Print this pageThe Influence of Tectonics, Biota Systems and Diagenetic Fluids in Controlling Carbonate Reservoir Development in the Sarawak Basin
The Miocene carbonates have trapped >40 TCF of gas in the Central Luconia Province, but the onshore carbonate potential remains unknown. These carbonates grew in different palaeo-ecological settings that influenced biota systems variability, internal architecture and characteristics of the reservoirs. These differences were further complicated by diagenesis, resulting from multiple episodes of uplift, emergence and karstification. As a result, remarkable differences in reservoir quality were observed in these onshore and offshore carbonates.
The Early Miocene carbonates onshore, that extend into untested areas offshore, are composed of different biota systems and much more controlled by clastic input. They were subjected to compressional tectonics and have undergone five phases of calcite cementations and dolomitization with multiple compositional zonings. Each phase of diagenetic cement shows very little variations in elemental, carbon and oxygen isotopic compositions. Melting temperature of inclusions suggests that the fluids were trapped from evaporitic waters during burial or mixed marine/meteoric waters at low temperature (<50oC). These cementations completely obliterated the pores and made the reservoir quality very poor.
In contrast, the Middle-Late Miocene carbonates offshore, which all have similarity in term of biota systems and reservoir architecture that were more controlled by sea-level fluctuation, have undergone more complex diagenetic pattern. Most buildups have been affected by eight calcite cementation, dolomitization and dedolomitization phases together with three major dissolution episodes. Each diagenetic phase shows large variations in elemental compositions and isotope ratios, suggesting significant changes in palaeo-fluid systems and their sources during the burial. The existence of high temperature minerals and high homogenization temperature (130oC) indicate that the late stage diagenetic fluids and dissolution have associated with hydrothermal origin, migrated upward through fault plain and was responsible for significant porosity and permeability enhancement of the reservoirs.
AAPG Search and Discover Article #90100©2009 AAPG International Conference and Exhibition 15-18 November 2009, Rio de Janeiro, Brazil