FACIES CHARACTERIZATION OF CARBONATE RESERVOIR, BATURAJA FORMATION, PASIR BUNGA FIELD, MUSI PLATFORM, SOUTH SUMATRA BASIN, SUMATRA INDONESIA

• Main Author: NENGAH SADIARTA, I
• Format: Theses
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/69658
• Institution: Institut Teknologi Bandung

Seven wells (PB-1 – PB-6 and PB-8) in the Pasir Bunga field have been used for studying of facies characterization on carbonate reservoir of Baturaja Formation. Pasir Bunga field based on its oil water contact (OWC) can be divided into northern and southern parts in which the northern part has 41.6 feet-thick oil column and the southern part has around 27.5 feet-thick oil column. The limestone of the Baturaja Formation is characterized by four types of carbonate facies that are coral boundstone (coral framestone), packstone (coral rudstone and foraminiferal packstone), wackestone (coral floatstone) and mudstone. Bioclasts are dominated by corals and followed by red algae, echinoderms, bivalve/molluscs, larger foraminifera, small benthonic foraminifera, planktonic foraminifera, brachiopods, bryozoan, ostracods and other indeterminate type. Planktonic foraminifera are only present in the wackestone and mudstone from open marine platform. While coral rich-packstone, wackestone and coral boundstone were deposited in a complex of carbonate mudbanks draped over an open marine platform (mostly Lower part of Baturaja Formation) and a low relief basement high during Early Miocene marine transgression. v Based on GR log characters, Baturaja Formation can be divided into Lower and Upper Baturaja Formation. Further analysis on log characters is focused on Upper Baturaja Formation and reveals that Upper Baturaja Formation can be divided into three depositional cycles, which are marked by shale breaks or maximum flooding surfaces. Diagenesis study on cored intervals in the PB-2 (interval 3144’-3276’) and PB-4 (interval 3162’- 3289) wells shows that the diagenetic environments can be divided into freshwater phreatic zone in the upper part and mixing zone in the lower part. The diagenetic environment on cored interval in the PB-3 (interval 3216’-3228’) is generally of freshwater phreatic zone with locally mixing zone environment. Measured porosity on core #1 and core #2 of the PB-2 well is variable ranging from poor (7%) to excellent (37%), however the mean measured porosity is very good (23%). Permeability measurement results on the corresponding samples are also variable ranging from very low (0.029md) to high (919md). While visible porosity by thin section observations is also variable ranging from poor (2%) to excellent (32%). Measured porosity on core #3 of the PB-2 well that is dominated by intercrystalline pores ranges from poor (9.4%) to excellent (30.3%), with mean porosity is very good (24.2%). Permeability measurement is generally very low; but moderate permeability (80.7md) is measured at sample 3275’. Measured porosity in the PB-3 well is in the order of good (17.2%) to excellent (36.2%), with permeability in the order of low (1.47md) to moderate (167md). Visible porosity by thin section observations on some corresponding samples is in the order of 20 to 40%. Measured porosity in the PB- 4 well is variable in the order of very poor (3%) to excellent (35.3%), however, the mean porosity is very good (29.2%). Permeability measurement ranges from very low (0.003md) to moderate (84.8md). Visible porosity by thin section observations is variable, ranging from nil to 35%. The discrepancy between measured porosity and visible porosity by thin section observation can be explained by the presence of microporosity associated with lime mud matrix that is below the resolution of the petrographic microscope. The pore system in the PB-2 and PB-4 is dominated by moldic porosity, followed by variable amounts of vuggy, intercrystalline, interparticle and fracture porosity. Interparticle pores are dominant in the packstone facies in the PB-3 well. vi The main diagenetic factors that have enhanced the value of porosity and permeability in the limestone from the Upper Baturaja Formation are leaching of skeletal aragonite (corals), brecciation and fracturing, and leaching of calcite with enlarging of pores and widening of fractures. Otherwise, the diagenetic factors that have reduced the value of porosity and permeability are pressure solution, coral mold collapse and cementation by calcite. In general, the reservoir quality of the limestone from the Upper Baturaja Formation has been mostly experienced by diagenesis rather than facies. However, facies plays a role in enhancing the intensity of the diagenesis.