The Potential of the Oldest Shale Oil in China: Mesoproterozoic Hongshuizhuang and Xiamaling Shales in the Yanshan Basin

Shale oil exploration and development have made significant
breakthroughs
in Paleozoic marine shales and Mesozoic–Cenozoic lacustrine
shales. However, the shale oil potential of older Precambrian sediments
remains poorly constrained. Our two newly drilled boreholes reveal
shale oil shows in the Mesoproterozoic Hongshuizhuang and Xiamaling
shales in the Yanshan Basin, China. In order to further evaluate the
shale oil potential, multiple experimental methods were employed,
including TOC and thermal maturity analyses, routine and step-by-step
Rock-Eval pyrolyses, X-ray diffraction, field emission scanning electron
microscopy, low-temperature nitrogen adsorption, and hydrocarbon vapor
adsorption. The results show that Hongshuizhuang shale, which is characterized
by type II kerogen and low-medium maturity of R0 = 0.65–0.92%, is a set of good source rocks, while
the source rock quality of Xiamaling shale is highly variable. Moreover,
the hydrocarbon generation potential of prokaryote-dominated Mesoproterozoic
source rocks is comparable to that of eukaryote-dominated Phanerozoic
source rocks, implying a favorable factor for shale oil accumulation
in Precambrian formations. The main storage space for shale oil in
both formations is provided by inorganic pores, in which the adsorbed
oil is predominantly retained in micropores to fine mesopores (<15
nm) and free oil mainly stored in mesopores to macropores. Adsorbed
oil exists as a multilayer oil film, with its average thickness increasing
as a Langmuir adsorption curve with pore size. The evaporative hydrocarbon
content of the Hongshuizhuang Formation ranges from 2.53 to 12.53
mg/g and is dominated by adsorbed hydrocarbons (mean 6.01 ± 2.10
mg/g), while the evaporative hydrocarbon content of Xiamaling Formation
is much lower, with an average value of 2.4 ± 2.4 mg/g. TOC content,
mineral composition, thermal maturity, pore volume and surface area
are important factors affecting the degree of shale oil enrichment.
The compilation and comparison show that the Hongshuizhuang Formation
is a promising target for the oldest shale oil exploration. Furthermore,
the results suggest that the combination of step-by-step pyrolysis
and hydrocarbon vapor adsorption is a practical method for estimating
both free and adsorbed hydrocarbon contents and revealing their storage
space within shale nanopores.