STRUCTURE AND HYDROCARBON PROSPECTIVITY OF THE LENGGURU FOLD BELT, IRIAN JAYA

摘要

Regional balanced cross-sections across the Lengguru Fold Belt indicate two stages of Late Miocene-Pliocene compressional deformation, responsible for kilometre scale shortening of the crust. In the Late Miocene, fault-bend-fold and fault-propagation-fold structures developed in the Mesozoic to Tertiary section, just above the top of basement. From the foreland to the hinterland, in the northeast, these structures make up the Strongly Folded Zone, the Imbricate Zone, and the Distal Facies Zone. The formation of structural hydrocarbon traps may have occurred during this deformational regime. The Pliocene event involved inversion of basement, elevating the overlying thin-skinned fold belt. Inversion may have been accommodated along extensional faults previously active during the Late Paleozoic or Mesozoic. Pre-Tertiary extensional faulting is consistent with an abrupt thickening of distal sediments along the proposed shelf edge in the north-eastern portion of the fold belt. Temperature gradients in the fold belt are low such that there is considerable potential for oil in deep traps. However, Pleistocene extensional faults may have breached some hydrocarbon traps. Restored sections reveal burial of the Mesozoic reservoir by 4-5km of Tertiary sediments prior to compression, such that porosity reduction is also a risk. The Lengguru areas with most potential are considered to be those with preserved highs that were not buried so deeply, at the northern end of the fold belt, where sediments onlap onto the exposed Bird's Head basement high. This area is adjacent to the recently discovered ～14TCF Tangguh Gas Project.