Bulletin of the Geological Society of Malaysia, Volume 25, Dec.1989, pp. 119 – 161
1 Sarawak Shell Berhad, Lutong, 98009 MIRI, Sarawak, Malaysia.
2 PETRONAS Carigali Sdn. Bhd., (Baram Delta Operations), P.O. Box 1452, Lutong, 98008 MIRI, Sarawak, Malaysia.
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 fields 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 ftI2195-2941 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 ftI49 m thick) overlain by generally thinner, fining upward/retrogradational sequences (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).
The individual Betty reservoirs are interpreted as representing the repeated build-out and gradual retreat of wave-/storm-dominated sand bodies (shoreface and/or shoreface-connected bars). They probably accumulated in a coastal to inner-shelf environment, which was marginal to the axial part of the palaeo-Baram Delta. Complete coastal progradation never occurred in this area in Upper Cycle V times with the environment remaining essentially sub-littoral.
Three main types of vertical facies sequence types are recognized with distinctive gamma ray log profiles. These sequence probably reflect fluctuations in sediment supply and repeated base level changes (mainly subsidence-related), in which the latter was probably significantly influenced by movements along the nearby Betty Growth Fault. The preservation of both progradational and retrogradational deposits, including the development of thick amplified sequences, is indicative of the high subsidence and sedimentation rates within the Baram Delta Province.
Hydrocarbons are trapped within at least twenty-one stacked sand bodies separated by sealing shales. The bulk of the hydrocarbons are encountered in a single structural block where trapping is a result of anticlinal dip closure and updip seal against the Betty Growth Fault. Only minor hydrocarbons are present in subsidiary fault blocks behind the Betty Growth Fault. Within the Betty structure oil-bearing reservoirs decrease in thickness and frequency with depth, while both associated primary gas caps and unassociated gas reservoirs increase in depth (down to 9500 ftI2895 m sub-sea). This reflects the thermal maturity profile of oil and gas migration in this area; later expulsion and migration of gas has led to the preferential displacement of oil by gas in the structurally deep reservoirs.
Finally, the fields geological model is discussed in relation to production performance and to reservoir management.