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The Jerudong anticline provides an outcrop example of multiple phases of diapir growth and interaction with the country rock. The N–S-trending anticline overlies a high-angled basement fault zone that was episodically active, moving predominantly sinistrally in response to transpressive stresses. The earliest stage of deformation in the study area was mid-Miocene E–W to NE–SW-trending growth faulting and shale-diapir growth. Early normal faults exerted an important influence on the large- and small-scale bedding geometries and facies changes of syn-tectonic shallow-marine, shoreface and tidal strata. In particular there are laterally rapid facies changes and differential rotation of strata across growth faults. Diapir activity is indicated by the presence of shale dykes which commonly intrude normal fault planes, and concordant shale intrusions. Shale intrusions, associated minor thrust faults and normal faults were rotated during a late Miocene–early Pliocene phase of folding related to transpression. Few shale dykes were intruded during this phase, probably because the horizontal principal stresses were relatively large compared with the pore-fluid pressure. At the end of the transpressive phase, shale dykes were intruded into steeply dipping beds as a result of stress relaxation. Continued uplift and erosion elevated overpressured horizons to a point where hydraulic fracturing reached the surface and Holocene-age mud volcanoes were developed.

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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.