CARBONATE DEVELOPMENT ON THE 'TN' FIELD IN THE LEMATANG TROUGH, SOUTH SUMATRA BASIN

摘要

The Early Miocene (N4-N5) "TN" reef complex is one of the seismically well defined Baturaja carbonate build-up located at the local high of the Lematang Trough), South Sumatra Basin, onshore South Sumatra. A different approach was used to solve the problem of determining these carbonate by using 3-D seismic and well log data, fully integrating the interpretation of the seismic facies, acoustic impedance, porosity analysis and lithofacies distribution. Based on the available cores, well logs and seismic data analysis, the Baturaja carbonate sequence can be divided into two sub-sequences: sub-sequence 1 is the carbonate platform facies and sub-sequence 2 is the carbonate build-up facies. The "TN" reef complex can be divided into three lithofacies: coral floatstone - packstone and grainstone facies forming the reef flat environment, packstone - wackestone facies forming the back reef sand environment, and wackestone -mudstone corresponding to a lagoonal environment. The diagenetic processes of the "TN" reef complex plays an important role in maintaining the reservoir quality. The carbonate in this area has the porosity with an average 6.8 % - 9.6 % mouldic / vuggy and intercrystalline, microfracture type porosity in several areas with permeability ranging between 0.32 and 1.68 md it is concluded that the top of the carbonate build-up has medium quality. Based on the acoustic impedance and the porosity, the "TN" reef complex has been mapped in detail by taking 25 milliseconds below the top of the Baturaja carbonate. From those maps, the lithofacies of the carbonate was interpreted. The reef flat facies is dominated by low acoustic impedance (porous lithology; coral floatstone, packstone and grainstone facies), while the back reef sands are dominated by medium acoustic impedance (packstone, wackestone) and the lagoonal facies is dominated by high acoustic impedance value (tight lithology; mudstone and wackestone). The result of the study shows that the Baturaja carbonates have 2 sub-sequences and the distribution of the lithofacies can be determined. The reservoir quality is facies dependent and can be predicted from lithofacies maps. The packstone-wackstone and coral floatstone-packstone-grainstone with mouldic and vuggy secondary porosity constitutes the best conditions for hydrocarbon accumulation.