Relationship between acoustic impedance and porosity of Pratu Tao Formation in the Greater Pratu Tao area, Phitsanulok Basin, Thailand

Abstract

The Pratu Tao Formation is a tertiary formation of the Phitsanulok Group located in the central plain of Thailand. Thai Shell Exploration and Production Co., Ltd. was granted the S1 concession in March 1979, covering part of the Phitsanulok Basin. The exploration results presents that the Pratu Tao Formation has been proved successfully as a potential petroleum reservoir. This research aims at quantitative and qualitative analyses of the relationship between acoustic impedance and porosity of Pratu Tao Formation. The data employed in this study is composed of 11 wells with wire-line geophysical logs of gamma ray, density, neutron and caliper. The 3D seismic data with pseudo acoustic impedance (Runsum) volume is also employed in this study. The logging data acquired in 1980 s have been limited in use due to poor data quality cause from poor borehole condition, whist the other good data acquired in 2001 are appropriate for this study. The result of quantitative study presents that there is a linear relationship between acoustic impedance derived from the wire-line geoplysical logging and porosity data. However, no linear relationship was observed between pseudo acoustic impedance (Runsum) from seismic and porosity data, the results indicates that the use of Runsum of predict a reservoir distribution cannot be applied in this seismic survey. The result of qualitative study reveals that the values of plotting acoustic impedance versus depth in the X-Y pattern can be used to distinguish lithlogical units. From such new result, the Pratu Tao formation can be subdivided into 4 distince units (unit 1, 2, 3 and 4) based on slopes of shale impedance trrend. Thsi can be used as the supporting information for further detailed study of depositiongl environment and sequence stratigraphy of the Pratu Tao Formation.