Characterization and Comparison of Outcrop and Subsurface Unconventional Shale Samples

Abstract Subsurface unconventional shale samples are always scarce. Outcrop analogs are often used as an alternative to enhance the understanding of the corresponding reservoir formation. One assumption is usually made that rock composition and properties between the outcrop and subsurface samples remain the same or similar, despite differences in their burial and diagenetic histories. This paper presents a comparative case study to investigate the similarities and differences in rock properties between outcrop and subsurface samples from the same formation. Two subsurface and two outcrop samples from the Lower Silurian Longmaxi formation in Sichuan Basin of China were characterized to determine their mineralogical, geochemical, petrophysical, elastic and mechanical properties. Micro-CT images showed that one subsurface sample was drilled normal to the bedding, while other three samples were parallel to the bedding. Two subsurface samples differ in their mineralogy – the horizontal sample is clay-dominant, while the other one is predominantly comprise of quartz, dolomite and calcite minerals, very similar to two outcrop samples. All four samples are thermally immature and their Tmax is less than 435 °C. Subsurface samples have the highest TOC of 3.75% but relatively lower HI and OI. Other pyrolysis parameters are very similar between subsurface and outcrop samples. All samples have very low porosity of less than 2.5% and permeability of less than 9 nD, although subsurface samples have a relatively higher value. The discrepancy in mineralogical composition, especially the clay content, results in different elastic and mechanical behavior of outcrop and subsurface samples. The subsurface sample is highly anisotropic in both compressional and shear wave anisotropy due to the large amount of clay minerals, while one outcrop sample exhibits the strong shear wave anisotropy only and the other one is almost isotropic. Subsurface samples have lower values of Young's modulus, peak stress, Mohr-Coulomb failure parameters and unconfined compressive strength than outcrop samples.