Abstract
The St. Joseph field is a large, structurally and stratigraphically complex oil field situated offshore West Sabah. The paper describes the main geological characteristics of the field, highlighting the benefit of 3D seismic and core/well log data in developing a comprehensive three-dimensional geological model, which is being used to guide field development. Structurally the field is situated along a major Lower Pliocene wrench fault zone (Bunbury-St. Joseph ridge) and comprises three distinctive areas : (1) NW Flank is a structurally simple area, dipping uniformly (at ca. 20°) to the NW, which contains the majority of recoverable oil reserves (ca. 95%). (2) Crestal Area is a structurally complex zone characterised by intense faulting, steeply dipping beds and incomplete stratigraphic sequences. Minor oil reserves are present which were discovered by the first exploration well (SJ-1) in 1975. (3) SE Flank is an area of moderate structural complexity with negligible oil reserves. 3D seismic data has significantly increased the quality of the structural definition of the field as a result of: (1) ability to map individual intra-Stage IVC reservoir intervals, (2) better fault definition, particularly in the structuraly complex areas, and (3) identi fication of stratigraphic features, such as slump scars. Practical benefits include im proved definition of the boundary between the NW Flank and Crestal Area, better delineation of the NE extent of the field (by faulting/slump scars) and improved location of development wells and drilling jackets. Stratigraphically the main NW Flank reservoir (Upper Sand Unit) comprises a complex sequence of shallow marine sandstones and shales (late Miocene, Stage IVC), which display marked lateral variations in sand development, reservoir quality and shale layer thickness/continuity. Sedimentological studies of ca. 1600 ft of core (including 800 ft of continuous core from well SJ-7) and palaeontological data indicate deposition in a storm/wave-influenced shallow marine (neritic) environment. The main reservoir units comprise several stacked coarsening/fining upward sequences reflecting repeated progra dation and transgression of coastal/delta front sand bodies. This subdivides the Upper Sand Unit into thirteen distinctive sub-units, which can be correlated field-wide. Log calibration of the four main facies types and correlation of the genetic sequences have enabled construction of a three-dimensional reservoir geological model of the Upper Sand Unit. This thick (700 - 900 ft I 27 - 43 m ), heterogeneous sequence is effectively a single, connected reservoir but with predictable lateral variations in sand and shale distribution:
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