THE INFLUENT OF MINERAL DISSOLUTION PROCESS TO THE SECONDARY POROSITY OF THE PRE-TERTIARY BASEMENT ROCK OF SOUTH SUMATRA BASIN

PROCEEDINGS JOINT CONVENTION BALI 2007
The 32^nd HAGI, The 36^th IAGI, and The 29^th IATMI Annual Conference and Exhibition

THE INFLUENT OF MINERAL DISSOLUTION PROCESS TO THE SECONDARY POROSITY OF THE PRE-TERTIARY BASEMENT ROCK OF SOUTH SUMATRA BASIN

Euis Tintin Yuningsih
Department of Geology, University of Padjadjaran
ABSTRACT

The objective of the research is to identify the dissolution process of the minerals composed of the Pre- Tertiary basement rock of South Sumatra Basin petrographically and its relation with the formation of secondary porosity. 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 South Sumatra Basin. Pre-Tertiary basement rock is dominated by andesite in JSB-3, granite in JSB-4 and granodiorite in JSB-6. Weakly – strongly hydrothermal alteration occurred in all samples. Microscopic 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 classifying as outer or sub propylitic – phyllic.

Dissolution occurred in the margin of plagioclase specialty the weakly zonal of plagioclase that associated with alteration minerals of sericite and fine muscovite as well as calcite, which is partially dolomitized. Dissolution in muscovite partially associated with dolomitized calcite, otherwise K-feldspar dissolution associated with perthite texture. Secondary porosity resulted from primary mineral alteration, mostly found in plagioclase as pin points and spongy pore system related with replacement by alteration minerals of calcite and sericite, while in K-feldspar partial dissolution occurred in perthite texture. In addition, secondary porosity also obtain from dissolution of alteration mineral from muscovite and biotite. Moreover, secondary porosity also occur as dissolution process from secondary minerals that filled the fracture system within the rock.

Key words : dissolution, secondary porosity, hydrothermal alteration, replacement,