Abstract

The Pertamina Chevron and Texaco PSC Coastal Plains Pekanbaru Block (CPP) is located in the eastern portion of the Central Sumatra Basin adjoining the Bengkalis-Panjang straits. Fifteen oil fields within the concession are operated by PT Caltex Pacific Indonesia. Our synthesis of CPI's extensive exploration and development data helps delineate structural elements fundamental to the Paleogene and Neogene framework, the order in which they developed, and their influence on localizing source rocks and the migration/trapping of hydrocarbons.

Mapped structures are diverse, multiply reactivated, and ordered. The various geometrical combinations involve extensional rifting, transpressional and transtensional wrenching, and compressional thrusting and reverse faulting. Cenozoic structural development involved three geometrically and kinematically distinct episodes of faulting and folding. These include: 1) Eo-Oligocene rifting (F1) along old basement breaks of N-NNE strike and reactivation of WNW-trending regional basement arches; 2) ±early Miocene (F2) crustal sagging, regional dextral wrenching, local counter-clockwise kinking (15–25°) of older N-S faults, and development of N0-20°E-striking transtensional fracture zones; and 3) Middle Miocene to Recent (F3) WSW-directed thrusting and reverse faulting along older wrench faults of NNW-strike, SSW-verging monoclinal flexuring across older NW-WNW-trending basement breaks, and transtensional rifting along elements of N-NNE-strike. The recognition that fault type, fold style and size of structural closure vary in a predictable manner through time and space has important exploration and development ramifications.

An assortment of structural traps were formed during each of the three respective deformational episodes. The oldest (F1, 45–28 Ma) is related to the N-NNW-directed collision of India with SE Asia commencing at 40–45 Ma. Resulting Eo-Oligocene fracture sets are oriented parallel and perpendicular to the WNW-ESE-trending Indochina-Australian transform plate boundary. N-NNE-directed lateral compression is inferred from the pronounced set of NW-WNW-trending anticlines and pre-Cenozoic paleohighs (F0) that culminate regularly at 35–40 km spacing. These F0-1 elements often have thrusts and reverse faults along their flanks. Coeval F1 extension is recognized in the transverse direction by the shallow NNE-trending Kempas-Sembilan-Padang half-graben system. The much larger N-S-trending Bengkalis trough is primarily extensional in nature with deep source rock depocenters situated laterally to an E-ENE-trending moderate relief accommodation zone. WNW-trending F0-1 nuclear uplifts and NW-trending anticlinal culminations are the most important hydrocarbon traps once overprinted by younger compressional episodes of deformation and produce between 150–750 MMBOIP.

F2 (28-13 Ma) deformation is characterized by sag phase tectonism overprinted by fault and fold styles dominated by right-lateral strike-slip along N-NNW-striking faults and reactivated older F1 extensional faults of N-NNE strike. Individual fault blocks typically have transverse fault spacings of 10–12 km; right-lateral offsets of 5–10 km; and have an accumulative dextral strike-slip of up to 43 km across the block. The right-lateral F2 shear couple accentuated older F0-1 highs. Where dextral motion is transferred to NNE-striking F1–2 fractures, the resulting geometry is releasing and a complex array of listric and planar normal faults, grabens, and half-grabens develop. F2 structural closures are often small, equidimensional, involve non-sealing fault arrays, and trap <50 MMBOIP.

The final F3 (13-0 Ma) episode of major compressional deformation commenced concurrent with active seafloor spreading in the Andaman Sea; regional uplift, major arc volcanism, and N35°W ±10°-striking right-lateral strike-slip faulting along the Barisan Mountains; and penetrative compressional upthrusting across the North and Central Sumatra Basins. Geometric and kinematic constraints suggest that the maximum horizontal stress continued its progressive relative shift toward more northeasterly trends and was NE-SW-directed during F₃ deformation. Compressional F3 structural traps are favorable habitats for moderate-size oil accumulations (50–350 MMBOIP); whereas, transtensional structural traps are small, contain on average 25 MMBOIP or less, and represent about 5% of the hydrocarbons present in the block.