Clay Minerals in North Sea Sandstones
The most abundant clay-mineral cements in North Sea reservoir sandstones are kaolinite, illite and mixed-layer minerals. Authigenic chlorite commonly is present but rarely is abundant. Permian and Triassic sandstones were deposited in an arid to semiarid climate and contain mostly illite/smectite with subordinate kaolinite. Isotopic and textural evidence suggests that authigenic kaolinite in these sandstones formed after tectonic uplift and meteoric-water flushing. In fluvial and shallow marine Lower and Middle Jurassic reservoirs, kaolinite is the dominant clay mineral and feldspar dissolution forms abundant secondary porosity even in the shallowest reservoirs. Stable-isotope analyses of authigenic kaolinite suggest crystallization at relatively low temperature and from pore water of meteoric origin. Upper Jurassic sandstones representing shallow marine facies also contain secondary porosity, due to feldspar leaching, and authigenic kaolinite. In turbiditic sandstones and sandstones interbedded with the main source rock (Kimmeridge Clay Formation), however, diagenetic kaolinite is rare. The sandstones also show little evidence of feldspar leaching, probably because this distal facies was not effectively leached by meteoric water. Organic acids or carbon dioxide that may have been released from source rocks seem to have had little effect on adjacent sandstones in terms of feldspar leaching and precipitation of kaolinite. Dissolved feldspar and authigenic kaolinite are also relatively uncommon in the Cretaceous and Tertiary turbiditic sandstones. The main control on feldspar leaching and distribution of authigenic kaolinite appears to have been the degree of meteoric-water flushing, which depends on climate, depositional environment and continuity of sandstone beds. Later tectonic uplifts may also have caused meteoric-water recharge from exposed areas into the basin.
The geochemistry of formation water from reservoir rocks suggests that pore waters in North Sea reservoirs are mostly in the stability field of illite during burial diagenesis. In Jurassic reservoirs, illite can be observed to replace kaolinite in the deeper wells if K-feldspar is available as a source of potassium. A strong increase in the degree of illitization can be observed below 3.7 to 3.8 km burial depth in several oil fields. K/Ar dating of illites from reservoirs in the North Sea and Haltenbanken areas give a wide range of estimated ages, many between 30 and 50 Ma. Many of these sandstones would only have been buried to about 2 km during those early Tertiary times. If these dates for illite precipitation are correct, they might indicate high geothermal gradients at that time. The present depth distribution of illite suggests, however, that illitization of kaolinite takes place at greater depth (3.5–4 km). illite may also form from a smectite precursor and illite of this origin is particularly abundant in Triassic and Permian reservoirs.
Chlorite may replace kaolinite starting at about 90 to 100°C, but the amount is limited by the supply of iron and magnesium from dissolving mafic minerals and rock fragments. Co-existing illites and chlorites appear to follow regular compositional trends with increasing burial temperatures. These trends are in covariance with the stability of the endmember components of the clay minerals. Crystal-size distributions may be explained in terms of Ostwald-ripening mechanisms. Chlorite crystals show compositional variations from core to rim.
Burial diagenesis in the North Sea basin is interpreted to be relatively isochemical and our diagenetic models do not require large-scale transport of solids in solution. Theoretical models of corapactional poreawater flow and observed compositional stratifications of the pore water in this basin also constrain large-scale mass transfer by advection.
Figures & Tables
Origin, Diagenesis, and Petrophysics of Clay Minerals in Sandstone–This volume grew out of a symposium held at the 27th Annual Meeting of the Clay Minerals Society in Columbia, Missouri on October, 1990. The symposium was designed to present a current synthesis of research devoted to the origin, diagenesis and petrophysics of clay minerals in sandstones. International authors demonstrate a multidisciplinary approach to interpreting the origin and diagenesis of clay minerals in sandstones and to evaluating their influence on reservoir quality. This volume bridges a gap between petrographic and geochemical researchers and reservoir geologists and engineers.