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Introduction

Carbonate reservoirs contain a majority of remaining proven oil reserves, yet are much more difficult to evaluate than their siliciclastic counterparts (Palaz and Marfurt, 1997; Eberli et al., 2003; Sayers and Latimer, 2008; Fontaine et al., 1987). Many aspects of carbonate rocks make their seismic signature complex and difficult to interpret both qualitatively and quantitatively. Because carbonate rocks generally have higher seismic velocities than siliciclastics, horizontal and vertical resolution is commonly low.

Carbonate sediments are also more prone to complex, rapid diagenetic alteration, where heat and pressure change rock chemistry, after deposition and continuing through the burial process (Vanario et al., 2008). These diagenetic processes significantly affect the acoustic properties of carbonate rocks. Postdepositional alteration such as karst processes, where weathering creates steep sided valleys and cave networks in carbonate strata, or dolimitization, where calcium carbonate is replaced by calcium magnesium carbonate (dolomite), can further complicate already heterogeneous deposits. Carbonate heterogeneity exists at different scales. Carbonates often posses larger-scale voids, caves, and fracture networks, accompanied by small scale features such as microfractures, intergranular porosity, and chemical alteration (Lucia, 1999).

The acoustic properties of carbonate rocks are not a simple function of mineralogy and porosity. Recent advances in rock mechanics have shown that carbonate rocks' acoustic properties also depend on their pore type, size, shape, and distribution (Eberli et al., 2003; Wang, 1997; Weger et al., 2009; Adam and Batzle, 2008). Heterogeneities in carbonates scatter seismic energy, attenuating high frequency signal and reducing resolution. High impedance contrasts typically exist between carbonate structures and surrounding rocks, leading to a strong reflective interface that can generate multiple reflections. Impedance contrasts for strata within the carbonate body tend to be relatively weak, and horizontal homogeneity means that reflections rarely have strong lateral continuity.

Rock physics models developed for siliciclastics often fail to effectively describe carbonate systems (Baechle et al., 2008; Sayers, 2008). This makes relating velocities from core, sonic logs, and seismic very difficult, as these are sampled with different frequency waves. As a result, seismic images of carbonate deposits are usually not easily interpreted, especially at the reservoir scale (1-5 km). In addition, because of the intertwined control factors on the seismic response of carbonates, quantitative interpretation of the seismic signal is even more challenging. Janson et al. (2010) and Janson and Fomel (2011) used outcrop analogues and synthetic models to better understand the seismic response of carbonate deposits.

The difficulties associated with reflection imaging in carbonates encourage us to explore alternative imaging approaches, such as diffraction imaging. Seismic diffractions are a fundamentally different phenomenon than seismic reflections (Klem-Mus-atov, 1994). They occur when seismc waves scatter from small-scale features. Diffractions may be caused by geologically significant features including voids, faults, fractures, karsts, pitchouts, salt flanks, and other small-scale heterogeneities (Khaidukov et al., 2004; Moser and How-ard, 2008; Klokov and Fomel, 2012; Fomel et al., 2007; Harlan et al., 1984). Rays associated with seismic diffractions take more diverse paths than those associated with reflection events, and thus can contain more information about the subsurface (Neidell, 1997). These more diverse ray paths enable super-resolution with seismic diffraction imaging (Khaidukov et al., 2004).

Seismic diffraction imaging can highlight features commonly observed in carbonates, such as karsts, voids, and small scale heterogeneities, with high resolution. These characteristics make seismic diffraction imaging well suited for use with carbonate imaging targets, where reflection resolution is typically limited. In this paper, we use two synthetic models to illustrate how seismic diffraction imaging can better constrain void geometry and detect heterogeneous zones that may not be immediately apparent in conventional reflection imaging.


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Next: Synthetic Models Up: Decker et al.: Carbonate Previous: Decker et al.: Carbonate

2015-03-25