Adsorbed and free hydrocarbons in unconventional shale reservoir: A new insight from NMR T1-T2 maps.

Characterizing free and adsorbed hydrocarbon is crucial for the resources assessment and mobility evaluation of shale oil plays. The complex feature of hydrogen-bearing components in shale reservoirs requires new analytical methods to study the occurrences of different states of hydrocarbons. In this study, forty core samples from five shale oil research wells in the Jiyang Depression of China were analyzed using various analytical tools including nuclear magnetic resonance (NMR) T 1 -T 2 mapping, Leco total organic carbon (TOC) analysis, routine and special programmed Rock-Eval pyrolysis, solvent extraction and fractionation, as well as mercury injection capillary pressure (MICP) experiments. Additional experiments were conducted on a powdered shale for NMR T 1 -T 2 maps under four states: as-received, after being dried at 110 °C, after special programmed pyrolysis to 350 °C, and after solvent extraction. The results showed that residual water signals in the NMR T 1 -T 2 map disappeared after being dried at 110 °C. Free oil signals decreased dramatically when the sample was thermally treated to 350 °C, while adsorbed oil signals decreased sharply after solvent extraction. A good accordance was observed between the organic hydrogen T 2 spectra and three modes of Rock-Eval pyrograms, representing free hydrocarbons, adsorbed hydrocarbons, and hydrocarbons from kerogen/bitumen decomposition. A positive correlation was also found between the intensity of NMR T 1 -T 2 map signals and the free and adsorbed hydrocarbon contents from geochemical analysis. T 2 cut-off values of adsorbed hydrocarbons and free hydrocarbons were determined to be 0.2 ms and 1 ms, respectively. Core samples with a high oil saturation index, porosity, and relative content of hydrocarbons display a low T 1 /T 2 ratio indicative of a high shale oil mobility. As a complementary method to the Rock-Eval analysis, NMR T 1 -T 2 mapping is a non-destructive technique for the characterization of adsorbed and free hydrocarbons at the core scale with limited sample preparation. The results of this study are helpful for NMR characterization of shale reservoirs. • The organic hydrogen T 2 spectra were conducted from the NMR T 1 -T 2 maps of shale. • Two T 2 cut-off value sets were determined: >1 ms for free oil, at 0.2–1 ms for adsorbed oil, and <0.2 ms for kerogen remnant. • Good positive correlations between NMR T 1 -T 2 map and Rock-Eval pyrograms were revealed. • The T 1 /T 2 ratio of free oil could be used as an indicator of shale oil mobility. [ABSTRACT FROM AUTHOR]