Abstract

Structurally, a typical continental rift system is composed of coalescing half-grabens that commonly alternate in the sense of asymmetry along the strike of the rift. Internally this geometry creates horizontal components of motion which may be expressed as shearing and rotation along accommodation zones that separate individual half-graben units. Within an intracratonic setting, sedimentation is typically nonmarine during the period of active graben formation; later regional subsidence may give rise to marine transgression within rifts located near cratonic margins. The best potential source rocks in nonmarine rifts are lacustrine shales or claystones with organic contents that can range up to 20%. The volume of oil generated may be very large for a depocentre of limited areal extent. Long distance migration from oil kitchens (20 km) is not common in continental rift settings. Synrift reservoir quality is highly dependent on local sedimentary provenance and depositional environment. There may be sharp facies variations within the rift, and aspect ratios of these facies may approach 1:1. Seals may be either lacustrine shales or evaporites deposited under hypersaline, closed drainage conditions. Potential traps are series of tilted blocks controlled by commonly listric, down-to-the-basin faults, folds on the down-thrown side of growth faults, intrabasinal highs along accommodation zones, buried paleotopography, horst blocks, drape folds over underlying structure or diapirs, “Sunda folds” over reactivated basement faults, and stratigraphic traps related to lacustrine turbidites or sand pinchouts. Basin size is typically 20–60 km in width and 70–300 km in length. Research indicates that this type of fluvio-lacustrine dominated rift system is present both on and offshore in many areas of the world. Examples are discussed from East Africa and Southeast Asia.