Do Organo-mineral Interactions Dominate Organic Carbon Properties from Deposition to Production in Unconventional (Shale) Reservoirs?

Production surveys from unconventional (shale) reservoirs show significant variability between wells and even between frac stages in single wells. This variability is inconsistent with the homogenous source/reservoir concept of a resource play and implies geological variability as an influence in gas and oil production. One little considered, though potentially significant geological consideration specific to unconventional reservoirs, is the intimate (nano-scale) relation between organic carbon and reactive mineral surfaces typical of modern sediments. Shale is characterized by a significant percentage of clay minerals with reactive surfaces known to preserve, catalyse, or polymerize organic molecules. These organo-clay composites have different thermal stability than discrete organic particles and may influence the timing of hydrocarbon generation and over pressure through mineralogical transformations such as illitization. Data presented here from classic source intervals including the Monterey Fm., Pierre Shale, Woodford Fm. and Green River Fm, show a strong first order relationship between mineral surface area (MSA) and TOC that indicates the majority of organic carbon occurs as sub-micron scale organo-mineral composites and not as particulate organic compounds. This relationship was initially established in the depositional environment where mineral surfaces acted to preserve and stabilize organic matter. Proportionality was maintained through burial. Illitization of the Woodford Fm., maintained proportionality between TOC and MSA however the loss of MSA with the collapse of interlayers increased the TOC:MSA and expelled organic matter from interlayer sites. These data show that the initial controls of clay mineral delivery to marine environments (provenance and continental climate) determine TOC in sediment and will continue to influence hydrocarbon production through burial and diagenesis to well-completion. These findings also raise the question how gas/oil are distributed in self-sourced shales if most organic matter occurs as mineral associated and at molecular scale. Micro-porosity or local migration to interbedded permeable silt intervals provides two alternatives.
 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California