| Summary: | 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 (747 m) thick sequence (between 7200-
9650 ft/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/49 m thick) overlain by generally
thinner, fining upward/retrogradational sequences (ca. 20-50 ft/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%
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