Many oil reservoirs have passed their production life and oil production is reducing. These fields may be termed as depleted oil fields. Depending on the type of reservoir, a great portion of initial oil in place (30-80) is still left as the residual oil (Bauer et al. 2022). Current climate policies which are against carbon dioxide (CO2) producing fuels make it less and less attractive to produce this remaining oil by employing various enhanced oil recovery technologies until abandonment. These reservoirs in particular may be well-suited for alternative forms of production or for the use of new fuels for field rejuvenation since there is typically sufficient infrastructure in the form of platforms, pipeline networks, and wells, and there is sufficient understanding regarding the subsurface behavior (Veshareh et al. 2022). Hydrogen, as one of the green and sustainable fuels, can be produced under subsurface conditions (Nicole et.al, 2022; Bhutto et.al, 2013; Hajdo et.al, 1985; Veshareh et.al, 2022). Especially, hydrogen production in oil and gas reservoirs has been extensively researched (Kapadia 2009, 2013; Murthy et. al. 2014; Strem et al. 2021; Wang & Gates, 2020; Bauer et al. 2022). Two main techniques can be applied to produce hydrogen from depleted oil reservoirs, namely, 1) Thermal hydrogen production by in-situ combustion; 2) microbiological hydrogen production by dark fermentation. Since microbiological hydrogen production requires less energy, it is a very promising method. Using microorganisms that use a carbon source as a substrate, hydrogen can be produced (Sivaramakrishnan et al. 2021). This is known as biohydrogen. Depending on the substrate and the microbe, biohydrogen can be produced by a variety of methods, such as bio-photolysis, indirect photolysis, dark fermentation, photo fermentation, and microbial electrolysis. Because dark fermentation has the potential to produce more hydrogen, with simpler operating restrictions, and can improve pH conditions, this method of hydrogen production has garnered a lot of attention (Supriya, 2019; Sivaramakrishnan et al. 2021). Many other techniques for pretreatment, including thermochemical, ultrasound, thermal, alkali/acid, chemo mechanical and heat shock have been used for hydrogen generation. However, these techniques either are challenging to deploy especially in offshore settings in Malaysia, difficult to use, or produce hazardous chemicals that are harmful for the environment. Therefore, there is a strong interest in the biological production of hydrogen from reservoirs by manipulating the growth of existing hydrogen-producing microbes. This manipulation involves selecting specific substrates or nutrients that may already be present in the fluids. Recognizing the interconnected nature of biological processes with thermal, hydraulic, mechanical, and chemical processes has led to an enhanced comprehension of the importance of hydrogen-metabolizing microorganisms in subsurface environments (Lu et al. 2011). Microorganisms can be found in a variety of environments, microbial populations in samples from various oil and gas fields have been extensively reported in subsurface environments (Silva et al. 2013). Native microbes play an important role in the deep biosphere and can survive in these oil reservoirs' extreme conditions. Oil exploration by humans has impacted the microbial dynamics in these environments. According to a recent study, members of the families Thermotogales, Clostridia, Deferribacteres, and Proteobacteria dominated the microbial community composition in offshore oil fields that used nitrate-altered seawater as injection fluid (Vigneron et al. 2017). The fermentation and degradation of detrital organic matter, acetogenesis and the degradation of aromatic hydrocarbons, and nitrate reduction and oxidative processes such as the oxidation of hydrogen sulphide or sulphur are the primary metabolic processes of these populations. Oil production causes significant systematic changes in microbial communities Vigneron et al. (2017). Copyright © 2024, Offshore Technology Conference.