Facies and Reservoir Rock Type Analysis of Kujung Unit-I Formation in “Betta” Field, East Java Basin, Indonesia

Abstract

Kujung Unit-I formation in Betta field is proven to have produced 1.4 – 9.1 MMCFGPD gas which is confirmed to be 100% C1. Where the condition makes kujung Formation Unit I as a secondary target of the reservoir observed. Given the reservoar carbonate rocks are known for their high degree of complexity. So, to understand it requires more detailed attention. Therefore, this study aims to find out the relationship of qualitative analysis (analysis of deposition fasies) and quantitative (reservoir rock type) of carbonate rock reservoirs. Analysis of deposition fasies at research intervals (SB-4 to MFS-4), resulted in fasies associations in the form of interior carbonatreef mound platforms, lagoons/open shelf, barriers (sand shoals – reef shoals), slopes, and deep shelf. The direction of deposition is known to be getting deeper to the Southwest (basinal). Of the three wells, which show the best porosity value of 30-35% are ASA-2 and ASA-3. Then a good permeability value is shown in the ASA-4 well (22 – 32 mD). From the analysis of reservoar rock type (RT) with property value parameters, four groups of rocks produced which are the best (RT-1) showed interconnected pore size (R35) 2.62 – 3.17 μm with flow unit (FZI) 1.7 – 2.7. Then the worst reservoar rock type shows (R35) 0.7 – 0.9 μm (FZI) 0.006 – 0.01. The intervals that produce gas 1.4 – 9.1 MMCFGPD on asa-3 wells are filled by wackstone limestone litofasies – packstones deposited in the open shelf zone, and classified as 4th type rock reservoar (RT-4).