Making diagenesis obey thermodynamics and kinetics: the case of quartz cementation in sandstones from offshore mid-Norway (original) (raw)
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Precipitation rates for quartz cement in sandstones
ABsraacr: Precipitation rates for quartz cement in quartz-rich Jurassic sandstones from the Norwegian shelf have been determined by combining petrographic data, fluid-inclusion data, and temperature-history modeling. Thin-section petrography enables the number of moles of quartz cement precipitated in a sample and the surface area available for precipitation to be determined. Meusurement of homogenization temperatures for fluid inclusions located at the boundaries between quartz clasts and quartz overgrowths permits the temperature of initial quartz cementation to be found, and this temperature may be translated to a date by constructing a temperature-history curve for each sandstone. Since quartz cementation has continued up to the present in the studied sandstones, precipitation rates for quartz cement per unit time and surface area can be calculated.
Quartz cementation mechanisms and porosity variation in Baltic Cambrian sandstones
Sedimentary Geology, 2007
Quartz is an import cementing material in siliciclastic sandstones that can reduce porosity and permeability severely. For efficiently predicting and extrapolating petrophysical properties such as porosity and permeability, the controls on the occurrence and the degree of quartz cementation need to be better understood. Toward this end, a succession of Cambrian marine quartz arenites in Lithuania, at a present burial depth ranging from about 1 to 2 km, has been studied. In the central part of the Baltic Basin these quartz arenites are heavily cemented by quartz and the degree of quartz cementation is the main control on reservoir properties. On a regional Baltic Basin scale, the amount of quartz cement, and inversely the porosity and permeability, are correlated to burial depth and palaeotemperature. Porosity of the sandstones in the Baltic Basin flanks is around 23-28% and decreases toward the west to 15-18% in the central part and to 2-4% in west Lithuania. However, superposed on general trends between petrophysical properties and depth, large variations exist even between closely spaced wells making forward prediction of reservoir quality difficult. Evidently, factors other than general physical conditions and overall chemical conditions related to burial depth must have locally influenced the processes of quartz cementation and controlled the location and amounts of quartz cement. Such local factors are detrital composition, sedimentary structures and reservoir architecture, inherent to depositional facies. Clayinduced chemical compaction and pressure dissolution of detrital quartz at shale-sandstone contacts and within thin shale lamina is probably the main process yielding silica for local quartz cement. The supply of silica for quartz cement in the sandstones is thus dependent on the number of thin clay lamina and clay intercalations within the reservoir sandstones. The sandstone/shale ratio and thickness of sandstone bodies control the location and the degree of quartz cementation and thus reservoir quality.
Duration of quartz cementation in sandstones, North Sea and Haltenbanken Basins
Marine and Petroleum Geology, 1992
Evidence from fluid inclusions, petrography and burial history modelling is used to estimate the duration of quartz cementation in sandstones. The homogenization temperatures of primary fluid inclusions in quartz cements from samples from the Brent Group (North Sea) and Haltenbanken (offshore Norway) show variations (4o) of 17.4 and 20.2°C. These are interpreted to reflect durations of less than approximately 9 and 17 Ma. Low variances are general features of such data sets and suggest that quartz cementation usually takes place as events with durations of the order of 10 Ma or possibly less. Confining the age of quartz cement growth using a combination of geological, petrographical and radiometric dating methods that are independent of fluid inclusions also suggests that quartz cements grow over periods substantially shorter than the age of the sandstone. It is not easy to reconcile this deduction with the suggestion that burial itself causes cementation.
Chemical Geology, 2002
Oxygen isotope microanalyses of authigenic quartz, in combination with temperatures of quartz precipitation constrained by fluid inclusion microthermometry and burial history modelling, are employed to trace the origin and evolution of pore waters in three distinct reservoirs of the Brae Formation in the Miller and Kingfisher Fields (North Sea). Oxygen isotope ratios of quartz cements were measured in situ in nine sandstone thin sections with a Cameca ims-4f ion microprobe. In conjunction with quartz cement paragenesis in the reservoirs, constrained from textural and cathodoluminescence (CL) microscopy studies, pore water evolution was reconstructed from the time of deposition of the sandstones in the Upper Jurassic until the present.
Geological Quarterly, 2010
Quartz cementation is a major parameter controlling the reservoir properties of the Middle Cambrian quartz arenites of the central and western parts of the Baltic Basin. Marked local variations in the porosity and permeability severely complicate oil exploration and exploitation in West Lithuania. Commonly, the porosity of the oil reservoirs is 6-8%. Therefore even minor changes in the porosity have a considerable impact on the potential of oil fields. A predictive model of the quartz cementation is proposed, based on kinetic modelling results. The precipitation rate-limiting model effectively explains sharp variations in quartz cementation controlled by grain-size changes. The model was further improved by incorporating the sorting factor. Even so, the amount of quartz cementation is overpredicted by 4-7% in some intervals, implying that the precipitation rate-limiting model is too simplified. A good correlation was obtained between stylolite spacing and quartz cementation, the ove...
Thermal controls on reservoir quality prediction in deeple buried sandstones
Proceedings, 1996
Reservoir quality prediction often utilises porosity /permeability`rela tionships as a function of depth, ma inly based on models for mechanical compaction, compensat ing for any overpressure effects. Pe trographic obse rvations, however, indicate that quartz overgrowths are the dominant porosity-reducing cement in deeply buried sandstones of the Norwegian continental shelf. These overgrowths result from a quartz redistribut ion mechanism consisting of quartz dissolution at mica-rich stylolite interfaces coupled to the diffusion of aqueous silica away from these interfaces into inters tylolite regions, where it is precipitated as cement. The rate of the overall dissolution-transport-precipitation processes is strongly controlled by the precipitation-step. Since mobilisation of silica at the stylolites/mica-quartz interfaces is mainly insensitive to pressure (Bjorkum, 199 6) the temperature becomes the most important parameter for porosity reduction due to quartz cementation (Oelkers et al., 1992 ; 1996; Walderhaug, 1994). In general, the total amount of quartz overgrowth, and therefore porosity reduction due to this quartz cementation, increases non-linearly with increasing temperature and increasing stylolite .density, but decreases linearly with increasing grain size, due to surface area considerations. The effect of hydrocarbons on this process in water wet reservoirs, with commonly observed irreducible water saturations, is minor and varies non=linearly with stylolite frequency due in part to transport considerations. This conclusion is supported by independent observations from other North Sea data bases (Giles et al ., 1992). A computational algorithm has been developed and used to characterise the quantity and distribution of quartz cement as a function of burial rate, temperature, stylolite frequency and grain size. Taking into account both petrographic observations and experimentally measured precipitation rates, mechanical compaction dominates as the porosity-reducing mechanism at temperatures of less than approximately 90°C, while quartz redistribution dominates over compaction at higher temperatures. The algorithm, optimised using data acquired from the Norwegian continental shelf, allows for accurate predictions of porosity in other basins, having dramatically different burial histories and temperature gradients.
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.