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Linking Diagenesis to Sequence Stratigraphy: An Integrated Tool for Understanding and Predicting Reservoir Quality Distribution
S. Morad*,ā , J.M. Ketzerā” and L.F. De RosĀ§
*Department of Petroleum Geosciences, The Petroleum Institute, P.O. Box 2533, Abu Dhabi, United Arab Emirates; E-mail: [email protected] ā Department of Earth Sciences, Uppsala University, 752 36, Uppsala, Sweden
ā”CEPAC Brazilian Carbon Storage Research Center, PUCRS, Av. Ipiranga, 6681, Predio 96J, TecnoPuc, Porto Alegre, RS, 90619-900, Brazil; E-mail: [email protected] Ā§Instituto de GeociĆŖncias, Universidade Federal do Rio Grande do Sul - UFRGS, Av. Bento GonƧalves, 9500, Porto Alegre, RS, 91501-970, Brazil; E-mail: [email protected] Abstract
Sequence stratigraphy is a useful tool for the prediction of primary (depositional) porosity and permeability. However, these primary characteristics are modified to variable extents by diverse diagenetic processes. This paper demonstrates that integration of sequence stratigraphy and diagenesis is possible because the parameters controlling the sequence stratigraphic framework may have a profound impact on early diagenetic processes. The latter processes play a decisive role in the burial diagenetic and related reservoir-quality evolution pathways. Therefore, the integration of sequence stratigraphy and diagenesis allows a proper understanding and prediction of the spatial and temporal distribution of diagenetic alterations and, consequently, of reservoir quality in sedimentary successions.
Introduction
The diagenesis of sedimentary rocks, which may enhance, preserve or destroy porosity and permeability, is controlled by a complex array of inter-related parameters (Stonecipher et al., 1984). These parameters range from tectonic setting (controls burial-thermal history of the basin and detrital composition of clastic sediments) to depositional facies and palaeo-climatic conditions (Morad, 2000; Worden & Morad, 2003). Despite the large number of studies (e.g. Schmidt & McDonalds, 1979; Stonecipher et al., 1984; Jeans, 1986; Curtis, 1987; Walderhaug & Bjorkum, 1998; Ketzer et al., 2003; Shaw & Conybeare, 2003) on the diagenetic alteration of sedimentary rocks, the parameters controlling their spatial and temporal distribution patterns in paralic and shallow-marine and particularly in continental and deep water sedimentary deposits are still not fully understood (Surdam et al., 1989; Morad, 1998; Worden & Morad, 2000, 2003).
Diagenetic studies have been used independently from sequence stratigraphy as a tool to understand and predict the distribution of reservoir quality in clastic and carbonate successions (e.g. Ehrenberg, 1990; Byrnes, 1994; Wilson, 1994; Bloch & Helmold, 1995; Kupecz et al., 1997; Anjos et al., 2000; Spƶtl et al., 2000; Bourque et al., 2001; Bloch et al., 2002; Esteban & Taberner, 2003; Heydari, 2003; Prochnow et al., 2006; Ehrenberg et al., 2006a).
The sequence stratigraphic approach, nevertheless, allows the prediction of facies distributions (Posamentier & Vail, 1988; Van Wagoner et al., 1990; Emery & Myers, 1996; Posamentier & Allen, 1999), providing information on the depositional distribution of primary porosity and permeability (Van Wagoner et al., 1990; Posamentier & Allen, 1999). Depositional reservoir quality is mainly controlled by the geometry, sorting and grain size of sediments. Sequence stratigraphy enables prediction of the distribution of mudstones and other fine-grained deposits that may act as seals, baffles and barriers for fluid flow within reservoir successions and as petroleum source rocks (Van Wagoner et al., 1990; Emery & Myers, 1996; Posamentier & Allen, 1999).
