Micritic Sandstone in Kerek Formation and Its Potential as Unconventional Reservoir (Micritic Sandstone Formasi Kerek dan Potensi Sebagai Unconventional Reservoir)

Rahmawati, D.

Nazzar, M.A.A.* Handayani, E.

Geological Engineering Dept. Faculty of Engineering, Gadjah Mada University

Jalan Grafika 2 Yogyakarta, 55284 *presenter

Kerek Formation was outcropped and represented as Micritic sandstone in Sumberlawang, middle Java. The method used in this research is surface mapping with 2 lines of measured statigrapich section. In the research area, there are some facies that is similar to the Kerek formation, they are the facies interbeding between fine tufaceous micrite, muddy micrite, and micritic sandstone (Mount, 1983)-or also called as A Fasies, and fasies muddy micrite with Intercalation of micritic sandstone (Mount, 1983)—or also called as B Fasies. The research focused on A facies, because it contains Micritic Sandstone which is commonly known as calcareous sandstone in the field. This rock has a very big amount of silisclastic material which has sand size, good porosity, and it spreads widely, Petography analysis was done in order to know about the porosity and the composition of the rock. Paleontological analysis—along with sedimentary structure, was also done as a control of the depositional environment and sedimentary mechanism. Micritic Sandstone has pethographic character; appearing as grey in the parallel nicol and dark in the cross nicol, its grain size is between 0,2-0,4 mm, with plan-convex grain contact, fabric grain supported, with the porosity of the interparticle and moldic type, with opaque composition, organic skeletal, Homblende, Micrite, and Diopside. The porosity analysis on the sample showed amount above 20%, which is good enough as reservoir. The most Interesting part comes from the fasies analysis where micritic sandstone was deposited by turbidy current mechanism. This studies was meant to reveal the potential of micritic sand stone as Unconventional turbidit reservoir. Formasi Kerek tersingkap di daerah pemetaan, yaitu di Kecamatan Sumberlawang, Sragen, Jawa Tengah, di daerah ini terdapat anggota dari Formasi Kerek berupa Micritic Sandstone. Metode penelitian yang digunakan adalah surface mapping, dengan Measured Statigraphic Section sebanyak 2 jalur. Di daerah penelitian terdapat fasies yang berkesebandingan dengan formasi Kerek, yaitu fasies perselingan antara fine tufaceous micrite, muddy micrite, micritic sandstone (Mount, 1983), atau yang disebut fasies A, dan fasies muddy micrite dengan sisipan micritic sandstone (Mount, 1983), atau yang disebut fasies B. Penelitian detail dilakukan pada fasies A, karena terdapat Micritic Sandstone dengan nama lapangan batupasir tufan gampingan yang memiliki material silisiklatik berukuran pasir sangat melimpah, persebaran luas, serta porositas tinggi. Penelitian petrografi dilakukan untuk memahami porositas dan komposisi dari batuan, serta dilakukan juga analisa paleontologi sebagai kontrol terhadap lingkungan pengendapan dan mekanisme sedimentasi, dibantu dengan struktur sedimen. Micritic Sandstone memiliki karakter petrografi berwarna abu-abu kecoklatan di nikol sejajar dan hitam di nikol bersilang, berukuran kristal 0,2 – 0,4 mm, hubungan antar butir plain conveks, fabric grain supported, dengan tipe porositas interpartikel & moldic, dengan komposisi mineral opak, organic skeletal, hornblenda, mikrit, dan diopsid. Pengukuran porositas pada sampel menunjukan angka diatas 20%, yang memiliki arti cukup baik sebagai reservoir. Hal yang menarik didapat melalui analisa fasies, dimana micritic sandstone terendapkan melalui mekanisme turbidit, studi ini dimaksud untuk mengungkap potensi micritic sandstone sebagai Unconventional Turbidit Reservoir.