--> --> Outcrop Analogues for Subsurface Sand Body Geometries in Regressive and Transgressive Mahakam Delta Successions, Indonesia

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Outcrop Analogues for Subsurface Sand Body Geometries in Regressive and Transgressive Mahakam Delta Successions, Indonesia

Abstract

Detailed sedimentologic, stratigraphic, and biostratigraphic analysis indicates that both regressive and transgressive deltaic successions comprise the continuous, nearly 400 m thick vertical succession of middle Miocene, Mahakam Delta Province Balikpapan Formation that is exposed in the 600 m wide Grand Taman Sari Circuit outcrop. There are two major lithofacies associations: 1) a 115 m thick, fluvial-dominant regressive succession, and 2) an overlying 280 m thick transgressive shallow marine succession that becomes increasingly marine-upward. The shallow marine succession is tide-dominant and includes progradational and retrogradational parasequences within highly aggradational, retrogradational parasequence sets. In the regressive succession, individual fluvial channel sandstones are up to 5 m thick and are occasionally stacked to a total thickness of 18 m. Fluvial channel sands are less than 300 m wide so even stacked sands cannot be correlated across the outcrop. Conversely, there are back-filled distributary channel sandstones in the transgressive succession that are up to 20 m thick and laterally continuous across the entire outcrop. Both types of channel sandstone have very good reservoir potential with respect to porosity and permeability but they generate significantly different sand body geometries. The back-filled distributaries sandstones will be as wide as the distributary they fill (typically about 1 km in the Mahakam Delta Province) and have its relatively straight, shoreline-perpendicular shape while the much narrower fluvial channel sandstones will have the shape of a meandering channel. Although both types of sandstone have similar thicknesses in the Grand Sari Taman Circuit outcrop, the thickness of back-filled distributary sandstones is not related to channel width so thickness can be highly variable and markedly different from fluvial channel sandstones within the same succession. It may be difficult to distinguish the two types of channel sandstone in the subsurface because: 1) they can occur in very close vertical stratigraphic proximity (< 25 m in the Grand Taman Sari Circuit outcrop), and 2) they generate nearly identical well log signatures, as both fine-upward over very similar stratigraphic thicknesses. Integrated sedimentologic and biostratigraphic analysis, coupled with seismic horizon slicing, may provide adequate resolution.