Enhanced porosity preservation by pore fluid overpressure and chlorite grain coatings in the Triassic Skagerrak, Central Graben, North Sea, UK

Abstract

Current understanding of porosity preservation in deeply buried sandstone reservoirs tends to be focused on how diagenetic grain coatings of clay minerals and microquartz can inhibit macroquartz cementation. However, the importance of overpressure developed during initial (shallow) burial in maintaining high primary porosity during subsequent burial has generally not been appreciated. Where pore fluid pressures are high, and the vertical effective stress is low, the shallow arrest of compaction can allow preservation of high porosity and permeability at depths normally considered uneconomic. The deeply buried fluvial sandstone reservoirs of the Triassic Skagerrak Formation in the Central Graben, North Sea, show anomalously high porosities at depths greater than 3500 metres below sea floor (mbsf). Pore pressures can exceed 80 MPa in the upper part of the Skagerrak Formation at depths of 4000-5000 mbsf, where temperatures are above 1408C. The Skagerrak reservoirs commonly have high primary porosities of up to 35%, little macroquartz cement and variable amounts of diagenetic chlorite grain coats. This research sheds light on the complex controls on reservoir quality in the fluvial sandstones of the Skagerrak Formation by identifying the role of shallow overpressure in arresting mechanical compaction and the importance of chlorite detrital grain coatings in inhibiting macroquartz cement overgrowth as temperature increases during progressive burial.Gold Open Access: This article is published under the terms of the CC-BY 3.0 license.

Importance of vertical effective stress for reservoir quality in the Skagerrak Formation, Central Graben, North Sea

“…The positive effect of overpressure or low VES towards retardation of mechanical compaction has been previously observed by modelling studies of (Lander and Walderhaug, 1999;Paxton et al, 2002), .Furthermore, high primary porosities, maintained by shallow overpressure have been documented for the Skagerrak Formation sands of the Judy Field (Nguyen et al, 2013;Stricker and Jones 2016).…”

“…This can be observed in the Judy field, where IGV values are higher than expected (>26% , Table 5), due to shallow overpressure development and a constantly reduced VES accrual rate (Figure 7), resulting in the experience of lower maximum VES and a low mechanical compaction state for the present-day burial depth (e.g. Nguyen et al, 2013;Stricker and Jones, 2016). A similar correlation can be made for the present-day IGV values and the experienced maximum VES of the Heron and Skua fields, where IGV values are slightly higher than the expected 26% (Table 5 & Figure 8).…”

“…Limiting the accrual rate or reducing the VES during burial by pore fluid overpressure can slow down or arrest mechanical compaction and reduce its effect on porosity loss, leading to the maintenance of primary porosity to depth (Bloch et al, 2002;Nguyen et al, 2013;Stricker and Jones, 2016). Even though this effect has been well known since Terzaghi's introduction of the effective stress concept, the impact of low VES due to overpressure has often been overlooked or underestimated in reservoir quality studies (e.g.…”

The role played by carbonate cementation in controlling reservoir quality of the Triassic Skagerrak Formation, Norway

“…16) is most likely attributed to a combination of factors including chlorite coats, overpressure, cement types, and matrix content. Chlorite has been widely reported as an important grain coating clay mineral inhibiting quartz overgrowth and help preserve porosity (Bloch et al, 2002;Ehrenberg, 1993;Stricker and Jones, 2016;Taylor et al, 2010). Bloch et al (2002) and Ajdukiewicz and Lander (2010) (Stricker et al, 2016a).…”

Overpressure preventing quartz cementation? - A reply

“…Maast (2016), however, insists in his comment that pore fluid overpressure does not play a role and that reduced VES does not benefit the maintenance of reservoir quality. We will address this comment by not providing a recount of our petrographical observations and modelling (Stricker et al, 2016b) but will refer to further observations and analyses from the Triassic Skagerrak Formation in the North Sea (see Stricker et al, 2016a;Stricker and Jones, 2016). …”