Abstract

The Sanga Sanga Block contains four large to giant hydrocarbon fields in mid-to upper Miocene deltaic sandstones of the Mahakam Delta, eastern Kalimantan (Indonesia). These fields occur in the northeast-trending Mahakam fold belt, which is characterized by long, tight, fault-bounded anticlines and broad synclines and cored by overpressured shales. Onshore sections of the fold belt are strongly deformed, uplifted, and eroded, whereas the eastern offshore sections are little deformed and buried by the progradational delta wedge. Section balancing of depth-converted seismic lines, together with scaled analog modeling, was used to develop a new tectonic model of inverted delta growth faults for the evolution of the Mahakam fold belt. Section balancing shows that the fault-bounded anticlines of the Sanga Sanga Block are formed by contractional reactivation of early delta-top extensional growth faults. The change from gravity-driven extension to regional contraction occurred at around 14 Ma. Anticlinal folds controlled local sedimentation patterns and influenced the distribution of the reservoir channel sands in the main hydrocarbon fields. Scaled analog models of progradational loading above a ductile substrate produced delta-top extensional growth faults and "depobelts," together with delta-toe fold-thrust. Contraction inverted the extensional growth faults and depobelts, producing tight, fault-bounded anticlines. The results support the model of delta inversion and, thus, the most viable explanation for the geometric, kinematic, and mechanical evolution of the structures in the Sanga Sanga Block. The inverted delta model has applications to other hydrocarbon-bearing deltas around Borneo and in other contracted delta systems.

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