Late Burial Dissolution in the Kerendan Carbonate Platform, Oligocene, Central Borneo
Kerendan is an isolated Oligocene carbonate platform in the subsurface of Central Kalimantan, Indonesia. Depositionally, Kerendan had coral boundstones at the platform margin, high-energy grainstones at the platform rim, and low-energy, micritic facies in the platform interior. Porosity preferentially occurs in the platform-rim grainstones and platform-margin coral boundstones. Fabric-selective microporosity occurs in calcareous grains on the platform rim. Vuggy porosity and collapse-related fractures formed on the platform margin during nonfabric-selective dissolution. Four observations suggest that much of that nonfabric-selective porosity is related to dissolution by acidic waters compacting out of adjacent shales during moderate to deep burial (200-3,000 m; Saller and Vijaya, 2002). (1) The nonfabric-selective dissolution and associated porosity are petrographically late. (2) Saddle dolomite is associated with the vuggy dissolution in Kerendan. Saddle dolomite generally precipitates in deep burial environments (>60 degrees C; Radke and Mathis, 1980). Dolomite cements that partially fill vugs have δ18O values of approximately -8.0‰ (PDB) supporting dolomite precipitation during deep burial (>2,000m). (3) Vuggy porosity is greatest near the margins of the Kerendan platform where acidic waters emanating from shales would have been most abundant. (4) Kerendan formation waters have oxygen isotopic compositions of +6.2 and +8.0‰ (SMOW) which is consistent with waters being released during the smectite to illite transition (Land, 1983) during deep burial. Those isotopically heavy waters were probably expelled from the adjacent shales during deep burial compaction. Waters probably became acidic because of reactions involving the maturation of organic material in the shales during burial. Those acidic waters were then expelled from the shales during compaction causing the observed nonfabric-selective porosity. Late carbonate dissolution in the Kerendan platform by fluids expelled from adjacent shales during deep burial is consistent with all of our subsurface data.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013