Abstract

The Natuna D-Alpha Block is located in the Natuna Sea in Indonesian waters, northwest of Kalimantan. The thick Upper and Lower Terumbu Formation carbonates of the L-Structure were deposited mainly during Late Miocene time. The reef-complex has a gas column 5,250 feet thick with CO₂ content ranging from 67 per cent in upper zones to 82per cent near the base. The purpose of this seismic stratigraphy analysis is to develop the depositional history of the Natuna D-Alpha Block and its relationship to porositydistribution. Initial studies were conducted and presented by Sangree (1981) of Esso Exploration Inc. at the 10th Annual Indonesian Petroleum Society Conference. This work is an extension of that study with emphasis on the seismic stratigraphy of the entire L-Structure reef-complex.

Lower Terumbu Formation history begins with development of a broad carbonate platform. Faulting continued through much of this platform stage, resulting in gradual tilting of the fault block and restriction of deposition to the eastern edge of the block. Rise in relative sea level then resulted in rapid carbonate build-up in the L-Structure vicinity, with accompanying shedding of clastic carbonate grains into deep water on the southwestern edge of the build-up. When relative sea level subsided, a portion of the structural high was subjected to erosion.

Deposition of the Upper Terumbu Formation began with the next cycle of relative sea level rise, resulting in the formation of several over-lapping reef-complex layers. Seismic reflection patterns of these layers allow each to be divided into a reef and an off-reef facies. Currently available well date suggests that the reef facies has higher porosity than the off-reef facies.

Further drilling evaluation of the structure is planned and we will continue to use the well data to test our working hypotheses of porosity distribution based on seismic facies interpretation.