Assessing the impact of bioturbation on reservoir quality: Integrated spontaneous imbibition, pressure decay profile permeability and pore-throat characterization study of the Cretaceous Bhuj formation, Kachchh basin, India

Abstract

The Lower Cretaceous Bhuj Formation in Kachchh basin is well known for its hydrocarbon potentials including proven gas reserves in Kachchh offshore. In the onshore these reservoir equivalent rocks outcrops and includes sandstones (quartz arenites), with higher degree of bioturbations. The present work, deals with determining of impact of variable bioturbation degree sandstones reservoir quality. The petrographic properties were integrated with spot permeability and spontaneous imbibition study to understand petrophysical contrast between burrow and non-burrowed part.In moderately bioturbated samples (BI-3), spot permeability for burrows was 18 times higher than non-burrowed matrix. While in higher bioturbated, samples (BI-4) the permeability contrast between burrows and non-burrowed area was moderately lower (10.26 times). Similarly in highly bioturbated sandstones of BI-5 the permeability contrast between burrow and non-burrow was less up to 5.45 times. Thus, highly bioturbated sandstones appears to be less heterogeneous as compared with moderately bioturbation sandstones. To support this study, qualitative and quantitative investigation of the pore network, pore structure and pore connectivity through Scanning Electron Microcopy (SEM) indicated a larger proportion of intergranular pores in burrow fill than in the surrounding matrix for all types of bioturbation classes. High pressure Mercury Injection (HPMI) data for quantitative validation of these inferences also estimated a higher presence of connected macropore volume in burrowed regions of Bioturbation Index 4 and 5. In contrast, the Bioturbation Index-3 indicated highest connected mesopore volume for the non-burrowed area. The imbibition characteristic corroborated with the better pore connectivity and larger pore throat size in the burrow volume than the non-burrowed matrix estimated from other studies. The Bioturbation Index 4 and 5 had higher initial imbibition rates and imbibition capacities than Bioturbation Index-3. The well-developed and connected pores in the mesopore region tend to form a large transition zone, while a higher proportion of connected pore volume in Bioturbation Index 5 is exhibited in a higher late imbibition rate. In essence, a conceptual model is presented to describe spontaneous imbibition in bioturbated media, in order to understand the enhanced petrophysical quality in bioturbated sandstones.