Poro-perm evolution in Oligo-Miocene coastal sandstones: Constraining the relative influence of sedimentary facies, mineralogy, and diagenesis on analogue reservoir quality of the Nyalau Formation, Borneo

Abstract

The application of enhanced recovery techniques to improve oil recovery requires a critical understanding of the depositional and post-depositional factors that impact fluid flow in sandstone reservoirs. This study combined facies, petrographic, mineralogical, and helium porosimetry analyses to examine the quality and producibility of the subsurface reservoir equivalents (Cycles I-III) of the Nyalau Formation in the underexplored Onshore Central Sarawak, Borneo. The Nyalau Formation consists of medium-to fine-grained, moderate-to well-sorted, and sub-rounded to sub-angular sublitharenite to lithic subarkose sandstones. Three facies associations and eight reservoir facies interpreted to have been deposited within a coast-to-coastal barrier depositional environment were delineated. The sandstones have undergone a wide range of diagenetic alterations, with the primary and secondary pore spaces predominantly occluded by authigenic quartz overgrowths and clay mineral cement. The helium porosity (ɸH) and Klinkenberg permeability (KK) in the reservoir facies widely vary, ranging from 3.0 to 22.14% and 0.003 to 8.283 mD, respectively. However, the weak correlation between ɸH and KK (R2 = 0.35) and petrographic datasets indicate limited diagenetic and mineralogical controls on the porosity-permeability (poro-perm) parameters, consistent with relatively high heterogeneous reservoirs. We inferred that the heterogeneous reservoir properties and fluid flow in the studied analogue reservoir sandstones are dominantly influenced by depositional processes, as shoreface sandstones typically yield better reservoir quality than those deposited within the intertidal reaches. Our data indicate that diagenetic modification and bioturbation exerted lesser control on fluid-flow quality. Nevertheless, the disparity of poro-perm values in samples of the same facies suggest that localised post-depositional modifications can disproportionately affect reservoir quality, regardless of uniformity or variations in depositional conditions.