Pre-requisites, processes, and prediction of chlorite grain coatings in petroleum reservoirs: A review of subsurface examples

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Abstract

Deeply buried reservoirs containing chlorite-coated quartz sand grains commonly have higher than expected porosity and permeability, although prediction of such positive anomalies still remains elusive. A total of 54 published examples based on information and data from 62 scientific papers was collated. Quantification of some of the most common parameters including depositional environment, age and latitude of sand deposition, effect on reservoir quality and chemical composition of chlorite is presented.

The dataset indicates that chlorite-coats are found in sandstones deposited in a range of depositional environments, but most commonly occur in delta-related environments (44%), with fluvial environments the second most common (19%). Age relationships indicate that there is an overall exploration bias in published examples, with chlorite-coats becoming increasingly common through time. The latitude at the time of deposition of sands with chlorite-coats is wide (60°N–60°S), and indicates that temperate and tropical climates are important for the generation of this clay mineral. Chlorite can have a variable effect on reservoir quality, but is typically positive. Iron-rich chlorites occur overwhelmingly in coastal environments, while mixed iron- and magnesium-rich chlorites are principally found in marine and terrestrial sandstones.

Analysis of these factors suggests that hinterland geology, basinal soil development and geochemical weathering, and proximity to river systems are essential to the formation of chlorite precursor phases. These characteristics have been combined to define situations where chlorite-coats were more likely or more unlikely to form. These parameters will provide insights into the formation of chlorite and to further refine predictive models for the presence or absence of chlorite-coated sandstones.

Highlights

► Chlorite-coats in petroleum sandstone reservoirs can have a positive effect on reservoir quality. ► We examine common parameters of chlorite-coated reservoir sandstone examples to better predict where chlorite may occur. ► Chlorite-coats predominantly occur in delta environments and other fluvial associated environments. ► Other factors such as latitude and age of sand formation are also important. ► Hinterland geology, soil development and proximity to river systems are key to chlorite precursor formation.

Introduction

Grain-coating chlorite can help to preserve open pore networks in deeply buried petroleum sandstone reservoirs by moderating the effects of authigenic quartz cement growth on detrital grains (Anjos et al., 2003; Ehrenberg, 1993; Ramm and Ryseth, 1996). Where found as a grain-coating mineral, chlorite can reduce the nucleation area for overgrowths; models of quartz cementation indicate that coat coverage is a key factor in the inhibition of quartz overgrowths (Lander et al., 2008). Pore-filling chlorite can also reduce porosity and permeability by decreasing pore throat diameters (Islam, 2009; Nadeau, 2000; Pay et al., 2000; Porter and Weimer, 1982). Prediction of chlorite occurrence, whether grain-coating or pore-filling, still remains elusive. Previous attempts to predict the occurrence of chlorite have focused primarily on using depositional environment although it must be noted that chlorite is considered to be the result of burial diagenetic transformation of precursor minerals. The previous work suggests that river-influenced deltaic and shelf environments and saline-influenced desert environments would lead to iron- and magnesium-rich chlorite respectively (Bloch et al., 2002; Ehrenberg, 1993; Kugler and McHugh, 1990).

A detailed evaluation of the distribution of different chlorite types and their effects on petroleum reservoir sandstones of varying ages and depositional settings has not been published previously, and this work is timely as industry increasingly explores for deeply buried reservoirs. This review paper gathers data and information from the literature on chlorite-bearing petroleum sandstone reservoirs. The data and information is then be used to establish any patterns in the type, occurrence and effect of chlorite. The ultimate objective is to improve the prediction of reservoir quality within deeply buried sandstone reservoirs.

Section snippets

Chlorite-coat formation: pre-requisites

Chlorite (general formula: (Mg,Al,Fe)6 [(Si,Al)4O10](OH)8) is a 2:1:1 clay mineral composed of a tetrahedral–octahedral–tetrahedral structure, inter-layered between these structures is a octahedral sheet composed of cations and hydroxyls. Chlorite can have a variable chemistry, but the two main types are iron-rich and magnesium-rich chlorite. Identification is normally achieved through X-ray diffraction microprobe analyses (Worden and Morad, 2003).

The concept of open-system burial diagenesis

Dataset

To better understand the controls on the formation of chlorite-coatings in sandstone reservoirs, and to assess the effect these factors may have, a number of question have been defined, that will be discussed by reference to published literature:

  • 1)

    In which depositional environments did the chlorite-bearing sandstone form?

  • 2)

    What effect do diagenetic chlorite-coatings have on the reservoir quality of the sandstone?

  • 3)

    At what age and latitude were the chlorite-coated sandstones deposited?

  • 4)

    What is the

Results

The dataset (see Supplementary Data Tables) is divided into three geographical groupings – “North America”, “Northwest Europe” and the “Rest of the World”, which reflect historical exploration trends as the majority of examples come from mature provinces of North America and Northwest Europe. This approach highlights that more chlorite-coated reservoir sandstones may be discovered during future exploration of frontier areas and deeper targets, making the search for common controls on chlorite

Discussion

Each parameter previously outlined is discussed with respect to its effect on the formation of chlorite and how this may aid the prediction of chlorite in subsurface exploration.

Conclusions

  • Chlorite-coats have formed in a range of depositional environments but are most commonly found in coastal settings and particularly in deltaic environments. Fluvial environments are the second most common setting for chlorite-coats. The role of rivers is crucial to the development of chlorite as these arteries provide the supply of precursor material that proceeds to form chlorite during mesodiagenesis.

  • Latitude at the time of deposition indicates a wider range (60°N and 60°S) than predicted for

Acknowledgements

This work was carried out as part of the consortium research project BASIC, which was sponsored by BP, Chevron, ConocoPhillips, ENI, ExxonMobil, Petrobras, Shell and Statoil. The comments and suggestions of S. Morad, R.H. Lander, L.F De Ros and an anonymous reviewer are gratefully acknowledged.

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