THE INFLUENT OF INTENSITY AND TYPE OF ALTERATION TO THE SECONDARY POROSITY OF THE PRE-TERTIARY BASEMENT ROCK BASED ON PETROLOGICAL STUDY FROM JSB-3, JSB-4 AND JSB-6 WELLS, JAMBI SUB BASIN, SOUTH SUMATRA

• Main Author: TINTIN YUNINGSIH, EUIS
• Format: Theses
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/69663
• Institution: Institut Teknologi Bandung

Study area located at the Jambi Sub Basin, South Sumatra. To the north the Jambi Sub Basin is bounded by the Tigapuluh Mountain, to the west by the Barisan Mountain Ranges, to the south by basement high partly shown as the Duabelas mountain, to the east by the basement high partly shown as Belitung, Bangka, Singkep and Lingga Islands. Lithology of the pre-Tertiary basement at the Jambi Sub Basin is dominated by andesite in JSB-3, granite in JSB-4 and granodiorite in JSB-6. Magma crystallization in the study area indicates fractionation crystallization mechanism represented by phorfiritic and glomerophorfiritic textures (at JSB-3), there are also magma mixing process during magma generation indicated by unbalancing texture in plagioclase. Chemical rock analysis confirm that the lithology are intermediate – acid magmas, calcalkaline, medium – high K, metalluminous, subduction-related (orogen) at active continental margin. Granitoid at JSB-4 and JSB-6 shows magnetite series and I type. This Mesozoic granitoid probably as extension from the Thailand and Burma granite province, that underline the fact that there has been a history of subduction-related magmatism along Southwestern edge of Sundaland since earliest Mesozoic times. Hydrothermal alteration occurred in JSB-3, JSB-4 and JSB-6 has weakly – strongly intensity. Microscopically observation indicates that alteration mostly through replacement process of primary minerals followed by filling of secondary minerals through pore and fracture. The type of alteration is outer/sub propylitic – phyllic and based on its episode, there are two mineral alteration assemblage, there are formation of mineral assemblage of chlorite – illite – calcite in the first episode and formation of mineral assemblage of sericite ± quartz in the second episode. Secondary porosity resulted as dissolution process in rock forming minerals especially secondary minerals and porosity also resulted in the empty fracture or associate with the filled fracture partly. Generally, alteration intensity has influent to the secondary porosity of the dissolution process, where the increasing alteration intensity which is replace primary minerals cause the increasing of secondary porosity. Beside that, the type of alteration also has influent to the secondary porosity of the dissolution process. Rocks fracturing in the study area formed at least in three different events. Fracture porosity dominantly resulted in the empty fractures that formed in the last event (third), otherwise at the previous events this fracture filled by the secondary minerals such as carbonates (calcite and dolomite), sericite, quartz, chlorite and opaq minerals.