Depositional and Structural History of the Lagonoy Gulf Basin, Luzon, Philippines

Generoso P. Revilla

Department of Energy

Recent seismic data evaluation reveals the presence of about 3.2 km thick Eocene to Recent sediments in Lagonoy Gulf Basin. Three major episodes that affected the basin development were also identified.

The first episode during the Eocene is marked by left-lateral strike-slip faulting that resulted in the formation of a transtensional basin where about a 1.5 km thick sedimentary unit was deposited in southern offshore Catanduanes Island. This event was accompanied by carbonate deposition including reefal development that is also indicated by the seismic interpretation.

The second episode from Late Oligocene to Late Miocene is characterized by an extensional event that resulted in the development of the Basin Fault and small half grabens that served as basins for Oligocene-Miocene rock units. These rock sequences are exposed on the islands of Cagraray, Batan and Rapu-Rapu.

The last episode is marked by the Pliocene tectonic event of transpressional and transtensional phases that resulted in the folding and faulting of the pre-Pliocene rock units. This episode is accompanied by clastic sedimentation and minor synsedimentary faulting.

Two major normal fault systems define the Lagonoy Gulf Basin. The northern boundary is defined by the Minas Fault, which marks the southern boundary of Catanduanes and Caramoan Peninsula. The southern limit is marked by the Batan Fault. The development of a northwest dipping half graben, that is now the present Lagonoy Gulf Basin, can be attributed to the Batan Fault.

There is a petroleum system in the Basin as evidenced by the existence of Eocene and Miocene mature source and reservoir rocks. Source rock maturation and migration occurred in the Pliocene prior to movement of the Batan Fault. It is believed that hydrocarbons are still being generated in the Basin.