Insights in the pore structure, fluid mobility and oiliness in oil shales of Paleogene Funing Formation in Subei Basin, China

Pore structure is an important factor influencing reservoir properties of oil shales. Routine core analysis, thin section, scanning electron microscope (SEM), mercury injection capillary pressure (MICP) tests, Nuclear Magnetic Resonance (NMR), and computed tomography (CT) were used to provide insights into the pore throat distribution in oil shales of member 2 of Paleogene Funing Formation (E(1)f(2)) in Subei Basin, China, with the special aim to unravel the effect of pore structure on fluid mobility and oiliness. The relationships between NMR parameters, petrophysical property and capillary parameters are investigated. The results show that rock composition in the oil shales consist of quartz, feldspar, carbonate particles, clay minerals and organic matters. Pore systems consist of large-scale interparticle pores, intragranular dissolution pores, and small-scale micro pores within clay minerals and the organic matter pores. Four types of pore structure (Type I, Type II, Type III and Type IV) are divided according to the patterns of capillary curves, capillary parameters, NMR T-2 (transverse relaxation time) spectrum and BVI (bulk volume of immovable fluid) values. From Type I to Type IV pore structure, the maximum mercury saturation (SHgmax) and mercury extrusion efficiency are decreasing, and the SHgmax are less than 50% averagely, indicating the oil shales are characterized by very poor pore connectivity. The T-2 spectrum changes from bi-modal behavior to unimodal and the right peaks become lower or even disappearing from Type I to Type IV pore structure. Fluid mobility is not primarily controlled by pore size, but dependent on the content of short T-2 components (< 1 ms). The micro-fractures as well as particles (mainly dolomites) filling them emit strong blue fluorescence. The edges of particles emit strong blue fluorescences and indicate that the interparticle pores are oil bearing. The carbonate particles are oil-wet, and intragranular dissolution pores within carbonate particles are fluorescent. Authigenic clay minerals are abundant in intercrystalline pores, and organic matters are abundant in organic pores, and they emit scattered strong blue fluorescence. Pore structure controlled fluid mobility and determined the microscopic oiliness and macroscopic oil bearing property or hydrocarbon productivity. The comprehensive study above gains insights into the microscopic pore structure and their controls on fluid mobility and oiliness in shale oil reservoirs, and this may have implications for resource potential evaluation and effective exploitation of oil shales.