Abstract: Discovering Large Gas Fields in Crystalline Basement: Unconventional Exploration in Sumatra

In 1991, Gulf Indonesia and its partners discovered South Sumatra Basin’s first major gas field at Dayung in the Corridor PSC. A key feature of this field is that most of the reserves are held within fractured basement rocks of pre-Tertiary age. 

The South Sumatra Basin is considered one of Indonesia’s most mature hydrocarbon provinces. Prior to 1990, the majority of gas discoveries were made in conventional reservoirs containing accumulations of less than 100 BCF. This small field size rendered major gas development projects economically unattractive.

In 1991, Gulf Indonesia and its partners discovered the basin’s first major gas field at Dayung in the Corridor PSC. A key feature of this field is that most of the reserves are held within fractured basement rocks of pre-Tertiary age. Over the next 8 years, Gulf found 7 more large pre-Tertiary gas fields in the area, containing nearly 15 TCF of raw, recoverable gas reserves. Ongoing drilling, seismic, geoscience activities have clarified the relationship between these unique reservoirs and the suprajacent Tertiary Petroleum System.

The growing inventory of pre-Tertiary reservoirs range in age from Triassic to late Cretaceous. The granites possess an unusual matrix porosity in addition to significant fracture porosity. Defining the fracture network has helped in positioning wells for optimum gas deliverability.

Traps containing the gas fields were created through a combination of early Tertiary trans-tensional faulting and late Tertiary compressional folding. Gravity along with 2D & 3D seismic data have proved effective in locating and detailing these traps, many of which are filled to structural spill point and contain gas columns up to 1000 meters in height.

The pre-Tertiary gas play is significant in providing sufficiently large gas reserves to support economic development. High, infrastructure-related capital costs can be borne by long term contracts for sales volumes that exceed 1 TCF of gas. New pipeline infrastructure also opens the way for future sales from smaller fields in the basin. Current and future gas sales aid in strengthening Indonesia’s economy through generation of foreign income and displacement of coslty imported liquid fuels.

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