Elsevier

Marine and Petroleum Geology

Volume 92, April 2018, Pages 319-331
Marine and Petroleum Geology

Research paper
Porosity, permeability and compaction trends for Scandinavian regoliths

https://doi.org/10.1016/j.marpetgeo.2017.10.027Get rights and content

Highlights

  • Unique measurements of porosity and permeability for weathered basement rocks with increasing confining pressure.

  • New compaction models for incoherent saprock and saprolites from offshore Norway (Utsira High), on-land Norway and Sweden.

  • New input parameters to be used in reservoir modelling and basin modelling for weathered basement.

Abstract

Weathered crystalline and metamorphic basement are proven as hydrocarbon reservoir rock in many areas of the world, including the Norwegian Continental Shelf. The reservoir properties of these rocks vary laterally and vertically as a function of the burial history, initial basement lithologies, sub-aerial exposure to weathering processes, chemical weathering, fault patterns, and these properties are relatively unknown. In this work, laboratory measurements are performed on different regolith types applying varying confining pressures representative for different degrees of basement weathering and burial depth. Porosity and permeability are measured on coherent samples (altered basement) and incoherent samples (disintegrated basement rock and saprolites) from the Utsira High (offshore Norway), and outcrop samples from Bømlo (southwestern Norway) and Ivö Klack (southern Sweden). Coherent samples from Utsira High (well 16/1-15 and 16/1-12) give porosities between 5.10% and 7.95% and permeabilities between 1.08 and 3.30 mD at 5.1 MPa, and indicate a relation between observed micro-fractures and decreasing permeabilities. In general, permeability increases with increasing amount of micro-fractures. The permeability is varying from 0.15 mD at 30 MPa to 3.30 mD at 5.1 MPa. This work presents the first published compaction curves for weathered basement (saprock and saprolites), and to our knowledge first permeability-porosity relationships for saprock and saprolites from three locations (Bømlo, Ivö Klack and Utsira High). The new porosity and permeability measurements of weathered basement rocks can be used as input parameters for reservoir modelling, basin modelling, field planning and hydrocarbon exploration in areas with regoliths. It can also serve as important input into hydrogeology, geothermal modelling, planning of tunnels, and as help to understand landslides and hillslope gullies in weathered basement.

Introduction

During the last years, fractured and weathered crystalline and metamorphic basements along the continental margin of Norway have become a substantial focal point for hydrocarbon exploration in new areas. The recent discoveries on the Utsira High (Fig. 1a) with the Edvard Grieg, Rolvsnes and Johan Sverdrup fields show that deeply buried regolith profiles with weathered and fractured Caledonian granitic and gabbroic rocks can act as petroleum reservoirs (Riber et al., 2015, Riber et al., 2016, Riber et al., 2017). These phenomena are not restricted to the Norwegian continental shelf. Naturally fractured and weathered basement reservoirs occur globally (Petford and McCaffrey, 2003), as proven by many discoveries, e.g., the Lancaster field west of Shetland (Trice, 2014), the La Paz field in Venezuela (Koning, 2003) and the Bach Ho field in Vietnam (Cuong and Warren, 2009).

In general, the basement-cover-interfaces in the upper crust can contain regoliths, which are composed of weathered residuum of the underlying basement rocks. At present day surface conditions, commonly a few metres to over 150 m thick lateritic regoliths are widespread in inter-tropical regions of the world (Butt et al., 2000). The thickness and degree of weathering depends on the age of the land surface, tectonic activity, climatic history and the nature of the bedrock (Butt et al., 2000). This lateral and vertical inhomogeneity of regoliths makes any kind of exploration challenging (Butt et al., 2000). In a petroleum system, a regolith might serve as a carrier, a reservoir and/or as a sealing unit. The migration operates on scales ranging from micro-to basin-scale (Bethke, 1985, Ge and Garven, 1994). Furthermore, the petrophysical, morphological, mineralogical and geochemical characteristics of the regolith might differ with respect to the degree of weathering (e.g., depth in weathering profile) and its burial and exhumation history (Riber et al., 2017). Riber et al. (2015) found that reservoir quality in crystalline rocks from 18 different wells located on the Utsira High (Fig. 1c) varied greatly as a function of type and degree of alteration. Moreover, it is well known that a petroleum system involving weathered basement (e.g., Lancaster Field) is dominantly controlled by the fracture systems (Trice, 2014). Thus, the study of the regolith behaviour in a petroleum system requires specific approaches, which differ from conventional used techniques in petroleum exploration (Riber et al., 2016). This includes establishing experimental compaction trends for regoliths assuming burial depth in a km-scale. Moreover, working with subsurface data limits the accessibility and thus analogue outcrop studies are inevitable.

