Abstract

The shallow gas zone in the Pantai Pakam Timur (PPT) field, located in Northern Sumatra, Indonesia, has recently become an important target for development. However, only two wells were drilled in peripheral part of the field. In this situation the method of Geostatistics is hardly applied because of less control points, but there is a new suitable method to estimate reservoir properties under the condition of such few control points (GDI: Geology Driven Integration Tool). To compensate few controls, GDI creates pseudo-wells by Monte Carlo Simulation method with regional geological constraints in its regulation, and generates theoretical seismic traces from them. Then the suitable seismic attributes are selected after checking the proportionality with the given reservoir property. Finally the artificial neural network (ANN) is applied to detect the weighting factors, which relate the selected seismic attributes to the given physical reservoir properties. We apply this method to the 2-D seismic records in the PPT field to extract successfully the distribution of porosity and thickness of the gas sandstone reservoir. The most prospected area is figured out in the southern part of the field, where the net thickness of gas zone is estimated to increase 27 meters with fairly higher porosity of 28%, which can be fairly confirmed by the well proposed and drilled by this study. Once getting the distribution, it is easier to calculate the total rock volume of the target reservoir under non-homogenized situation, and hence to progress on estimating more precise volume of reserves in place. Thus this method has an advantage in estimating reservoir characters from a few well data in the early development stage, or even in the late exploration stage. It is certainly important for asset managing that new idea should save the cost even in the stage of exploration.

Introduction

The Pantai Pakam Timur (PPT) gas field is located about 17 km to the Northwest of Medan, Northern Sumatra (Fig. 1), and was discovered in 1979 where two main reservoir horizons were recognized. It was under development mainly for the deep reservoirs for which some economical difficulties have been found in the later stage. The total of 6 wells have been drilled since 1979 in the study area,.

In recent years, the shallow gas zone (called here as 1275 m zone) becomes important as a developmental target, even though the reserve may be critical. The wells PPT-5 and PPT-6 were drilled at the almost same locations as PPT-2 and PPT-1, respectively, for developing the shallow gas reservoir as a target. These wells were successful in finding gas columns of 5 m+ at PPT-5 and 19 m at PPT-6. However, we need more development wells to be drilled for efficient recovery.

This is the most suitable setting to apply Monte Carlo Simulation for estimating the reservoir characteristics over the field, where the usual geostatistics method cannot be applied because of too small number of known data points. The objective of this study is to estimate lateral distribution of reservoir properties, using seismic data as useful guide in interpretation under this condition. Reservoir characterization software (de Groot et al., 19961) was applied for this purpose, and the reserves will be re-evaluated.

Geological Background of PPT Field

Seismic facies of the target gas reservoir has to be characterized and correlated over the field with consideration of the regional litho-facies in mind. To do this, the synthetic seismograms of the wells PPT-6 are over-plotted on the appropriate seismic sections (Fig. 2) to find out the suitable seismic wavelet for the study.

On the seismic sections there is a small fault between PPT-5 and PPT-6. However, the reservoirs between both can be believed to communicate each other, hence it is interpreted that blocks have a same gas/water level. Therefore, we neglect the fault on the following maps for this study.

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