Pre-drill predictions versus post-drill results: use of sequence stratigraphic methods in reduction of exploration risk, Sarawak Deep-water Blocks, Malaysia

Limited well control, great distances from age-equivalent producing fields, and a largely unknown stratigraphy necessitated use of sequence stratigrap hic methods to assess exploration risk associated with reservoir, source an d seal distribution in the Mobil-operated Deep-water Blocks of Sarawak, Mal aysia. These methods allowed predictions to be made and reservoir risks to be halved in each of the locations drilled in 1995. Predictions regarding r eservoir and stratigraphy proved correct, as the Mulu-1 and Bako-1 wells pe netrated numerous high-quality, thick sandstone reservoirs in the Middle to Lower Miocene section. Shallow marine sandstones dominate the vertical suc cession in both wells, with characteristic aggradational, upward-coarsening log motifs, Cores display classic wave-generated stratification and hummoc ky cross-bedding. Evidence, such as marginal-marine to neritic microfauna i n cuttings of both wells, supports these interpretations. Lack of hydrocarb on charge in the two wells may be due to their position relative to coaly h ydrocarbon source beds. These prospects have high trap and seal integrity, being well defined on seismics as high relief horst blocks covered by a ver y thick shale-prone section. The Mulu-1 well, for example, is located at le ast 20-30 km down stratigraphic dip from mapped coeval lower coastal-plain deposits. Amplitude anomalies on the flank of the Mulu horst are probably d erived from transported organics buried in deep Plio-Pleistocene kitchens i n the northwest portion of the Mobil blocks. Remaining potential of mapped prospects is high and efforts continue at characterizing the petroleum syst em of the Deep-water Blocks. Seismic attribute and interval velocity analys es provide new clues to the location of probable coaly source rocks, especi ally when viewed in their regional and sequence stratigraphic context. Futu re work is planned and will serve to reduce risk to acceptable levels and s upport further drilling in this prospective hydrocarbon province. (C) 1999 Elsevier Science Ltd. All rights reserved.