Parigi carbonate buildups : Northwest Java Sea

Abstract Detailed analyses of cores from Parigi-age carbonate buildups provide insights into the depositional history, diagenesis and reservoir quality of these Upper Miocene gas reservoirs. The buildups range in thickness from 4 to 1103 feet and are composed of fine-to medium-grained, moderately-sorted skeletal-foraminiferal wackestone, packstone and grainstone with interbedded claystone. Eight carbonate lithofacies are present in the KLY-1 and EJ-4 wells and elsewhere in the Java Sea. These are: 1) bioclastic peloidal packstone to grainstone, 2) bioclastic wackestone to packstone, 3) foraminiferal wackestone to packstone, 4) red algal wackestone to packstone, 5) dasycladacean/cryptalgal-laminated wackestone to packstone, 6) dolomite/calcareous dolomite, 7) argillaceous mudstone/clay-stone, 8) glauconite/peloid mudstone. Over the area covered by the ARCO contract, the Parigi carbonates vary in thickness, lithology and faunal content. The KLY-1 core records deposition of thick, shallow water carbonates on a rimmed carbonate shelf. Deposition of these rocks was diachronous with thinner, argillaceous, deeper water shelf sediments seen in the EJ-4 core. Up to four marine transgressive episodes can be recognized. The major diagenetic processes to affect these rocks include cementation, replacement and dissolution. However, diagenetic alteration, with the possible exception of local dolomitization, did not alter primary depositional textures to a great extent. The reservoir quality is primarily controlled by the original depositional facies of the carbonates. The major pore system in the Parigi is interparticle microporosity within the carbonate mud matrix of wackestones and packstones. Early and late cementation partly or wholly occluded isolated (ineffective) moldic pores, thus having little effect on the rock's overall flow properties. Locally, dissolution has created and enhanced both porosity and permeability in carbonate mud by enlarging the micropores and increasing their interconnectivity.