RESERVOIR CHARACTERISATION AND SOURCE ROCK STUDIES OF MESOZOIC AND TERTIARY SEQUENCES, JAISALMER BASIN, RAJASTHAN

Abstract

Understanding the fundamental aspects of the petroleum system is crucial for oil and gas exploration and extraction. How and where hydrocarbons originated and knowing about the reservoir properties helps in understanding where they can accumulate, along with evaluating how much reserves can be produced from a particular reservoir unit. Source rock evaluation and reservoir characterisation provide valuable information ready to be used for upgrading areas under investigation, concentrating exploration activities in specific places, and reducing risks. This thesis aims to characterise the reservoir facies by integrating the results obtained from microfacies analysis, associated clay mineralogy, and diagenetic factors with petrophysical, well log, and core analysis data of Mesozoic and Tertiary sequences, Jaisalmer Basin, Rajasthan, India. Furthermore, the quality of source rock, its capacity for hydrocarbon generation, kerogen classification, and sediment maturity across the spectrum from the lowermost Lathi to the uppermost Bandah Formation are ascertained via rock-eval pyrolysis and vitrinite reflectance investigations. The Rajasthan Basin is a Category-I Basin in the north-western part of India with an established commercial production of oil and gas and comprises Jaisalmer, Bikaner-Nagaur, and Barmer- Sanchor basins having different tectonic settings and ages. The spotlight on the Jaisalmer Basin primarily emerged due to hydrocarbon discoveries across the adjacent Pakistan border in the year 1914 and subsequently in the Indus Basin. The Jaisalmer Basin can be partitioned into four distinct tectonic units: the Kishangarh shelf, Jaisalmer Mari High, Shahgarh and Miajlar depressions, spanning from north to south. The basin is characterised by sedimentary deposits from Neoproterozoic to Recent epochs with sedimentary thickness ~ 7 km, exhibiting ~ 3° to 5° dip towards NW. This study undertook microfacies analysis, examinations of bulk and clay mineralogy, as well as scanning electron microscope (SEM) investigations. These endeavours encompassed the Jaisalmer Formation of the Middle Jurassic through to the Bandah Formation of the Middle Eocene, aiming to unravel the reservoir attributes inherent in these formations. In the subsurface, the Jaisalmer Formation is represented by lower clastic and upper carbonate sections. The limestone is oolitic and shows the tight nature of reservoir facies, leading to poor reservoir characteristics. The studies from the Baisakhi-Bhadesar (Upper Jurassic) Formation have calcareous quartz arenite as a dominant microfacies and reveal tight nature with negligible porosity. The decline in intergranular porosity primarily arises from authigenic kaolinite and extensive calcite cementation, which occupy the pore spaces. Pariwar Formation is an arenaceous sequence having shale bands in between and has multiple reservoir units. The sedimentological investigations reveal three distinct sedimentary facies within the Pariwar Formation: coarsegrained massive sandstones with intermixed bioturbated facies, thinly bedded sandstones, and massive fine-grained cherty sandstones. The coarse-grained massive sandstone facies are deposited in a foreshore environment. Notably, hydrocarbon production has been successfully established from the H2 and H4 reservoir units within this formation. Detailed petrographic studies conducted on selected samples indicate the prevalence of quartz arenite and calcareous quartz arenite as dominant microfacies, exhibiting moderate intergranular porosity in the reservoir sections. However, it is noteworthy that eogenetic calcite cementation significantly contributes to the decline in both porosity and permeability. Primary porosity is retained at many intervals, but occlusion is observed due to compaction and authigenic formation of chlorite and kaolinite. Petrophysical investigations conducted on the H2 and H4 reservoir units in fields such as Bankia, Kharatar, Ghotaru, Sadewala, and Manhera Tibba within the Jaisalmer Basin suggest that the uppermost H2 reservoir unit represents reservoir characteristics ranging from fair to good across different fields within the basin. Analysis based on the Winland plot indicates that the majority of pores in the examined samples exhibit meso and macro dimensions, indicative of enhanced flow efficiency. Cross plots generated from digital electrolog data provide insights into the mineral composition, outlining an envelope encompassing five principal minerals: quartz, glauconite, mica, cemented calcite, and kaolinite. Point distribution indicates the development of better sandstone facies within the Pariwar Formation compared to the Lower Goru Formation. Goru Formation is divisible into two Formations, namely Upper Goru and Lower Goru. Petrographic and SEM studies demonstrate that the Upper Goru Formation is devoid of reservoir characteristics; however, the Lower Goru Formation has different reservoir sand units having calcareous quartz arenite and glauconitic quartz arenite along with quartz wacke as the dominant microfacies with moderate reservoir properties. The Lower Goru Formation has two reservoir units, viz., G2 and G4, which are hydrocarbon-bearing in the different fields of the Jaisalmer Basin. Through comprehensive petrographic and petrophysical analyses, it becomes evident that the reservoir facies of G2 and G4 within the Lower Goru Formation, as observed in fields such as Bankia, Kharatar, Ghotaru, and Sadewala, exhibit average porosity levels ranging from fair to good. Simultaneously, the permeability of these reservoir sections tends to range from poor to fair. In the Tertiary, the Sanu Formation is an arenaceous sequence that has three main gasbearing reservoirs: D2, D4, and D6. D2 and D6 reservoir units are fine-grained, subangular to subrounded detrital quartz grain with calcareous and ferruginous matrix having good porosity, while the D4 unit is carbonate mudstone and wackestone. Khuiala Formation has two main reservoirs, viz., B4 and C2-C4. B4 unit is carbonate mudstone, while C2-C4 limestone is mainly packstone and algal wackestone with good porosity resulting from dolomitisation and solution activity. The reservoir facies in the Bandah Formation have lower porosity than the Sanu and Khuiala reservoirs.