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).

This paper was prepared for presentation at the 1998 SPE Asia Pacific Conference on Integrated Modeling for Asset Management held in Kuala Lumpur, Malaysia, 23-24 March 1998.