Microfossil assemblages as proxies for precise palaeoenvironmental determination — an example from Miocene sediments of northwest Borneo

Abstract

Abstract Hydrocarbon reservoirs in northwest Borneo are often developed in ‘paralic’ depositional settings, although current exploration is evaluating relatively deep-water turbiditic plays. In the absence of conventional core, and with only ambiguous wireline log and seismic signatures being available, the use of microfossil data is considered to determine precise depositional setting. This is important because different depositional settings imply different reservoir qualities in terms of architecture, connectivity, heterogeneity and poroperm characteristics. Equivalents of the reservoir succession are well exposed in northwest Brunei, and contain well-preserved sedimentary features and ichnofossils to determine precise depositional setting. Microfossil assemblages (both palynomorphs and foraminifera) have been sampled from each depositional environment identified at outcrop and by using an iterative approach, and incorporating data on modern distributions, diagnostic microfossil assemblages and taxa have been identified which can be used as precise palaeoenvironmental proxies. By using this approach distal turbidite, proximal turbidite, open shelf with slumping, open shelf, lower shoreface, upper shoreface, tidal flat with tidal channels, lower distributary channel, lagoon-distributary channel margin and upper distributary channel depositional environments can be recognized.