Abstract
This study uses a combination of sedimentological analysis, X-radiation techniques, electron
microscopy, and routine core analysis methods to investigate multi-scale variations of reservoir
properties from a tight-oil reservoir. Higher reservoir quality, defined by porosity and permeability
values, is mostly associated with lower values of X-ray attenuation index, and corresponds well
with lighter-colored (relatively clean sandstone) portions of the highly bioturbated lithofacies
investigated. Physicochemical properties of the samples are highly influenced by its mineralogy,
which is dominated by quartz and illitic clays (85-90% wt.), and can be captured using mainly the
mass concentration of Si, Al, K, Fe, and Rb. Equally important, connectivity to main flow-paths
at the reservoir scale can also be extracted from these interpretations.
Evaluation of the three-dimensional distribution of reservoir properties at the core-scale
indicate contrasting variations of these properties within elementary lithological components
(ELCs). These ELCs are present at the cm- to sub-cm scale and are moderated in part by the effects
of synsedimentary bioturbation. In order of decreasing reservoir quality, ELCs identified were
defined as: a) SS1 – relatively clean sandstone, b) SS2 – argillaceous sandstone, c) SH – mudstone,
and d) siderite- and pyrite nodules/concretions and dense mineral burrow fillings. Despite the high
degree of bioturbation observed, sand-filled structures of biogenic origin were found to be poorly
connected as confirmed with fluid flow numerical simulations. Also, a geometric averaging
algorithm was observed to offer a better representation for the upscaling of simulated horizontal
permeability datasets, whereas both geometric and harmonic averaging work similarly well for the
vertical measurements on multi-scale virtual subsamples. Anisotropy of horizontal permeability
was largely influenced by the presence of relic bedding structures. Pore size distribution obtained for ELC SS2 samples spans from ≈120 μm to ≈20 Å (diameter), with accessibility ratio higher than 90%. Comparison of incremental pore volume curves from mercury porosimetry and small-angle and ultra-small angle neutron scattering (SANS/USANS) highlights the potential influence of clay-hosted slit pores in controlling the fluid-flow process in these tight rocks. Moreover, discrepancies observed among the different analytical techniques
highlights the influence of subtle compositional variations in the analysis of porosity and pore
accessibility from SANS/USANS datasets.
