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Mediterranean Sapropels: Analogues for Petrographic Assessment of Organic Matter in Oil and Gas Shales


High-resolution electron microscopy of organic matter-hosted pore systems in unconventional reservoirs has yielded observations of organic matter (OM) that fills pores of submicron size in mudrocks. Interpretation of such small OM features cannot readily benefit from the conventional light microscope observations on which most knowledge of OM petrography is founded, leaving much ambiguity with respect to the interpretation of OM as kerogen (deposited OM) versus locally migrated bitumen (a diagenetic product). We illustrate these ambiguities with examples from the Eagle Ford Formation of South Texas. Mediterranean sapropels (Pliocene – Pleistocene) provide an interesting analogue for refining our understanding of the SEM petrography of OM, as their thermal immaturity allows inspection of petrographic textures without overprints related to maturation. Kerogen in these sapropels includes discrete particles some of which display micron-size primary intragranular pores. The most abundant organic matter takes the form of microns-thick flaky or stringy material, largely non-porous and internally amorphous, lacking the well-defined shapes of discrete particles. This dominant OM has been highly ductile in compaction and is thoroughly pervaded into intergranular spaces, causing substantial compactional porosity loss. Some of the larger mineral-hosted pores remain open, unfilled by ductile kerogen. Silt- and clay-size mineral crystals are highly admixed into most of the sapropelic OM, suggesting that the absence of detrital mineral admixture may be one criterion for discriminating kerogen from bitumen. It is clear, however, that petrographic discrimination of bitumen from ductile dispersed amorphous kerogen using SEM data will be challenging.