Occurrences and characterisation of textural and mineralogical heterogeneities in analogue reservoir sandstones: case study of the onshore Central Sarawak, NW Borneo

Abstract

The plethora of micro (μm)–to–mega (≥ 10 to ≤ 100 m) scale heterogeneities in marginal marine siliciclastic reservoirs makes their petrophysical analysis often cumbersome, and thus yield unrealistic reservoir parameters. Micro-scale heterogeneities typically occur at the microscopic scale, while the mega-scale is observable at the outcrop scale. A good understanding of the possible heterogeneities within sandstone reservoirs can enhance their quality assessment significantly. In this study, heterogeneities in analogue marginal marine sandstones of Nyalau and Balingian formations, NW Borneo, were qualitatively characterised using integrated field geology and geochemical analyses. The outcrops are typically characterised by an upward-coarsening sequence, consisting of sandstone and argillaceous lithounits. The sandstones are moderate- to well-sorted, fine- to medium-grained with varying kurtosis and skewness. Estimated matrix composition in thin section ranges between 15 and 67%, thus classifying the sandstone as feldspathic litharenites to sublithearenite. Skolithos (Ophiomorpha) and Planolites are the commonly observed ichnofacies with bioturbation index of 0 to 90%. Three mineralogical relationships (denoted as MF) were defined based on quartz-feldspar-clay contents. Of these, reservoir units characterised by the MF-1 have better reservoir potential than the other two. Scanning and electron microscopy (SEM) revealed that the secondary pores created from grain dissolutions are partly to completely filled by networks of illite strands, kaolinite/dickite sheets, chlorite flakes, pyrite and very rarely laumontite. An integration of SEM, XRD and mineralogical evolution trend suggests that most of these pore-modifying minerals are of authigenic origin. This research concluded that pore-destroying alterations predominate pore enhancement modifications in the sandstones. Hence, fluid flow parameters and by implication production from the sandstone may be significantly impacted in a negative sense.