The lack of relationship between overpressure and porosity in North Sea and Haltenbanken shales

Abstract

All rights reserved. No part of this publication may be reproduced or transmitted, in any form or by any means, without permission.Cover: Inger Sandved Anfinsen. Printed in Norway: AiT e-dit AS, Oslo, 2008. Produced in co-operation with Unipub AS. The thesis is produced by Unipub AS merely in connection with the thesis defence. Kindly direct all inquiries regarding the thesis to the copyright holder or the unit which grants the doctorate. Unipub AS is owned by The University Foundation for Student Life (SiO) III CONCLUSIONS / IMPLICATIONSThis dissertation was motivated by the realization that four concepts, which are too simplistic, are widely assumed in the petroleum industry:First, mechanical compaction and associated disequilibrium compaction are frequently assumed to be the main mechanism for overpressure formation, although data are rarely given to support this assumption. This thesis concludes that neither the North Sea nor the Haltenbanken shales compacted mechanically at moderate to deep burial. Therefore, high overpressures in these rocks were not caused by disequilibrium compaction, but more likely by diagenetic processes that were largely unaffected by fluid pressures. Traditional seismic and log-based pore pressure detection methods in these areas should be expected to result in under-prediction of fluid overpressures because the porosities are not higher in overpressured shales than in normally pressured shales.Second, observations of zods (zones of deteriorated seismic signals; at times termed gas chimneys) are often interpreted as evidence of hydrocarbon leakage. This thesis concludes that the occurrence of zods may identify hydrocarbon leakages and where pressure compartments leak. However, prior to interpreting these zones as hydrocarbon leakage, the interpreter must be aware of the various geological processes and non-geological origins that could cause such velocity variations: (a) hydrocarbon leakage, (b) leakage of water with dissolved gas (that could create an inhomogeneous gas saturation), (c) fault or fracture zones themselves, (d) fluid leakage above fault(s) or fault junction, or (e) data quality issues. As a result, applications of zods in hydrocarbon prospect evaluation should be performed more carefully than what is often seen in the industry today.Third, the consequences of high overpressures are often assumed to be hydrocarbon leakage through the caprock -either because of hydro-fracturing or because high water pressure force oil or gas through membrane seals. This thesis concludes that high overpressures are compatible with hydrocarbon preservation. Vertical water leakage from the apex of a trap may take place while oil and gas are retained by capillary forces within the structure. This result is consistent with the fact that several of the largest oil fields on IV the Norwegian Continental Shelf (Statfjord, Gullfaks, Snorre, Visund, and Kvitebjørn) are highly overpressured and leaky, and yet contain vast amounts of oil and gas.Finally, vertical leakage is often assumed to occu...

Factors controlling the thermo-mechanical deformation of oil shales: Implications for compaction of mudstones and exploitation

“…Wangen (2000) showed that overpressure also results from cementation of pore space. This is consistent with Teige et al (1999) and Nordga˚rd Bola˚s et al (2004) who showed that porosity does not always show a positive correlation to overpressure in mudstones. It therefore becomes apparent that the void ratio change used as an index of compaction is problematic because precipitation of minerals may reduce porosity without compaction while compaction may proceed but porosity increases due to dissolution.…”

Compaction of diagenetically altered mudstones – Part 1: Mechanical and chemical contributions

“…Previous studies have observed the lack of a relationship between porosity and effective stress in mudstones buried into the diagenetic regime (e.g., Hermanrud et al 1998;Teige et al 1999). Others have reported density increases in mudstones that are not related to effective stress but are coincident with smectite-to-illite transformation (e.g., Peltonen et al 2009;Lahann and Swarbrick, 2011), which confirms that chemical compaction may take place in association with clay diagenesis.…”

Pressure–temperature–time–composition (P–T–t–X) constraints of multiple petroleum charges in the Hild field, Norwegian North Sea

“…12B). The lack of correlation between overpressure and porosity has been demonstrated for a wide range of North Sea and Haltenbanken shales (Hermanrud et al, 1998;Teige, Hermandrud, Wensaas, & Nordgård Bolås, 1999). Teige et al therefore suggest that high formation pressure is likely to have been generated after the rocks had been normally compacted and/or that thermally controlled mineral reactions dominate the porosity reduction at temperatures and depths where diagenesis is quantitatively important.…”