Geological Factors Controlling Gas Volumes in Miocene Carbonate Buildups - Offshore Sarawak

Lawrence R. Smith

Over 170 independent Miocene carbonate buildups are located within the Luconia Province of offshore Sarawak, Malaysia. They vary in size and shape from small isolated pinnacles through tabular to broad low relief platforms, distributed across nearly 10MM acres of the shallow Miocene shelf. Among the 70-odd buildups drilled to date, over half are gas-bearing. Discovered volumes exceeds 40 TCF of gas-in-place, with gas columns ranging from a few meters to greater than 500 m. Geological factors suspected of controlling the occurrence and volume of gas within each buildup include caprock, source, stratigraphic and structural variables. The source for gas is the (largely uncontrolled) Cycle I/II coastal plain sequence. In practice, only caprock and seismically-derived stratigr phic and structural factors provide sufficient geological/geophysical evidence to characterize and distinguish buildups at the prospect level. Useful caprock-related discriminators include clastic/seismic facies, drape, and pore pressure - each reflecting the sealing capacity of the caprock as measured by the gas column height in the reservoir. Factors related to the gas-focusing effectiveness of bedding below the reservoir include the relief and structural configuration of the pre-carbonate section, the areal extent of the drainage area, and faulting. As prospect discriminators, these structural factors are limited by possible velocity distortions and poor seismic resolution beneath the carbonate section. Five gas accumulations discovered by Occidental since 1992 are compared to show th variety of discernible factors controlling their in-place volumes.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California