Depositional and postdepositional processes of an oil-shale analog at the microstructure scale: The Lower Cretaceous Agrio Formation, Neuqu´en Basin, northern Patagonia

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

AAPG BULLETIN

AMER ASSOC PETROLEUM GEOLOGIST

Lugar: Tulsa; Año: 2020

0149-1423

The Lower Cretaceous Agrio Formation is a source rock consideredto be a possible unconventional reservoir. However, no detailedmicrostructure characterization of this hydrocarbon playhas been performed. An exceptional oil-prone level is determinedthrough outcrop information, petrography, x-ray diffraction, geochemicalanalysis, and x-ray micro?computed tomography (mCT).Microscopic studies reveal a lenticular fabric and calcareous nannofossilssuggesting high carbonate productivity in the water columnand algal-derived organic matter. The high organic mattercontent points to anoxic?euxinic conditions consistent with pyriteframboids, authigenic fluorapatite, and an elevated concentrationof redox-sensitive elements. However, burrows reveal the activityof infaunal organisms, thus rejecting the idea of persistent bottomwateranoxia. The bioturbated, calcareous plankton and carbonaterichmudstone represents accumulation in a distal outer rampsetting with calcareous plankton blooms and the co-occurrence ofbenthicmicrobial mats. Bottom currents, probably as a consequenceof storm-related processes, were involved in detrital transport andoxygenation pulses. A three-dimensional (3-D)?microstructure reconstructionreveals three main mCT density-dependent phases