Acoustic properties in travertines and their relation to porosity and pore types

Highlights

• Acoustic velocities were measured in 60 continental carbonate plugs.

• Acoustic velocity is dependent on pore types, pore size and distribution.

• Seismic reflectors relate to geobody boundaries.

• Relations of acoustic velocity in marine and continental carbonates were examined.

• Velocity decrease in continental carbonates is slower with increasing porosity.

Abstract

Sonic velocities of Pleistocene travertines were measured under variable confining pressures. Combined with petrographical characteristics and petrophysical data, i.e. porosity, permeability and density, it was determined that travertine porosity, pore types and cementation control compressional-wave (V_p) and shear-wave velocity (V_s). At 40 MPa confining pressures, V_p ranges between 3695 and 6097 m/s and V_s between 2037 and 3140 m/s. Velocity variations in travertines are, as with all carbonates, primarily linked to sample heterogeneity, i.e. differences in fabric, texture and porosity. They thus not necessarily emanate from changes in mineralogy or composition. Body wave velocities have a positive correlation with sample density and an inverse correlation with porosity. The travertines, sampled in extensional settings with normal faulting activity, define a specific compressional-wave velocity (y-axis) versus porosity (x-axis) equation, i.e. (log(y) = −0.0048x + 3.7844) that differs from the V_p-porosity paths defined by marine carbonates. Acoustic wave velocities are higher for travertines than for marine carbonates. Travertine precipitates form rigid rock frames, often called framestone, with large primary pores. Marine carbonates on the other hand often consist of (cemented) transported sediments, resulting in a rock frame that permits slower wave propagation when compared to the continental limestones.

Acoustic velocity variations are linked to variations in pore types. Mouldic pores (macropores) show faster wave propagation than expected from their total porosities. Microporosity, interlaminar and interpeloidal porosity result in slower acoustic velocities. Framework pores and micro-moulds are associated with lowered acoustic velocities, while vug porosity is found above, on and below the general velocity-porosity trend. Not only the pore type, but also pore shapes exert control on body wave velocities. Cuboid-and rod-like pore shapes increase the velocity, while plate-and blade-like pore shapes have a negative effect on the velocity. The study demonstrates how seismic sections in travertine systems can contain seismic reflections that are not caused by non-carbonate intercalations, but relate to geobody boundaries, in which the seismic expression is function of porosity, pore types and shapes. This study provides and relates petrophysical data, i.e. porosity, permeability and acoustic velocities of travertines and is of importance for the interpretation of seismic reflection data in subsurface continental carbonate reservoirs.

Introduction

Carbonate deposits gained interest due to their potential as reservoir rocks, e.g. the supergiant fields in the Middle East (Nurmi and Standen, 1997) and offshore Brazil (Thompson and Oftebro, 2011, Wright, 2012). The genesis of continental carbonates is associated with physico-chemical and biological precipitation that largely influences the rock texture and petrophysical properties (Toumelin et al., 2003). This causes petrophysical heterogeneity that can become even more pronounced due to diagenetic overprinting (with cementation, dissolution, recrystallisation, dolomitisation, fracturing, etc.). The studied continental carbonates in Turkey and Hungary are calcareous spring deposits, i.e. travertines (Pentecost, 2005), associated with ambient and hydrothermal fluids (El Desouky et al., 2014, Sierralta et al., 2010). Their deposition is controlled by a complex interplay of physico-chemical, biological and hydrological factors, including CO₂ degassing, the dominating process for travertine precipitation (Fouke, 2011, Guo and Riding, 1998, Kele et al., 2011, Özkul et al., 2013, Özkul et al., 2014, Pentecost, 2005). Travertines can form in a thin water film in sub-aerial conditions, or in lake, marsh and fluvial environments. These precipitates have been widely studied (Alonso-Zarza and Tanner, 2010, Ford and Pedley, 1996, Pedley and Rogerson, 2010, Pentecost, 2005), however, a detailed investigation of the rock petrophysics was seldom accomplished. This paper aims to investigate the behaviour of acoustic waves in travertines.

The dependency of sonic velocities on lithology, rock texture and fabric is the key to understand acoustic logs and seismic sections in sedimentary systems. Compressional-wave velocity and bulk density are used to calculate acoustic impedance. In carbonate lithologies that consist purely of calcite, grain density variations are limited, meaning that other parameters cause variations in body wave velocity. These parameters are of importance for the interpretation of seismic reflection and geophysical data. The control that is exhibited by porosity on acoustic velocity was already reported in the late fifties (Biot, 1956; Gassmann, 1951). Causes for velocity variations in pure carbonates were part of several studies especially from the early nineties onwards. It was concluded that not only porosity, but also the entire rock fabric and its texture are of importance (Anselmetti and Eberli, 1993, Wang et al., 1991). Moreover, carbonates are prone to diagenesis, which can easily alter porosity, crystal morphology, the rigidity of the solid framework, etc. (Anselmetti and Eberli, 1993, Braaksma et al., 2003, Verwer et al., 2008).

In this study, compressional-wave velocity (V_p) and shear-wave velocity (V_s) are measured on travertine samples from three different locations, under confining pressures that approach in situ subsurface conditions. Sonic velocity measurements were done in combination with petrography, X-Ray Diffraction (XRD) and micro-Computer Tomography (μCT) analyses.