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Thomas A. Ryer1, Jurgen Meyer2, Mark Bagge1, Nick J. Comrie-Smith3, Gertjan Van Mechelen3
(1) Schlumberger, Houston, TX
(2) Schlumberger, Jakarta, Indonesia
(3) Conoco, Indonesia, Jakarta, Indonesia

Abstract: Sequence stratigraphy and depositional history, Upper Sandy Member of Gabus Formation (Miocene), Kerisi-Hiu area, West Natuna Basin, South China Sea, Indonesia

Clastic reservoirs deposited on prograding delta fronts, in channel belts, and in aggraded valleys contain substantial gas reserves within the upper part of the Gabus Formation on the Kerisi and Hiu structures in Block B, West Natuna Basin. A single well was drilled on each of the two anticlinal structures. Well data is augmented by exceptionally good 3D seismic data. Seismic attribute maps reveal the fine detail of the depositional fabric, particularly in the shallowest strata of the Gabus Formation at depths of 3300 to 4200 feet.

The Upper Sandy Member of the Gabus Formation in the Kerisi-Hiu area includes high-stand deposits of an earlier sequence, a sequence boundary on which at least 120 feet of erosional relief developed, and the aggradational, transgressive-phase deposits of a later sequence. Delta-front sandstones contain gas on the Hiu structure. Two east-trending valleys were incised across them during a pronounced but apparently short-lived fall of lake level. The aggradational fill of these valleys is sand-rich and gas-charged on the Kerisi structure. Once the valleys were filled, unconfined rivers deposited an interval of coastal-plain strata with a lower content of sand. Delta-front strata that probably constitute a back-stepping parasequence set accumulated during general southwestward transgression of the lake. A short-lived, minor lowering of lake level resulted in incision of a dendritic network of both large and small streams into the uppermost parasequence. A subsequent rapid rise of lake level resulted in filling of this drainage network with mud and, finally, transgression of the shoreline.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana