Determination of in situ hydrocarbon contents in shale oil plays: Part 3: Quantification of light hydrocarbon evaporative loss in old cores based on preserved shales

Petroleum resource assessment obtained from laboratory tests on old core samples shelved in the open environment tend to underestimate in situ resources. Therefore, hydrocarbon loss and its restoration need to be investigated. The hydrocarbon loss was studied via the difference of programmed pyrolysis parameter (Rock-Eval derived S1) between preserved/fresh cores and their replicas that had been long-term exposed to ambient conditions. However, the in situ hydrocarbon potential is still seemed to be underestimated, because the effect of sample crushing and crucible waiting on pristine S1 detection did not receive enough attention. To solve this issue, 50 preserved shale core samples with Ro from 1.00% to 1.51% were collected from the K1qn1 Formation of northern Songliao Basin, Eastern China. All the samples were subjected to four parallel experiments: Dean-Stark extraction and nuclear magnetic resonance (NMR) T1-T2 mapping, using large rock pieces 3-5 cm in size, and thermal desorption gas chromatography (TD-GC) and Rock-Eval analysis, using 60-mesh powdered samples. In addition, a time series of TD-GC and Rock-Eval analyses were conducted on a preserved shale sample to investigate hydrocarbon loss during sample crushing, Rock-Eval crucible waiting process, and elapse-time exposure to the open environment. Rock-Eval pyrolysis was also conducted on one month-exposed shale cores and solvent-extracted samples. The results show that the total hydrocarbon content of large rock pieces is significantly greater than that of powdered samples. Sample crushing or grinding leads C8_ light hydrocarbons to evaporate, and results in a loss of approximately 35% of S1. Rock-Eval crucible waiting process causes C14_ hydrocarbons to evaporate, and results in an additional 25% loss of S1. To compensate for the sample crush induced S1 loss, an exponential distribution model was proposed based on the C8+ hydrocarbons in the TD-GC S1 fingerprints. Sample maturity, pristine light hydrocarbon (C14_) proportions, organic matter richness, and oilbearing pore size primarily control the hydrocarbon loss of old shale samples. The higher the proportion of C14_ and the lower the abundance of organic matter, the higher the amount of hydrocarbon loss. Finally, S1 restoration coefficient charts for large rock pieces and powdered samples with Ro ranging from 1.00% to 1.42% are provided. It highlights that previous studies have severely underestimated the in situ S1 when using the Rock-Eval method. This research has great implication for estimating in situ hydrocarbon potential using conventional coring samples.

Automated summary

The petroleum resource assessment obtained from laboratory tests on old core samples tends to underestimate the in situ resources. This study investigates the hydrocarbon loss and restoration by comparing preserved and exposed core samples. It is found that previous studies have severely underestimated the in situ hydrocarbon potential due to factors such as sample crushing and crucible waiting. A new restoration model is proposed to compensate for these losses.