T.P. Harding, 1983. "Structural Inversion at Rambutan Oil Field, South Sumatra Basin", Seismic Expression of Structural Styles: A Picture and Work Atlas. Volume 1–The Layered Earth, Volume 2–Tectonics Of Extensional Provinces, & Volume 3–Tectonics Of Compressional Provinces, A. W. Bally
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A seismic profile across the Rambutan oil field of the South Sumatra basin demonstrates structural inversion and is also used to illustrate seismic techniques for identifying superimposed deformation. Structural inversion on a large scale is the tectonic conversion of a previously low area, such as a graben, into a high region. When it occurs on this scale, it includes the deformation identified as "basin inversion" by other workers. On a smaller scale, it is the reactivation of faults in an opposite dip-slip sense, for example, normal slip followed by reverse slip on the same fault.
In the discussion that follows, I first outline the geologic history and the structural setting of the region in which the Rambutan oil field occurs. Next a structural interpretation of the Rambutan profile and its documentation of superimposed deformation are described. Lastly, possible mechanisms for the inversion of the Rambutan structure are discussed.
The South Sumatra basin (Figure 1) is Cenozoic in age and has had three general tectonic phases. During the earliest phase of tectonism, normal faults were active from latest Paleocene until, locally, as late as early Miocene (Harsa, 1975, Figure 3a). Eocene to early Miocene deposits are thickest within the down-dropped fault blocks. This normal faulting was followed by a tectonically quiescent phase from early Miocene to about early Pliocene. From Pliocene to the present, the basin has experienced basement-involved, backarc contraction.
The last tectonic phase produced the en echelon fold set that dominates the exposed structures of the basin and provides most of the hydrocarbon traps. Many of the fold closures are offset on their flanks by high-angle reverse faults, some of which are inverted normal faults that had their inception during the first tectonic phase.
The Rambutan oil field produces small amounts of oil from lower Pliocene and upper Miocene sandstones closed in one of these folds. The field is located near the boundary between two different structural provinces within the en echelon fold set: the Benakat "Gulley" and the Musi Platform (Figure 2; also de Coster, 1974, Figure 6). The Benakat Gulley is a trough of thick and deformed Cenozoic sediments; the Rambutan field lies in its western portion. The gulley and adjacent areas on the northeast (Abab Shelf) are underlain by buried grabens and half grabens, some of which have been inverted into a large, surface anticlinorium. Several major fields (e.g., the Talang Akar, Benakat, and Limau Trend fields, shown in Figure 2) produce from fold culminations on the anticlinorium. The Musi Platform adjoins the Benakat Gulley on the west and is a stable region essentially devoid of early normal faults and later compressive folds.