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