Increasing the producibility of petroleum from shale is a key challenge for this decade and beyond. While understanding of producing petroleum from shales has advanced rapidly during the past decade, many unknowns remain. In addition, fundamental differences remain between high-thermal maturity shale gas systems (gas-window shales) and oil-window shales. Although it is shown that oil is produced from the shale matrix similar to gas shales, it is not known what improvement to recovery factors should be expected due to the fundamental differences and uniqueness of shale oil systems.
Some of the challenges in early exploration of shales in the oil window are related to the loss of oil from rock samples (cuttings, core), sample processing, storage conditions, sample preparation, oil type, API gravity, gas-oil ratio (GOR), rock lithofacies, and analytical conditions. It is shown that old cuttings may lose up to 300% of their free oil content simply due to evaporation, even in tight shale with black oil having a GOR of about 500 scf/bbl. When cuttings are compared with RSWC or core chips, the loss increases to almost 500%.
Projection of oil content to match measured GOR values of oils or even extracts of organic-rich tight shales allows prediction of this oil loss—this impacts calculations of original oil in place (OOIP) and, hence, hydrocarbon recovery estimates from such systems.