Sedimentology and reservoir geology of the Betty field, Baram Delta Province, offshore Sarawak, NW Borneo

Abstract

The Betty field is a moderate-sized oil field situated in the Baram Delta Province, offshore Sarawak. The field displays many of the characteristics that are typical of this Tertiary deltaic province, notably: (1) the structure is a result of the interaction of delta-related growth faulting and later Pliocene compressional folding, (2) the reservoirs comprise Miocene shallow marine sandstones and shales, which accumulated during repeated phases of small-scale progadation and retrogradation within a major regressive clastic wedge (comprising the wave-dominated palaeo-Baram Delta), and (3) the hydrocarbons occur in numerous vertically-stacked sands separated by sealing shales and trapped by a combination of fault seal and dip closure. This paper discusses these aspects of the Betty field in more detail, particularly the nature and origin of the reservoirs, and relates this geological framework to the field's development and production performance. Structurally the field is relatively simple, consisting of a NE-SW trending anticline which is bounded to the south by a major E-W trending growth fault (Betty Growth Fault). The anticline is a result of rollover associated with growth faulting combined with Pliocene compressional folding along the NE-SW trending Baronia-Betty-Bokor anticlinal trend. The Betty reservoirs occur within a ca. 2450 ft (7 4 7 m) thick sequence (between 7200-9650 ft I 2195-2941 m sub-sea) of Late Miocene, Upper Cycle V clastic sediments, which accumulated in a wave-/storm-dominated, inner neritic to nearshore/coastal environment within the palaeo-Baram Delta complex. The sand bodies are mainly characterized by numerous, composite and/ or amplified coarsening upward/progradational sequences (ca. 160 ft I 49 m thick) overlain by generally thinner, fining upwardlretrogradationalsequences (ca. 20-50 ft I 6-15 m thick). The sand bodies are vertically heterogeneous but display high lateral continuity with excellent field-wide correlation, which is consistent with the inferred high wave-energy depositional setting. Vertical heterogeneity is reflected in variations in the thickness and frequency of shale layers, and in the distribution of four distinctive reservoir facies of varying rock quality: (1) poorly stratified sandstone (porosity ca. 23%; permeability ca. 1200 mD), (2) bioturbated sandstone (22%; 500 mD), (3) laminated sandstone (19%; 90 mD), and (4) bioturbated heterolithic sandstone (17%; 50 mD).

Coupled ‘storm‐flood’ depositional model: Application to the Miocene–Modern Baram Delta Province, north‐west Borneo

“…Early models of the BDP therefore postulated a single‐sourced wave‐dominated delta model, based on the analogous modern Baram delta (e.g. Ho, ; Scherer, ; Johnson et al ., ). However, a wave‐dominated delta front fed by multiple rivers is likely to show much greater lateral variability in sand distribution, as a result of spatial and temporal variations in sediment transfer across the multiple drainage basins; this is supported by numerous subsurface studies of reservoir‐scale stratigraphic architecture (e.g.…”

“…However, a wave‐dominated delta front fed by multiple rivers is likely to show much greater lateral variability in sand distribution, as a result of spatial and temporal variations in sediment transfer across the multiple drainage basins; this is supported by numerous subsurface studies of reservoir‐scale stratigraphic architecture (e.g. Atkinson et al ., ; Johnson et al ., ; Tan et al ., ; Lambiase et al ., ; Mulcahy et al ., ; Hadley et al ., ) and by more recent quantitative evaluation of the modern coastal system (Ahmadi et al ., ).…”

“…Both types of marine flooding surface are known to be laterally extensive from subsurface data, and are the main cause of the ubiquitous vertical stratigraphic compartmentalization within the BDP (e.g. Johnson et al ., ; Hadley et al ., ).…”

“…), including previous facies‐related work (e.g. Atkinson et al ., ; Johnson et al ., ; Tan et al ., ; Lambiase et al ., ; Mulcahy et al ., ; Hadley et al ., ; Lambiase & Cullen, ; Lambiase & Suraya, ; Abd. Rahman et al ., ).…”

“…Within both types of parasequence, and their subsurface equivalents (e.g. Atkinson et al ., ; Johnson et al ., , ), trough cross‐bedding is absent and vertical transitions between facies associations may either be gradational or abrupt. Abrupt contacts are particularly common within Type A parasequences between FA‐1 and FA‐2 (for example, Unit 2, Figs and ), FA‐1 and FA‐3 (for example, Unit PS1, Fig.…”