Abstract

The Krisna Field, located in the ILAPCO Southeast Sumatra Productiion Sharing Contract Area, produces oil from early Miocene, lower Batu Raja carbonates. Upper Batu Raja carbonates and Talang Akar sandstones also produce oil in the Krisna Field, but will not be discussed herein. The high porosities found in the lower Batu Raja carbonates are of secondary origin, and were formed by the diagenetic leaching of originally aragonitic skeletal material. Leaching took place in a freshwater phreatic environment, indicating that part of the area was repeatedly exposed during mid and late lower Batu Raja time.

Porosity, which was developed by the action of fresh water on packstone/wackestone carbonate facies, is distributed field-wide as a continuous, lensoid body, a factor which contributes to reasonably uniform production characteristics. Fracturing of the tight carbonates and weathering at the basement-sediment interfaces provided additional means of communication for reservoir fluids.

Seven major carbonate depositional facies have been recognized, and to a large extent they controlled diagenetic effects. Correlation of primary facies has been achieved by combining analyses of logs, petrography, and using time-lines derived from 'caliche'-like horizons.

Some implications of this model for furture secondary recovery and exploration programs are : (i) The continuous, lensoid distribution of the porosity contributes to good primary and secondary reservoir recvery. (ii) Some wells on the crest of the structure have reduced 'net pay'. This is because the thicker carbonate sections were developed away from the crest, behind the fringing reefedge, which lay at some distance from the shoreline. Thus, exploration targets confined to reef core, or even lagoonal areas, could be missed by wells drilled solely on structural crests. (iii) Lagoonal areas are shown to have significant exploration potential, despite originally low effective porosity and permeability of the sediments.