Abstract

Almost half of Indonesia's, and indeed the world's, hydrocarbon production comes from carbonate reservoirs. Carbonate sedimentation was common in the tropical marine waters of numerous basins that developed in SE Asia during the Tertiary. These carbonates form targets for hydrocarbon exploration, since they occur in transgressive successions, often overlying potential source rocks and overlain by sealing lithologies. Carbonate exposures, studied on the Mangkalihat Peninsula in Borneo, were deposited throughout the Cenozoic, in a range of depositional environments. These include mixed carbonate-clastic shelves, a variety of platform top and deeper water platform margin environments. An understanding of the carbonate depositional environments, spatial facies relationships, controls on deposition and diagenesis is essential in order to evaluate the reservoir potential of these carbonates.

On the low energy, inner parts of the carbonate platform, leaching of aragonitic bioclasts resulted in some secondary porosity development (5-15%), but low permeabilities. Along the high energy platform margins well-developed shallow marine cements caused almost total occlusion of primary porosity. The best reservoir characteristics occur in moderate energy platform top lithologies, deposited landward of the platform margin, and redeposited facies reworked from the seaward side of the platform. These lithologies have porosities of 5-35% and moderate to good permeabilities. Good primary porosities result from little micrite in these moderate energy deposits, and leaching of aragonitic bioclasts provides additional secondary porosities. Some development of isopachous or blocky cements renders these lithologies resistant to compaction and therefore poroperms are likely to be preserved during burial. This study stresses the importance of detailed depositional and diagenetic evaluation of carbonates and provides analogue models for subsurface carbonates in the offshore area to the northeast of the Mangkalihat Peninsula.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah