Sem Petrography of Eastern Mediterranean Sapropels: Analogue Data For Assessing Organic Matter In Oil and Gas Shales

Abstract

Abstract Examination of organic-matter-hosted pore systems in unconventional reservoir rocks has drawn attention to the appearance of sedimentary organic matter (OM) as seen in high-resolution SEM images. Field-emission SEM imaging of eastern Mediterranean sapropels (Pliocene–Pleistocene age from Ocean Drilling Program drill sites) was performed on samples prepared by Ar-ion cross-section polishing. The immaturity of marine kerogen in these rocks allows inspection of petrographic textures without overprints related to thermal maturation (hydrocarbon generation). Kerogen in these samples includes discrete particles that in some cases contain primary intragranular pores. The most abundant organic matter is in the form of microns-thick flaky or stringy material, largely nonporous and internally amorphous, and lacking the well-defined shapes of discrete particles. This dominant OM has behaved in a highly ductile manner in compaction and is now highly pervaded into intergranular spaces between silt- and clay-size grains. A portion of the larger mineral-hosted pores remain open, unfilled by ductile OM. Minor nanometer-scale pores of uncertain origin occur in the dominant OM. Silt- and clay-size mineral crystals are distributed in organic matter of most types, and it is difficult to fully ascertain if the OM/mineral mixing has occurred during compaction versus during aggregation during gravity settling (marine-snow sedimentation) or through sediment accumulation in OM-rich microbial mats. Results indicate that sediments rich in marine kerogen are subject to substantial compactional porosity loss in early burial. These observations of marine detrital organic matter in its early diagenetic state provide a useful baseline for interpretation of petrographic features in more mature OM-rich mudrocks. It is clear that petrographic discrimination of diagenetically generated organic matter (bitumen and related OM types) from ductile dispersed amorphous detrital OM (kerogen) using SEM data will be challenging in the absence of compositional information.