Facies-related diagenesis in the Main Claymore Oilfield sandstones

Abstract

Four major sedimentary sequences of the Triassic and Upper Jurassic of the Main Claymore Oilfield of the North Sea each contain a characteristic suite of diagenetic minerals and fabrics. (1) Triassic Skagerrak Formation fluvial sandstones contain early authigenic pore-lining smectite, together with kaolinite and chlorite which form grain replacements and pore fills. Quartz and feldspar overgrowths are minor. Ferroan dolomite forms a late diagenetic patchy poikilotopic cement. Smectite is converted to illite-smectite in a 5 m thick zone beneath the sub-Jurassic unconformity. Smectite formed early in diagenesis prior to oil migration and destroyed permeability. Thus oil is not found in these sandstones although they occur in the oilzone. (2) The Piper Formation (late Oxfordian/early Kimmeridgian) paralic deposits mainly contain authigenic, pore-lining illite-smectite, vermicular kaolinite grain replacements and pore fills. Quartz overgrowths are generally well developed. (3) The Kimmeridge Clay Formation (early Kimmeridgian/early Volgian) comprises thin marine sandstone turbidites, contained within a thick siltstone/shale sequence. In the sandstones (the 'Ten Foot Sandstone') discrete double-ended quartz crystals (1-20 #m) developed prior to quartz, K-and Na-feldspar overgrowths. Only minor kaolinite and lllite-smectite are present. Late diagenetic dolomitic occurs as a patchy poikil0topic cement and as clusters of pore-filling rhombs. (4) The Claymore Sandstone Member (early to middle Volgian) thick marine sandstone turbidites are interbedded with thin siltstones/shales. Sandstones have well-developed quartz, K and Na-feldspar overgrowths, and kaolinite and illite-smectite occur as grain replacements and rarely as pore fills. Late-diagenetic dolomite and ferroan dolomite form poikilotopic cement and clusters of pore-filling rhombs. The major factors which control diagenetic features are depositional environment and associated porewater together with original mineralogy. Burial history and textural features of the sandstones also have important influences. Reservoir quality is controlled by a complex interplay of these features.

Dissolution of apatite in North Sea Jurassic sandstones: implications for the generation of secondary porosity

“…whereas marine sandstones of, for example, the Claymore Field (Spark & Trewin, 1986) or the Fulmar Field (Stewart, 1986) show comparatively little secondary porosity. This gives strong support to Bjorlykke (1980Bjorlykke ( , 1983Bjorlykke ( , 1984, who regards much of the secondary porosity in North Sea sandstones to have resulted from the introduction of meteoric waters early in diagenesis.…”

Diagenetic Evolution Control on Reservoir Quality of the Oligocene Weizhou Formation in Weixinan Sag of Beibuwan Basin, Northern South China Sea

“…In addition, the effect of chlorite coating in sandstones from different places is well known [3][4][5][6][7][8][9][10]. Micro-quartz [12,13], illite [2,11] and kaolinite [14] were also documented.…”

“…There have been many studies on the effect of chlorite coats on reservoir quality [3][4][5][6][7][8][9][10]. In addition to chlorite coating, clay grain coats include illite coating [2,11], quartz overgrowth coating [12,13] and kaolinite coating [14]. Additionally, organic acids are also important to the dissolution of reservoirs, which result from the maturation of organic matter in shales or mudstone adjacent to sandstone [15].…”

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

“…Ramm and Forsberg (1991) reported in a study of Upper Jurassic sandstones of the Gyda area that sandstones containing microcrystalline quartz overgrowths had higher porosities (around 20-22% at 4.1-4.2 km) than sandstones that contained macroquartz overgrowth. In addition, Spark and Trewin (1986) reported porosity preservation in the Ten Foot Sandstone of the Claymore field, North Sea, which they attributed to early formation of discrete quartz crystals. Spark and Trewin (1986) also suggested that the sandstones were supersaturated with respect to silica because discrete crystals rather than syntaxial quartz overgrowths developed.…”

“…In addition, Spark and Trewin (1986) reported porosity preservation in the Ten Foot Sandstone of the Claymore field, North Sea, which they attributed to early formation of discrete quartz crystals. Spark and Trewin (1986) also suggested that the sandstones were supersaturated with respect to silica because discrete crystals rather than syntaxial quartz overgrowths developed. They considered the source of silica to be a result of dewatering of adjacent shales.…”