Recovering original type and abundance of organic matter in spent source rocks: A review and advances in elemental proxies

Recovering the original organic matter properties (e.g., organic matter type and abundance) of source rocks during high matura-tion to overmaturation (spent source rocks) is the foundation for accurately understanding their original hydrocarbon generation potential, but it is extremely challenging due to the lack of effec-tive proxies. In this paper, the research status of this field is systematically reviewed, and it is found that a series of evaluation methods proposed meet critical challenges, due to the harsh con-ditions, complicated calculation, uncertainty of parameter setting, high test cost and limited application, and more. In contrast, inor-ganic elemental geochemical methods show unique advantages. Based on a case study in the overmature source rocks of the lower Cambrian Qiongzhusi Formation in the intracratonic rift of the Sichuan Basin, South China, the recovery model was established by the element geochemistry method. These data, combined with organic geochemical data, such as RockEval pyrolysis parameters and measured total organic carbon contents, were used to deter-mine the original total organic carbon contents based on an improved mass balance model. Based on further comparison with existing examples, the general recognition is summarized. The inorganic geochemical method is able to reliably determine the original hydrocarbon generation potential, but the applica-bility of the elemental proxies needs to be carefully considered, and the corresponding indexes should be selected according to the depositional setting of the source rocks. This would allow accurate evaluation of conventional hydrocarbon resources and prediction of unconventional shale oil or gas sweet spot reserves.