SEDIMENTOLOGY, STRATIGRAPHY, AND GEOCHEMISTRY OF THE AGRIO FORMATION, NEUQUÉN BASIN, ARGENTINA: ASSESSING HYDROCARBON POTENTIAL AND INTERNAL VARIABILITY IN AN ANCIENT OFFSHORE MARINE MUDSTONE SYSTEM

The mudstone-dominated Early Cretaceous Agrio Formation is the youngest of the three main marine source rock intervals of the Neuquén Basin, Argentina, yet its unconventional hydrocarbon potential remains relatively understudied. In order to be able to locate and extract shale hydrocarbons in a responsible, sustainable, and economic way, an improved understanding of offshore mudstone dominated depositional mechanisms and high-resolution facies models need to be developed and better constrained. The upper and lower members of the Agrio Formation were deposited in a storm-influenced, mixed siliciclastic-carbonate ramp environment. We conducted programmed pyrolysis and carbon isotope geochemical analyses on 46 samples from 395 m of section measured through the lower and central members. Total organic carbon (TOC) values through this interval range from 0.2 – 17.22% with an average of 3%. Program pyrolysis results show type II to III kerogen with samples in the mature oil window. Organic carbon isotope data reveals a high terrigenous input of organic material, and a 5.6‰ positive excursion in the terrestrial δ¹³C_org curve associated with the interval where TOC reaches the maximum 17.22%. 300 m of section through the lower member and 116 m of section through the upper member were measured at two separate localities; El Portón and Puerta Curaco. We recognize eight facies and five facies associations (FA). The facies identified are black fine mudstone, grey fine mudstone, medium mudstone, structureless coarse mudstone, bioclastic coarse mudstone, laminated coarse mudstone, grainstone, and fine-grained tuff. FA1 and FA2 consist of low and high frequency interbedding of fine, medium, and structureless coarse mudstone; representing a basinal and distal outer ramp depositional environment respectively. FA3, FA4, and FA5 are respectively indicative of deposition in the proximal outer ramp, transition to distal mid ramp, and mid ramp environments. The formation exhibits high frequency stratigraphic variability between the multiple facies associations and, thus, depositional environments. This variability resulted from the low gradient mixed siliciclastic-carbonate ramp depositional environment in which small shifts in eustasy, accommodation, or sediment supply can lead to a dramatic shift in bathymetry.