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ABSTRACT

The Aman Trough of Central Sumatra, Indonesia has two associated major hydrocarbon accumulations, Minas and Duri fields. As with most maturely explored basins, more recent exploration activities have involved smaller structural closures, often testing new or different exploration concepts. Sidingin Gas Field, discovered in 1989, is located on an isolated fault block on the northern end of the Aman Trough. Wells in the field encountered reservoir stratigraphy quite different from that encountered in most wells in the Aman Trough.

The stratigraphy of the North Aman Trough demonstrates several scales of stratigraphic development: (1) a 2nd order tectonic (syn-rift) cycle, encompassing the entire Pematang Group (Oligocene), composed of stacked fluvial-lacustrine-fluvial members; (2) a series of higher order (3rd order?) sequences, best observed in the fluvial strata, composed of basal coarse fluvial sandstones overlain by floodplain shales; (3) parasequences of individual sandstone/shale couplets.

The fluvial rocks in the North Aman Trough and in the Sidingin field are interpreted in terms of regularly varying and repeating base-level cycles. The sedimentology and stacking patterns can be explained in terms of lowered base-level producing highly erosional coarser-grained fluvial channel sands, likely autocyclic in origin, giving way to individual channel and splay sands deposited as base-level rose, and finally capped by a muddy floodplain shale interval. In many cases, the tops of the shale packages are capped with a paleosol, interpreted as a remnant subaerial exposure surface consistent with a relative lowering of base-level.

Although well logs through Sidingin field display somewhat similar log character to wells within the Aman Trough proper, evaluation of seismic facies and petrography yielded different stratigraphic interpretations. The reservoir in Sidingin field is interpreted as an alluvial fan and fan delta interval, sourced by highlands associated with the rift basin border fault. The facies successions from oxidized, proximal alluvial fan to intercalated fan delta–lacustrine shale strata suggest that base-level variations related to the local structuring controlled stratigraphic development and distribution in the Sidingin field area.

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