Relationships between natural fractures and chemical composition: Marcellus Shale, Appalachian Basin

Abstract

Within mudrock and shale reservoirs, brittle zones undergo plastic deformation during hydraulic stimulation, creating numerous artificial fractures through which hydrocarbons can permeate. Natural fractures in mudrock and shale reduce the tensile strength of the host rock, and because of this, are hypothesized to become reactivated during hydraulic stimulation. Combined, brittleness and natural fractures contribute to creating more abundant and complex fracture networks during hydraulic stimulation. Research efforts towards quantifying rock brittleness have resulted in numerous mineral/compositional-based indices, which are utilized during petrophysical analysis to predict zones most conducive to hydraulic stimulation. In contrast, investigations on the relationship between chemical composition and core-scale natural fractures are limited. High-resolution energy-dispersive X-ray fluorescence (XRF) data, calibrated with a wave- dispersive XRF, were collected from a Marcellus shale core. Core-scale natural fractures were characterized in terms of length, width, in-filling material or lack thereof, and orientation. Natural fracture data were transformed into a continuous P10 curve, the lineal fracture intensity, which is expressed as fractures per a one foot window. Utilizing these datasets, we investigated the relationship between rock composition and natural fracture intensity. Regression analyses recorded positive relationships between natural fracture intensity and calcium, silicon/aluminum, and total organic carbon, and negative relationships with silicon and aluminum. Aluminum recorded the strongest (negative) relationship (r2=0.379) with natural fracture intensity. Least partial squared analysis, a multivariate method, was used to assess the degree to which natural fractures can be predicted by chemical composition, and recorded an r2=0.5255. This study illustrates that, while numerous factors are responsible for natural fracture genesis, such fractures predictively concentrate in areas of similar chemical composition, largely in zones depleted in aluminum.

AAPG Datapages/Search and Discovery Article #90335 © 2018 AAPG 47th Annual AAPG-SPE Eastern Section Joint Meeting, Pittsburgh, Pennsylvania, October 7-11, 2018