Forced Regressive Wedge of the Early Cretaceous Sembar Formation, the Potential Reservoir Body Encased Within Outershelf Sediments; Lower Indus Basin, Pakistan
Authors: Ahmad Shakeel , Ghazi Shahid , Shahzad Rana Faisal , Ali Iftikhar , Iqbal Shahid , Akram Shakeel , Qureshi Muhammad Akram , Awan Uzair Hamid , Khan Imran , Ahmad Ghayoor 
Published: October 2023

The Sembar Formation (Hauterivian to Berriasian) is considered a significant source of rock in the Lower and Middle Indus Basins of Pakistan. The sediments are mostly composed of shales with subordinate sandstones and siltstones. The main objective was to identify potential reservoir sand bodies within the Sembar Formation (source rock) in the Lower Indus Basin, Pakistan by utilizing integrated sequence stratigraphic analysis in conjunction with forward stratigraphic modelling. Previously, the Sembar Formation has extensively been evaluated for its source potential but its reservoir perspective has not been evaluated yet. This research was carried out to determine reservoir potential and the possibility of stratigraphic mechanism within the Sembar Formation. A forced regressive wedge with the possibility of good reservoir quality has been identified with the help of integrated sequence stratigraphic analysis. These depositional features may have an effective stratigraphic trapping mechanism owing to the four-way sealing phenomenon.

The depositional sequences were identified in the Sembar formation coupled with their calibration with the global sea level curves. These calibrated sequences were then interpreted on available well and seismic (2D/3D) data followed by forward stratigraphic modelling of the potential system tract (Semb3-FSST) identified on seismic data.

The palaeogeography to simulate the deposition of Semb3-FSST was developed based on seismic and well data. The digital sea-level curve of Haq, (2014) was used as input for the simulation of this package. The simulation was reiterated until it matched the real data analysis. Finally, facies fractions were extracted from the modelling results.

The Sembar Formation can be divided into two main depositional trends i.e. prograding (lower half) and retrograding (upper half) on a long-term scale (2nd order sequence). Four 3rd order depositional sequences were identified including sequences Semb-0, Semb-1, Semb-2 and Semb-3 from bottom to top respectively. Semb-0, Semb-1 & Semb-2 sequences are present within the prograding part while the Semb-3 sequence is in the retrograding part of 2nd order sequence.

Seismic data represents that the Sembar Formation has well-developed clinoforms dominated by river systems, prograding from east to west. A forced regressive wedge has been identified just above the Semb-2 sequence in the falling stage system tract of the Semb-3 sequence (Semb-3-FSST). The forced regressive wedge may have good reservoir sands sourced by the erosion of the proximal shelf top during the rapid sea fall below the shelf break.

Forward Stratigraphic Modelling results of Semb-3-FSST reveal that it consisted of down-stepping clinoforms (each clinoform deposited during a period of about 0.6 m.y). The force regressive wedge has down-stepping clinoforms with more fractions of coarser sands concentrated in the topsets and upper parts of the foresets in each clinoform while its toesets have higher concentrations of shales/finer sediments.