Gas Reservoir Delineation of Pantai Pakam Timur Field, North Sumatera - Indonesia

Abstract

This paper will demonstrate a preliminary synergy of geophysics-geology study in delineating gas reservoir at Pantai Pakam Timur field. The objective is sandstone unit of lower Keutapang formation, on seismic section shown by amplitude anomaly brightspot at 1300 msec. The study is conducted by utilising preserved amplitude of 2D seismic data and well data of four wells. Amplitude distribution, amplitude variation with offset (AVO), and tuning effect are seismic parameter that used for improving accuracy of investigation. Well data contributes in providing pay thickness information, through well log analysis, and production test result of one of the wells for convincing gas presence in this reservoir. Seismic amplitude distribution analysis shows the amplitude value various to the pay thickness of the reservoir. AVO analysis shows strong negative value of AVO gradient on high amplitude area. These phenomena related to the change of porosity or gas content, and then the area with AVO strong anomaly can be mapped as area of gas reservoir. Tuning effect analysis is considered necessary since the seismic data show the dominant period is thicker, while the objective reservoir is thinner. It means the pay of all wells below tuning thickness. In this case, the stronger amplitude may indicate thicker pay. However, another possibility is due to lithology change. Introduction Pantai Pakam Timur field is one of gas producing structures in North Sumatra area, Indonesia, under Pertamina EP Rantau (Fig. 1). A restudies is conducted to delineate another gas reservoir that is known as sandstone unit of lower Keutapang formation (Fig. 2). There are four wells has been successfully produced and only in the last two wells gas is produced from this reservoir. Previously, from the others gas is being produced through the deeper reservoir that is known as Belumai formation. Two dimensional (2D) seismic data are reprocessed using preserved amplitude method to get better seismic data quality. Structural interpretation that has been done using the same seismic data, shows the reservoir is formed by an anticlinal with NW-SE trend (Fig. 3). It consists of two closures, northern and southern, that separated by normal fault in between, while they are both bounded by thrust fault to the West. According to the data availability, synergy of geophysics-geology in planning phase (Fig. 4) is adopted to conduct this study. This phase consists of detailed geophysical works includes interpretation of fine-grid seismic data for improved mapping and aquifer by geologists. Data Preparation Seismic Reprocessing. Two dimensional (2D) seismic data is reprocessed using preserved amplitude technique and the results is very good. This is performed with two main purposes; the first is to get true amplitude value of seismic data, and the second is to save them in SEG-Y format, so that can be used for workstation interpretation. These seismic data come from different vintages (1977–1980). Production geologists here by have been involved in each step of reprocessing as quality controller (Fig. 5). Reprocessed seismic sections have higher resolution than original sections. There are only small mi sties at tie points (Fig. 6). The horizon of gas reservoir is indicated by brightspot seismic phenomenon at 1300 msec with little structural relief. Geological Analysis. Detail correlation of the gas reservoir zone among wells (Fig. 7) shows interesting characters. It is part of sandstone package, which is known as the basal part of Keutapang formation. The basal sandstone package consists three units of sandstone, the lowermost unit is about 23 m, thick and has coarsening upward log type. The middle unit is typically serrated type of log, and mostly has more clay impurities. The upper unit that has blocky to serrated log types, has better reservoir quality than two lower sand units. The gas producer is found in this upper unit. The Keutapang formation in fact has two groups of sand packages. The lower package is where the gas reservoir belonged, while the upper package lies at a depth of 830 m. In seismic profile these two groups of sandstone are easily recognized (Fig. 8). Detail study of all seismic profiles has been done in attempt to delineate these sandstone distribution. We conclude that the sandstone of gas reservoir is more or less spread all over this field, with various thickness, apparently it is thicker on the south and thinner to the north-east. Methodology Three seismic methods are applied in this investigation. They are amplitude distribution, AVO analysis, and tuning effect analysis. P. 507^