ABSTRACT: Strontium Isotope Chronostratigraphy and Diagenesis of the Batu Raja Limestone, Offshore Northwest Java, Indonesia

Nathan R. Miller, John G. Kaldi

Strontium isotope chronostratigraphy was applied at Bima field of offshore northwest Java, Indonesia, as a dating tool to construct a geochronologic framework and as an indicator of diagenetic alteration. The field is situated on the western edge of the Seribu platform, a major horst block separated from the adjacent Sunda basin by the Seribu fault. The main reservoir at Bima lies within the Batu Raja Formation, a transgressive shallow-water platform carbonate that blankets the Java Sea.

Measurements of ⁸⁷Sr/⁸⁶Sr from seven wells across the platform margin indicate that deposition of the Batu Raja limestone initiated in the late Oligocene (26-27 Ma) and ceased in the early Miocene (21-22 Ma). A eustatic sea level drop at about 21 Ma exposed the Bima Batu Raja carbonate platform to meteoric diagenesis and formed the porous/permeable reservoir facies. The ⁸⁷Sr/⁸⁶Sr ratios of the majority of Bima samples follow the normal Tertiary trend of increasing upward. However, samples from zones which were significantly affected by early meteoric diagenesis have anomalously low ratios. Also, altered samples have lower ⁸⁷Sr/⁸⁶Sr values with increasing proximity to the Seribu fault. Migration of low ⁸⁷Sr/86Sr early Tertiary marine formation waters from the Sunda basin, up the fault and into the porous/permeable horizons, is the likely mechanism for rock alteration. An association of altered samples with hydrocarbon pay zones suggests that migration of isotopically low cementing fluids was related to hydrocarbon emplacement.