Elastic properties of lacustrine, palustrine, and pedogenic carbonates: petrographic implications and insights from analytical modeling

The development of lacustrine, palustrine, and pedogenic carbonates through a continuum of sedimentary and diagenetic environments results in highly heterogeneous pore networks, which remain poorly characterized in palustrine and pedogenic facies in terms of their influence on petrophysical properties. These preclude any straightforward correlations between their physical properties and geological characteristics. This study investigates the diversity of elastic properties in fifty Cenozoic lacustrine, palustrine, and pedogenic micritic carbonates of the Paris Basin using samples from three boreholes and ten outcrops. This provides: i) a detailed petrographic framework, ii) ultrasonic measurements of seventy-six 1-inch plugs covering seven facies, and iii) a modeling approach based on the effective medium theory and Gassmann predictions. This study exhibits a broad range of porosities (1.7–40.5%), alongside unusually high P-wave velocities for a given porosity (4.1–6.5 km.s−1). The seven studied facies (wackestones with intraclasts, shell-rich floatstones, wackestones with root traces, peloidal grainstones, in-situ brecciated limestones, laminar limestones, nodular brecciated limestones) cannot be discriminated based on their elastic properties. Nevertheless, palustrine and pedogenic samples have similar properties, and about half of them display higher velocities than lacustrine facies. Nine diagenetic pathways were identified in these shallowly buried facies (maximum depth of ∼ 200m), which synthesize multiple calcite cementation, dissolution, and/or silicification phases, as well as cracks, but do not account for the observed acoustic dispersion. Studied rocks show six dominant pore types: microporosity, microbial framework, gastropod shells or roots moldic pores, root-related framework, and vuggy pores. They are the results of close relationships between depositional environment and diagenetic processes and provide a better explanation for the dispersion of acoustic data. The difference of acoustic values in palustrine and pedogenic versus lacustrine facies is explained by the prevalence of framework porosity inherited from root systems and vuggy pores from significant dissolution phases in palustrine and pedogenic carbonates. Samples with these pore types behave like an effective medium composed of slightly deformed spheres (aspect ratio close to 1), resulting in a stiffer response than standard differential effective medium models predict. Similar elastic behavior is found only in travertines in the literature. As often in pedogenic deposits, velocities are reduced when complete silicification occurs due to the quartz–calcite compressibility difference. In contrast, the micropores and moldic pores, frequently found in lacustrine facies, form a network resembling crack-like inclusions (aspect ratio ∼ 0.3), consistent with typical carbonate behavior. This study thus underscores significant contrasts in elastic properties between lacustrine and palustrine carbonates, offering new perspectives for distinguishing them across different scales and for investigating subsurface reservoir characteristics.