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Solid Bitumen in Shales: Facies and Maturity Effects on Microstructures

Abstract

Solid bitumen is in the focus of recent research work on shale oil/gas plays as it might lower effective gas permeabilities due to the occlusion of pore throats, but also provide important hydrocarbon storage space and form migration pathways if a porous network of oil-wet pores is present. However, despite its significance for the quality of a shale reservoir, ambiguities about the various definitions of (solid) bitumen in organic geochemistry and petrography, as well as regarding the actual onset of compositional and microstructural transformations during thermal maturation and accompanying hydrocarbon generation, exist. In an attempt to visualize different solid bitumen types and transformation stages over a broad maturity range (0.5 - 2.7 %Ro) in shales of varying primary kerogen composition, we reviewed optical and scanning electron microscopy data of shales with a Cambrian to Triassic age with emphasis on the petrographic characteristics of solid bitumen. We were able to identify in-situ pre-oil solid bitumen as well as migrated post-oil solid bitumen at various maturity stages. We found solid bitumen to be surprisingly mobile already at marginal to early oil window maturity, arguing for significant influence of primary migration. Onset of porosity development in solid bitumen differs considerably between predominantly oil-prone (e.g., alginite) and gas-prone (vitrinite-rich) kerogen compositions; furthermore, solid bitumen (pyrobitumen at advanced maturity) in rocks with a terrestrially dominated facies seems to be considerably less mobile compared to pre- and post-oil solid bitumen in oil-prone rocks. In most samples, several solid bitumen populations with varying reflectance were observed, complicating its use as a petrographic maturity indicator. Microstructural features such as irregularly distributed spongy porosity or detrital and authigenic mineral inclusions in the sub-micrometre scale were found to have a great influence on texture and reflectivity in lower-resolution optical microscopy. The formation of authigenic minerals (quartz, various carbonate phases, magnetite in Cambrian samples) was observed frequently in post-oil solid bitumen of oil-prone rocks; this points to a close genetic relationship between transformation products formed during hydrocarbon generation (e.g., acetate, carbon dioxide and methane) and the dissolution and precipitation of minerals during diagenesis.