Lithogeomorphological facies analysis of Upper Miocene coal-prone fluviodeltaic reservoirs, Northern Malay Basin

Abstract

The Group E stratigraphic unit is a significant gas producer in the Northern Malay Basin. However, due to the thinly bedded nature of the sandstone reservoirs, thick shale, and abundant coal beds, accurate seismic attributes interpretation of lithology and fluid prediction has been a daunting task. To address this problem, we have conducted an integrated seismic sedimentology workflow using spectral decomposition, color blending, waveform classification, prestack seismic inversion, and stratal slicing to characterize the lithogeomorphological facies of the coal-bearing reservoirs. On spectral decomposition and waveform classification maps, we clearly identified depositional elements such as the distributary channel, distributary mouth bar, subaqueous levee, and interdistributary fill. We computed the elastic properties through prestack seismic inversion to obtain good lithology discrimination between coal and gas-charged sandstone. Both lithologies are characterized by low acoustic impedance, but the compressional to shear velocity ratio ([Formula: see text]) of coal is high compared to gas-charged sandstone. The current interpretation indicated that the Group E interval was deposited in a delta plain setting. The varying flow directions of the distributary channels in the area support the hypothesis that describes the Malay Basin during Miocene time as a narrow gulf, connected to an open sea to the south and flanked by deltas and fan deltas.