IPA12-G-095 PROCEEDINGS, INDONESIAN PETROLEUM ASSOCIATION Thirty-Sixth Annual Convention & Exhibition May 2012 SUCCESS STORY WITH LOW RESISTIVITY SAND IN AN EXPLORATION BLOCK, WESTERN EDGE OF CENTRAL SUMATRAN BASIN Heri Setiawan* Panca Widiantoro* Hendarman* Made Primaryanta* ABSTRACT The exploration area of focus for this study is located at the western edge of the Central Sumatran Basin. Three exploration wells were drilled to 6500 ft (MD). The wells discovered oil in the Early Miocene Lower Sihapas Formation estuarine sandstone. The gross reservoir interval was up to 140 ft. thick, exhibiting low resistivity typically between 6 – 8 Ohm meters. Each well tested up to 3000 BOPD of 44 API oil with zero water and minimal gas. The resistivity of the Lower Sihapas oil-filled sands is lower than the resistivity of underlying Upper Pematang water-filled sands. This paper will explain the exploration and development strategy for these low-resistivity sands, which can be applied to other similar areas. Keywords: Success story, low resistivity zone, exploration block, Central Sumatran Basin INTRODUCTION This exploration block is located at the Western Edge of the Central Sumatran Basin (Figure 1). The Central Sumatran Basin is one of a series of rift basins that occupy a back-arc position along the leading edge of Sundaland. It is the most prolific oil producing basin in Indonesia. Three exploration wells drilled in the same structure all discovered oil in the Lower Sihapas Formation, which is a prolific reservoir in the Central Sumatran Basin. The Lower Sihapas Formation comprises siliciclastic sediments deposited in an estuarine environment. Tidal influence is confirmed from conventional core sedimentologic structures such as ripple current, shale drapes, cross lamination and wavy lamination in Well-X. * Mosesa Petroleum The resistivity of the Lower Sihapas oil-filled sands is lower than the resistivity of underlying Upper Pematang water-filled sands. Fluid type in the sands is confirmed from cuttings, MDT samples and well-tests. This paper will discuss the causes of low and high resistivities in sands, formation pressures, fluid sampling, petrophysical estimates, and the exploration and development strategy applicable to this field and perhaps to similar fields. METHODOLOGY We analysed data from mud-logs, wireline logs, formation pressures, fluid samples, cores, and production tests to understand the cause of low resistivity in oil sands and high resistivity in water sands. This improved our choice of parameters for petrophysical calculations, and helped to define the exploration and development strategy in this field. RESULTS a. Drilling Result Review In May 2011, exploration wells Well-X and Well-Y were drilled in the Z structure. Oil in Zone A was inferred from cuttings and sidewall cores (poor to moderate oil shows), and from MDT samples (100% oil with 44 0 API). See Figure 2 and Figure 3. Conventional core was cut in Zone A in Well-X with 87% recovery. Slab core fluorescence photographs (Figure 4) show that Zone A has poor-moderate oil shows. Routine core analysis revealed porosity in the range of 12-22 %, permeability in the range of 17-1700 md. Special core analysis (SCAL) was performed to determine petrophysical parameters, relative permeability and capillary pressure. Water saturation derived from SCAL was in the range of 20-30%. Unusually, resistivity in the Back to Session