Geochemical modelling of diagenetic reactions in a sub-arkosic sandstone

Abstract

A reaction path model was constructed in a bid to simulate diagenesis in the Magnus Sandstone, an Upper Jurassic turbidite reservoir in the Northern North Sea, UKCS. The model, involving a flux of source rock-derived CO2 into an arkosic sandstone, successfully reproduced simultaneous dissolution of detrital K-feldspar and growth of authigenic quartz, ankerite and illite. Generation of CO2 occurred before and during the main phase of oil generation linking source rock maturation with patterns of diagenesis in arkosic sandstones and limiting this type of diagenesis to the earlier stages of oil charging. Independent corroborative evidence for the model is provided by formation water geochemical data, carbon isotope data from ankerite and produced gas phase CO2 and the presence of petroleum inclusions within the mineral cements. The model involves a closed system with respect to relatively insoluble species such as SiO2 and Al2O3 but is an open system with respect to CO2. There are up to seven possible rate-controlling steps including: influx of CO2, dissolution of K-feldspar, precipitation of quartz, ankerite and illite, diffusive transport of SiO2 and Al2O3 from the site of dissolution to the site of precipitation and possibly the rate of influx of Mg2+ and Ca2+. Given the large number of possible controls, and contrary to modern popular belief, the rate of quartz precipitation is thus not always rate limiting.

CO2 degassing and trapping during hydrothermal cycles related to Gondwana rifting in eastern Australia

“…As discussed above, interaction between CO 2 -rich rich hydrothermal fluids and siliciclastic rocks will enhance the cation release leading to formation of a widespread clay, carbonate and quartz mineral association (Huang and Longo, 1994;Barclay and Worden, 2000;Kaszuba et al, Battaglia et al, 2007). Assuming the illitic clays and carbonates were formed in a similar temperature range from roughly 80°C (smectite-rich) to 170°C (illite-rich) and from fluids with a similar origin we can estimate the oxygen isotope composition of fluids from which the carbonates precipitated (Table 2).…”

“…At the earliest stage of the reaction, kaolinite will precipitate from HCO 3 À1 -rich acidic fluids. As the reaction proceeds, depending on the availability of cations (particularly K + ), precipitation of illite-smectite will follow in a slightly acidic to neutral environment (e.g., Barclay and Worden, 2000;Battaglia et al, 2007). The remaining, more cation-rich fluids will lead to deposition of carbonate minerals and silica (e.g., Barclay and Worden, 2000;Kaszuba et al, 2006) (see below).…”

Contrasting red bed diagenesis: the southern and northern margin of the Central European Basin

“…Liewig and Clauer 2000;Wilkinson and Haszeldine 2002). Meshwork illite precipitation is most likely related to increasingly acidic/CO 2 -rich pore waters during early stages of oil charging (Cookenboo and Bustin 1999;Barclay and Worden 2000).…”

Diagenesis of tight oil sand reservoirs: Upper Triassic tight sandstones of Yanchang Formation in Ordos Basin, China

“…However, the amount of pressure dissolution of quartz is difficult to quantify. Furthermore, the K‐feldspar dissolution can generate excess SiO 2 (Barclay & Worden, ; Thyne et al, ), which may also be a likely source for quartz cement as shown in Figure . Moreover, as Figure a shows, there is external silica source for precipitation of quartz cement, the catagenesis of adjacent shales may be a probable source (Foscolos & Powell, ).…”