ARCHITECTURAL AND CHARACTERISTIC BASEMENT FRACTURES IN THE WESTERN INDONESIA
Benyamin Sapiie, Alfend Rudyawan, Indra Gunawan

Prodi Teknik Geologi ITB

Abstract

Fractured reservoirs present challenging problems to the petrophysicist and reservoir engineer. In fractured reservoirs, the oil and gas occur within an extensive fracture network on various scales rather than within the matrix porosity of the formation. As a result, fractured reservoirs are more complex and expensive to evaluate than conventional ones. Therefore, an outcrops study can help ramify some of the issues, particularly regarding physical properties, geometry, and distributions of the fractures. This paper delivers the results of extensive fieldwork of fractures characterization and mapping in the basement rocks, particularly granitic complex across Sumatra, Batam, and Bangka Island. The study covers detailed structural analysis, petrography, and porosity measurements. as well as comparing the results to several basement fractures reservoir fields in the Central and South Sumatra Basin. Fractures are observed in granite to granodiorite, andesite to basalt, metamorphic rocks such as phyllite, slate, metasediment, and sedimentary rocks, including limestone, sandstone, and mudstone. Fractures attributes such as length, spacing, and apertures were observed systematically using scanline and window mapping. In addition, complex cross-cutting relationships were carefully mapped to have a better understanding in terms of deformation sequences. The primary orientation varies from each lithology and age. However, the majorities are NE - SW, NW - SE, E - W, and N - S trends, mostly like known pre-existing faults orientation with the basement rocks in the subsurface. Within the same tectonic block, fracture density tends to be greater in metamorphic rocks, igneous rocks, and sedimentary rocks. It also varies within the same lithologies with different ages, indicating different phases of tectonic deformation. Fracture intensity and density also decrease relatively from the eastern part of Sumatra to the western part. However, they are comparably high in the proximity of the suture zone and regional faults. Based on the measurements on granitic rocks, fracture intensity and density tend to be higher in rhyolite or aplite than in granite. This evidence specifies the role of grain size and textural composition on intensity and density and that rocks with the same composition will have higher intensity in the finer-grained rocks. Fracture density numbers on granitic rocks are higher than those observed in both outcrop and subsurface, indicating lithology control dominates fracture behaviors.

Keywords: Fractured Basement Reservoir, Fracture density, Fractures intensity

Topic: Solid Earth Sciences