The Society of Petroleum Geophysicists (SPG) Ahmedabad Chapter, organized a geological field excursion to Lower Reaches of Narmada Valley near Bharuch on 8 March 2025.

A total of 40 geoscientists from IRS, SST, RFS and WLS of ONGC Ahmedabad participated in the geological field excursion. Dr. Prabhin Sukumaran, Asst. Professor, Charotar University of Science and Technology, with over 18 years of experience in studying Quaternary landforms and fluvial processes acrossIndia, guided the participants during field trip. The field trip was flagged off by ED-Chief of Institute,IRSOm Prakash Sinha on 8 March. The geological excursion aimed to give participants a first-handexposure to geomorphology and fluvial processes.

A glimpse of the participants undertaking the field trip to Narmada Valley

Fluvial systems play a crucial role in forming reservoirs. Understanding how sediments are deposited, eroded, transported and redeposited during the Geological ages is essential for predictingthe reservoir extent and facies heterogeneity while preparing the sand model. Detailed knowledge of fluvial process at different geographical and time scale helps in constructing accurate reservoir model, which are vital for effectivereservoirdevelopment and management.

The lower Narmada Valley (LrNV) is an excellent natural laboratory for studying fluvial processes. It features a variety of landforms like point bar, channel bar, palaeobanks and floodplains. The valley’s geological and geomorphological features have been extensively studied, providing a wealth of data for training and research.

The river’scourse downstream of Kevadiya, known as LrNR, is divided into three segments based on dominant sediment types: gravel, sand, and clay rich deposits of channels. Situated along the eastern margin of south Cambay Basin, the LrNV exposes a range of geological formations, including Precambrian metasedimentary rocks, Cretaceous Bagh and Lameta beds, Deccan basalts, Tertiary sediments, and Quaternary deposits. The Quaternary sediments overlie Precambrian rocks, Bagh beds, Deccan basalt, and Tertiary sediments.

Google earth map of the visited locations

General Stratigraphic Setup at Uchediya

The LrNV contains higher and lower erosional surfaces along with alluvial plains. The late Holocene floodplain is relatively flat and gently dips northward, intersected by transverse-flowing number of tributaries like Kaveri and Karjan etc. The region also features well-preserved paleo- and neo-landforms within the floodplain, which offer insights into past fluvial activities.

In the morningsession Dr. PrabhinSukumaranexplain about the fluvial depositional systems and their characteristics in detail. The spatial and temporal extent, variation in reservoir facies and its impact on porosity and permeability. After briefing session, the team moved to first point to see the palaeobanks deposits.

Briefing Session by Dr. Prabhin

Location-1 (Palaeobank section):At the first location on the southern bank of Narmada a section with Gravel bed at the bottom overlain by a 5-6mts siltstone bed devoid of any sedimentary structures are seen which has been dated as 74000years old. The gravel deposits are represented by high energy alluvial fan deposits by small tributaries flowing across the Narmada whereas massive siltstone deposits are believed to be aeolian origin.

Participants interacting with Dr.ParbhinSukumaranat Location-1 (Palaeobank Section)

It is located at southern erosional bank of Narmada near Uchediya. This is the section where 2-3mts sand is sandwiched within 3-4mts of Clay layerat top and bottom that helps the sand bed to stabilise. A representative litholog of the section is given below.

Uchediya Section Location-2

Participants at Uchediya Section site at Location-2

Later participants took a walk along the Kaveri Rivera small tributary of Narmada. Kaveri river is a perfect example of fluvial processes happening present day. Various depositional landforms such as point bar; channel bar and lateral bar are visible at smaller scale compared to Narmada. Meandering processes such as river erosion and deposition are also seen while walking through the river.It was also demonstrated that the anthropogenic action also impacted present day fluvial processes which also resulted in different geomorphological fluvial features.

Participants taking a walk along the Kaveri River at Location-3

During the break, asummary of fluvial processes was explained to participants about the visited locations,at the scale of Narmada and Kaveri River.It was emphasized that the fluvial processesmay be anything in between and beyond.It was also explained to relate it while preparing sand maps during G&G assignments. Also briefing was given about the next location to be visited.

Location-4 Post-lunch team moved to the point bar near Moti Coral village on the northern bank of Narmada. The participantstook a number of traverses along and across the point bar and observed the variation of facies along and across the point bar.

Participant walking across the point bar on the northern bank of Narmada and observing the variation in facies at Location-4

Dr. Prabhin Sukumaran SPG Ahmedabad for giving him this opportunity to lead an enthusiastic group of professionalsin the field.

The day long field excursionenhances participants’ practical knowledge of fluvial processes and their sedimentary products. Such first-hand exposure in field will add value to reservoir characterisation and reservoir model building through the integration of geological and geophysical data.