Reservoir quality of deeply buried, Upper Jurassic sandstones of the South Viking Graben : A sedimentological, petrophysical and modeling approach

Abstract

Quartz cement, precipitated as syntaxial overgrowths on detrital quartz grains, is the dominant porosity-destroying process in deeply buried quartz-rich sandstones and has proven to be predominantly a pressure insensitive, precipitation rate controlled process in North-Sea reservoir sandstones. Grain-coats are the most important porosity-preserving mechanism in North-Sea sandstones buried to great depths (>4km), because they cover detrital quartz surfaces and thus block precipitation of syntaxial quartz overgrowths.

Petrophysical and petrographic data evaluated in this study indicate that grain-coats are common in Oxfordian to Tithonian sandstones located in the eastern parts of the South Viking Graben. Grain-coating micro-quartz is especially abundant, grain-coating illite occur in variable amounts. Grain-coating micro-quartz appears in deep marine as well as shallow marine sands and does not appear to be facies dependent. Grain-coating illite may depend on facies to a larger degree.

Grain-coating micro-quartz is common in the Upper Jurassic of the North Sea and is generated from transformation of siliceous sponge spicules from the ancient sponge Rhaxella Perforata. Though ubiquitous in Oxfordian to Tithonian sandstones of the eastern parts of the South Viking Graben, no micro-quartz was observed in the time-equivalent sandstones of the western South Viking Graben (Brae Formation), which implies that Rhaxella needed a relatively sediment starved environment to thrive.

Modeling calibrated to petrophysical and petrographic porosity-depth, mineralogical and textural data indicates that grain-coats may cause porosities 10-15% above the expected in deeply buried sandstones in the study area and preserve porosities above reservoir cut-off to burial depths of 5000-5500 meters (TVDRSF). These results however would be affected by overpressure variations. Current modeling algorithms (i.e. exemplar) does not account for mechanisms such as grain-crushing and very high silica super-saturations that will allow quartz cementation to continue despite optimally coating of detrital grains. Very high overpressures are present in the Upper Jurassic of the eastern South Viking Graben, where grain-coats are abundant. This will certainly prevent fracturing and most likely limit silica super-saturations, meaning that the conditions for porosity preservation are optimal in these sandstones and that the modeled depths of reservoir cut-off are realistic.