Rock Physics-Based Pore-Filling and Seismic Amplitude Modeling of Barail Sandstone from Assam Geologic Province, India

This study attempted a rock physics template (RPT) based approach to model the impact of mechanical compaction and chemical diagenesis on overall porosity reduction of the Barail Sandstone across the Assam Shelf and Schuppen Belt area. Then, a sensitivity analysis was carried out based on the probabilistic amplitude variation with angle (AVA) approach to study the impact of diagenesis, rock frame constituents, and pore fluids on seismic amplitude response. The RPT depth trend suggests a significant upliftment (similar to 2500 m) within the supra-thrust region, while the Assam Shelf and sub-thrust part of the Schuppen Belt were normally compacted throughout the geologic time. The geologically constrained RPT was used to (i) identify the primary process for chemical diagenesis as feldspar dissolution and (ii) estimate total cement, non-contact cement (NCC), and contact cement (CC) values. Laboratory-based analysis of rock samples validated the modeled cement volumes. The RPT-based CC (mode value) estimates were observed as 3%, 5%, and 7%, whereas the NCC varied as 6%, 8.5%, and 11% sequentially from extensional through supra-thrust to sub-thrust regions. The extensional and supra-thrust regions dominantly exhibited class IV AVA for oil and gas saturated rocks mainly due to the presence of coal and the relatively under compacted nature of the Barail Sandstones. A change in AVA class was observed to class II and IIP within the sub-thrust region due to enhanced cementation. The forward modeling revealed that a small amount (2%) of coal presence could potentially change the AVA class from II to IV, and an increase in CC from 5 to 7% could cause the AVA class to change from IV to II. This study efficiently modeled the digenetic effects and seismic amplitude in this tectonically complex region.

Automated summary

This study used a rock physics template (RPT) approach to model the impacts of mechanical compaction and chemical diagenesis on porosity reduction in the Barail Sandstone. This study also analyzed the effects of diagenesis, rock frame constituents, and pore fluids on seismic amplitude response using a probabilistic amplitude variation with angle (AVA) approach.