Understanding Hydraulic Head Levels and Hydrostatic Regimes in Identifying Apparent Subnormal Pressure, Virgin Pressure Variations, and Estimating Hydrocarbon-Water Contact Depth in the Mutiara Field, Lower Kutei Basin, East Kalimantan, Indonesia

Abstract The reservoir in the study area consists of multiple Miocene deltaic sandstones reservoir trapped within the Samarinda Anticlinorium. This research aims to investigate the presence of subnormal virgin hydrocarbon pressures (below hydrostatic pressure, equivalent to 8.3 ppg) in the saddle and northern regions of the field. An additional challenge is encountered in situations where reservoirs data from wells only have hydrocarbons at the deepest levels. Understanding the hydrostatic regime can help in identifying the possible hydrocarbon-water contact depth within the reservoir. The study focuses on three primary analyses: 1) evaluating hydraulic head levels and the hydrostatic regime, 2) mapping hydraulic head levels from the southern to the northern field, and 3) using pressure gradients across multiple reservoirs to estimate the hydrocarbon-water contact depth. The hydrostatic regime was constructed by considering only undepleted pressures, leading to the assessment of hydraulic head depths. A combination of hydrostatic regime, pressure gradient, and log analysis was employed to estimate the possible depth of hydrocarbon-water contact depth. The research utilized data from 1202 pressure tests and fluid analyses from 36 wells spanning the southern to northern Mutiara field. A key finding of the study is that the hydrocarbon reservoir, under virgin conditions, is not experiencing subnormal pressure. Instead, the apparent subnormal pressure is due to the hydraulic head level being lower than the ground level, and in some cases, below sea level. This discrepancy arises from variations in hydraulic head levels across the field. The study also revealed a significant difference in virgin pressures between the southern and the saddle-to-northern regions, driven by differences in hydraulic head levels. Mapping of hydraulic head levels showed a clear trend: the highest levels were in the southern region, the lowest in the saddle, and levels increased again toward the north, reflecting the depth of the top structure of the reservoir. Pressure gradient analysis across data from three reservoirs demonstrated that this method can effectively estimate the hydrocarbon-water contact depth in each reservoir, even when hydrostatic gradient derived from different depth of reservoir. The innovation of this approach lies in its ability to estimate the hydrocarbon-water contact depth without requiring direct water data from the reservoir. This is particularly valuable, as determining the hydrocarbon-water contact depth is essential for in-place volume estimation and perforation interval determination.