DIAGENETIC STUDY AND POROSITY DEVELOPMENT WELL 'IR' IN BATURAJA FORMATION SOUTH SUMATERA BASIN BASED ON PETROGRAPHY, CORE ANALYSES, AND PETROPHYSICAL CALCULATION

摘要

South Sumatra Basin is an asymmetric basin located to the east of Barisan Mountains, to the north and west of the Lampung High, and to the south of Tigapuluh Mountains. Stratigraphy comprises Lahat, Talang akar, Baturaja, Gumai, Air Benakat, Muara Enim, and Kasai Formations. Baturaja Formation consists of Early Miocene carbonate buildups formed by clastic limestone and organic limestone (reef) deposited in shallow marine environment during marine transgression phase and it is divided into Upper and Lower Baturaja (BRF members). The BRF limestone facies has been affected by some diagenetic processes such as cementation, replacement, dissolution, and compaction. The most common cementation agent is calcite commonly filling fossil chambers and some fractures, with minor pyrite (0-0.5%). Relatively significant replacement process has caused some matrix and unstable grain alteration or replacement commonly by calcite (8.00%-42.75%), with lesser micrite (3.50-12.50%) and rare dolomite (1.2%, only at depth 1534.0 m). Less significant compaction has affected this facies as evidenced particularly by the occurence of minor local stylolites and fracture grains. Several core samples in BRF were collected for analyses including biostratigrapy ; permeability is fair to good (18-22 mD), visible porosity is generally poor to good (1.50-20.00%), represented by secondary vugs (7.50-18.50) and oversized- fracture (1.50-4.00) pore types. Oil in pores 2.09- 2.80% and total water in pores 61.16-64.10%, grain density 2.735-2.759 gr/cc and comprising mud- wackestone facies. The range of observations may be related to heterogenity of the limestone facies. Biostratigraphic analyses revealed planktonic zone in N4 and nannoplankton zone in NN1 (inner-outer neritic). Combination of wireline log and petrography data will used to define the porosity and assess the digenetic processes. Based on the understanding of reservoir character heterogenity on BRF, information about variation of porosity development and diagenetic level in carbonate reservoir can be achieved.