Depositional and Diagenetic Effects on Reservoir Properties in Carbonate Debris Deposits: Comparison of Two Debris Flows within the Berai Fm., Makassar Strait, Indonesia

Tanos, Chrisna A.¹; Kupecz, Julie¹; Warren, John K.²; Lestari, Sri¹; Baki, Alaa³
¹Exploration, Pearl Energy, Jakarta, Indonesia.
²Department of Geology, Chulalongkorn University, Bangkok, Thailand.
³PT Eksindo Pratama, Tangerang, Indonesia.

After the successful discovery and appraisal of the "M" field, interpreted as a debris flow carbonate reservoir in the Oligocene - Early Miocene Berai Formation of the South Makassar Basin, a subsequent exploration well in an adjacent feature (NW-1) was unexpectedly dry. The dry NW-1 well was a surprise as previous work indicated that this was a favorable structural location to drill.

This study has shown that the Berai limestone has a complex depositional framework and that it has experienced a multistage diagenetic evolution. In order to reduce the risk in any future exploration drilling, this integrated study was done, using data from all the existing wells in the "M" Field and the NW-1 well. Stage 1 of this study incorporated available cores and thin sections, and stable C and O isotopes. The dry NW-1 well, comprising clasts of reefal carbonate platform host, was cemented under marine conditions prior to re-sedimentation, as evidenced by thin section and isotopic data. In contrast, the productive well (M-4), also a carbonate debris flow, though of a different provenance, exhibited little syn-depositional cementation. Importantly, it experienced a phase of post-depositional leaching that significantly enhanced the reservoir quality. Depleted oxygen isotopic data from M-4 are consistent with dissolution/reprecipitation by higher temperature fluids in a post-depositional setting, while NW-1 isotopic data indicate little diagenetic alteration.

In stage 2 of the study, results from the isotopic study were used to high-grade samples for cathodoluminescence petrography, which were in turn used to select sampling sites for total Sr and 87Sr/86Sr analyses. Drill cuttings from the sections above and below the reservoir were analyzed for biostratigraphy and quantitative mineralogy; these data were used to aid in identification of major flooding surfaces and condensed intervals, and used as correlation tools to help constrain timing of debris flow deposits. These intervals will be incorporated into the seismic interpretation.

AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012