Abstract

Seismic amplitudes and well data can be used to make limited but useful lithology predictions in the Ngimbang Formation of the Sakala Timur, Satanger and Sailus PSC areas.

For sandstone, limestone, shale and coal, zonal average's of velocity, density and acoustic impedance in ten wells were crossplotted against maximum burial depth and overpressure. For sandstone, porosity and fluid fill type were also crossplotted. Only a limited range of sandstone porosities depending on depth are discriminable from shale, and fluid fill effects are of the same magnitude as small changes in porosity. Sandstones of economic interest and shale are discriminable from coal or limestone.

Seismic amplitudes in uniformly processed, high quality 1991 seismic data were calibrated by least squares well matching. High quality matches were obtained and showed that the assumptions required of the seismic data for successful acoustic impedance estimation were fulfilled.

Acocustic Impedances were estimated using a modified version of Galbraith and Millington's (1978) method. For approximately 1000 km of the 19298 km of available data, depth dependent compaction corrections were applied to impedance logs extrapolated along seismic structure and highcut filtered to obtain the required low frequency model.

Colour coded acoustic impedance sections were produced by merging the low frequency models with the zero phased and amplitude calibrated seismic data. The results were interpreted using crossplot data and simulations of the effects of limited resolution in support of regional and prospect specific studies.

A blind test successfully predicted a new intermediate porosity sandstone in a well 70 km away from the control well. Sensitivity tests showed that facies changes in the thin beds concerned here are only a weak source of error in the extrapolation because they have only a weak contribution to the low frequency model. Coals are the main cause of ambiguity: subresolved thickness variations between 5 and 10 ft give a range of apparent AI extending up to values expected for high porosity sandstone. Sidelobes with amplitudes up to 2000 ft.gm/s.cm3 can occur immediately adjacent to coals, and this figure is approximately the smallest change in impedance which can be relied on.