Seismic attributes and spectral waveform-constrained porosity-controlled dynamical simulations of lower-Cretaceous shale gas-bearing Lowstands prograding wedges and deep-water submarine fans, NE-Indus rifting delta

Abstract

Quantitative seismic reservoir simulations are an accurate estimation tool for quantitative imaging of lowstand prograding wedge systems (LPW) of deep-water shale gas-bearing submarine fans. These LPWs are filled with coarse-grained sandstone lenses deposited during a significant sea-level fall, followed by a minimal rise. The key implication for imaging these LPWs is whether the shelf-break was developed. These are the key concerns that decide whether the shale gas systems are developed at the inner shelf edge or outer shelf-edge deltaic systems. During the development of stratigraphic traps, the sea level has a strong influence on the facies migrations and depositions. Additionally, the Indus Delta is dominated by the divergent plate margins. Therefore, there must be normal fault systems. These normal fault systems have played their role either in providing the lateral seals or the migration pathways for exploration of LPWs from this zone. Hence, the exploration becomes very ambiguous due to the influence of sea level and tectonics. Therefore, poor frequency-controlled sub-seismic data are constrained to quantify the LPW's reservoir rock types, thicknesses, location, inclination of LPW, fluid dynamics impedances contrasts (IDS) of low-frequency anomalies of hydrocarbon-bearing sandstones, and stratigraphic pinch-out zones. This research utilizes spectral attributes consortium and spectral waveform-constrained porosity-controlled quantitative dynamical simulations (PHVAS) on a gas field, NE-Indus Onshore. The 28-Hz envelope sub-bands-based (28-ESB) horizon slice have imaged 13 m thick and coarse-grained sandstones-bearing LPW at amplitude mapping, but failed to predict the inclined surfaces of LPW from the mid-to-upper slope geomorphology. Conventional PHVAS simulations with an R² < 0.70 produced LPW thicknesses of 20–44 m and impedance values of −0.883 gm./cc∗m/s, but remained poor in resolving the top sealing layer. 28-ESB PHVAS shows R² > 0.95 at seismic-based porosity [SBP] [%] of 25–30 %, images 42-48-m thick [STS] for organically-rich shale-plugged LPW at <2° inclination and highest sedimentary influx at −0.303gm./cc.∗m/s [SIS]. Western zones at 28-ESB experience −0.28 and −0.303gm./cc.∗m/s SIS and 28-Hz low-frequency anomaly below LPW, implicating regionally developed hydrocarbon-bearing progradational-to-aggradational sedimentary traps along horizontally-fluctuating SIS during fall-to-standstill sea-level. Eastern zones at 28-ESB PHVAS experience −0.2 and −0.169-gm/c.c.∗m/s SIS, 8-10-m STS lateral transgressive seal at 5–10 % SBP, which are inclined at >5°, implicating rising sea-level and stratigraphic pinch-out. PHVAS has also predicted no-fault and shelf-break exposure along this shale gas system. This indicates that the LPW was developed in the upper slope zones of LPW. This implies that the vertical and lateral migrations were categorically controlled by the vertical and lateral changes in the rock type within the stratigraphy of LPW. Consequently, this workflow serves as an analogue for exploring global basins.