Imaging of Stratigraphic Pinch-Out Traps Within the Lower-Cretaceous Shaly-Sandstone System, Pakistan, Using 3D Quantitative Seismic Inverted Porosity–Velocity Modeling

Abstract

Incised valleys form excellent stratigraphic pinch-out traps. Traditional seismic data analysis techniques fail to predict quantitatively the porous and low-velocity sand-fills for incised valleys. The 3D quantitative seismic inverted porosity–velocity (3DQSIPV) analysis was applied in the Indus Basin, SW Pakistan. The reflection strength attribute better portrayed the reservoir sandstone and faults compared to seismic amplitude attribute. The sweetness-based continuous wavelet transform authenticated the development of the stratigraphic play. The 17 Hz amplitude delineated the non-porous seal and porous reservoirs of sand-filled incised valley and strand plain, and faults. The integrated model of seismic attributes categorizes the reservoir and seal constituents. The petrophysical modeling corroborated the gas-bearing “sweet-spots” within the stratigraphic-based dynamical system. The facies modeling predicted the for coarse-grained sandstone and fine-grained shales, depositional environments, fluctuations of sea level and their impacts on the overall development of stratigraphic plays. The predicted density and P-wave velocity for the sandstone-filled incised valley of the lowstand system tract were ~ 1.4–1.75 g/cc and ~ 3217–3802 m/s, respectively. The predicted density and P-wave velocity for the sealing shales facies of strand plain of transgressive system tract were ~ 1.9–2.1 g/cc and 2.55–2.7 g/cc and 3900–4700 m/s, respectively. The 3DQSIPV predicted > 25% porosity and ~ 3300 m/s velocity of reservoirs in the west. The eastern zones shows < 12% porosity and high velocity of ~ 4580 m/s. Cross-plots of porosity, velocity, and thickness showed correlation coefficients of R² > 0.90 for inverted velocity. This workflow may serve as an analogue for the remaining oil and gas fields of the Indus Basins of Pakistan and similar geological settings of divergent plate margins.

Appendix

Strand Plain

A strand plain or strand plain is a broad belt of sand along a shoreline with a surface exhibiting well-defined parallel or semi-parallel sand ridges separated by shallow swales. A strand plain differs from a barrier island in that it lacks either the lagoons or tidal marshes that separate a barrier island from the shoreline to which the strand plain is directly attached. In addition, the tidal channels and inlets that cut through barrier islands are absent. Strand plains typically are created by the redistribution by waves and longshore currents of coarse sediment on either side of a river mouth. Thus, they are part of one type of wave-dominated delta (Fitzgerald et al., 2007; McCubbin, 1982).

Incised-valley

An incised valley forms when a river has cut into its floodplain or underlying strata sufficiently that even at the flood stage, the flow does not overtop the banks. The formerly active floodplain is left abandoned to serve as interfluves (Posamentier, 1999). Valley incision can occur as a result of base-level fall, tectonic tilting of an alluvial plain, and/or decrease in fluvial discharge to form underfit streams. During the stage in which valley incision of the underlying substrate occurs, fluvial sediment is transported downstream, beyond the confines of the valley. Filling occurs mainly during the turnaround and rise of the base level. The fill of a valley can be quite complex, both laterally and vertically. Laterally, the same meandering-river processes that were described earlier in this chapter occur in incised valleys, so the resulting deposits are equally complex. The horizontal transect of a valley fill also varies in the downstream direction, with primarily fluvial deposits upstream and more marine-influenced deposits downstream (Bowen & Weimer, 2003). Vertically, the ideal valley-fill sequence consists of a basal fluvial lag, which grades upward into fluvial sandstones, and these are overlain by estuarine sandstones and mudstones and then shelf mudstones. However, the vertical sequence will exhibit a preponderance of fluvial strata upstream and more estuarine strata downstream, closer to the marine environment.