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Compaction State of Aggregate Grains in Carbonaceous Mudstones: Evidence from Microfabric Studies of the Upper Cretaceous Colorado Group, Western Canada Foreland Basin


Intraparticle microporosity within clay aggregate grains is a principal contributor to storage capacity in self-sourcing carbonaceous mudstone reservoirs of the Upper Cretaceous Colorado Group in the Western Canada Foreland Basin. Prediction of the volumes, phase distribution and deliverabilities of hydrocarbons from these reservoirs requires a comprehensive understanding of the physicochemical processes governing the formation and preservation of this mode of microporosity. FIB-SEM studies of microfabric in the Colorado Group mudstones reveal that the intraparticle microporosity occupies submicron-scale interstitial voids between clay platelets arranged in both edge-to-face and face-to-face stacking arrangements within discrete spatial domains. The local domains, which are typically scaled between 2 to 5 microns in diameter, are interpreted as microflocs. These microflocs are themselves component grains (“zero-order aggregates” of Krone (1963)) of larger, higher-order aggregate particles. One salient feature of the microflocs is that the particles commonly retain an approximately equant cross-sectional form, regardless of the burial depth which ranges from hundreds to several thousands of metres in the sample suite. This suggests that at least some of the flocs have not been subjected to significant compaction following deposition and burial. This observation stands in stark contrast with the prevailing expectation of flattening of flocs due to dewatering at very shallow burial depths, and may reflect early diagenetic cementation resulting from oxidation of high concentrations of disseminated organic matter in the consolidated gel below the sediment-water interface. The conditions required to prevent early compactional loss of intraparticle microporosity in clay aggregate grains are governed by the depositional processes that impart the primary sediment microfabric, in concert with net flux of labile organic matter and pore water composition below the sediment-water interface. Ultimately, these factors respond to basin-scale allogenic drivers (e.g.: climate, tectonism, eustacy) that modulate the character of the sedimentary record. Prediction of the spatial and temporal preservation of inherited microporosity, therefore, must be informed by the context of basin evolution.