Use of Outcrop as Substitute for Subsurface Shale: Current Understanding of Similarities, Discrepancies, and Associated Challenges

Abstract

Full coring in shale reservoirs is challenging, time consuming, and risky due to fragility. Therefore, shale outcrop analogs are often used as an alternative to enhance the understanding of corresponding subsurface reservoir formation multifaceted mineralogy and rock characteristics. As consequence, many researchers perform their fundamental experimental studies on the core samples collected from outcrops when the subsurface unconventional shale samples are scarce. An assumption is often made that the rock composition and properties of outcrop and subsurface samples remain the same which raises some serious concerns and demands a thorough review of published research to check the validity of this approach to characterize shale reservoirs. The high degree of lateral and vertical heterogeneity and anisotropy, weathering, contamination, and postdepositional tectonic activities further add to the major challenges. In this literature-based study, the authors revisit the current understanding and challenges of using shale outcrops to complement the available subsurface samples and use them for the preliminary evaluation of underexploration and undeveloped unconventional shale reservoirs. The similarities and discrepancies of mineralogical, petrophysical, geochemical, and geomechanical properties are discussed. Finally, some concluding remarks and recommendations are presented.

Experimental and Modeling Study of Water Imbibition and Flowback in Shale: Prediction of Relative Permeability and Capillary Pressure Curves

“…The experiments were conducted on a Mancos outcrop shale sample, provided by Kocurek Company . The sample length and diameter are 7.5 and 3.81 cm, respectively.…”

Shale’s Pore Structure and Sorption-Diffusion Characteristics: Effect of Analyzing Methods and Particle Size

“…The core plugs and shale chunks were supplied by Kocurek Company from different quarry outcrops of the Marcellus and Mancos formations in the United States. The coring operation was performed deep enough into the formation to acquire unweathered core samples . The pulverized samples below 50 μm were used to determine mineralogy and total organic carbon.…”

“…The coring operation was performed deep enough into the formation to acquire unweathered core samples. 42 The pulverized samples below 50 μm were used to determine mineralogy and total organic carbon. The mineralogy was characterized by the Panalytical Xpert3 X-ray diffraction system, applying Cu Kα radiation at 40 kV and 40 mA.…”

Standardized field methods for fracture-focused surface processes research

“…The fracture characteristics of outcrops have long been employed as proxies for subsurface fracture networks, and there is a reasonably large body of literature addressing these relationships and their potential pitfalls (e.g., Ukar et al, 2019;Al-Fahmi et al, 2020;Sharifigaliuk et al, 2021). Overall, we emphasize that the researcher should be aware that for any outcrop of in situ bedrock, tectonic stresses are likely not the only cause of fractures observed there.…”