depths of 3.4-4.4 km. The porosity in these zones is 5-15% higher than expected based on average porosity-depth trends from Brent and Haltenbanken sandstones. We propose that the high porosity is due to continuous grain coatings of euhedral microcrystalline quartz crystals that are 0.1-2 µm thick. The distribution of microcrystalline quartz coatings is controlled by the presence of siliceous sponge spicules (Rhaxella), which implies a sedimentological control on the reservoir quality. We present a thermodynamic model showing how continuous microcrystalline quartz coatings inhibit development of no mal macrocrystalline quartz overgrowths sourced mainly from stylolites. High porosities in parts of various Upper Jurassic oil fields (Ula and Gyda) have previously been explained by inhibition of quartz cementation by early hydrocarbon charge. We suggest that the microcrystalline quartz coatings provide a more plausible explanation.

Physical constraints on hydrocarbon leakage and trapping revisited

In a water-wet petroleum reservoir with a water-wet seal, a continuous water phase will extend from the reservoir into the seal, and the pressure difference between the water phase in the uppermost pores of the reservoir and the water phase in the lowermost pores of the seal can therefore only be of an infinitesimal magnitude. This implies that any overpressure in a water-wet reservoir will not contribute to pushing the hydrocarbons through a water-wet seal, and overpressured water-wet reservoirs should therefore not be considered more prone to capillary leakage than normally pressured reservoirs. Within a water-wet petroleum reservoirs, the overpressure in the hydrocarbon phase relative to the water phase is balanced by the elastic forces at the fluid interface (interfacial tension). The overpressure in the hydrocarbon phase relative to the water phase therefore does not increase the risk of hydrofracturing the reservoirs seal. This implies that the risk of hydrofracturing should not be increased as a function of hydrocarbon column height, and should not be considered to be higher for gas than it is for oil. When an upward-directed hydraulic gradient is present from a reservoir unit into the overlying seal, water will continuously move upwards from the reservoir unit and into the seal if both rocks are water-wet. This movement of water may lead to exsolution of gas above the reservoir unit, and the presence of free gas may be detected as gas chimneys on seismic sections. This mechanism will operate regardless of whether or not a hydrocarbon accumulation is present below the gas chimneys, and fracturing of the reservoir units seal or capillary leakage of hydrocarbons are therefore not necessary conditions for the development of gas chimneys.

The Effect of Stylolite Spacing on Quartz Cementation in the Lower Jurassic Stø 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.

Making diagenesis obey thermodynamics and kinetics : the case of quartz cementation in sandstones from offshore mid-Norway

Calculation of the quantity and distribution of quartz cement as a function of time and temperature/depth in quartzose sandstones is performed using a coupled dissolution/diffusional–transport/precipitation model. This model is based on the assumptions that the source of the silica cement is quartz surfaces adjoining mica and/or clay grains at stylolite interfaces within the sandstones, and the quantity of silica transport into and out of the sandstone by advecting fluids is negligible. Integration of the coupled mass transfer/transport equations over geologically relevant time frames is performed using the quasi-stationary state approximation. Results of calculations performed using quartz dissolution rate constants and aqueous diffusion coefficients generated from laboratory data, are in close agreement with both the overall porosity and the distribution of quartz cement in the Middle Jurassic Garn Formation only after optimizing the product of the effective surface area and quartz precipitation rate constants with the field data. When quartz precipitation rate constants are fixed to equal corresponding dissolution rate constants, the effective surface area required to match field data depends on the choice of laboratory generated quartz rate constant algorithm and ranges from 0.008 cm−1 to 0.34 cm−1. In either case, these reactive surface areas are ∼2 to 4 orders of magnitude lower than that computed using geometric models.

The effect of temperature on sealing capacity of faults in sandstone reservoirs: Examples from the Gullfaks and Gullfaks Sor fields, North Sea

