Abstract

The Natuna D-Alpha "L" structure is a large late Miocene carbonate reef complex reservoiring a proven gas column 5,250 feet thick. CO₂ content ranged from 67% in upper zones to 82% near the base. Gas water contact is as yet unproven. The discovery well was drilled by AGIP in 1973. Esso Exploration acquired the permit in 1980 and shot a 1,400 mile grid of seismic over the block. The Arang formation is considered the source of methane; CO₂ content is believed to have been introduced from deep igneous activity.

Arang and Barat-Gabus shale (middle and lower Miocene) is widespread and relatively uniform in thicknees over the block, suggesting stable nonmarine to shallow marine shelf conditions. Post-Arang normal faulting associated with regional tectonics resulted in rotation and downfaulting of the "L" structure. Lower Terumbu (upper Miocene) carbonate bank development began on both the updip portion-of the "L" structure and on the broad shelf area to the west. However, carbonate deposition on the western shelf was limited by shallow water depths, while the "L" structure accumulated a thick carbonate unit as the block continued to founder.

Further downfaulting of the "L" structure in upper Terumbu time (upper Miocene and early Pliocene) resulted in widespread neritic carbonate deposits, with local reefing on the western shelf area, and with local buildups of several reef complexes on the crest of the "L" structure.

Seismic facies and structure maps of the upper Terumbu carbonates have been used to develop well locations for evaluation of the stratigraphic model of the "L" structure in favorable facies.