Impact of petroleum expulsion and evaporation losses on shale oil assessment using pyrolysis techniques

Shale reservoirs contain petroleum that undergoes losses through oil expulsion and evaporation, with uncertain implications on shale oil reserves and mobility evaluation. This study focuses on understanding the influence of these losses on open-system pyrolysis, the primary method for assessing shale oil content and mobility. Comprehensive analyses were performed on 26 lacustrine shale samples from the Chang7 Member, encompassing total organic carbon (TOC), programmed pyrolysis, solvent extraction, and gas chromatography-mass spectrometry (GC-MS). The research examines the impact of petroleum losses on petroleum fractions, the oil saturation index (OSI), and multi-step pyrolysis outcomes. The findings indicate that Chang7 shale with TOC content above 7% experiences more substantial petroleum losses than those with TOC below 7%. Petroleum losses result in significant fractional distillation of crude oil in the shale, leading to the deterioration of shale oil properties and a notable reduction in free oil content in S1, ultimately causing a decline in OSI values. As a result, Chang7 shale with TOC below 7% demonstrates a higher potential for shale oil recovery, whereas Chang7 shale with TOC exceeding 7% should be considered more significant as a hydrocarbon source rock. Moreover, multi-step pyrolysis is not suitable for assessing petroleum mobility in shale with low free oil content (OSI values below 100 mg/g). This limitation arises from the loss of free low-boiling components in Chang7 shale, and the currently detected low-boiling pyrolysis products may predominantly exist in an adsorbed state. The graphical abstract shows shale's petroleum loss using two parameters: petroleum expulsion efficiency (PExEf) and low-carbon-number n-alkanes loss (PCnLoss). As loss rises (PExEf, PCnLoss), pyrolysis parameter oil saturation index (OSI) drops, indicating reduced free oil. Results highlight a significant adverse impact of oil loss on pyrolysis evaluation and utilization.image