Pustaka Ilmiah Universitas Padjadjaran

Abstrak

The Influent Of Intensity And Type Of Alteration To The Secondary Porosity Of The Pre-Tertiary Basement Rock Of South Sumatra Basin

Study area located at the Jambi Sub Basin, South Sumatra. The area is bounded by the Tigapuluh Mountain in the north, the Barisan Mountain Ranges in the west, the basement high partially shown as the Duabelas mountain in the south, and, the basement high partially shown as Befitting, Bangka, Singkep and Lingga Islands in the east. The samples are obtained from pre-Tertiary igneous rock at JSB-3, JSB-4 and JSB-6 wells that represent a part of pre-Tertiary basement rock from Jambi sub Basin, South Sumatra. 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. Weakly — strongly hydrothermal alteration occurred in all samples. Petrography study 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 classified as outer/sub propyhtic — phyllic, which can be divided into two alteration minerals assemblages; the chlorite — illite ¬calcite group in the first episode; and the sericite ± quartz group in the second episode. Secondary porosity is resulted from the dissolution process in the rock forming minerals especially secondary minerals; and in the empty fracture or partially associated with the filled fracture. The type of alteration has influent to the secondary porosity of the dissolution process. The increasing of alteration intensity which is replaced primary minerals, also caused the increasing of secondary porosity