Olav Walderhaug

Kinetic Modeling of Quartz Cementation and Porosity Loss in Deeply Buried Sandstone Reservoirs

A mathematically simple kinetic model simulates quartz cementation and the resulting porosity loss in quartzose sandstones as a function of temperature history. Dissolved silica is considered to be sourced from quartz dissolution at stylolites or individual quartz grain contacts containing clay or mica, and diffuses short distances to sites of precipitation on clean quartz surfaces. The modeled sandstone volume is located between stylolites, and no quartz dissolution or grain interpenetration takes place within this volume. After quartz cementation starts, compactional porosity loss is typically minor, and porosity loss within the modeled sandstone volume is therefore considered to be equal to the volume of precipitated quartz cement. The quartz cementation process is mod led as a precipitation rate-controlled reaction where quartz precipitation rate per unit time and surface area can be expressed by an empirically determined logarithmic function of temperature. When the sandstones temperature history is known, precipitation rate per unit time and surface area can be expressed as a function of time, and the amount of quartz cement precipitated within a certain time interval can be calculated by multiplying the precipitation rate function with the surface area available for quartz precipitation and integrating with respect to time. Because quartz surface area will change as quartz cement precipitation proceeds, the calculations are performed for short time steps, and quartz surface area is adjusted after each time step. The total amount of quartz cement p ecipitated during a sandstones burial history and the corresponding porosity loss are found by taking the sum of the increments of quartz cement precipitated during each time step. The effect of variation in parameters such as grain size, detrital quartz content, abundance of clay or other grain coatings, prequartz cementation porosities, and temperature history is easily simulated with the presented algorithm. This flexibility is illustrated by presenting calculated histories of quartz cementation and porosity loss for sandstones with a range of grain sizes, framework grain compositions, degree of clay coat development, prequartz cementation porosities, and temperature histories.

Predicting porosity through simulating sandstone compaction and quartz cementation

Presently available techniques for predicting quantitative reservoir quality typically are limited in applicability to specific geographic areas or lithostratigraphic units, or require input data that are poorly constrained or difficult to obtain. We have developed a forward numerical model (Exemplar) of compaction and quartz cementation to provide a general method suited for porosity prediction of quartzose and ductile grain-rich sandstones in mature and frontier basins. The model provides accurate predictions for many quartz-rich sandstones using generally available geologic data as input. Model predictions can be directly compared to routinely available data, and can be used in risk analysis through incorporating parameter optimization and Monte Carlo techniques. The diagenetic history is modeled from the time of deposition to present. Compaction is modeled by an exponential decrease in intergranular volume as a function of effective stress. The model is consistent with compaction arising from grain rearrangement, ductile grain deformation, and brittle failure of grains, and accounts for the effects of fluid overpressures and stable grain packing configurations. Quartz cementation is modeled as a precipitation-rate-controlled process according to the method of Walderhaug (1994, 1996) and Walderhaug et al. (in press). Input data required for a simulation include effective stress and temperature histories, together with the composition and texture of the modeled sandstone upon deposition. Burial history data can be obtained from basin models, whereas sandstone composition and texture are derived from point-count analysis of analog thin sections. Exemplar predictions are consistent with measured porosity, intergranular volume, and quartz cement fractions for modeled examples from the Quaternary and Tertiary of the Gulf of Mexico Basin, the Jurassic of the Norwegian shelf, the Ordovician of the Illinois basin, and the Cambrian of the Baltic region.

Temperatures of Quartz Cementation in Jurassic Sandstones from the Norwegian Continental Shelf--Evidence from Fluid Inclusions

ABSTRACT Measurement of homogenization temperatures for 274 aqueous and 366 hydrocarbon fluid inclusions trapped within quartz overgrowths in Jurassic sandstones from the Norwegian continental shelf indicates that quartz cementation of the studied sandstones has taken place at temperatures between 75°C and 165°C. Inclusions trapped at temperatures below 100°C are common in samples that have spent several tens of millions of years within the temperature interval 75-100°C, whereas sandstones that have passed more rapidly through this range of temperatures seldom contain inclusions trapped below 100°C, probably due to inclusions not having had time to form in the sandstones that were heated most rapidly. Different heating rates after sandstones enter the temperature range wher quartz cementation takes place may also give rise to a correlation between homogenization temperatures and present formation temperatures, since inclusions of a given size that start to form simultaneously in different sandstones become closed at the highest temperatures in the sandstones that are heated most rapidly. Such a correlation may be misinterpreted as indicating stretching of inclusions and resetting of homogenization temperatures. Quartz cementation at temperatures above 75°C is compatible with dissolution of quartz clasts at stylolites and grain contacts and/or quartz produced by clay-mineral reactions in shales being the dominant sources of quartz cement. However, lack of a viable mechanism for transport of large amounts of quartz from shales and into sandstones, combined with the common occurrence of stylolites in the studied cores, suggests that dissolution of quartz at stylolites, and to a lesser extent at grain contacts, were the most important sources of quartz cement. Homogenization temperatures approximately equal to or slightly below present formation temperature are common in the studied sandstones despite the presence of hydrocarbons. This suggests that quartz cementation is still in progress, which in turn implies that hydrocarbon emplacement has not stopped quartz cementation. Probable continued quartz cementation after hydrocarbon emplacement can be explained by dissolution of quartz at stylolites and grain contacts and subsequent short-range diffusion to sites of precipitation through water films coating grains.

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.

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 Statoils 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.

Carbonate porosity creation by mesogenetic dissolution: Reality or illusion?

Many authors have proposed that significant volumes of porosity are created by deep-burial dissolution in carbonate reservoirs. We argue, however, that this model is unsupported by empirical data and violates important chemical constraints on mass transport. Because of the ubiquitous presence and rapid kinetics of dissolution of carbonate minerals, the mesogenetic pore waters in sedimentary basins can be expected to be always saturated and buffered by carbonates, providing little opportunity for the preservation of significantly undersaturated water chemistry during upward flow, even if the initial generation of such undersaturated pore water could occur. A review of the literature where this model has been advanced reveals a consistent lack of quantitative treatment. In consequence, the presumption of mesogenetic dissolution producing a net increase in secondary porosity should not be used in the prediction of carbonate reservoir quality.

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 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.