Core-Based Calibration of Image Log and Dipmeter Data in Shelf Reservoirs: Their Utility in Separating Shoreface from Tidal Dominated Sands in Subsurface, Offshore Brunei Darussalam

Lai Quoc Lap¹ and John K. Warren^2
1 Universiti Brunei Darussalam, N/A, Brunei
² Universiti Brunei Darussalam, Tungku Link, Brunei

Middle-late Miocene to early Pliocene aged, clastic sediments of offshore Brunei host multi-million barrel oil and gas fields in shallow marine sediments, including wave- and tidal-dominated deposits. Calibration of FMI/FMS and dipmeter (SHDT/OBDT) data to core from different stratigraphic levels in number of producing fields and to equivalent outcrops in onshore Brunei clearly show that image data is the most useful wireline tool available for depositional interpretation. The combination of calibrated image and conventional logs can separate tidal from shoreface reservoirs in un-cored intervals.

Boundaries between two the main lithologies (sand and mud) across thicknesses ranging from cm to meters can be distinguished on FMI/FMS. Mm to cm-scale wavy-lenticular beds and thinly interbedded sand/shales can be detected, while at larger-scales parallel-lamination to planar-crossbedding, lateral-accretion and channel-fill crossbedding structures are also recognizable. However, finer-scale structures (e.g. thin lamination, trough-cross foresets…) can only be seen if there is a sufficient resistivity contrast. SHDT pseudo-images are also capable of detecting changes of lithology, cm wavy-lenticular bedding and large-scale structures, but with lower levels of confidence. In contrast, the OBDT tool poorly resolves almost all sedimentary features and is only useful for structural interpretation, such as defining regional dips.

At meter-scale, shoreface successions commonly show blocky to funnel-shapes on the gamma log, while images are either quite massive or variably bioturbated defining dm-spaced parallel to crossbedded layering and some minor mud-drapes. Tidal successions are generally indicated by a higher-frequency “spiky” to serrated bell-shaped gamma signature, with FMI/FMS or SHDT logs indicating abundant cm-spaced bedding, along with wavy-lenticular and thinly interbedded sand/shale successions. Channel-fills are detected by decreasing-up dip-motifs or large-scale lateral-accretion sets and higher bioturbation intensities toward bed tops. However, caution must be taken when attempting an image-based separation between transitional shelf deposits and tidal-flats as they show similar diagnostics.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005