Depositional and post-depositional processes of an oil-shale analog at the microstructure scale: The Lower Cretaceous Agrio Formation, Neuquén Basin, northern Patagonia

COMERIO M.; DIANA E. FERNÁNDEZ; RENDTORFF, N.; CIPOLLONE, M; ZALBA P.; PABLO J. PAZOS

AAPG BULLETIN

AMER ASSOC PETROLEUM GEOLOGIST

Lugar: Tulsa; Año: 2020 vol. 104 p. 1679 - 1705

0149-1423

The Lower Cretaceous Agrio Formation is a source rock considered to be a possible unconventional reservoir. However, no detailed microstructure characterization of hydrocarbon play has been performed. An exceptional oil-prone level is determined through outcrop information, petrography, X-ray diffraction, geochemical analysis and X-ray microtomography (μCT). Microscopic studies reveal a lenticular fabric and calcareous nannofossils suggesting high carbonate productivity in the water column and algal-derived organic matter. The high organic matter content points to anoxic? euxinic conditions consistent with pyrite framboids, authigenic fluorapatite, and an elevated concentration of redox-sensitive elements. However, burrows reveal the activity of infaunal organisms, thus rejecting the idea of persistent bottom water anoxia. The bioturbated, calcareous plankton and carbonaceous-rich mudstone represent accumulation in a distal outer ramp setting with calcareous plankton blooms and the co-occurrence of benthic microbial mats. Bottom currents, probably as a consequence of storm-related processes, were involved in detrital transport and oxygenation pulses. A 3D46 microstructure reconstruction reveals three main μCT density-dependent phases: (1) a low47 density phase composed of organic matter and calcite; (2) an intermediate-density phase dominated by calcite and minor organic matter; and, (3) a high-density phase composed of authigenic minerals (pyrite/marcasite and fluorapatite) and silt-sized detrital quartz and plagioclase. This analysis showed that burial diagenesis did not create enough pore space to be evaluated through μCT and that, besides mineralogy, particle-size distribution, particle arrangement (sorting) and the local compaction degree affect the 3D reconstructed structure. The analyzed sample is located towards the carbonaceous end member of the shale spectrum and exemplifies new approaches for the characterization 54 of carbonate oil shales in the unit and similar deposits. From the perspective of unconventional exploration, the evaluation of depositional and post-depositional controls on the resultant microstructure might contribute to a better understanding of potential target intervals within the Agrio Formation.