Although sequence stratigraphic models can predict facies and depositional porosity and permeability distribution in sedimentary successions, particularly in deltaic, coastal and shallow-marine deposits (Emery & Myers, 1996), they cannot provide direct information about the diagenetic evolution of reservoir quality. As most of the controls on early diagenetic processes are also sensitive to relative sea-level changes (e.g. pore water compositions and flow, duration of subaerial exposure), diagenesis can be linked to sequence stratigraphy (Tucker, 1993; South & Talbot, 2000; Morad et al., 2000, 2010; Ketzer et al., 2002, 2003). Hence, it is logical to assume that the integration of diagenesis and sequence stratigraphy will constitute a powerful tool for the prediction of the spatial and temporal distribution and evolution of quality in clastic reservoirs, as it has already been developed for carbonate successions (Goldhammar et al., 1990; Read & Horbury, 1993 and references therein; Tucker, 1993; Moss & Tucker, 1995; South & Talbot, 2000; Bourque et al., 2001; Eberli et al., 2001; Tucker & Booler, 2002; Glumac & Walker, 2002; Moore, 2004; Caron et al., 2005). This approach can also provide useful information on the formation of diagenetic seals, barriers and baffles for fluid flow, which may promote diagenetic compartmentalization of the reservoirs. A limited number of studies has been undertaken that illustrate how the spatial distribution of diagenetic features in various types of sedimentary successions can be better understood when linked to a sequence stratigraphic framework (Read & Horbury, 1993 and references therein; Tucker, 1993; Moss & Tucker, 1995; Morad et al., 2000; Ketzer et al., 2002, 2003a, 2003b, 2005; Al-Ramadan et al., 2005; El-Ghali et al., 2006, 2009).
Carbonate sediments are more reactive than siliciclastic deposits to changes in pore-water chemistry caused by changes in relative sea-level versus rates of sediment supply (i.e. regression and transgression) (Morad et al., 2000). Therefore, the distribution of diagenetic alterations can be more readily linked to the sequence stratigraphic framework of carbonate than of siliciclastic deposits (Tucker, 1993; McCarthy & Plint, 1998; Bardossy & Combes, 1999; Morad et al., 2000). Cool-water limestones are commonly composed of low-Mg calcite and thus are less reactive than tropical limestones, which are composed of the metastable aragonite and high-Mg calcite. In tropical carbonate rocks, particularly, the distribution of diagenetic alterations can be recognized within third (1ā10 Ma) or fourth (10s ky to 100 ky) order cycles of relative sea-level change (Tucker, 1993), whereas in siliciclastic deposits only alterations relative to third order cycles can be recognized (Morad et al., 2000). Less commonly, however, diagenetic alterations can be correlated to smaller cycles (parasequences; Van Wagoner et al., 1990) within third order sequences (Taylor et al., 1995; Loomis & Crossey, 1996; Klein et al., 1999; Ketzer et al., 2002). The low rates of subsidence in marine epicontinental environments (Sloss, 1996) render linking diagenesis to sequence stratigraphy difficult.
In the following discussion, definitions of the diagenetic stages eodiagenesis, mesodiagenesis and telodiagenesis sensu Morad et al. (2000) will be applied to clastic successions, whereas the original definitions of these stages (Choquette & Pray, 1970) are applied to carbonate successions. According to Morad et al. (2000), eodiagenesis includes processes developed under the influence of surface or modified surface waters such as marine, mixed marine-meteoric, or meteoric waters, at depths <2 km (T < 70 Ā°C), whereas mesodiagenesis includes processes encountered at depths >2 km (T > 70 Ā°C) and reactions involving chemically evolved formation waters. Shallow mesodiagenesis corresponds to depths between 2 and 3 km and to temperatures between 70 and 100 Ā°C. Deep mesodiagenesis extends from depths of ~3 km and temperatures ~100 Ā°C to the limit of metamorphism, corresponding to temperatures >200 Ā°C to 250 Ā°C and to highly-variable depths, according to the thermal gradient of the area. Telodiagenesis refers to those processes related to the uplift and exposure of sandstones to near-surface meteoric conditions, after burial and mesodiagenesis. In the original definitions of Choquette & Pray (1970) there is no depth or temperature limit between eodiagenesis and mesodiagenesis, but only a vague effective burial limit, defined as the case-specific depth below...
