Publications

Significant shale diapirism in the Baong Formation has been observed in the seismic data and in the outcrops in the northern onshore of the North Sumatra Basin. These diapirs appear to be forming from the deep formations (2000 m+) underlying the upper Miocene to Pliocence Keutapang sandstone packages. Shale diapir is characterized by the appearance of chaotic seismic reflectors that intrude on the Keutapang and Seurula Formations up to the surface as nonconformities. At the bottom of the structure, the shale diapir (> 3000 ms), interpreted as strong reflectors in the Belumai Formation, are relatively undisturbed by tectonic movement. The Belumai Formation clastic rocks with a carbonate platform are considered the slip plane of the shale diapirism deformation processes. Outcrop observations in the field indicate that the Baong Formation consists of strongly deformed mudstone (scaly clay), inserted as thin sandstone, that is likely splitting (fissile) and prone to deformation. The shale diapir structures are associated with transpression tectonic activity on the age of the Plio-Pleistocene in the North Sumatra Basin. The Baong Formation has been interpreted as marine sediments and organic rich material. Oil seepage was found at the top of the anticline where the Baong mudstone formations were exposed at the surface. Several wells were drilled on the anticlinal flank targeting the lower Keutapang sandstone but problems occurred when the wells penetrated the thick shale of the Baong Formation to a depth of 800–1000 m TVD and the mudstone was the dominant lithology in the objective zone. Based on seismic and well log data, subsurface geological maps, and oil seepage findings, appropriate modeling has been conducted to determine the structural restoration of the shale diapirism. Some structures are potential candidates for * PT Pertamina EP nonconformity correlation to the reservoir trapping of 750–2000 m, with a dominant composition of clay, shale, and mudstone. Mulhadiono et al. (1982) Middle Baong sandstone and Kower Keutapang sandstone. Many of the compressional ridges from the Sumatra subduction zone and reactivated fault trends may, in fact, be cored by the ‘shale flowage’ of the Baong Formation. Much of this diapirism seems to have been active from the Late Miocene to the Pleistocence period. The reconstruction of the three-dimensional model of the seismic and geological maps has allowed for a complete dimensional image that has an afforded a comprehensive understanding of the evolution and possible hydrocarbon trapping capacity of the shale diapirs in the North Sumatra Basin.