Unconventional resources are hosted in fine-grained reservoirs that are enriched in organic carbon. The play can be either thermogenic, where the shale has been deeply buried and thus matured to oil and gas level, or biogenic, where the shale has been degraded by microbes whereby gas was generated. A systematic description of the shale layers is a pre-requisite for a uniform and scientific-based assessment of the shale resource systems. As a first important step, screening criteria are provided below for evaluating any shale. The selection criteria in the table cover both thermogenic and biogenic shale-gas plays.
For thermogenic plays, the selection criteria are chosen to ensure that the right TOC type is present (marine types I-II) in significant volumes (>2% and 20 m thick), that the unit has a significant thickness (>20 m), that the thermal maturity is high enough to ensure generation of HC (at least oil maturity), and that the present-day reservoir has maintained its integrity (present-day depth at least 1.5 km and low to medium structural complexity). Present-day depth larger than 7 km is used to exclude shales that are not within reach of the well bore.