Abstract

Early exploration efforts in the Barito Basin focused on the large surface defined thrust structures in the northern areas. Despite the large initial discoveries in the Lower Tanjung Formation of the Tanjung Oil Field in 1939, no sustained exploration program has focused on this known prolific sedimentary section, and the poor understanding of its stratigraphy has produced only limited successes since.

Recent exploration efforts have defined a period of early Tertiary rifting giving rise to a series of northwest to southeast trending horsts and grabens across the basin. These early Tertiary structural elements have been overprinted by a Neogene compressional regime which continues to the present day. This more recent compression has produced a left-lateral reactivation of the earlier normal faults, giving rise to the recent structural configuration of the basin.

Major thickness and facies changes with four distinct stages of deposition can be recognized and correlated across the basin in the Tanjung Formation. These variations result primarily from the topography produced by the early Tertiary rifting.

The terrestrial coals and organically rich shales of the Lower Tanjung Formation are prolific hydrocarbon source rocks, and have produced the oil emplaced in the Tanjung Field. These source facies are at optimum maturity levels over large areas of the basin.

At least five of these early Tertiary rifts have been identified. In terms of migration and entrapment, each must be considered a separate self-contained basinal depocenter. Only the Tanjung Field graben, with over 600 MMBOOIP, has been adequately tested.