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Characterisation and Modelling of Fine-Grained Limestones: Lessons From Four Decades of Progress in Hydrocarbon Reservoirs


A vast majority of the limestone deposits around the world, in both marine and lacustrine settings, are made up of fine-grained and lithified carbonate muds (particle sizes <10μm, generally called micrite). Such deposits host a variably developed micropore and nanopore-dominated pore systems (pore sizes <10μm and <1μm, respectively), which have storage capacities (ie. porosity volume) locally as good as coarser grained and macroporous deposits, and can retain permeabilities that make them viable hydrocarbon reservoirs. Such deposits have been described in most of the key worldwide hydrocarbon plays, either as supplemental secondary reservoirs within conventional reservoirs (eg. North Sea chalk, Shuaiba and Mishrif Formations in the Middle East), or as the main reservoir in their own right in unconventional plays (eg. Niobrara chalks). In addition to the interest for these deposits from a hydrocarbon production perspective, other applications include the assessment of their storage capacity for both CO2 and industrial waste. In this context, the understanding of the long-term diffusion of fluids through this very small and complex core system is key. In addition to their particular pore-scale properties, microporous reservoirs tend to display different mechanical properties compared to macroporous carbonates. Despite almost 40 years of international research, the characterisation and the origin of these microporous deposits remain challenging, mainly due to the high resolution observation tools required to analyse these micron-scale facies. The aim of this work is to present recent advances in the characterisation and prediction of microporous and nanoporous carbonate deposits. Clearly defined and readily applicable petrographical classifications provide the basis for a good understanding of how the sediments evolved through time and space. In a particular this paper presents a refined understanding of the sedimentological formation of these facies, which, in turn is critical for constraining the variations in reservoir properties and their distribution within microporous reservoirs. Finally, this work will establish criterion to explain how, why and where microporous and tight carbonates are formed within the carbonate sedimentological column, with the aim toto provide a framework for their upscalling and exploration, with a particular attention to the influence of exposure surfaces and associated diagenesis affecting neritic shallow marine limestones.