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