Evolution of a Forearc Basin, Luzon Central Valley, Philippines

Abstract

The Cenozoic history of the 14 km-thick Luzon Central Valley sequence illustrates the development of a forearc basin. Forearc basins are important both as major sediment traps and as sites of hydrocarbon accumulations. The Luzon basin is floored by oceanic crust on the seaward (western) side and older accreted terranes on the arc (eastern) side. Initial sedimentation on this oceanic crust occurred during early Tertiary northward translation and emplacement of the crust as an ophiolite along a strike-slip or oblique-slip zone. The basal sediments consist of pelagic limestones and thin ash layers overlain by sandy turbidites derived from uplift and progressive dissection of the ophiolite. A sequence of arc-derived sediments at least 26,000 ft (8 km) thick was shed into the astern (arc) side of the basin during late Paleogene to Quaternary convergence along the western margin of Luzon. By the middle Miocene, the Central Valley became a continuous, elongate basin fringed by extensive shelf deposits along both the uplifted seaward and arc sides of the basin. Detritus shed from both flanks filled the subsiding basin and resulted in progressively shallower depths. Nonmarine deposition began in central portions of the basin in the Pliocene and migrated with time both north and south along the basin axis. Late Miocene to Holocene movement along the Philippine fault zone caused uplift and folding of adjacent parts of the basin. Exploration models for the Central Valley predict gas-prone hydrocarbon generation in central portions of the basin at times that coincide with and postdate the formation of both structural and stratigraphic traps. Previous drilling in the basin has either been in areas with thermally immature source rocks or has failed to reach prospective intervals where thermal maturation is inferred. The hydrocarbon potential of the Central Valley has not been determined adequately.