Geochemical Dynamics in Organic-Rich Mudstone Reservoirs: Interactions and Implications
Authors: Nair Amrita , Fernandez Carlos , Ling Mei , Gupta Ramesh 
Published: February 2025

This paper explores the intricate geochemical characteristics of organic-rich mudstone reservoirs, focusing on key factors such as thermal maturity, wettability, and adsorption properties. These reservoirs, often referred to as unconventional hydrocarbon systems, are composed of complex organic and inorganic phases that interact dynamically over geological timescales. Understanding these interactions is crucial for optimizing hydrocarbon recovery and developing effective reservoir management strategies. One of the central themes of this study is thermal maturity, which governs the transformation of organic material into hydrocarbons. As mudstones undergo burial and increased temperature exposure, organic matter, primarily kerogen, is thermally degraded into oil and gas. The degree of thermal maturity affects the composition, phase behavior, and mobility of hydrocarbons within the reservoir. High-maturity shales tend to exhibit lower oil viscosity and increased gas content, which influence the overall fluid transport and production efficiency. Moreover, thermal maturity impacts the wettability of the rock, altering the interaction between hydrocarbons and the mineral matrix, which in turn affects fluid retention and migration pathways. Another key factor considered is wettability, which plays a significant role in determining fluid distribution and flow behavior within mudstone reservoirs. Wettability refers to the preference of a solid surface to be in contact with either water or hydrocarbons. The organic-rich nature of these mudstones contributes to a highly heterogeneous wettability profile, with variations between oil-wet, water-wet, and mixed-wet conditions. These differences impact capillary pressures, relative permeability, and ultimately, the efficiency of hydrocarbon recovery. Experimental studies using contact angle measurements and nuclear magnetic resonance (NMR) techniques provide valuable insights into how wettability changes with thermal evolution and fluid exposure. In addition to wettability, adsorption properties are critical in dictating hydrocarbon storage and transport mechanisms. Due to their high surface area and microporous nature, organic-rich mudstones can adsorb significant quantities of gas, particularly methane, through physical and chemical interactions. Adsorption is influenced by factors such as mineral composition, organic matter content, and pressure-temperature conditions. Computational approaches, including molecular dynamics (MD) simulations and density functional theory (DFT) calculations, have been instrumental in elucidating the fundamental mechanisms governing gas adsorption in these complex systems. By integrating experimental findings with computational studies, this paper provides a comprehensive synthesis of the geochemical processes affecting organic-rich mudstone reservoirs. Insights from contemporary research are discussed, offering a nuanced understanding of the interplay between thermal maturity, wettability, and adsorption properties. These findings have important implications for optimizing hydrocarbon extraction strategies and improving reservoir management in unconventional energy resources.