Fractured Basement Reservoirs Characterization in Central Sumatera Basin, Kotopanjang Area, Riau, Western Indonesia: An Outcrop Analog Study

Holis, Zaenal¹; Sapiie, Benyamin²
¹Exploration Division, BPMIGAS, Jakarta, Indonesia.
²Geological Engineering Department, Bandung Institute of Technology, Bandung, Indonesia.

Fractured basement reservoirs in Central Sumatera Basin would be a future Indonesian oil and gas development. An analog outcrop model need to be conducted as a first step toward predicting subsurface fracture system attributes. Kotopanjang area was chosen for this study because of good quality outcrops of pre-Tertiary basement of Central Sumatera Basin. The Central Sumatra Basin is one of the most hydrocarbon prolific Indonesian Tertiary back-arc basins today. It was formed as pull-apart basin related to NW-SE trending dextral strike-slip faults. It experienced three tectonic deformation phases: Mesozoic compressional, Eocene-Oligocene extensional, and Pliocene-Pleistocene compressional tectonic. The objective of this study was to conduct fractures characterization on basement outcrops and to obtain empirical and functional relationships between the fracture attibutes.

The methods used in this study included field geological mapping and scanline sampling, data sorting, data calculation, statistical analysis, and interpretation. Scanline sampling was conducted on pebbly mudstone (Carbon-Early Permian). Data calculation results were plotted in graphical form, analyzed statistically, and interpreted geologically. The result would be useful in predicting: (i) fracture zone width, (ii) geometry of fracture zone, and (iii) fracture porosity and permeability.

The study area is dominated by NW-SE and NE-SW trending basement structures. The fractures observed in study area include fault-related fracture system. Two damage zones can be observed in scanline sampling: Damage Zone#1 and Damage Zone#2. Damage Zone#1 shows that the fractures are related to dextral strike-slip fault. Damage Zone#2 shows that the fractures are related to normal fault. Both of damage zones indicate several high strain zones with average intensity 3 to 5 fractures per meter. Rose diagram demonstrated three main fractures orientation: NE-SW, NNE-SSW, and WNW-ESE trending fractures. NE-SW trend consists of two average strike orientations N 215 °E and N 235 °E represented by conjugate fractures system. NNE-SSW trend with average strike N 185 °E is represented by joints and veins. WNW-ESE trend is represented by veins. It can be summarized that main stress controlling all the fractures is NNE-SSW direction. Fracture spacing, length, and thickness cumulative distribution plot demonstrated that all of fractures follows Power-Law distribution with fractal dimension (D) 1.0 to 1.94.

AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012