RESERVOIR CHARACTERIZATION AND COMPARTMENT IN POSTRIFT DEPOSITED, UPPER TALANGAKAR FORMATION, BELUT FIELD, SOUTH SUMATRA BASIN

Abstract

Belut field is one of Pertamina EP assets which has been in production for more than 50 years. This field is located about 120 km west of Palembang City, South Sumatra. The main reservoir is sandstone and limestone which was deposited at the beginning of the Miocene and is included in the upper part of the Talangakar or Transgressive Member (TRM) group. The discovery of hydrocarbons in a more down flank (north) area originated from step out drilling in the study area. Based on the drilling, oil is obtained from the sandstone reservoir with a greater pressure than in the existing area. Currently, Belut area is already in the development stage, so a more comprehensive reservoir model is needed for further development strategies. The data used in this study are core rock data from three wells, wireline data from 47 wells, fossil analysis, borehole images, and three-dimensional seismic data (3D Karbela) as well as reservoir pressure data. The first stage of the research was to determine lithofacies and facies associations from core rock data and biostratigraphy. Furthermore, determine parasequence marker ce and sequence by the electrofacies pattern for each well. Sequence stratigraphic correlation was performed to determine the distribution pattern of depositional facies. Reservoir characterization in three dimensions was carried out by modeling facies, shale volume, porosity, permeability, rock type, and saturation. Based on the correlation of sequence stratigraphy and seismic stratigraphy, the lateral distribution of the facies indicates a southward basinward direction. In this study, the southern area will be dominated by distal or marine deposits. On the other hand, the further north, terrestrial deposits will be found which are more sand prone in nature. An increasing in sea level during filling of basins (retrogradational) causes changes in facies vertically, hence it made channel, delta, to shallow marine deposition systems. The pressure data shows the northern part (after datum) has a pressure of 0.85 - 0.99 (SG equivalent) while the existing area has a pressure of 0.67 - 0.78 (SG equivalent). The difference in pressure is due to differences in genetic facies in the same sequence between the northern and southern areas. Based on the analysis of formation pressure and the discovery of hydrocarbon potentials in the down flank area, it shows that there is a compartment between the north and south areas (stratigraphic trap).