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Challenge the Norms: Fluvial Channel in Shallow Marine Environment. Is it Possible? A Case Study of A-1 Well in Central Luconia, Offshore Sarawak, Malaysia — An Integrated Approach Combining Seismo-Stratigraphy and Seismic Attributes Analysis

Wafa, Sharifah S.; Mustafa, Mohd Sharir S.; Kamarudzaman, Khairul Amry A.; Sapri, Dayang Hasspariah H.; Mahmud, Othman Ali A.

Central Luconia Province is an overly matured basin that is well known for its Miocene carbonate plays as well as the pre and post carbonate structural plays.Therefore,new approach has been adopted to look at the shallow stratigraphic plays.

Recently, A-1 well drilled by PETRONAS in July 2012 has resulted to a hydrocarbon discovery in shallow reservoirs between the depth of 800m and 1300m.Multiple shallow stratigraphic plays were mapped, and two of them were identified as channel plays of Cycle V/VI Sand namely as CH-1 and CH-2.These reservoirs are stratigraphically sitting right above the Central Luconia Carbonate platforms, which were previously interpreted to be deposited in a shallow marine environment. Now the questions arise how is it possible to have fluvial channel in the area interpreted as shallow marine environment? To unveil the answer, an integrated approach was made by combining seismic attribute analysis, recent well data and sequence stratigraphic study.Several seismic attribute maps were generated such as RMS and Maximum Amplitude using Pre Stacked Time Migration Near and Far Stack cube.Time slice taken from Spectral Decomposition shown fluvial channels geometry.In addition,based from seismo-sequence stratigraphy study,several seismic facies and reflection terminations clearly suggest the presence of fluvial channel within the area.This evidence is further supported by the log motif from well data which shown a sharp erosive base fining upward Gamma ray log motifs and the presence of coal as indicated by spikes in wireline logs as an evidence of subareal exposure of Lower Coastal Plain depositional environment.The presence of alternating marine and fluvial deposits in the 1000m of the basin indicates exposure and submergence of the shelf numerous times during the Miocene.It is believed that immediately after the deposition of the Miocene carbonate in shallow marine environment a significant short drop of relative sea level has exposed entirely of the shelf,erosion occurred and fluvial channel were formed.This short drop of relative sea level will be immediately followed by transgressive and highstand systems tract periods.Hence,this process eventually return the the area to shallow marine again.

Ultimately, this study has unveiled the complexity of depositional environment within the area.By applying this concept, it basically opens up new opportunities in chasing the fluvial channels for future prospects in Offshore Sarawak.


AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013