Origin and timing of late diagenetic illite in the Permian–Carboniferous Unayzah sandstone reservoirs of Saudi Arabia

Abstract

Diagenetic illite is one of two major porosity occluding cements in the Unayzah sandstone reservoirs (Permian-Carboniferous) of Saudi Arabia. The other is diagenetic quartz. This article focuses on the origin of illite and its timing. Illite has been formed by a reaction of detrital K-feldspar and early diagenetic kaolinite as temperatures increased due to burial. When either of the two reactants is exhausted, illite ceases to precipitate. There is no evidence that hydrocarbon emplacement, deep brine migration, or unique thermal events are factors in illite precipitation. Modeling of illite precipitation as a kinetically controlled reaction using burial histories of the samples studied generally yields a reasonable match between measured and modeled ages and amounts of illite. This lends further support to the gradual formation of illite over a time-temperature interval. Although quartz overgrowths and diagenetic illite may occur in the same thin section, they appear to be mutually exclusive locally. Quartz overgrowths do not occur on detrital quartz grains that are coated with diagenetic illite, and illite is rarely observed on quartz overgrowths. Therefore, it appears that not only does diagenetic illite inhibit nucleation of quartz overgrowths but quartz overgrowths may also inhibit precipitation of diagenetic illite. The two cements appear to compete for surface area on uncoated detrital quartz grains.

Diagenetic evolution and its effect on reservoir-quality of fan delta sandstones during progressive burial: Evidence from the upper part of the fourth member of Shahejie formation, Bonan sag, Jiyang depression

“…9). This distribution of kaolinite with depth suggests that it mainly comes from relative early stage feldspar dissolution when temperature is less than 125 °C, which is in consistent with previous research [23]. Smectite will be transformed to illite progressively through illite/ smectite mixed layers in alkaline condition with high concentration of K + .…”

Petroleum reservoir quality prediction: overview and contrasting approaches from sandstone and carbonate communities

“…Compaction simulations generally consider mean grain size and sorting, variations in the mechanical properties of framework grains and pore-filling materials, as well as effective stress. Additionally, coupled effect-oriented compaction and cementation models incorporate kinetic models for quartz overgrowth cements (Walderhaug 1994b(Walderhaug , 1996, plagioclase albitization (Perez & Boles 2005), the reaction of kaolinite and K-feldspar to form fibrous illite (Franks & Zwingmann 2010;, and the reaction of volcanic rock fragments and other labile mineral grains to form grain-coating chlorite (Bonnell et al 2014). Other diagenetic processes such as carbonate cementation and replacement may be considered in a simulation but currently are not predicted a priori except where they occur as a by-product of albitization and volcanic rock fragment alteration.…”

Pressure and thermal regimes and systems in the sedimentary sequence of central and eastern Saudi Arabia

“…It is also interesting that the porosity and permeability of the Paleozoic sandstones are significantly reduced at depths exceeding 12,000 ft. due to cementation with quartz overgrowth and illite (Carney et al 2002 andZwingmann 2010). This is the same depth at which the OPR 1 starts which indicates that the gas generation aided by the reduced porosity and permeability of the reservoirs are the likely factors involved in generating overpressures in the OPR 1.…”

“…The sandstone units have relatively high porosity and permeability on the western areas near the outcrops forming prolific regional aquifers. Towards deeper areas in the east, the porosity and permeability decrease due to compaction and further deteriorate due to quartz cement and diagenetic illite formation (Carney et al 2002, Franks and Zwingmann 2010, and Arouri et al 2010. The Paleozoic sandstone units are the primary targets for oil in central Arabia and sweet gas in eastern and northwestern Saudi Arabia.…”