A SEQUENCE STRATIGRAPHIC CASE STUDY OF AN EXPLORATION PERMIT IN DEEPWATER SABAH: COMPARISON AND LESSONS LEARNED FROM PRE-VERSUS POST-DRILL EVALUATIONS

摘要

Sequence stratigraphy is a seismic-based evaluation technique commonly applied to frontier exploration areas with limited well data, that attempts to subdivide sedimentary deposits into unconformity bound units linked to variations in sediment supply and the rate of change in accommodation space. Correlatable sequence boundaries and maximum flooding surfaces are assumed to represent time lines, which can constrain depositional units within an established chronostratigraphic framework. A pre-drill sequence stratigraphic study was completed prior to the drilling of three exploration wells in "Block X" offshore deepwater Sabah. Regional structural mapping from 3D seismic established the kinematic and deformation history, while seismic attribute analyses aided in the interpretation of turbidite sedimentary fairways. The reconstruction of depositional environments in key reservoir intervals was facilitated by the framework of sequence stratigraphy. A feature of the seismic data in the basin is the deterioration of imaging over the large anticlinal traps, due to steep dips and the common occurrence of shallow gas over structural crests. As reservoir presence is the main geological risk for hydrocarbon exploration in the basin, the sequence stratigraphic approach allows the prediction of reservoir in target intervals to be extended into the poor data areas over the major thrust-bounded anticlines. The first well drilled was a significant oil discovery, but the following two wells found no reservoir. Post-drill evaluation has revealed that the latter two wells were probably located in a slope by-pass setting, while the first well was more optimally located in a mid-fan environment on the basin floor, beyond the base of the main palaeoslope. Confirming the conclusions of similar studies elsewhere, the location of the toe-of-slope break during periods of major sediment influx is therefore shown to be the single most important criterion in de-risking reservoir sand presence in such deepwater settings.