Simulating the stratigraphy of meandering channels and point bars of Cretaceous system using spectral decomposition tool, Southwest Pakistan: Implications for petroleum exploration

Abstract

Fluvial channels (FC) and point bars sandstone (PDs) sedimentary fills form primary brilliant stratigraphic petroleum systems within the transgressive system tract (TST). They are developed during the rapid rise and subsequent standstill of sea-level, which fills the reservoirs with FC and PD. These petroleum systems are ultra-thin-bedded (UTB) due to the very small vertical thickness of TST within the complete depositional sequences. Hence, they create hurdles in the prediction of lithology and porosity for these stratigraphic traps. The continuous wavelet transform (CWT) is the only sub-surface seismic imaging tool of spectral decomposition that can resolve the thin-beds of petroleum-bearing stratigraphic traps. This method removes the tuning effects of ambiguous lithology and fluids that masks the reservoir zones, and hence, generates false bright spots. This method decomposes the conventional seismic data into its constituent frequency components without fixing the vertical time window for the stratigraphic systems. These frequency components are further used for the prediction of lithology and porosity for resolving sub-surface depositional systems. Therefore, the CWT has delineated the fluvial sandstone facies, fractures connectivity, and architecture of the fluvial depositional system during the sea-level fluctuations. The 48-Hz has imaged the regional standstill of sea level, which have accumulated the porous sedimentary facies of coarse-grained FC andPDs. A very local sea-level rise was imaged in the eastern to the central part of this petroleum system. This sea-level fluctuation has implicated that there were minor cycles of transgressive sea-level, which has developed a pure stratigraphic play. The trace envelope attribute experienced a poor correlation coefficient with R2<0.2. The 48-Hz CWT experienced a correlation coefficient of R2>0.91 for FC and PDs. The conventional inverted reservoir simulation has imaged the low-frequency variance inside ~15-31% porous zones of FC and PDs. The 48-Hz volume has imaged the laterally continuous and hydrocarbon-bearing reservoirs with >25% porosity. 48-Hz CWT model also has imaged the coarse-grained FC of aggradational meandering channel stream, which were implicated to develop during stand-still of sea-level. These sandstone lenses were enclosed at the top by progradational shale facies of the highstand system tract (HST) and bottom and lateral by the shales of the TST. This workflow serves as an analogue within the SE-Asian sedimentary Basins and similar worldwide geological basins.