Evaluation of gas content in organic-rich shale: A review of the history, current status, and future directions

Shale gas is being hailed as the green energy of the future due to high heating value, low carbon emissions, and large reserves. Gas content of shale is a key parameter for evaluating the shale gas potential and screening for the shale gas sweet spots. Although the concept of gas content has been well defined, obtaining a reliable gas content data still remains a challenge. A significant barrier is the method for evaluating the gas content. In this paper, we provide a review of the long-established and recently developed gas content evaluation methods. In the first part of this review article, the history of gas content evaluation methods is summarized since 1910s, relied on published and unpublished literatures as well as our own experiences. Then, the fundamental contents and concepts involved in gas content evaluation are introduced to provide a clear theoretical foundation for the methods. In the third part, eleven evaluation methods, including four direct methods and seven indirect methods, are systematically reviewed. In each method, its application to evaluating the gas content is presented, the key advances are highlighted, and the advantages and limitations are discussed. Finally, future directions are discussed to promote creative thinking across disciplines to develop new methods or improve current methods for evaluating the gas content more accurately and efficiently.