Closed-System Burial Diagenesis in Reservoir Sandstones: Examples from the Garn Formation at Haltenbanken Area, Offshore Mid-Norway

Abstract

The Middle Jurassic Garn Formation of the Haltenbanken area has been studied using mineralogical and geochemical data from 21 wells, ranging in burial depths from 2.0 to 4.1 km relative to seafloor (RSF). K-feldspar and plagioclase contents show variations on a regional scale both laterally and as a function of burial depth. The content of pore-filling authigenic illite increases sharply, and the content of K-feldspar and kaolinite decreases in Garn sandstones presently at depths greater than 3.6-3.7 km RSF (120-130؇C). The depletion in Kfeldspar below 3.7 km RSF is not accompanied by lower potassium values in the bulk chemical composition (wt % K 2 O). This suggests that the potassium released during K-feldspar dissolution is retained in the sandstones and is precipitated as illite. The variations in bulk contents of potassium and sodium are therefore considered to be related principally to primary variations in sandstone mineralogy.The shallower sandstones (Ͻ 3.7 km RSF) with average wt % K 2 O greater than 0.95 (K/Al molar ratio Ͼ 1/3) have a K-feldspar:kaolinite ratio greater than one. The deeply buried (Ͼ 3.7 km RSF) sandstones with similar potassium contents contain excess K-feldspar and most of the kaolinite is illitized. However, deeply buried sandstones containing an average of 0.38 wt % K 2 O (K/Al molar ratio Ͻ 1/4) contain a significant amount of kaolinite but negligible K-feldspar. This suggests that the K-feldspar:kaolinite ratio before the onset of illitization was less than one, and hence that the kaolinite-illite reaction has been restricted by an insufficient supply of potassium (absence of K-feldspar). This illustrates how illitization of kaolinite depends upon K-feldspar as a local source of potassium. Prediction of illitization in sandstones, therefore, must be based on integration of models for provenance, facies, and early diagenesis in addition to burial and thermal history. The formation of pore-filling authigenic illite in these sandstones is an important influence on the total reservoir quality.

Diagenetic paths in the margin of a Triassic Basin: NW zone of the Iberian Chain, Spain

“…However, no replacements of kaolinite by illite have been observed, as occurs in other depocentral areas (Marfi l and Buendia 1980;Arribas 1984Arribas , 1987. The generation of authigenic illite at the expense of kaolinite and K-feldspar has been described by several authors (Bj0rlykke 1980;Ehrenberg and Nadeau 1989;Bj0rlykke et al 1995;Chuhan et al 2000Chuhan et al , 2001. These authors consider that this reaction takes place during late diagenesis in a relative close system at 120°C 140°C, and at burial depths greater than 3.7 km (Chuhan et al 2000).…”

Open-system chemical behaviour of Wilcox Group mudstones. How is large scale mass transfer at great burial depth in sedimentary basins possible? A discussion

“…There is evidence that even sandstone sequences are geochemically rather closed systems. The illitization of kaolinite depends on the local presence of a source for potassium such as K-feldspar, indicating that potassium is not supplied from K-feldspar a few metres away (Chuhan et al, 2001). The potential for mass transport is much lower still in mudstone sequences than in sandstones, due to low diffusion coefficient.…”

Evolution of provenance in the NE Atlantic rift: The Early–Middle Jurassic succession in the Heidrun Field, Halten Terrace, offshore Mid-Norway

“…However, given that palaeogeographic reconstructions depend heavily on a knowledge of the location of sediment source areas, there is surprisingly little published information concerning the provenance of the EarlyMiddle Jurassic succession. Most of the studies carried out on sandstone compositions have been undertaken to evaluate the nature and extent of diagenetic modification (Bjørlykke et al, 1986;Saigal et al, 1988;Ehrenberg and Nadeau, 2002;Ehrenberg et al, 1998;Chuhan et al, 2000Chuhan et al, , 2001, although the role of provenance as a possible control on diagenesis has been considered in some of these papers (Chuhan et al, 2000;Ehrenberg et al, 1998). Apart from provenance interpretations made on the basis of depositional facies trends (for example, the Tilje Formation is interpreted as having a Scandinavian source by Martinius et al, 2001, on the basis of the disposition of facies associations), the only published provenance information documents Sm-Nd model age data (Ehrenberg et al, 1998;Martinius et al, 2001).…”