Kantu Formation on Ketungau Basin study area overlaid the Semitau Complex unconformably., with a fault as lithology boundary. Kantu Formation is composed of quartz sandstone, claystone, siltstone with intercalations of conglomerate and coal.
Based on petrography analysis, samples from eastern part of the area are sandstone composed of 65%-75% quartz grain showing sutured contact and secondary quartz outgrowth, 10-15 k-feldspar of which part has been dissolved, 5-10% polycrystalline silica composed by polycrystalline quartz, 3% opaque minerals grain-sized 0,1mm–0,7mm, 5-10% muddy-colored clay matrix present among the grains which begin to recrystallize to sericite, 5% alteration minerals composed of clay mineral present as alteration from k-feldspar, 10%-15% cement composed by iron oxide and silica, and 15% porosity consists of intergranular, intragranular, and microvug as results of dissolution.
Samples from western part of the area are sandstone composed of 70%-80% quartz grains showing point to long contact texture, 5%-10% k-feldspar, 5-10% clay matrix of which part has been recrystallized to sericite, 5% cement composed by iron oxide and clay minerals, and 10% porosity consist of intergranular, intragranular, and microvug.
Sandstone samples from western part of the area indicate eodiagenesis-mesodiagenesis stage with medium compaction degree, marked by long contact texture. Sandstone samples from eastern part of the area indicate mesodiagenesis-telodiagenesis stage, marked by sutured contact and the presence of clay minerals as alteration from k-feldspar. Diagenesis process of sandstone from both parts began with cementation of secondary quartz outgrowth and compaction, which reduced the primary porosity. Cementation by clay minerals, and iron oxide can also reduce the porosity. Nevertheless, sandstone indicates generation of secondary porosity as a result of k-feldspar dissolution.
Keywords : diagenesis, Kantu Formation, porosity, sandstone
Authors : C. R. Kusuma, M. F. Nugroho, M. Z. Yantu, R. A. Yulman, N. I. Basuki