Biogenic Gas Generation From Shallow Organic-Matter-Rich Shales

Abstract

Understanding both physical and biological processes of gas generation and movement in immature organic matter rich shales is essential to optimize gas production from this resource. As yet, there is no complete description that accounts for the many gas transport modes in these systems. Field production data reveals that gas production from these reservoirs declines initially and then stabilizes after a specified time. The stabilized rate is controlled by the contributions of biogenic gas generation, desorption of gas from kerogen, diffusion and transport of gas through nanometer to potentially even micron scale pore systems in coarser grained intervals where physics of transport differ in mechanism. One factor that is ignored is the biogenic gas generation rate and whether this is significant in recharging the resource during a resource well lifetime. This paper presents a modified gas material balance to account for biogenic gas generation. The results of the theory are then compared to gas production data obtained from a shallow shale reservoir in Western Canada. Production from biogenic shale gas reservoirs tends to be at low rate but is stable over extended periods of time. However, it is not yet well understood what the dominant gas transport mechanisms are so that production from these reservoirs can be improved and optimized. Although the gas material balance approach is well established, it has not been used to constrain biogenic gas generation rates. The analysis conducted in this research can be used to optimize productivity from these fields. The new theory helps reveal the key physical controls on gas production from organic shales and means to enhance it. The theory developed in this study is novel and significant because it further develops the underlying theory for biogenic gas transport and production from organic shales, a potentially massive gas source in Western Canada and worldwide.

Miocene Biogas Generation System in the Carpathian Foredeep (SE Poland): A Basin Modeling Study to Assess the Potential of Unconventional Mudstone Reservoirs

“…The numerical analysis was intended to (i) investigate the dynamics of generation, expulsion, and migration of biogenic gas, referring to structural, lithological, petrophysical, geochemical, thermal, and other factors; and (ii) identify the conditions that facilitate gas accumulation in unconventional reservoirs in mudstone-claystone formations. The identification of crucial elements of the model of the Miocene biogenic system was based on up-to-date results of geological, structural, petrophysical, and geochemical studies [36][37][38][39][40][41] and supported by publications dealing with the theory of biogenic reactions [33,34,[42][43][44].…”

“…In some intervals of the Miocene profile, appropriate conditions for biogenic generation were present, while for intervals buried deeper, it could already be too hot for bacterial activity, but thermogenic reaction could have started. The mechanism of microbial gas generation has already been described in a number of papers, e.g., [34,42,43]. According to those authors, the organic matter dispersed in buried sediments was subjected to transformations: The less resistant molecules (e.g., nucleic acids and proteins) were quickly decomposed by bacteria, whereas the more resistant compounds were transformed under both pressure and temperature into protokerogen and then kerogen, the latter resistant to bacterial activity [34].…”

Comparison of biogenic gas fields in the Western Canada Sedimentary Basin and Qaidam Basin: implications for essential geological controls on large microbial gas accumulations

“…The SAGF petroleum system is inferred to be a self-sourced microbial system in thermally immature source rocks (Cokar et al, 2010;Fishman et al, 2012). The SAGF petroleum system is inferred to be a self-sourced microbial system in thermally immature source rocks (Cokar et al, 2010;Fishman et al, 2012).…”

“…Porosity and permeability data (Table 1) indicate some conventional reservoir quality, but these are localized areas within an otherwise tight, continuous reservoir (Cokar et al, 2010). Porosity and permeability data (Table 1) indicate some conventional reservoir quality, but these are localized areas within an otherwise tight, continuous reservoir (Cokar et al, 2010).…”

“…Like many unconventional shale gas accumulations, the SAGF and Sanhu Sag biogenic gas accumulations are self-sourced and self-contained systems (Fishman et al, 2012;Cokar et al, 2010;Zhang et al, 2013). Biogenic gas is generated from organic-rich mudstone/shale that becomes trapped in reservoirs that are interbedded or encased by the source rocks.…”