Abstract: Burial Hydrodynamic Trapping: a Working Model for the Giant Peciko and Tunu Gas Fields (Mahakam Delta Basin, Indonesia)

Yves Grosjean

The Tunu and Peciko fields, located in the gently folded offshore part of the Mahakam Delta, hold more than 17 TCF of gas in place in numerous thin upper Miocene delta-front sands with multiple fluid contacts. Gas fill extends in the direction of the clastic influx downdip from low relief culminations,

toward the main kitchen of the basin centered on a large syncline. Both fields are unfaulted, and there is no direct relationship between the fluid contacts and structural contours or facies boundaries. Lateral sealing is importantly assisted by hydrodynamic factors related to burial.

Deltaic sediments have accumulated since early Miocene in a regressive wedge more than 8000 m thick, made of eastward-prograding depositional sequences. Periods of active uplift in the hinterland and high subsidence offshore were synchronous with compressive syndepositional structural growth. Liquid-rich gas was sourced by the proximal coals and organic shales; the distal shales are organic lean and overpressured.

A deep hydrodynamic system evolved during burial as compaction waters were expelled by the distal shales. The active, though very slow, circulation was forced toward the clastic influx between regionally extensive shale layers. It became impeded by migrating gas as small columns built up to break through minor lateral capillary barriers (i.e., the transitional facies between delta front sands). The gas saturation caused a reduction of the overall relative permeability to water, and pressure increased in the aquifers on the distal side. This hydrodynamic factor added to the retention, and very large gas columns were eventually trapped. The fluid contacts are dynamically controlled; they were mapped using high-accuracy reservoir pressure measurements.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994