Tectonically-subsidized shallow-marine Cretaceous gas-bearing stratigraphic traps, shelf-slope basin, NW-Arabian Sea: Seismic attributes and broadband porosity-constrained seismic acoustic impedance variability dynamical reservoir simulations

Abstract

Quantitative inverted reservoir simulations-based seismic reservoir characterizations facilitate accurate imaging of stratigraphic traps, e.g., incised-valleys infills (IVFs) of Lowstands systems tract (LST). These systems are sourced by fluvial channel sandstone bodies (CH) and meandering point bars (PBARS), which are developed during sea-level standstill along the parallel (P) and divergent (D) seismic reflection configurations (SRF). From the well control points, poor frequency seismic and well logs create hurdles to quantifying the seismic stratigraphic and sedimentological constraints. These constraints are comprised of lithology-impedance contrasts for paleo-erosional and depositional sediments, paleo-dense fracture networks, inclination of stratigraphic traps, and lateral changes in thicknesses of thin-bedded prograding parasequence sets (PPS) and aggrading parasequence sets (APS). Accurate prediction of these constraints may lead to locating direct hydrocarbon indicators (DHI). During the development of IVFs, there are also varieties of SRFs, which serve as the key constituents for developing petroleum. These SRFs are not resolvable at the full-spectrum seismic due to sub-seismic deficiency of poor seismic frequency resolution. These full-spectrum seismic provide poor indicators for sources of sedimentary influxes, which are restricted to predicting source-reservoir and basins' configurations. These implications are not feasible to predict using full-spectrum seismic and well-log data due to the poor resolution of subsurface systems. Consequently, this study utilizes state-of-the-art 29 Hz average energy waveforms and broadband porosity-constrained seismic acoustic impedance variability dynamical reservoir simulations to quantify the Cretaceous system of Northwest Onshore, Arabian Sea. The 29 Hz waveform could image 15–18 m thick sandstone-filled channels inside IVFs. However, this attribute failed to predict the pore spaces due to tuning effects. Average energy-based dynamical simulations at (R) 2 = 0.73 have simulated 24–62 m thick depositional and 17–18 m thick erosional facies from 25 % seismic-based porosities (Ps). ∼ 45 m thick en-echelon architecture was also resolved and simulated by dynamical simulations at 10 % Ps inside the IVFs. Yet, the 29 Hz waveform-based dynamical simulations show unique seismic stratigraphic and sedimentological expressions of IVFs. This simulation has also simulated lateral changes in the lithology-impedance contrasts from PPS-APS, which indicates that P was a key component for gas generation at the continental shelf of the basin. A 39 m thick P gas-bearing thin-bed trapped CH lens was simulated at 25 % Ps. 25 % Ps also simulated a 21 m thick erosional zone D along the shelf-to-slope location. A continuous shoreline trajectory was simulated along this trapped lens inside dense-fractured networks. This trajectory has generated strong lateral lithology-impedance contrasts for APS-to-PPS along the shelf-to-slope basin. The inclination of the deposited trapped lens was <1° with a low-gradient and high-sinuosity index (S.I) of 2.2. This 2.2 high S.I. implicates the PBARS being the key source of sedimentary influxes for the IVF stratigraphic trap. APS with 0.938–1.094 dB's spectral amplitudes implicate highly-fractured sedimentary facies, and hence, the development of tectonic subsidence. These stratigraphic reservoir architectures have simulated 51 to 52 m-thick regionally developed gas-bearing pore spaces, which serve as DHIs along shelf-to-slope basins. Consequently, this research endeavours have vital tectonics-stratigraphic implications, which may serve as an analogue for global extension basins.