Puji Astuti, Tri Rani (U. Gadjah Mada) | Surjono, Sugeng Sapto (U. Gadjah Mada) | Warmada, I Wayan (U. Gadjah Mada) | Kusuma, Didit Putra (U. Gadjah Mada) | Tsukada, Yasumoto (Hokkaido U.) | Otake, Tsubasa (Hokkaido U.) | Sato, Tsutomu (Hokkaido U.)
The Middle – Late Eocene sandstones of Batu Ayau Formation have been examined for the potential as a reservoir based on subsurface data and outcrop samples taken along Ritan and Belayan’s River. Petrographic study, XRD, SEM/EDS, porosity measurement, and quantitative determination of reservoir properties were carried out in this study. The sandstones are fine- to coarse-grained, moderately well to well sorted litharenite with subordinate lithic arkose, subarkose, sublitharenite and feldsphatic litharenite. The framework compositions of all sandstones are entirely quartz and plagioclase with trace amounts of K-feldspar and muscovite. The diagenetic processes include compaction, cementation, overgrowth of autigenic minerals (smectite), and dissolution due to alteration of feldspars. The SEM photomicrographs exhibit four types of cement such as mordenite, kaolinite, smectite and overgrowth quartz which are present over the entire samples. Quartz and kaolinite occur as pore-lining and pore-filling cements which locally developed as vermiform and accelerated the minor porosity loss due to pore-occlusion. Result of XRD analysis for the whole work are consistent with the qualitative result of petrographic analysis. Result of XRD analysis with ethylene glycol solvation for clay fraction (<2 µm) indicate a progressive alteration of smectite to illite. Based on the Watanabe’s Diagram, the presence of smectite layers in interstratified illite/smectite continously decrease from 100% to 65% with increasing the burial depth. The result indicate that the paleotemperature for sandstone diagenesis around 60-90°C. This is interpreted to be the result of temperature controlled chemical compaction, i.e. transformation from smectite to illite, feldspar dissolution and quartz precipitation. The laboratory analysis as well as field data demonstrate the sandstones of Batu Ayau Formation were subjected to mesodiagenesis and result in good to tight ranges of porosity.
Keyword: sandstone reservoir, Batu Ayau Formation, diagenesis, smectite, illite