CHARACTERIZATION OF HYDROCARBON RETENTION IN LACUSTRINE SHALES DURING THERMAL MATURATION BASED ON SEMI-CLOSED SYSTEM PYROLYSIS

Hydrocarbon generation, expulsion and retention have always been considered as important research topics for shale oil/gas resource assessment. Semi-closed system pyrolysis experiment was applied to characterize hydrocarbon retention in lacustrine shales during thermal maturation, using an immature sample from the Es₃ Shale in Dongpu Depression, Bohai Bay Basin, China. Besides, analyses including Ro measurement, solvent extraction, kinetic study, open-system pyrolysis, thermovaporization-gas chromatography (Tvap-GC) and pyrolysis-gas chromatography (Py-GC) were also conducted on the products from semi-closed system pyrolysis experiments. Results show that retained oil in shales can be divided into free oil (pyrolysis S1) and sorbed oil (involved in pyrolysis S2). In the studied lacustrine shale, oil retention is dominated by sorbed phase, as the content of sorbed oil can be >4 times larger than that of free oil. As thermal maturity increases, the molecular weights of n-alkanes in free oil show more obvious reduction than sorbed oil. In addition, compared to sorbed oil, retention of free oil prefers a higher thermal maturity. During the thermal maturation process, residual hydrocarbon generation potential of the studied sample has always been dominated by gaseous hydrocarbon, and >90% of the potential loses before entering into gas window. Change of TOC in lacustrine shales, which can roughly reflect hydrocarbon generation process, is attributed to loss of hydrocarbon generation potential and formation of pyrobitumen during gas window. Although hydrocarbon retention in shales consists of many complex processes and is controlled by various factors, it can be generally considered as a result from balance among hydrocarbon generation, expulsion and retention during thermal maturation. A “retention ratio (RR)” index is thus suggested in this study, based on experimental results from semi-closed system and open system pyrolysis. The result of this research is expected to aid in reasonable shale oil/gas resource evaluation, because light hydrocarbon loss from high-mature shale is considered using RR.