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Controls of Hydrocarbon Retention in the Unconventional Barnett and Posidonia Shale Systems

Abstract

Hydrocarbons are retained in shales either in an adsorbed state on particle surfaces or in a free form within pores and fractures. The relative importance of the two mechanisms and the total retention capacity vary considerably at both reservoir and regional scales. Identifying the controls of hydrocarbon retention, and predicting sweet spots in heterogeneous media are key goals in exploration. Here we contrast two shale plays – the Barnett Shale, known for its heterogeneity with locally high productivity, and the Posidonia Shale (Germany), a lithologically homogeneous calcareous mudstone/marl with untested potential. Using a multiparameter approach involving geochemistry, mineralogy and petrophysics, well profiles have been characterized in detail. Organic matter properties i.e. richness, thermal maturity and kerogen composition control retention in both cases. A generally positive correlation is observed between organic richness and the amount of retained bitumen. As maturity increases the TOC normalized bitumen concentration (S1/TOC) first increases in the oil window, and subsequently decreases once the shale enters the wet-gas window. Interestingly, it is the “live” carbon, especially where aromatic, rather than “dead” carbon which preferentially retains hydrocarbons. Shale intervals enriched in free oil or bitumen are not necessarily associated with the organic richest layers. Indeed, oil crossovers (S1/TOC >100 mg/g TOC) generally occur within layers where biogenic matrices are also abundant. In the Barnett Shale for instance, the presence of sponge spicules directly controls the accumulation of intraformationally migrated oil, as demonstrated by compositional mass balance calculations. In the Posidonia Shale, oil crossovers occur in zones where biogenic calcite in the form of coccolithophores provides enhanced pore space. The overall porosity first decreases with increasing maturity, which can be ascribed to progressing compaction on the one hand and to pore plugging by bitumen on the other hand. Then, nano-pores formed by kerogen decomposition subsequently contribute to the later increase of porosity in the gas window. In summary, the combination of organic matter enrichment and abundance of porous fossils delineate the most productive intervals in both of the examples shown. The siliceous sponge spicule and calcareous coccolithophore enrichment layers in the Barnett and Posidonia Shale, respectively, represent the main reservoir facies.