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

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.

Marine connections of Amazonia: Evidence from foraminifera and dinoflagellate cysts (early to middle Miocene, Colombia/Peru)

“…The genus Trochammina is common in lower shore face deposits, on tidal flats, channels and mangroves (Simmons et al, 1999), and in supratidal and landwards zones (Debenay et al., 2000). Trochammina inflata is an agglutinated foraminifera characteristic for salt marshes and associated with marsh plants, but also occurs outside the marshes (Alve and Murray, 1999).…”

Depositional Environment and Sequence Stratigraphy of Miocene Deltaic Cycles Exposed along the Jerudong Anticline, Brunei Darussalam

“…A number of arguments strongly support an overall shelf interpretation: the basal slumps could be interpreted to indicate either a continental slope setting or a shelf environment close to a delta front. However, microfossil and palynomorph analyses of the slumped shales (Simmons et al 1999) and of the interbedded shales of the units above exclusively indicate shallow-marine to open-shelf conditions (Table 1). The absence of any background fauna living in water depths greater than 200 m strongly favors the shelf interpretation.…”

“…In some nodules fossil crabs are preserved. Palynological assemblages described by Simmons et al (1999) consist of moderately rich spores, occasional mangrove, back-mangrove, freshwater swamp and palm pollen, and very rare dynocysts. The foraminiferal assemblage is dominated by agglutinating foraminifera that indicate a ''normal'' open shelf setting; planktonic foraminifera are lacking.…”

Three‐dimensional restoration of original sedimentary geometries in deformed basin fill, onshore Brunei Darussalam, NW Borneo

“…Wilford, 1961;James, 1984;Sandal, 1996), as well as reports and publications providing detailed sedimentological and structural outcrop descriptions (e.g. Biostratigraphic ages of outcrops combine data of Liechti et al (1960), Sandal (1996), Simmons et al (1999), Back et al (2001) and palynological analyses of this study (Table1).The geophysical subsurface dataset consists of over 1400 km regional 2D seismic lines (eight di¡erent vintages between 1967 and 1987 of highly variable quality), and standard wireline logs of seven exploration wells. Biostratigraphic ages of outcrops combine data of Liechti et al (1960), Sandal (1996), Simmons et al (1999), Back et al (2001) and palynological analyses of this study (Table1).The geophysical subsurface dataset consists of over 1400 km regional 2D seismic lines (eight di¡erent vintages between 1967 and 1987 of highly variable quality), and standard wireline logs of seven exploration wells.…”