New Perspective for Malaysian Carbonate Exploration - With a Focus on Central Luconia, Sarawak, East Malaysia

Wan Mohd Zaizuri Wan Embong, Hamdan Mohamad, Nor Kartini Suriati Mansor, and Barry James Boyce
PETRONAS, Kuala Lumpur, Malaysia

More than 70% of the gas discovered in Malaysia is from carbonate reservoirs; and presently carbonates still contribute a large portion of new reserves discovered. The majority of these reservoirs are in Sarawak Basin Central Luconia Province (CLP); and these reefs are primarily middle Miocene CyclesIV/V.
In 1995, less than 30% of the Central Luconia Carbonate (CLC) prospects had been drilled because of ill founded exploration thinking. Downgrading was based upon: i. Carbonate reefs were thought to be small with limited reserves; ii. Most carbonates were believed to be over pressured with small gas columns; iii. Carbonate were believed to contain high CO₂ with significant H₂S concentrations; and iv. Thief sands breached the reservoir seal to allow hydrocarbons to escape from the reefs.
To date, only 45 gas fields have been discovered in CLC while more than 200 identified leads remain to be drilled. Recent discoveries of several reefs with significant gas caused questioning of historical exploration concepts to allow more drilling.
The latest CLP discovery successfully penetrated a 640 meter reef gas column noted the longest single pay column penetrated to date in Malaysia. This normally pressured reservoir had minimal H₂S and CO₂ contaminants and sand thieves were absent. Carbonates breached by thief zones, or transgressive lag deposits, are located inboard.
Future 3D seismic will mitigate uncertainties and delineate many carbonate play types, viz. pinnacle reefs, biostrome platform edges, dolomitization and recrystallization corridors, secondary porosity traps originating from dissolution enhancement and corrosion within the massive platform carbonates. Basically, 3D will increase the possibility of finding TCFs in larger reefs with potentially thick oil rims.

AAPG Search and Discovery Article #90072 © 2007 AAPG and AAPG European Region Conference, Athens, Greece