Permeability and porosity are key petrophysical parameters that are crucial input for fluid flow simulation models. Studies of crystalline basement rocks has been recently focused to fault-related rocks (e.g. Evans et al., 1997, Wibberley and Shimamoto, 2003, Mizoguchi et al., 2008). In this study, we examine crystalline basement rocks located at the interface between basement and sedimentary cover from outcrops and subsurface location across Scandinavia. Based on the degree of recognizable primary rock structures and the mechanical rock strength, a regolith can be divided into the altered coherent rock facies, saprock and saprolite facies (Velde and Meunier, 2008, Riber et al., 2017). Laboratory measurements of porosity and permeability are performed on samples of different regolith types (consolidated and unconsolidated) and under varying confining pressures representative for the different degree of basement weathering and burial depth, respectively.

Samples from exploration wells 16/-1-15 and well 16/1-12 are from the Utsira High (North Sea); outcrop samples are from Bømlo (southwestern Norway) and Ivö Klack (southern Sweden) (Fig. 1). The goal of this analysis is to establish compaction curves for saprock and saprolites, that to our knowledge never has been carried out, and to find porosity-permeability relationship. These new relationships, for porosity and permeability for weathered basement can potentially be used as input for reservoir and basin modelling in the oil industry, and make the base for further supplementing analysis of other locations in the future.

Section snippets

Utsira High (southern North Sea)

The Utsira High is located in the Southern Viking Graben, in the Norwegian North Sea ca. 160 km offshore southern Norway (Fig. 1a). The structure is a basement horst (Fig. 1c) comprised of granitic gabbroic rocks of Middle Ordovician to Middle Silurian U/Pb zircon ages (Slagstad et al., 2011). The top basement is located at depth of ca. 2 km (Ziegler, 1992). Sedimentary cover and low-thermochronological data indicate a complex burial history whereby the top basement was at surface/near surface

Methods

We used core flooding experiments and micro CT analyses to determine the effective porosity and permeability of each sample (Table 1). For measuring the effective porosity, we used three different methods: Helium porosity (He-porosity), water porosity and porosity from micro CT. Additionally, we measured Klinkenberg corrected air permeability (Klinkenberg, 1941) and water permeability at different confining pressures. Bulk mineralogy analysis were carried out using XRD for the incoherent

Results

The results are presented for coherent and incoherent samples (Table 1, Table 2). The coherent samples are the altered basement rocks from the wells 16/1-12 and 16/1-15 showing a different degree of micro fracturing (Fig. 4, Fig. 5). The degree of micro-fracturing is quantified by normalising it to the sample from well 16/1-12 1934 m, to get the relative difference in the amount of micro-fractures between the measured samples. The incoherent samples are one disintegrated basement rock (mixture

Discussion

Based on the degree of recognizable primary rock structures and the mechanical rock strength, a regolith can be subdivided into the altered coherent rock facies, saprock and saprolite facies (Velde and Meunier, 2008, Riber et al., 2017). The dissolution of labile minerals creates voids and becomes more important with progressive chemical weathering. Fractures are particularly important as pathways for the ingress of formation waters (e.g., Velde and Meunier, 2008, Borrelli et al., 2012,

Summary and conclusions

The porosity and permeability has been measured for weathered basement (coherent and incoherent samples) with different methods using standard He-porosity, X-ray micro CT, water porosity and air and water permeability on samples from three locations: two wells at the Utsira High (offshore southern Norway), and outcrop at Bømlo (southern Norway) and Ivö Klack (southern Sweden).

  • For coherent samples (weathered basements plugs from well 16/1-15 and well 16/1-12) the He-porosity varies from 5% to

Acknowledgement

This work is part of the research project BASE (Basement fracturing and weathering on- and offshore Norway – Genesis, age, and landscape development) lead by NGU, with SINTEF Petroleum as a research partner, and funded by Maersk Oil, Lundin Petroleum, Wintershall and AkerBP. Access to the core data from Utsira High at the Weatherford Laboratories, has kindly been granted by Lundin Petroleum. We are greatful for comments by Atle Rotevatn on an earlier version of the paper.

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