Loss of Organic Carbon From Source Rocks During Thermal Maturation: Past, Present, and Future
Much of this poster focuses on the work of Daly and Edman (1987) that used theoretical and experimental methods to demonstrate the loss of organic carbon from source rocks during thermal maturation. Then, using the theoretical and experimental data generated from this study, a simple technique that assumes only a basic knowledge of organic matter type and maturity level was developed for calculating initial TOC contents from measured residual TOC values. The purpose of this work was to clearly document the magnitude of organic carbon loss during maturation and provide a straightforward method for restoring initial TOC levels during basin evaluation. This study therefore represents one of the earliest accounts documenting the importance of TOC loss during maturation. At the time this work was done, although decreases in pyrolysis yields during thermal maturation were generally factored into source rock evaluation, reductions in TOC were often ignored. Unlike earlier work such as that of Dow (1977) and Durand‐Souron (1980), which indicates mixed opinions regarding the significance of TOC reduction during maturation and expulsion, Daly and Edman (1987) combined and compared the results of theoretical calculations derived from a van Krevelen diagram with the results of two different experimental approaches both of which measured TOC and pyrolysis data. This combined approach indicates that organic carbon loss during generation and expulsion may be as high as 70% in Type I kerogen, is approximately 50% in Type II kerogen and ranges from 12% to 20% in Type III kerogen. In addition, the combined approach provides a simple technique for calculating initial TOC contents from measured residual TOC values that extends beyond the limited thermal maturity levels (end of the oil window, approximately 1.4 %Ro, where pyrolysis yields become uniformly low) of charts yielding original kerogen quality published by Orr (1983) and Espitalie et al., (1984). TOC content is thus the least subjective and most quantitative measurement that can be made in evaluating rocks at elevated maturity levels, and, by incorporating the method used by Daly and Edman (1987), explorationists could have obtained a more accurate assessment of volumetric calculations regarding the amounts of hydrocarbons generated in areas that include mature and overmature strata. Based on the work of Daly and Edman (1987), TOC loss during thermal maturation was quickly incorporated into Platte River’s BasinMod 1D basin modeling program. Another notable development during this same time period includes among the first uses of well logs to identify and calculate total organic carbon in organic‐rich rocks (Passey et al., 1990). The methods and concepts developed in the 1980’s and 1990’s have often been updated and modified to fit the needs of exploration and production in unconventional reservoirs. Additional methods for the back calculation of original TOC have been developed, and work continues to be done on the use of well logs to determine TOC content, thermal maturity and porosity. For example, the skeletal density of kerogen varies with its maturity, and the typical range is from 1.1g/cc for low maturity kerogen up to 1.6g/cc for high maturity kerogen. Another point of interest is where does the TOC that is lost during generation and maturation go? Among the developments since Daly and Edman (1987) is more detailed work on the type of hydrocarbon (oil, volatile oil, condensate/wet gas, dry gas etc.) that is generated and expelled at a given maturity level. Furthermore, assuming 35% carbon loss due to generation, there is about a 9.8% porosity increase due to organic carbon decomposition that creates space for hydrocarbon storage (Jarvie, 2006). The creation and assessment of organic porosity is a focal point of interest for unconventional resource plays (Katz and Arango, 2018). With regard to future work, below are some of the topics that merit further investigation: 1) There will likely be continued work on the origins of organic porosity and improvements in back calculation of original TOC. 2) It is also likely that the use of logs, particularly NMR T1, T2 and diffusion data, will continue to be used to characterize porosity, subsurface fluid types, saturations and wettability. 3) While 3D seismic data is being used routinely by numerous companies to predict the mechanical properties and density of various formations, there has yet to be a direct link made between TOC loss during organic carbon conversion and the associated changes in rock properties (House and Edman, 2018).
AAPG Datapages/Search and Discovery Article #90349 © 2019 AAPG Hedberg Conference, The Evolution of Petroleum Systems Analysis: Changing of the Guard from Late Mature Experts to Peak Generating Staff, Houston, Texas, March 4-6, 2019