Mindanao is part an island arc system which includes the Philippines and eastern Indonesia. The geologic structure of the Island is reflected by the physiography. The structural features in eastern Mindanao are northerly aligned and are strongly influenced by the active Philippine rift; those of western and central Mindanao reflect a merging of diverse tectonic and volcanic trends. The mountainous areas consist chiefly of basement and Tertiary volcanic rocks, whereas intervening lowlands are underlain by Tertiary sedimentary rocks. There are two major sedimentary basins-the Agusan-Davao trough and the Cotabato basin-which contain 6,000-15,000 feet of marine strata of geosynclinal origin. The stratigraphic succession is composed mostly of Miocene and Pliocene rocks which a e characterized by abrupt lateral and vertical lithologic changes. In addition, a thick Eocene limestone is present in the Agusan-Davao trough. There are possibilities for commercial hydrocarbon accumulation in both basins. The sedimentary section contains probable source,cap, and reservoir rocks, and potential structural and stratigrahic traps are present.