Quantitative appraisal of tectonically-influenced hydrocarbon-bearing Late-Cretaceous fluvial depositional system, Southwest Pakistan using spectral waveform-based instantaneous lateral thickness variability static simulations

Abstract

Quantitative seismic reservoir characterization is among the finest advancements in seismic technologies for sub-surface exploration of fluvial depositional systems (FDSS). These FDSS energy resources are combinations of thinly distributed gas-bearing trapping configurations such as the meander sandy channels (CH) along with aggradational parasequences (PBARS) of transgressive system tract (TST), which are developed during the extensive rise of sea-level followed by the rigorous standstill, and hence, which fills the PBARS and CH stratigraphic traps with possible hydrocarbons due to vertical and lateral changes in the facies. These thin-bedded CH and PBARS are very sensitive to a certain frequency component, which bandlimited seismic amplitudes (product of density and velocity) fail to quantify the paleo-thickness, paleo-velocities, paleo-densities, paleo-inclinations, paleo-geomorphology, vertical and lateral extents etc. of the stratigraphic traps. These parameters provide deep insights for quantitative-based stratigraphic reservoir characterizations. This study utilizes the spectral acoustic waveform seismic components and broadband spectrally-decomposed acoustic spectral waveform-based instantaneous lateral thickness variability static simulations (SLTRS) to quantify the FDSS of Southwest Pakistan. The 9–57 Hz bandwidth processed frequency volume reveals a poor tuning frequency component of 28 Hz and lateral extent of the stratigraphic, which failed to predict the direct geomorphology of PBARS and CH. 43-Hz spectral waveform could reveal the geomorphology with parallel-to-wavy seismic reflections (SRCS) for indicating the presence of meandering channels and PBARS. However, this attribute failed to predict the stratigraphic pinch-out zones and exact paleo-inclination of the PBARS and CH. The SLTRS have resolved the parallel-to-wavy SRCS with seismic sedimentological constraints, i.e., lithology-impedance contrast, phase of hydrocarbon generation, tuning frequencies, and thickness of the stratigraphic reservoir configurations, which implicates the channels systems at the shelf position of the basin. These attributes were unable to be predicted using the bandlimited seismic amplitudes. The SLTRS have simulated the gas zone with 2.921 gm. /c.c simulated gas density [SGD], 7880 m/s simulated gas velocity [SGV], 19–25 m simulated gas thickness [SGT], and 1° simulated inclination of PBARS, which implicates the high sinuosity CH and PBARS stratigraphic trap [SAS]. Similarly, for transgressive-to-retrogradation sealing configurations, SLTRS have resolved 2.864 gm. /c.c [SGD], 7365 [m/s] [SGV], 12–14 m simulated seal thickness [SST], and >2° inclination, which implicates the transgressive seal. The vertical and lateral extend of the stratigraphic trap was 16 m (proximal locations) (eastern margins) to 60 m (basin wards) (western margins) of the gas field. The quantitative uncertainty analysis between the SGD [gm. /c.c], SGT [m] and SGV [m/s] show a firm R² > 0.90 with robust sea-level trends and the development of the aggradational to retrogradational parasequences. 43 Hz spectral waveform-based SLTRS have imaged very strong lateral amplitude attenuations observed from the 2.883 to 2.864 gm. /c.c [SGD], 22 to –21 m [SGT] on inverted density simulations, 7535 to –7365 m/s [SGV] on inverted velocity simulations, and 19 to –15 m [SGT], which confirms the development of the hydrocarbon-bearing PBARS during the standstill sea-level of TST. Consequently, these stratigraphic endeavours may serve as an analogue for the worldwide exploration of FDSS with similar geology and basin configurations.