Diagenetic Events as The Key Improvement of Carbonate Reservoir Quality in The S Field, Central Sulawesi, Indonesia
Deddy Hasanusi, Rahmat Wijaya, Agus Priyantoro,Neni Herawati, Indra Shahab
The Mentawa Reefal Build-up member and Minahaki carbonate sequences are the main hydrocarbon bearing reservoir of the S field, in the Block Central Sulawesi of the eastern Indonesia, especially in the Banggai-Sula basin. Carbonate reservoir of Mentawa member of Minahaki Formation commonly recognized as pinnacle reef build up type which were growth in the structural high areas and having orientations relatively northeast to southwest direction, following the tectonic setting in the areas. On the other hand, the platform carbonate which is most identified as globigerinids wackstone and globigerinids packstone facies spread out dominantly in the deeper areas enveloping the higher areas as the places of carbonate build-up (in back reef and open marine area). Combination of diagenetic events especially on the vadose zone (above water table) which are controlled by relative sea level drop and rock composition as indication of main control in improvement of the carbonate reservoir quality (both carbonate re-build-up and platform types) that initially by two events. First, presence of aggrading neomorphism / re-crystallization of the carbonate mud matrix to micro crystalline calcite (micrite) and – micro sparite that having crystal size range from 3 – 20 µ with average around 12 µ (for carbonate build-up) and 6 µ (for carbonate platform). Second, presence of intensive dissolution of both selective bioclast grains and carbonate mud matrix contents created secondary moldic, micro-macro vuggy and honeycomb porosities and it has maximum size reach-up 3.2 cm (measure from conventional core). Both re-crystallization and dissolution process was intensively well developed in the Mentawa reef build-up member of Minahaki Formation that resulted fantastically in increasing porosity and permeability value. The data measurement of routine core analysis of the carbonate reef build-up having porosity and permeability values ranges from good to excellent classes. Whereas, in the carbonate platform type, the porosity indicated good to very good classes, nevertheless the permeability value turn out to be commonly low and having a very contrast value. This condition was controlled by dominantly micro-intercrystalline and intraparticle porosity types. This phenomenon probably having relationship with composition and main diagenetic event that undergone of the both carbonate build-up and carbonate platform.
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