Lacustrine chalky carbonates: origin, physical properties and diagenesis (Palaeogene of the Madrid Basin, Spain)

The Palaeogene lacustrine chalky carbonates of the Madrid Basin are a peculiar type of very soft and friable carbonate facies with high porosity despite being covered by more than 800 m of sediment. Similar physical properties to those described in marine chalk reservoirs emphasize the interest in analysing and characterizing these carbonate facies within a lacustrine depositional system. Lithologically, they are calcitic and/or dolomitic poorly cemented carbonate muds with no significant amounts of skeletal debris. Clay minerals such as illite, smectite and palygorskite are present between the carbonate crystals. Palygorskite is the most common, covering the carbonate crystals and forming sheets between them. These lacustrine chalky carbonates were formed in the basinal areas of the lake as the result of inorganic carbonate precipitation and/or detrital sedimentation related to episodic reactivation of the adjacent fan systems. Their petrological, geochemical and physical properties indicate that few textural and compositional modifications occurred during diagenesis. Their main physical properties are a very low dry bulk and grain density (1.6–2.2 and 2.62 g/cm3, respectively) and medium to high porosity (10–40%) due to micropores ( 2 Am, 30%). The convergence of lacustrine sedimentation dynamics (rapid sedimentation), the original mineralogy of these calcareous lacustrine muds (relatively stable low-magnesian calcite and dolomite), the early formation of the palygorskite cement of these muds, and the retention of Mg-enriched fluids in the pore system, were decisive in the partial inhibition of calcite cementation, compaction and recrystallization. The chalky carbonates are also intercalated between impermeable littoral carbonate facies that impeded fluid flow through their pore systems