X-ray microtomography analysis to approach bioturbation's influence on minor-scale porosity distribution: A novel approach in contourite deposits

Abstract

Micro-CT analysis is employed for the first time to evaluate the effects of bioturbation on porosity distribution in contourite facies, namely: i) sandy clastic contourites from the Late Miocene Moroccan Rifian Corridor; and ii) dominant-calcareous contourites from Eocene- Middle Miocene Cyprus paleoslope. Porosity distribution is affected by bioturbation due to trace maker behavior and burrow infilling, but there is no clear, single relationship. Macaronichnus, especially M. segregatis, located in sandy clastic contourite facies would have the greatest impact, increasing porosity due to the grain-selective deposit-feeding behavior of the trace maker. Porosity values are up to three times higher in the burrow rim than in the infilling material, which favors a dual-porosity flow medium. Samples with M. s. degiberti record higher porosity values in the host sediment than in the burrow fill, but any increase of porosity in the burrow rim is not certain. This variable influence on porosity distribution (M. segregatis vs. M. s. degiberti) might be associated with different trace maker behaviors during feeding activities, or even with variable producers. Chondrites located in muddy chalk contourite facies has a neutral effect on porosity distribution, porosity data of the host sediment and the filling material being similar, in agreement with a comparable grain size. Thalassinoides located inside calcarenitic contourite facies has a minor effect on porosity distribution, showing similarities between passively infilled material and the host sediment. Our results also reveal the importance of secondary diagenetic overprints when homogenizing primary differences. Accordingly, understanding the impact of bioturbation on porosity distribution in contourite facies appears to be a key factor for evaluating the real potential of these unconventional reservoirs. This study represents a first step forward in discerning the relationship between contourite facies, ichnological features, petrophysical properties and reservoir geology.