A comparison of structural core analyses from the Gullfaks and Gullfaks Sor fields, northern North Sea, shows that there are important differences between the two fields that have serious consequences for field development plans. Although the general deformation characteristics are the same, the temperatures within the reservoirs on Gullfaks Sor have exceeded the critical temperatures for accelerated quartz dissolution and precipitation. This has led to reduced porosity and permeability within the reservoir and the many shear bands found within damage zones around the many larger scale faults. This has a larger effect on two-phase flow properties than accounted for in standard reservoir models and has a detrimental effect on the production from compartmentalized high-temperature reservoirs. Prior to the detailed core analyses carried out on Gullfaks Sor, it was assumed that faults in Gullfaks Sor and Gullfaks had similar effects on hydrocarbon flow. However, the important difference became obvious after the drilling of two producers within the Statfjord Formation reservoir in Gullfaks Sor. Due to rapid drop in pressure, the actual production rates are less than 15% of those planned. The drop in pressure likely is caused mainly by quartz cementation of the shear bands, which transforms the bands into low-porosity microporous rock in which the oil capillary entry pressure becomes so high that the oil phase is discontinuous in the field. Hence, during production, oil does not flow easily, resulting in a rapid drop in production.

Quantitative modelling of basin subsidence caused by temperature-driven silica dissolution and reprecipitation

Equations describing the rate of vertical thinning of sandstones due to precipitation of quartz cement produced at stylolites are derived for constant temperature and for a linear temperature change. All temperature histories can be approximated by a series of linear segments, and the equations therefore enable the diagenetic thinning of sandstones undergoing quartz cementation to be calculated as a function of temperature history. Application of the equations to interbedded sandstone–shale sequences indicate that for heating rates and geothermal gradients commonly encountered in sedimentary basins, thermally driven diagenesis leads to cumulative rates of sandstone thinning in the range of 1–10 m Ma−1. This implies that when effects of thermally driven shale diagenesis, mechanical compaction and isostatic adjustment are taken into account, thermochemical diagenetic thinning may explain a large proportion of the total generation of accommodation space in many basins. The equations describing the rate of sandstone volume reduction and, therefore, rate of fluid expulsion, could also be utilized for calculating overpressure development.

Correlation of calcite-cemented layers in shallow-marine sandstones of the Fensfjord Formation in the Brage Field

Shallow-marine sandstones of the Fensfjord Formation in the Brage Field contain numerous calcite-cemented layers with thicknesses up to 1.6 m. Most of the calcite-cemented layers are located within bioturbated coarse silt to fine-grained sandstone, and are similar to calcite-cemented layers with lateral extents greater than 3 km in the Bridport Sands in Dorset. Less common horizontally or hummocky laminated calcite-cemented intervals in the Fensfjord Formation resemble calcite-cemented concretions in the Bencliff Grit. The dominant source of Ca2+ in the calcite cement in the three formations is probably biogenic carbonate. Isotopic analysis indicates that calcite cement in the Fensfjord Formation contains a mixture of carbon from decomposition of organic matter and from biogenic carbonate. In the Bridport Sands carbon is probably predominantly derived from biogenic carbonate, whereas calcite cement in the Bencliff Grit contains a large proportion of carbon from fermentation reactions. Wireline log correlation, supported by core description and comparison with the studied outcrops, indicates that the most extensive calcite-cemented layers in the Fensfjord Formation have lateral extents of at least 6 km. Geochemical analysis of closely spaced samples from the correlated calcite-cemented intervals in four wells show that isotopic signatures and trace element contents are widely different within calcite-cemented intervals thought to belong to the same layer. Samples located along two vertical sections through a calcite-cemented layer in the Bridport Sands also have different isotopic signatures and trace element contents, even though the lateral separation between the two sample sets was only 30 cm. This suggests that geochemical analysis may be of limited value for correlation of calcite-cemented layers.

Isotopic composition of a calcite-cemented layer in the Lower Jurassic Birdport Sands, southern England; implications for formation of laterally extensive calcite-cemented layers

ABSTRACT 18OPDB and 13CPDB values have been measured on 107 calcite cement samples from a laterally extensive (> 3 km) and continuous calcite-cemented layer 0.5 m thick in the coastal exposures of the Lower Jurassic shallow-marine Bridport Sands in Dorset, southern England. The samples were taken from a two-dimensional grid with 10-cm horizontal and vertical spacing between samples and along individual vertical lines across the calcite-cemented layer. 18OPDB values vary between -4.8 and -9.2 and decrease radially outwards from points with lateral spacings on the order of 0.5-1 m in the middle of the calcite-cemented layer. The 18OPDB values therefore indicate that the calcite-cemented layer was formed by merging of concretions. All 13CPDB values measured are in the narrow range -2.2 to -0.5, which suggests that the dominant source of calcite cement in the layer was biogenic carbonate.