APPLICATION OF SEISMIC ATTRIBUTES FOR RESERVOIR CHARACTERIZATION, SOUTH PATTANI BASIN, GULF OF THAILAND

Abstract

Reservoirs of southern margin of Pattani Basin are mapped during company regular exploration evaluation but have not been looked into from a research oriented view. This study attempts to define the subsurface reservoir characters within Lower Miocene to Upper Miocene interval using 3D seismic attributes and well-log data. Reservoirs in this area are sometimes thin and have limited lateral extent which makes it difficult to predict the geometry and distribution of them. To predict sand character advanced imaging techniques such as : RMS (Root Mean Square) amplitudes, Spectral Decomposition and incoherency are used. At shallow stratigraphic levels, RMS volumes successfully resolved sand body geometries. But below 2000 meter due to a reduced acoustic impedance contrast between sand and shale RMS did not show distinctive images. On the other hand, using Spectral Decomposition CWT (Continuous Wavelet Transform) volumes can improve the image at deeper stratigraphic levels. It was also discovered that CWT outputs at 20 Hz can resolve thicker sands (>20m), while relatively thin beds and subtle features can be detected by higher frequencies. Therefore, these RMS and Spectral Decomposition imaging techniques were used jointly to examine the character of sands at different levels. Identified meander belts are N-S or NW-SE oriented and width of the belts vary from 200 m to 3 km. Based on the studied character of the sands they were divided into upper, middle and lower sand system. Upper sand system has well developed single meander belts in the above part and laterally migrated stacked sands at places in the lower part. Middle sand system shows more open marine influence as sands are distributed in a more scattered way with low sinuosity and as coal was observed in well logs. Lower sand system has thick stacked sands with moderate to low sinuosity and may have resulted from vertical aggregation of sands due to increased accommodation during syn-rifting.