Abstract

Three major Tertiary tectonic and stratigraphic episodes (F1, F2, and F3) characterize the sequential development of the Balam Depocenter.

Eocene (?) and Oligocene rifting (F1, ±45–25.5 Ma) resulted in an integrated set of N-NNW-trending half-grabens. In detail, the Balam depocenter is highly compartmentalized. Rift margin (border) faults strike either N-S (Manggala) or NNW-SSE (Jakun and Balam) and dip E-ENE at low-angles. The ENE-trending Antara-Nella Accommodation Zone (ANAZ) reflects the incomplete transfer of displacement between the Jakun and Balam border faults. The ANAZ subdivides the Balam Depocenter into shallow northern and deep central compartments. The distribution of reservoirs and source rocks within the basin are compromised by syndepositional activation of the ANAZ and F1 uplifts. Similarly, the Menggala South-Kopar Accomodation Zone (MKAZ) in the south subdivides Balam and Rangau Troughs.

The regional base Miocene unconformity marks the beginning of F2 tectonism (25.5–13.8 Ma). It cuts indiscriminately across bordering basement platforms as well as late F1 inversion structures and grades laterally into sag disconformities above F1 graben thicks. Marine and non-marine sequence stratigraphic models are necessary to understand the development of reservoir and seal geometries of these sediments. Isopachous and lithofacies distributions are suggestive of a N-S-trending incised braided stream system.

The final episode of structuring (F3, 13.8 Ma-Recent) is linked to widespread compressional inversion of specific F0-1-2 fault and fold trends across the Balam backarc setting. Inversion is also focused along framework elements of the WNW trend. The giant Bangko and Balam South fields are results of this F3 structural episode.

Fine-tuning of the Balam tectonostratigraphic model helped define, delineate, and predict several new unconventional hydrocarbon leads and prospects. A thorough understanding of multiple superimposed structural episodes and sequence stratigraphy is mandated in order to substantially reduce exploration risk.