Tight sandstone reservoirs
have ultralow physical properties and
strong heterogeneity, and there is a need to describe the corresponding
pore structure characteristics systematically to promote research
on unconventional reservoirs. The pore structure, controlled by the
diagenesis and volcanic activity of the tight reservoirs in the third
member of the Shahejie Formation (Es3) of the Gaoshangpu
structural belt in the Nanpu Sag, is studied by high-pressure mercury
injection, nuclear magnetic resonance, and constant-rate-controlled
mercury porosimetry. The results show that the Es3 reservoir
can be divided into three types: the pore radii of Type I reservoirs
range from 120 to 180 μm, and the throat radii are larger than
1 μm, resulting in good pore connectivity; pore radii of Type
II reservoirs are approximately 100 μm, and the throat radii
range from 0.1 to 1 μm, resulting in moderate pore connectivity;
and pore radii of Type III reservoirs are much smaller than 100 μm,
and the throat radii are smaller than 0.1 μm, resulting in worst
pore connectivity. The pore size of Type I reservoirs is most sensitive
to compaction, and the pore connectivity is mainly controlled by carbonate
cementation; the pore throat size and pore connectivity of Type II
reservoirs are seriously affected by clay cementation, and pores are
mainly formed by dissolution. However, the pore structure of Type
III reservoirs is the worst among those investigated in this study
but can be further improved by dissolution to a certain extent. Volcanic
activity controls cementation and affects dissolution, thus changing
the pore structure. A pore structure evolution model is established,
which can provide a reference for future oil gas exploration.