Acoustics and petrophysical investigations on upper cretaceous carbonate rocks from northern Lebanon

Highlights

• The present research is concerned with the study of the lithological, the acoustic, and the petrophysical properties of carbonate rocks from northern Lebanon.

• Highly heterogeneous rocks where many microfacies are recognized.

• The studied rocks have moderate porosity, very low permeability, and widely varying acoustic velocities.

• Porosity, grain size, pore geometry and some diagenetic processes impacted the measured acoustic properties.

Abstract

Carbonate rocks represent significant aquifers and reservoir rocks in many parts of the world. In this study, twenty-one carbonate samples collected from the exposed rocks at Batroun and Chekka, northern Lebanon, are studied to investigate their lithological characteristics, nannofossil content, and their acoustic and petrophysical properties. Four microfacies are recognized in the studied rocks including packstone, wackstone, wack-mudstone, and the wack-packstone. Nannofossils are detected in the top part of the studied section indicating marine-type depositional conditions for the upper Cretaceous carbonate rocks. The Petrographic analysis revealed also that the studied rocks have moderate to high porosity which is consistent with the measured porosity on core samples. The low porosity of some samples is induced by cementation, which also reduced their permeability. The bulk density varies widely because of the many diagenetic processes that impacted the studied rocks. Moreover, because the primary and secondary wave velocities (Vp, and Vs) are greatly influenced by porosity and density, the samples exhibit extreme variations in acoustic velocities from low values which characterize soft, highly-porous, rocks to higher velocities which are characteristic of compact, massive, and highly-cemented carbonates. Some relationships between the measured petrophysical and acoustic parameters are studied to investigate their mutual dependence, and discuss the impacts of rock composition and diagenetic alterations on the petrophysical properties of the collected rocks. The obtained results are also important in characterizing their mechanical strength, and in the analysis of seismic reflection profiles acquired for equivalent offshore limestone reservoirs in the Levant and the Eastern Mediterranean regions.

Introduction

Significant hydrocarbon reservoirs of enhanced porosity and permeability, resulting from prolonged diagenesis and karstification of carbonate rocks, are common in many sedimentary basins around the world (Mazzullo and Chilingarian, 1996; Dürrast and Siegesmund, 1999; Jin et al., 2017), including the supergiant oil fields of the Middle East (Nurmi and Standen, 1997). In addition, carbonate rocks represent significant, water-bearing, karst aquifers formed by fracturing and dissolution of limestone, dolomite or magnesite rock (Harmon and Wicks, 2006). Carbonate rocks are also important building stones whose durability is determined mainly by their geological history, petrophysics, strength and elastic characteristics (Cassar, 2016). In comparison to sandstone reservoirs, the exploration of carbonate rocks is challenging due to many intrinsic heterogeneities at various scales of observation and measurement (e.g., Soete et al., 2015). Such heterogeneities result mainly from variable lithology, sedimentary facies, chemistry/mineralogy, complex rock texture, pore geometry and connectivity, as well as subsequent diagenetic modifications (Kerans, 1988; Loucks, 1999, 2001; Loucks and Handford, 1992; Loucks and Mescher, 2001; Jin et al., 2017; Salah et al., 2020). These various variables affect also their acoustic properties and the relationships between various reservoir parameters (Neto et al., 2014).

The relationships between acoustic velocities, rock composition, and texture are fundamental to the understanding of sonic logs and seismic reflection profiles of sedimentary basins (e.g., Buckley et al., 2013; Soete et al., 2015). Acoustic impedance, an important rock parameter in reflection seismology, is calculated from the primary wave velocity (Vp) and the bulk density (ρ_b). Moreover, knowledge of both rigidity and incompressibility of a dry rock is fundamental for the shear wave velocity (Vs) prediction and fluid substitution (Neto et al., 2014). The Poisson's ratio (ν) is very sensitive to the presence of fluids, and is widely used in seismic exploration as a lithological gauge (Kearey et al., 2002). In addition to porosity and lithology, these elastic parameters are controlled also by the geometry of pores as well as their connectivity (Li and Zhang, 2011).

Carbonate rocks exhibit very wide ranges in their petrophysical and elastic properties (e.g., Anselmetti and Eberli, 1993, 2001). They are composed essentially of calcite with its well-known matrix density, which implies that other factors control the acoustic wave velocity variations in carbonate rocks. Porosity (ϕ), which is calculated as the ratio of the volume of the void part of a rock that is unoccupied by rock grains or mineral cement (V_pr) to the rock's total bulk volume (V_b), is the principal parameter affecting the acoustic properties of rocks (e.g., Zolotukhin and Ursin, 2000). It can be divided into primary porosity, secondary porosity, and effective porosity (ϕ_e). However, results of extensive research done in this field during the last decades indicated that beside porosity, the whole fabric of carbonate rocks controls the variations of acoustic wave velocities (e.g., Wang et al., 1991; Salah et al., 2018). Moreover, their high sensitivity to diagenetic changes may completely modify their porosity, crystal shape, and the rock rigidity (Verwer et al., 2008), which will, in turn, affect their overall elastic properties.

Diagenesis can enhance or diminish significant reservoir properties such as porosity and permeability. The initial porosity of shallow carbonate deposits may be as high as 50–60%. Unlike sandstone reservoirs, subsurface limestone reservoirs have much lower porosities, which decrease gradually with burial depth (Schmoker, 1984; Brown, 1997; Ehrenberg and Nadeau, 2005). Although mechanical compaction is a principal way by which sandstones lose porosity at depths less than about 2.5 km and temperatures of up to 90 °C, it is more difficult to assess its effects on carbonates because of the particular grain shape and the high diagenetic potential (Croizé et al., 2010). Meanwhile, if carbonate sediments are not cemented, laboratory investigations revealed that mechanical compaction has the greatest effect on porosity reduction (e.g., Chuhan et al., 2003).

Mineralogically, calcite, aragonite, and dolomite may co-exist in one carbonate rock with varying proportions. These minerals have different stabilities and respond differently to diagenetic processes. The early diagenetic processes may comprise dissolution of unstable components such as aragonite, dolomitization, and the precipitation of low Mg-calcite (Scholle and Halley, 1985). Dissolution, for example, may initially increase the pore volume, while late cementation can either reduce the pore size or result in complete pore closure and, therefore, decreased connectivity, and subsequently low permeability. Early diagenesis may add or remove significant rock constituents in a way that impacts mechanical compaction at shallow burial (e.g., Hamilton, 1976). Shallow platform carbonates, in particular, are very susceptible to early diagenesis (Friedman, 1964), which complicates the prediction of their petrophysical features (e.g., Croizé et al., 2010). Many authors addressed the dominant problems in carbonate petrophysical evaluation resulting from these highly variable rock properties and complex diagenesis (e.g., Lucia, 1999; Akbar et al., 2001; Kennedy, 2002; Cerepi et al., 2003).

The Late Cretaceous carbonate rocks outcropping at Chekka and Batroun areas, northern Lebanon (Fig. 1), which are essentially shallow platform deposits, are selected to study their facies, nannofossil content, and the impact of diagenesis on their petrophysical and elastic characteristics. The obtained results are useful in predicting the rock porosity and elastic properties of the carbonate sediments formed elsewhere at similar conditions, and the respective role of the different diagenetic processes in modifying these parameters. Moreover, the measured parameters including porosity, density, permeability, and the elastic properties are of great importance in characterizing the rocks’ mechanical strength and in the analysis of seismic reflection profiles acquired for equivalent offshore limestone reservoirs in the Levant and Eastern Mediterranean regions.