Geological Modeling of Carbonate Reservoirs

For JAPEX, Southeast Asia is one of the key regions for further exploration and development of oil and natural gas. An understanding of these sedimentary and diagenetic systems is essential for an accurate assessment of the properties and distribution of the abundant Neogene carbonate reservoirs in this region.

For five years from 2011 to 2015, we conducted an analog study in collaboration with Kumamoto University using boring cores and land surface samples collected from the Daito formation on Minami-Daito Island, Okinawa Prefecture, to gain insight into carbonate sedimentary and diagenetic systems.

Study Summary

The properties and distribution of carbonate rock reservoirs in Southeast Asia are complexly controlled by sedimentary and diagenetic processes. In order to understand the sedimentary system and lithological distribution, we chose to study Minami-Daito Island, Okinawa Prefecture, which has geological characteristics similar to carbonate rock reservoirs in Southeast Asia.

Minami-Daito Island is known as an oceanic island consisting of carbonate rocks of coral reef origin. It has a mortar-shaped topography consisting of a central lowland with an elevation of less than 30 meters and a ring-shaped plateau with a relatively high elevation. Most of the land surface to shallow subsurface of Minami-Daito Island is covered by the Upper Miocene to Pliocene Daito Formation, which originates from coral reefs.

Research Assumptions and Methods

• Use boring cores collected from four 50-meter-deep wells (MD-1 to MD-4) drilled on the island by Kumamoto University in 2013.
• Conduct petrological and geochemical analyses of sedimentary rocks, including core description and thin section observation, X-ray diffraction analysis, carbon and oxygen isotope analysis, and strontium isotope analysis, and comprehensively review the results.
  

Results of Analysis and Study

Sedimentary facies of the Daito Formation in Minami-Daito Island

The Daito Formation on Minami-Daito Island is composed mainly of shallow marine carbonate rocks, which can be divided into the following five lithologies (homogeneous or similar groups). Furthermore, from the combination of these lithologies, it was found that the Daito Formation can be subdivided into four sedimentary facies: the lagoon facies, the back reef facies, the reef facies, and the front reef facies.

1. Branching coral bafflestone /floatstone
2. Bioclastic grainstone/packstone
3. Rhodolith floatstone
4. Coral framestone /rudstone
5. Coralline algal bindstone

Sedimentary and Diagenetic Systems on the Daito Formation in Minami-Daito Island

The results of the study indicated that the morphology of the coral reefs comprising the Daito Formation on Minami-Daito Island may be closely related to relative sea-level changes. The study also clarified the distribution and formation time of dolomite, which affects the reservoir rock properties.

• The Daito Formation on Minami-Daito Island is divided into three sedimentary units: Unit I, Unit II, and Unit III. Each unit has an asymmetrical distribution of lithology that reflects the prevailing wind direction (the wind direction that blows most frequently during a particular period in the region).
• Based on the distribution of sedimentary facies and the morphology of the sediment, it can be assumed that Unit I and Unit III developed as atolls, whereas Unit II developed as a coral reef similar to a table reef.
• The coral reefs in Unit I and Unit III are thought to have grown upward in relation to the rise in relative sea level. In contrast, the reefs in Unit II grew laterally due to the stagnation of the relative sea level.
• Multiple dolomitization events occurred during or after the sedimentation of Units I-III. The dolomite content of the upper dolomite is higher on the ring-shaped plateau than on the central lowland, suggesting that dolomitization progressed mainly on the east coast (where the prevailing winds hit).

Sedimentary units and distribution of sedimentary facies in the Daito Formation on Minami-Daito Island (Source: *1)

Sedimentary units and dolomite distribution in the Daito Formation on Minami-Daito Island (Source: *1)

Research Results

The research has revealed the following three points about the Daito Formation on Minami-Daito Island:

1. Upper Miocene to Pliocene carbonate rocks originating from coral reefs have developed with changing reef morphology, reflecting relative sea-level changes.
2. Sedimentary facies and dolomitization are complexly distributed, reflecting coral reef morphology and prevailing winds.
3. Reconstruction of sea-level changes and marine environment at the time of sedimentation is important for the study of carbonate sedimentation systems.

These observations will be applied to the Kangean project in Indonesia, which we are participating in, as well as to the exploration and development of carbonate rock reservoirs in Southeast Asia, which were formed in a geological setting similar to this area.