Seismic attributes and quantitative stratigraphic simulation’ application for imaging the thin-bedded incised valley stratigraphic traps of Cretaceous sedimentary fairway, Pakistan

Abstract

Seismic attributes play a vital role in the stratigraphic reservoir characterization of incised valley systems. These systems are developed during the rapid fall and subsequent rise of sea level, which affects the reservoir connectivity within the thin-bedded stratigraphic traps. Hence, the prediction of vertical thickness, lateral extent, and angle of stratigraphically trapped porous reservoirs within intercalated shales become an obstacle for oil and gas exploration. Therefore, this study is focused on quantitative prediction of incised valley systems by applying the inverted seismic simulations and continuous wavelet transform-based (CWT) post-stack seismic attribute analysis on a gas field, SW Pakistan. The seismic amplitude has delineated limited dissemination sandstones and faulted scheme compared to the instantaneous frequency. The instantaneous frequency has detected a low-frequency anomaly below a 23 m thick sandstone-filled fractured reservoir, which has a lateral extent of 7.5 km. The acoustic impedance-based static reservoir model has resolved a 23 m thick incised valley stratigraphic sandstone lens with a lateral extent of 26 km within the low-impedance zones, which was equal to the tuning thickness threshold (TTT) of 26 m. The inverted velocity modelling has remained a very poor tool, which could image a 6 m thick sandstone reservoir. The response of the band-limited acoustic impedance-based thickness model (AITM) was better compared to the velocity model, which has predicted a 49 m thick sandstone lens. But, due to tuning effects of shale-bearing facies, the actual well locations couldn't be accurately imaged by AITM. The response of 19-Hz CWT-based processed AITM has predicted the exact drillable locations for a 69 m thick part of the hydrocarbon-bearing fluids and demarcated the gas-water-contacts (GWCs). These gas-bearing sandstones are encased within the high-impedance shales, which have proved as an analogue for regional lowstand depositional systems. The 19-Hz frequency has accurately demarcated the zones of regional subsidence and uplifting within the significant sea-level fluctuations. The subsidence zones have experienced a 69 m thick zone of gas-bearing fluvial channelized reserves during the falling stage of the sea level and subsequent growth. Therefore, the identified fluvial-dominated incised valley was developed during the falling stage of lowstand system tract (LST), and hence, the presented working scheme can be successfully applied for the exploration of remaining reserves within the Indus Basins of Pakistan and can act as an analogue to similar geological basins.