Controlling factors of petrophysical and sedimentary heterogeneities in Paleogene rocks beneath Paris, France

The Paleogene sedimentary series of the Paris Basin, characterized by mixed carbonate and siliciclastic deposits in continental and marine environments, exhibit significant lithological, facies, and diagenetic heterogeneities. However, the lack of data and conceptual frameworks linking petrophysical properties to geological knowledge of these sedimentary series remains a major limitation for predicting physical properties across different scales. Understanding and predicting reservoir heterogeneities is essential for addressing major societal challenges, such as subsurface planning and geothermal energy development in the Île-de-France region. To tackle this issue, a multi-scale study is conducted, integrating sedimentary geology and petrophysical analyses of the Paleogene series. The study focuses on key areas of the Paris Basin, defined by the alignment of the Grand Paris Express (GPE) project, a 200 km network of new metropolitan lines around Paris. These areas are ideal to lead to a very high spatial resolution characterization of the sedimentary system, thanks to the availability of an exceptionally dense dataset of core drillings.

The analysis of cores from 15 boreholes and gamma-ray logs from 34 additional boreholes allowed the identification of twenty-two facies grouped into seven facies associations, corresponding to seven depositional environments ranging from open marine environments to palustrine settings. Based on these observations, twelve transgressive–regressive cycles spanning the Danian to the Rupelian were identified and correlated along two multi-kilometer transects (≈20 km).

Petrophysical measurements performed on 633 samples reveal a strong heterogeneity within the studied Paleogene successions. For instance, palustrine limestone facies affected by intense recrystallization and silicification display relatively uniform P-wave velocities (4.1–6 km s⁻¹) and porosities (1.5%–12%). In contrast, marine limestones exhibit a wide range of P-wave velocities (0.9–5.6 km s⁻¹), partly related to facies variability and primarily controlled by porosity (5–46%), with decreasing velocities at increasing porosity. This parameter is mainly governed by the abundance and nature of diagenetic cements, with mosaic calcite cements (drusy, granular, blocky) leading to a stronger porosity reduction than isopachous cementation, while depositional facies exert a secondary control (e.g. Miliolid grainstone vs Bioclastic floatstone). Additional controls contributing to the petrophysical heterogeneity of marine limestone facies include pore spatial distribution and connectivity, pore size distribution, and pore type. Pore spatial organization exerts a first-order control on acoustic velocities and porosity: uniformly distributed and well-connected pore networks are associated with low P-wave velocities and high porosities, whereas patchy pore distributions lead to higher velocities and reduced porosity. Pore size also influences petrophysical properties, with macroporosity (>62 µm) generally associated with relatively high P-wave velocities and low porosities, while meso-microporosity (<62 µm) does not show a clear relationship with either parameter. Pore type also plays a significant role, as interparticle porosity is associated with low velocities and high porosities, in contrast to intraparticle, moldic, and vuggy porosities, which are characterized by higher velocities and lower porosities.

Although locally homogeneous from a petrophysical perspective, these sedimentary series display strong heterogeneities governed by multiple geological controls, emphasizing the key role of petrophysical characterization in reservoir prediction.