Shale
oil resource is abundant in the world, which has been the
dominant incremental oil production in recent years for several countries.
A determination of pore structure, as well as the state and distribution
of retained oil, not only is critical in identifying the controlling
factors for the differential accumulation of shale oil but also affects
the exploration and development of potentially favorable intervals
in shale-oil systems. Twelve lacustrine shale samples from Lucaogou
Formation (P2l) of Junggar Basin were selected to analyze
the influence of mineral compositions and organic matter abundance
on the pore system development and the retained oil distribution in
pore-scale. A series of analyses, including thin section petrography,
field emission-scanning electron microscopy (FE-SEM), quantitative
evaluation of minerals by SEM, solvent extraction, mercury intrusion
porosimetry, and nitrogen physisorption, were carried out to investigate
the linkage between pore structure and occurrence of retained oil.
The results indicate that the samples from Lucaogou Formation are
dominated by slit-shaped, mineral-related pores with a small amount
of organic matter-hosted pores. Feldspar has undergone an extensive
dissolution, as shown by the development of abundant dissolved inter-particle
and intra-particle pores, which significantly improve the reservoir
property. The development of quartz overgrowth had a negative impact
on the pore structure, and carbonates underwent processes of both
dissolution and cementation. The mineral sizes show a positive relationship
with pore diameter, and sedimentary microfacies close to the source
contained more coarse components such as shore bar and distal bar
are more favorable for oil exploration. Retained oil in pores with
diameters of 100–1500 nm primarily occur in the free state
as a thin oil film, whereas oil in pores with diameters smaller than
100 nm is adsorbed. Moreover, the retained oil volume is affected
by organic matter abundance, the storage type, as well as the material
source-reservoir configurations.
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