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.
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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
Until a few decades ago, structural and regional geology were traditionally the preserve of field geologists. They usually mapped areas of outcropping deformed rocks and supplemented their work by laboratory studies of rock deformation and by theoretical work. Structural geology became tied to the geology of uplifts, folded belts, and underground mines, all of which were accessible to direct observation. Since World War II we have witnessed a tremendous development of geophysics in oceanography and in petroleum geology. Academic geophysicists in oceanography led their geological colleagues into modern plate tectonics and industry geophysicists developed reflection seismology into a superb structural mapping tool that penetrated the subsurface.
Today we are facing a situation where instruction and textbooks in structural geology are almost entirely dedicated to rock deformation, analytical techniques in detailed field geology and summaries of plate tectonics. Illustrations based on reflection seismic profiles are virtually absent in textbooks of structural geology. These texts illustrate only the parts of the proverbial elephant, together with some conjecture, but without ever offering a glimpse of the whole elephant.
Some of the reason cited for the relative scarcity of published reflection profiles are: 1) the confidentiality of exploration data; 2) difficulties in the photographic reduction and reproduction of seismic profiles for a book format; 3) the two-dimensional nature of vertical reflection profiles; and 4) the obvious distortions in reflection profiles that are typically recorded in time.
The AAPG leadership felt that it was time to attempt to correct the situation and to produce this picture and work atlas. The first volumes, of what may become a series of volumes, are addressing an audience that includes: petroleum geologists concerned with structural interpretations; exploration companies that provide in-house training; the AAPG continuing education program; and academic colleagues interested in updating their curricula in structural geology by inclusion of reflection profiles from the “real world” in their teaching.
The atlas is not meant to be a textbook in reflection seismology (instead we listed some at the end of this introduction) nor a text in structural and/or regional geology. Our intent is simply to provide a teaching tool.