High Resolution Strontium Isotope Stratigraphy of L-III Reservoir of Mumbai High and adjoining Cluster 7 areas of Mumbai Offshore Basin, India

• Published in: Journal of the Geological Society of India 2024-08, Vol.100 (8), p.1157-1170
• Main Authors: Rathore, Shakti Singh, Gupta, Piyush, Sarkar, Argha Narayan
• Format: Article
• Language: English
• ISSN: 0016-7622; 0974-6889
• DOI: 10.17491/jgsi/2024/173964

High degree of reservoir heterogeneity has always been problematic in the identification and chronostratigraphic mapping of individual layers in a thick limestone sequence, especially when there is poor preservation of index fossils for thin sub-layers in the sequence. The multi-layered Miocene L-III reservoir of Bombay Formation in Mumbai High and adjoining southwestern area of the Mumbai offshore basin of India have suffered from similar issues due to lack of absolute ages and, therefore, the lack of highresolution chronostratigraphy for its individual thin limestone layers. The present study attempts to establish high-resolution chronostratigraphic age constraints of different sub-units, namely A, B, C, D, and E layers, of the L-III unit, using strontium isotope stratigraphy of carbonate (core and cutting) samples cut from 7 wells in Mumbai High and five wells in Cluster-7 area southwest to Mumbai High. The study provides a geochronological age range of ~2.1 Ma for the L-III unit, with the top of L-III in Mumbai High South assigned at 16.6 Ma based on Strontium isotopic analysis. The study has further helped in assigning absolute ages of individual layers of the L-III reservoir, with the A1 sub-unit at 16.6-16.8 Ma, A2 between 16.9-17.9 Ma, B sub-unit at 18.0 Ma and C & D sub-units between 18.2-18.3 Ma. Layer E of the L-III unit was dated to be 18.5-18.6 Ma, and the L-IV top has been assigned to be at 18.7 Ma. The study also correlated the ages obtained in the Mumbai High region with the studied wells southwest of Mumbai High and provided high-resolution strontium isotopic chronostratigraphy for equivalent sub-layers A to E in the Cluster-7 area SW to Mumbai High. These ages have provided new numerical age constraints for the L-III and its sub-units for the first time, are correlatable across different well sections, and are in perfect agreement with reported biostratigraphic ages for the Bombay Formation and the sub-units within. This has important implications for effective reservoir management and performance prediction for the hydrocarbon-producing pays in the studied L-III reservoir unit.