--> Geological Implications of Overpressure and Fracture Gradients in Deepwater Fold-and-Thrust Belt, Offshore Malaysia

AAPG/SEG International Conference & Exhibition

Datapages, Inc.Print this page

Geological Implications of Overpressure and Fracture Gradients in Deepwater Fold-and-Thrust Belt, Offshore Malaysia

Abstract

Abstract

The deepwater fold-and-thrust belt in offshore Malaysia is a promising hydrocarbon province with significant recent discoveries, where predictions of pore pressure and fracture gradient (PPFG) are critical in terms of safe drilling operations and geological risk evaluations. Complex thrust structures with high vertical reliefs due to strong lateral compression, overlain by young undeformed sediments, resulted in non-unique compaction trends and complicated overpressure distributions in this region. We conducted a regional PPFG study using 12 wells in order to geologically understand the overpressure mechanisms, to develop PPFG prediction methods, and to evaluate geological risk on top seal capacity for future exploration.

Velocity-density cross-plots revealed a dominant overpressure mechanism of disequilibrium compaction over the study area. Besides, these log (velocity and density) data exhibited two distinctive normal compaction trends (NCT) between highly deformed fold (pre-kinematic) sediments and undeformed drape sediments, which had significant impacts on shale pressure predictions both by conventional Eaton and equivalent depth methods. The estimated shale pressure curves for regional wells showed an interesting association with structural deformation; laterally compressed narrow structures tend to have shallower fluid retention depths (FRD) (i.e., higher overpressure) and gently deformed structures deeper FRDs (i.e., lower overpressure). Estimated fracture gradients, calibrated with measured leak-off pressure, indicated a normal (extensional) stress regime (overburden Sv = σ1) in shallow sections, in contrast to a compressional stress regime (fracture pressure close to Sv) in deeper parts of structures. These variable overpressure and stress regime distributions suggested strong influence of lateral compression (structural deformation) on PPFG development. We also evaluated top seal breach risk at target reservoir based on the fracture gradients and reservoir pressures, the latter of which are often higher than surrounding shale pressures in this region, due to lateral transfer mechanism. This analysis indicated PP-FG relationships control the seal capacities (exploration success) of structures, and thereby demonstrated PPFG is a critical factor for geological risk evaluations of prospects/leads in the study area.