Sequence stratigraphic modelling and reservoir architecture of the shallow marine successions of Baram field, West Baram Delta, offshore Sarawak, East Malaysia
Résumé
The Baram Delta Province is located in the northern part of Sarawak (West Baram Delta) and extends north-eastward into Brunei and further into the southwestern part of Sabah (East Baram Delta). The delta is a Neogene basin which developed over an accretionary wedge implying Cretaceous to Eocene sediments during the Late Eocene to Late Miocene times (Tongkul, 1991; Hutchison et al., 2000; Morley et al., 2003; Sapin et al., 2011). Facies and well log analyses were carried out on core and well data for the Late Miocene successions of Baram field, a medium-sized oilfield located in the northeastern flank of the Baram Delta Oil Province, offshore Sarawak. A numerical model of sea-level fluctuations and progradational basin-fill was generated using the Clastic Modeling Program (Hardy and Waltham, 1992a and 1992b; Waltham, 1992) software to evaluate the possible controls of sea-level changes in the development of the siliciclastic successions and their bounding surfaces. This model was based on four lines of evidence, namely core data, fieldwide wireline logs correlation, seismic sections and average thickness variations across the field. Cored intervals of the Upper Cycle V (Late Miocene) display reservoir successions dominated by thick swaley cross-stratified (SCS) sandstones, thin hummocky cross-stratified sandstones and other shallow marine, wave and storm-dominated facies, interbedded with laminated shelfal mudstones. The vertical facies organisation suggests deposition during shoreface progradation associated with a fall of relative sea level. Analysis and correlation of well logs reveal stacking patterns comprising three scales of depositional cyclicity: the parasequences (w10ew30 m thick), the parasequence sets (w45ew130 m thick) and the major cycles (w600e800 m thick). Field-wide, dip-oriented seismic sections show very well-developed horizontal to slightly upward convex layers traceable over great distances, which suggests a ramp-type margin, in which the basin floor dipped gradually seaward and lacked a distinct shelf-slope margin. The evidences gathered demonstrate that the deposition and build-up stratigraphy of the Late Miocene sedimentary successions could have been strongly controlled by superimposed short-term, medium-term and long-term sea-level changes. The simulated sea level and sedimentary basin-fill model, generated by the Clastic Modelling Program, match to the well log correlation. This model illustrates that high frequency sea-level fluctuations enable sands to be distributed over large areas within a shallow, low gradient shelf. Our study shows that integrated studies incorporating cores, well logs, seismic sections and simulated models can be employed as important tools for correlation and reservoir modelling.