Limitation of laboratory measurements in evaluating rock properties of bioturbated strata: A case study of the Upper Jubaila Member in central Saudi Arabia

Abstract

To understand the limitations of laboratory measurements in evaluating the rock properties of bioturbated reservoirs, this study integrates field investigations, computed tomography (CT) scanning, image analysis, spot permeability measurements and geostatistical modeling with previously published porosity and permeability results of Upper Jurassic bioturbated carbonate strata in central Saudi Arabia (Upper Jubaila Member). The porosity and permeability measurements from previous studies have indicated that the studied strata are very tight, with porosity ranging from 0.3 to 4.8% (mean [μ] = 1.2 ± 1.4) and permeability ranging from 0.0 to 2.0 mD (μ = 0.01 ± 0.03 mD). The field investigation results from this study showed that these strata were extensively bioturbated (10% to 50% burrow intensity) and dominated by Thalassinoides burrows. These Thalassinoides burrows had either open or partially filled shafts with diameters ranging from 2 to 5 cm in 2D outcrop faces. The results from CT scanning of a boulder-sized rock sample (~68 cm × 27 cm × 15 cm) with 20% bioturbated intensity from the studied strata showed that the Thalassinoides burrows developed a connected burrow porosity network within a tight matrix (spot permeabilities of the matrix ranging from 0.05 to 0.22 mD with μ = 0.07 ± 0.05 mD). Interestingly, however, in cropped volumes of the CT scans (with core plug and whole core sizes), the entire connected pore network of the Thalassinoides burrows is not represented. Instead, these volumes show isolated pores, erroneously indicating the absence of a connected permeability pathway for Thalassinoides burrows in the CT scanned sample. In 2D slices of the CT scans, however, Thalassinoides burrows show connected porous pore network from one end to the opposite end, suggesting a permeable pathway. The visual porosity estimated from these 2D slices has a μ value of 18 ± 4.8%, which is significantly larger than what was estimated for the Upper Jubaila strata in previous studies. Geostatistical models of the CT scanned sample estimated that the bulk permeability (permeability of a homogenized sample with a steady-state flow) was at least 1200 mD; this value is also significantly higher than previous estimates for the studied strata. This study highlights the inability of core plugs and full-diameter cores to capture the complete burrow porosity network, a limitation that can be problematic in the characterization of rock properties of bioturbated hydrocarbon reservoirs.