The diagenesis and porosity evolution of the
Middle Jurassic Shaximiao sandstones were analyzed
based on petrographic observations, X-ray diffractometry,
scanning electron microscopy observations, carbon and
oxygen stable isotope geochemistry, fluid inclusion microthermometry,
and thermal and burial history modeling
results. The point count data show that secondary pores
(av. 5.5 %) are more abundant than primary pores (av.
3.7 %) and are thus the dominant pore type in the Shaximiao
sandstones. Analysis of porosity evolution indicates
that alteration of sandstones mainly occurred during two
paragenetic stages. Mechanical compaction and cementation
by early chlorite, calcite, and quartz typically decrease
the depositional porosity (40.9 %) by an average of
37.2 %, leaving porosity of 3.7 % after stage I (\85 C,
175–145 Ma). The original intergranular porosity loss due
to compaction is calculated to be 29.3 %, suggesting that
mechanical compaction is the most significant diagenetic
process in primary porosity destruction. Stage II can be
further divided into two sub-stages (Stage IIa and Stage
IIb). Stage IIa (85–120 C, 145–125 Ma) is characterized
by late dissolution, which enhanced porosity by 8.8 %, and
the porosity increased from 3.7 % to 12.5 %. During stage
IIb ([120 C, 125–0 Ma), the precipitation of late chlorite,
calcite, quartz, and kaolinite destroyed 3.3 % porosity,
leaving porosity of 9.2 % in the rock today.