Seismic assessment of maturity and richness in carbonate source rocks Mita Sengupta*, David Jacobi, Shannon Eichmann, and Brad Wallet, Aramco Services Company Yazeed Altowairqi and Salma Alsinan, Saudi Aramco Summary Evolution of intra-kerogen porosity is an important physical consequence of thermal maturation. The properties of the fluid within the organic porosity also evolve as a function of maturation. Evolution of organic porosity and pore fluid affects the elastic properties of the source rock. It has been shown in the past that the acoustic impedance of organic- rich source rocks decreases with both increasing organic fraction and increasing maturation. Recently, it was also shown that the relative differences in maturation-induced bulk and shear softening, or the bulk-to-shear ratio can be a potential maturation indicator. Fluid evolution with maturation adds complexity to the problem, because the pore-fluid can be different at different stages of thermal maturation. We show that it is possible to model the effects of fluid evolution using the knowledge that the pore-fluid typically goes from heavy oils in immature rocks to lighter oil, condensate, and finally to gas, as the rock matures. The elastic response of the source rock is a function of its organic content, organic porosity, and pore-fluid properties, where the latter two properties are determined by the degree to which the kerogen has matured. We present a rock physics forward model and inversion scheme to predict and estimate organic fraction and maturity from seismic in source rocks. Using pre-stack seismic data, we demonstrate the estimation of organic content and maturity from compressional and shear elasticity. Introduction A source rock is a fine-grained sedimentary rock that is rich in organic matter and capable of producing hydrocarbons. Source rocks where the organic matter has thermally matured, but the hydrocarbons formed during maturation have not migrated, are known as unconventional reservoirs. Hydrocarbons trapped in tight source rocks are difficult to characterize and challenging to extract. To assess the generative capacity of source rocks, total organic content (TOC) and maturity measures such as Hydrogen Index (HI) are often utilized. It has been proven that seismic data can be used to estimate organic content (Hu et al. 2015; Prasad et al. 2011). Increases in organic content result in lower elastic moduli, velocities, and impedances, according to physical observations and effective medium theories (Mavko et al. 2009; Vernik 2016; Sayers 2013). TOC has also been predicted from acoustic impedance to assess the organic richness of source rocks away from wells. (Broadhead et al. 2016; Hakami et al. 2016). Organic content, maturity, and mineralogy of source rocks also affect the velocity-ratios (VP/VS) and Poisson’s ratio of organic-rich rocks (Sengupta et al. 2020). Elastic moduli of kerogen are difficult to measure (Ahmadov et al. 2009; Yan and Han 2013), especially its shear modulus. A few consistent observations and models can be found in the literature, that relate kerogen maturity to its VP/VS ratio (Behura et al. 2009; Ibrahim and Mukerji 2017). These authors suggest that while increasing maturity is expected to reduce both compressional and shear moduli of kerogen, the shear modulus reduces faster than the bulk. Some authors have explored the elastic response to maturation (Allan et al. 2016; Zargari et al. 2016), but seismic assessment of organic maturity, a challenging, multi-scale and multi-disciplinary problem, remains a topic of ongoing investigation. The formation of intra-kerogen porosity (Alsinan 2017), progressively increases as the rock matures. The pore-fluid within the kerogen also evolves along with maturity, from heavier oils to lighter oils, condensates, and finally to gas. These geochemical changes have an impact on elastic moduli of the organic content, and consequently on the elastic moduli of the entire rock composite. A rock physics forward model can use effective medium theories to predict compressional and shear moduli from organic content and maturation. Similarly, both organic content and organic maturity can be jointly derived from pre-stack seismic inversion products such as Acoustic impedance and VP/VS ratio. While we have used the most popular maturation index, richness indicator, and inversion products, our forward model and inversion scheme are flexible enough to accommodate others. Source Rock Properties Analysis of scanning electron microscope (SEM) images, micro-X-Ray fluorescence (XRF) images, well logs, and rock-eval analysis were integrated to understand the basic mineralogy, composition, and texture of our source-rocks of interest (Sengupta et al. 2019). The rock matrix is calcite dominated, with small amounts of pyrite, quartz, dolomite and other minerals. Layers of organic matter (kerogen) occur within the inorganic matrix. TOC and maturity from rock- eval analysis of core samples coupled with co-located sonic and shear velocities from well-logs were used to constrain model parameters to correlate to geochemical properties (such as mineralogy, organic matter, and thermal maturity) and the related geophysical responses (i.e., density, sonic, and shear velocity) were used to develop the rock physics model. Intra-kerogen porosity, clearly visible in the mature samples, showed that porosity evolution is an important consequence of organic maturation. We expect that the increase in kerogen porosity, that is a result of hydrocarbon 10.1190/image2022-3736741.1 Page 2283 Second International Meeting for Applied Geoscience & Energy © 2022 Society of Exploration Geophysicists and the American Association of Petroleum Geologists Downloaded 08/19/22 to 3.229.134.117. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/page/policies/terms DOI:10.1190/image2022-3736741.1