The Esso Exploration and Production Inc. Natuna D-Alpha block lies approximately 125 mi (200 km) northeast of Natuna Island in the Indonesian portion of the South China Sea. The block contains a large Miocene platform carbonate complex called the L-structure.

The Terumbu Formation L-structure, situated in front of and isolated from a much larger carbonate shelf, is similar to the carbonate atolls developed in front of the barrier-reef complex offshore of Belize. The Terumbu carbonate on the shelf and in the L-structure has been divided into upper and lower units on the basis of a prominent seismic reflector, and the L-structure carbonate has been divided into three main facies on the basis of wireline logs and seismic character. The facies are the deep-water detrital carbonate, the upper Terumbu platform carbonate, and the lower Terumbu platform carbonate.

The lower platform and upper platform have a similar log character. They have a 46-μsec/ft matrix interval transit time and a chaotic dipmeter response. The rocks are boundstones, grainstones, and packstones that have been homogenized by organic activity. They are separated by a hiatus overlying a thick, tight deep-water limestone that causes a prominent seismic reflector. Porosity in the platform is controlled by depth of burial, sea level fluctuations, and depositional texture.

The detrital facies is composed of fore-reef talus, pelagic, and hemipelagic carbonates. The facies has a matrix interval transit time of 49 μsec/ft and good dipmeter response, and it is composed of 25 to 75-ft (8 to 23-m) depositional pods that have a sigmoidal cross section. When stacked in sufficient thickness they have a similar cross section on seismic records and fringe the platform in a downlapping pattern. By mapping the Terumbu carbonate facies, the growth history of the L-structure is revealed.

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