Exploration and production companies in the South China Sea region have focused increasing attention on basement reservoirs. In the current study, a modest step was to taken to identify the fracture pattern and its associated physical properties, which help provide the potential zone of the formation sampling and pressure gradient analysis. An exploratory well located offshore Sarawak, Malaysia recently encountered oil in the meta-sediment basement—confirming the basement plays in this region.
Borehole microresistivity images and acoustic data with background geology characterized rock texture, lithofacies, and fracture network. Integration of sidewall core and microresistivity image data characterized litho-units having complex architecture. Open fracture zones were validated in images and acoustic logs. Borehole image-derived fracture properties calibrated with sonic-derived results were used to estimate gross voidage. Average porosity in fractured basement rock is usually low; hence, fracture transitivity/permeability is important for estimating fluid movement. To characterize basement rock, fractural properties and geometry, such as permeability, aperture, density, and flow potential, should be attributed properly with respect to the different facies association present.
The exploratory well was mainly drilled through the quartzite and phyllite section. Microresistivity image data helped decipher the different texture patterns with the two litho-units and also identify the internal structural features associated with each. The major types of structural features were identified, including partial fracture, open fracture, and closed fracture. Statistical analysis showed the major strike orientation for the open fracture was toward the NW-SE; whereas, the minor trend was NNE-SSW with a mean dip of 45° magnitude. Partial fractures also exhibited a similar trend, with the major strike NNE-SSW and minor trend NW-SE with mean dips of 52° magnitude. Average open fracture aperture was determined to be in the range of 0.2 to 0.4 mm, with an approximate secondary porosity range between 2 to 3% and permeability of 0.01 md.
This was the first integrated approach for evaluating a meta-sediments basement in the offshore Sarawak area. This methodology emphasized advanced integration and is essential for evaluating such complex reservoirs, providing a stepping stone for future well trajectory planning in the study area.