Sequence Stratigraphy of Miocene Tide-Influenced Sandstones In the Minas Field, Sumatra, Indonesia

Abstract

Abstract The Miocene Sihapas Group in the Minas field, Indonesia, is composed of a succession of tens-of-meters-thick, erosionally based, tide-influenced sandstone intervals interbedded with marine shale-dominated intervals. Despite more than 1600 wells drilled into this 12-km-wide by 30-km-long mature super-giant field, complex internal tidal facies architecture within these sandstone intervals makes predicting local connectivity within the reservoirs challenging. Past interpretations of Minas field suggested that reservoir sandstone intervals are incised-valley deposits, based on assumptions that abrupt coarsening at the base of reservoirs reflects river incision into the shelf and tidal reservoir facies formed in coastal embayments during transgression. In this paper a new interpretation is proposed that suggests that shorelines remained strongly tide-influenced throughout regressive–transgressive depositional episodes. Pronounced discontinuity surfaces in these deposits are interpreted to have formed by a variety of mechanisms: (1) tidal erosion of the sea floor in front of regressing shorelines; (2) lowstand fluvial valley incision; and (3) the reworking of deposits during transgression of the shelf. Consideration of a wider variety of regressive and transgressive tidal depositional settings and erosional processes allowed revision of Minas field stratigraphy to better explain the observed complex facies variations that control reservoir production behavior. Minas field is divided into five major reservoir intervals that overlie low-grade metamorphic basement. The older deposits, comprising the S and D reservoir intervals, contain multiple upward-coarsening tide-influenced regressive shoreline successions, locally cut out by tide-influenced channel sandstone bodies, and capped by thin beds of marine shelf transgressive sandstone and overlying marine shale. Seismic images indicate that these deposits filled erosional basement topography during a period of long-term relative sea level rise. The upper three reservoir intervals (B, A2, and A1) are each interpreted to be fourth-order regressive shoreline deposits that were variably reworked by tidal currents during transgression. Successive intervals record progressively lower average rate of accommodation development, more rapid falling-stage regression, deeper lowstand incision, and greater reworking during intervening transgressions. Together, these three intervals comprise a third-order, flooding-surface-defined, forward-stepping sequence set deposited on a low-gradient continental shelf. Paradoxically, because intervals deposited during more rapid progradation tend to be thinner, in most wells more regressive sandstone intervals contain proportionally more open marine shelf deposits. This reflects that valley incision is localized, and regressive deposits that formed in lower long-term accommodation growth rate settings tend to be more completely reworked by tidal currents during subsequent transgression.