Early Versus Late Dolomitization of the Miocene Luconia Platform: Insights from ‰13C, ‰18O, 87Sr/86Sr and Fluid Inclusions of Isolated Cement Generations

Warrlich, G.¹, C. Taberner², M. Esteban³, A. Dombrowski¹, J.H. Van Konijnenburg⁴, M. Rejas², M. Boya-Ferrero¹ (1) Shell International Exploration and Production B.V, Rijswijk, Netherlands (2) Institute of Earth Sciences, CSIC, Barcelona, Spain (3) Carbonates International Iberia, Mallorca, Spain (4) Sarawak Shell Berhard, Malaysia

Dolomitization of pinnacles from Luconia Miocene carbonate reservoirs (Sarawak, Borneo) has been interpreted as occurring during early diagenesis by either hypersaline brines or seawater. Nevertheless, petrographic evidence and cement stratigraphy support dolomitization and corrosion of calcite at burial stages as the main origin for secondary porosity generation in the reservoir. Dolomite cements post-dating fracturing have similar petrographic patterns to dolomite rhombohedra from pervasively dolomitized mud-support-ed fabrics in the pinnacles. Poikilotopic late calcite cements and sparry calcite filling frac­tures and porosity are mostly recorded below the oil-water contact. Primary fluid inclusions in dolomite crystals and late calcite cements record similar temperatures and salinities, comparable to bottom hole temperatures and salinities of formation waters. The ‰¹³C and ‰¹⁸O values of dolomite and late calcite microsamples display a parallel trend towards lower ‰¹³C and ‰¹⁸O suggesting similar parental brines, as well as the record of an increase in water:rock ratios and in the supply of organically-derived CO₂. Calculated ‰¹⁸O of parental fluids are similar to formation waters. ⁸⁷Sr/⁸⁶Sr of dolomite and late calcite cements have similar ratios to the original marine sediments (Early to Middle Miocene) sug­gesting significant rock buffering.

Integrated paragenesis, geochemistry and fluid inclusions of isolated dolomite and late calcite cements support late burial dolomitization and late corrosion by deep-seated fluids of similar chemistries and temperatures to those recorded in formation waters. Predicted porosity and permeability patterns are different when a late dolomitization model is applied to the Luconia carbonate reservoir. PETRONAS is acknowledged for permission to present these results.