Bulletin of the Geological Society of Malaysia, Volume 42, Dec. 1998, pp. 1 - 13
Sarawak Shell Bhd., EPG-PRO, Locked Bag #1, 98009 Miri, Sarawak
Abstract: The Luconia Province, offshore Sarawak, Malaysia, is one of the largest SE-Asian carbonate provinces with more than 200 platforms and gas reserves exceeding 40 Tscf. The application of the Sr-isotope technique integrated with core, log and seismic data significantly refines the stratigraphy of the province despite extensive diagenetic alteration. Based on the new data, a unified concept of platform evolution is tied to global sea-level variations and the regional distribution of reservoir architecture. Evidence is presented for major karstification during and after platform growth and its influence on the regional distribution of reservoir properties.
Sr-isotope analyses (ntotal = 137; data range: 0.7084-0.70903 for NBS987 = 0.710230) suggest that the carbonate platforms of the Luconia province are Early to Middle Miocene in age. The Sr-isotope signature is apparently unaffected by diagenetic stabilization from metastable carbonates to low-Mg calcite while dolomitization caused alteration of the isotope ratio. Growth and demise of the province can be correlated with a second-order eustatic sea-level cycle (TB2). Major karst horizons and flooding, aggradation and progradation packages are linked via step-changes in isotope signature to third order eustatic sea-level fluctuations. Simultaneous with the second order sea-level drop (late Middle Miocene) the influx of siliciclastics split the province into a southern part with low relief carbonate banks and a northern part with high relief platforms. All growth terminated at the end of the Middle Miocene. Low relief banks were buried while high relief platforms were karstified prior to drowning (Late Miocene-Pliocene). The regional distribution of pore types, porosity and permeability is linked to the duration of exposure and burial diagenesis. In the karstified platforms of the northern part drilling losses are common.