Scale dependence of petrophysical measurements in reservoirs with Thalassinoides: Insights from CT scans

Abstract

Subsurface reservoirs with Thalassinoides are well known. Using a CT scan of an 18.5 cm-diameter core (top cross section, 267.21 cm2; length, 26.5 cm) with Thalassinoides, this study demonstrates the scale dependence of sampling, and variables important in selecting representative sample dimensions for modeling these reservoirs. In the CT scan, 40 vertical square prism subsamples were extracted with cross sectional area ranging from 1.44 cm2 to 92.16 cm2. The results illustrate the scale dependence and sampling biases on Thalassinoides morphology, burrow percentage (BP), and connectivity of burrows from the top to bottom of subsamples (a proxy for permeability). Analysis of the entire scan shows a BP of ∼38%. The 40 subsamples vary in BP from 7% to 44% with the smallest cross sections showing the expected highest variance, but also showing the lowest mean BP. This illustrates that small-diameter vertical subsamples are likely to under sample Thalassinoides networks. In the full core, Thalassinoides connects across the top and bottom. Subsample cross sections >51.84 cm2 have 100% probability of showing this, whereas subsample cross sections <12.96 cm2 have 0% probability of showing this. To find the dimensions of representative samples to model Thalassinoides-bearing reservoirs, burrow diameter, BP, vertical length of sample, and core diameter must all be considered.