Various data including casting thin section

scanning electron microscopy (SEM)

XRD whole-rock mineral analysis

low-temperature nitrogen adsorption (LTNA) and high pressure mercury injection (HPMI) experiments are integrated to characterize the micro/nano-scale pore structure of the volcanic weathering crust reservoir in the Kalagang Formation (C

2

k)

Santanghu Basin

study in-depth the evolution mechanism of micro/nano-scale pores

and reveal the effect of pore throat structure on the oil-bearing property of reservoir in combination with oil/gas shows and formation testing data.The results show as follows.Four types of reservoir space

namely dissolution pores

residual pores

intercrystalline pores and fractures

are recognized in the volcanic weathering crust reservoir.The adsorption-desorption isotherm curves are dominated by hysteresis loops of Type H2 or near Type H2

and the efficiency of mercury ejection is quite low

varying from 24.26% to 44.91%

in samples as a whole

suggesting that the pores are mainly ink-bottle shaped.The reservoir is dominated by a pore size of below 1 μm

with micro-scale residual pores and dissolved pores developed locally.The lower content of pyroxene and the higher content of plagioclase in the original mineral composition of the volcanic rocks

will result in the development of low-content chlorite and high-content zeolite in the post-magmatic hydrothermal stage.Thereafter the volcanic reservoir can be effectively modified and get better in quality.The Swanson parameter and r

apex

can well demonstrate its matrix permeability.However

the Swanson parameter is closely related with the oil-bearing property of volcanic weathering crust reservoirs.When the Swanson parameter is greater than 2

the reservoirs are the oil layers; when it varies between 0.86 and 2

the reservoirs are poor pay zones; while when it is less than 0.86

the reservoirs are mostly dry layers.In conclusion

the Swanson parameter can be used as a key parameter to indicate the oil-bearing property of reservoir

and provide a basis for play fairway selection.