Sequence Stratigraphy of Tide-Influenced Reservoir Sandstones in Minas Field, Indonesia
Minas field in Indonesia is composed of a succession of tens of meter thick, erosionally-based, tide-influenced sandstones interbedded with marine shale intervals. Despite the 1600 wells drilled into this 15 by 30 mile mature super-giant, predicting local connectivity between reservoir layers and complex internal facies architecture within the sandstones remains challenging. In the past there was a tendency to interpret all basal erosion surfaces of sandstone layers as lowstand fluvial incised “sequence boundaries” and tide-influenced deposits as transgressive estuarine fills of valleys. A new depositional model is proposed recognizing the importance of erosion seaward of prograding tide-influenced delta fronts and deposit reworking during subsequent trangressions. This reinterpretation suggests the succession is not a regional transgressive succession of esturine filled valleys, but rather the main reservoir intervals comprise a forward-stepping succession of 3 sequences separated in most locations by marine shale. Successive sequences reflect progressively lower accommodation, greater falling stage deposition and lowstand incision, and greater reworking during intervening transgressions. The basal sequence (“B”) contains a 30 m-thick set of deltaic clinoform beds capped by a thick marine shale. These clinoforms are each several to 10 m thick, upward-coarsening beds that dip to the southwest at 1 to 4 degrees. The second sequence (“A2”) is a 15 m-thick, sharp-based deltaic sandstone locally incised completely by a 3 km-wide mud-filled tidal scour. Beds within the sandier regressive parts of this interval are nearly horizontal across the field. Upper parts of the tidal incision are filled by open marine shale and thin, discontinuous, bioturbated, shelf sandstones. The final sequence (“A1”) contains a deeply incised, tide-influenced fluvial filled valley, that trends nearly east-west. Moderately sinuous channels are observed within this fill. This valley fill, 40 m thick and 6 km wide) is capped by a glauconitic transgressive shelf sandstone. Falling stage deltaic deposits under valley interfluves were nearly completely reworked during transgressive shelf tidal reworking, leaving multiple Glossifungites surfaces overlain locally by up to pebble caliber marine lags. This revised stratigraphy better explains observed sedimentologic and ichnological variations within Minas field deposits.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009