Structural and stratigraphic configuration of the late Miocene Stage IVC reservoirs in the St. Joseph field, offshore Sabah, NW Borneo

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: