Description
The purpose of this seismic stratigraphy analysis is to develop the depositional history of the Natuna carbonate buildup and its relationship to porosity distribution. 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 were deposited mainly during late Miocene time. This large carbonate buildup has a gas column 1,600 m (5,250 ft) thick with carbon dioxide content ranging from 67 per cent in upper zones to 82 per cent near the base. 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 a rapid carbonate buildup with accompanying shedding of clastic carbonate grains into deep water on the southwestern edge of the buildup. When relative sea level lowered, 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 overlapping reef-complex layers. Seismic reflection patterns of these layers allow each to be divided into a reef and an off-reef facies. Current available well data suggests that the reef facies has higher porosity than the off-reef facies. Further drilling evaluation of the structure is planned and subsequent well data will test this hypothesis of porosity distribution based on seismic facies interpretation.
