Date of Graduation
Eberly College of Arts and Sciences
Geology and Geography
Amy L. Weislogel
Several biological and physico-chemical processes lead to the transformation of organic matter (OM) from simple organic compounds to a complex macromolecule and a mixture of hydrocarbons during the geological evolution of sedimentary rocks. These changes are controlled by biological activity in the early stages of burial, and by temperature and pressure in the later stages. The increasing temperature and pressure conditions with burial is termed as maturation, in which biomolecules are converted into petroleum. Three consecutive stages of maturation namely: diagenesis, catagenesis and metagenesis produce irreversible changes in the composition of sedimentary OM. The bulk characteristics (such as elemental composition) of OM starting from its deposition to the formation of oil and gas are well understood in the past using traditional methods such as source rock analyzer (SRA). However, a large knowledge gap still exists in understanding the processes involved in the transformation and evolution of OM during maturation at the molecular level. Understanding the molecular level properties of shale OM is important for precise estimation of hydrocarbons, increasing efficiency of HC recovery and production, designing effective CO2 sequestration and waste disposal strategies, and understanding mechanisms of contaminant release and sorption. The primary objective of my PhD dissertation is to understand evolution of sedimentary OM at molecular scale in Marcellus Shale. The Marcellus Shale is the largest natural gas producing reservoir in the United States and has been extensively drilled in the past decade. The amount of natural gas extracted from the reservoir has almost tripled due to advancements in horizontal 3 drilling technologies. As a result, thousands of wells have been drilled in areas of Pennsylvania, Ohio and West Virginia covering a wide range of maturation (from early oil window to overmature window) and paleo-depositional environments. Drilling of these wells provides access to several thousand feet deep core samples for biogeochemical analysis. In my study, I utilized core samples of Marcellus shale with variable maturity (ranging from 0.8 VRo to 3 VRo) and depositional environment. The variability in sources of OM, paleo-redox conditions and thermal maturation was determined using pyrolysis and biomarker proxies especially in samples from mature part of the basin. Once the information about source of OM and maturation was established, kerogen was extracted from the Marcellus Shale maturity series. Chemical composition and structural properties of the extracted kerogen was determined using advanced spectroscopic techniques. Using the distribution and changes in structural parameters of kerogen, an understanding was developed about kerogen cracking mechanism, its primary molecular components, and sources of hydrocarbon potential. Our findings indicated that previously published kerogen models significantly overestimated or underestimated HC potential, thermal maturity and other physicochemical properties of kerogen from Marcellus Shale. Reasons for such discrepancies are because the old models were derived from very limited number of shale samples and they do not account for heterogeneity in kerogen structure. This heterogeneity arises primarily due to variations in sources of OM (within a particular depositional environment), paleo-environmental conditions, and effect of differential kerogen cracking on thermal maturation. In our study, we developed new structural models and regression models of kerogen to determine thermal maturity and hydrocarbon potential of shales for almost the entire maturity window of HC generation (from peak oil window to dry gas window). Further, we compared the molecular structure of kerogen obtained from Marcellus Shale 4 with kerogen structures of other Shale Formations with similar kerogen type and thermal maturity. This comparison was done to examine the applicability and limitations of modelling physicochemical properties of shales based on kerogen “type”. STRUCTURE OF THE DISSERTATION I have divided my dissertations in four research papers focusing on research topics as follows: Chapter 1 is a research paper entitled ‘Testing Utility of Organogeochemical Proxies to Assess Sources of Organic Matter, Paleoredox Conditions and Thermal Maturity in Mature Marcellus Shale’. This research paper focuses on the efficacy of pyrolysis and biomarker proxies to determine source of OM, thermal maturation and paleo redox environment especially in mature shales. The highlight of this paper is that by using an improved methods of biomarker extractions and analysis, accurate information about the thermal and depositional history of mature shales like the Marcellus Shale can be determined. This paper is publised in the journal Frontiers in Energy Research. Chapter 2 is a research paper entitled ‘Molecular Characterization of Kerogen and its Implications for Determining Hydrocarbon Potential, Organic matter sources and Thermal Maturity in Marcellus Shale’. This paper focuses on using direct kerogen analysis to understand changes in structural properties of kerogen extracted from samples from three wells covering a thermal maturity range of 0.8 to 2.5 VRO. In this paper, molecular structural parameters of kerogen from upper and lower Marcellus Shale Formation of all the three wells were determined. Using these structural parameters, schematic models of unit kerogen were developed using theoritical calculations. This paper is publised in the journal Fuel. 5 Chapter 3 is a research paper entitled ‘Improved Kerogen Models for Determining Thermal Maturity and Hydrocarbon Potential of Shales’. In this research paper, we showed that the previous models based on structural parameters of kerogen overestimate the HC generation potential and underestimate the thermal maturity of Marcellus Shale samples. Further, by utilizing the kerogen molecular parameters determined in the Marcellus maturity series (covering thermal maturity from 0.8 to 3 VRo using core samples from 6 different wells), we proposed improved models for accurate estimation of thermal maturity and HC potential of shales like Marcellus Shale. This paper is in review in journal Nature Scientific Reports. Chapter 4 is a research paper entitled ‘Pitfalls in Modelling Physico-Chemical Properties of Shales Using Kerogen Type. In this research paper, we showed that the previous molecular models based on kerogen “type” does not incorporate heterogeneities that exists within a particular kerogen “type”. This could lead to underestimation/overestimation of several physicochemical properties of kerogen and shale which can have several implications. This paper is in review in journal Nature Scientific Reports.
Agrawal, Vikas, "Molecular Evolution of Sedimentary Organic Matter in Marcellus Shale." (2018). Graduate Theses, Dissertations, and Problem Reports. 8195.