Structural controls on sandstone compaction within the anticline crest and flank: An example from the Xihu Sag, East China Sea Basin

The tight sandstones of the fourth (E3h4) and fifth (E3h5) members of the Oligocene Huagang (E3h) Formation in the Xihu Sag of the East China Sea Basin are feldspar lithic quartzose, which are ideal for the investigation of compaction. Based on routine core analysis, petrology, fluid inclusions, and compaction numerical simulation, this study investigated the compaction characteristics, porosity evolution, and high-value porosity area of tight sandstone on a structural anticline two-dimensional (2D) profile. The textural characteristics of the sandstones are moderately well sorted and are generally subangular-subrounded. The E3h4 and E3h5 sandstones on the crest of the anticline develop primary intergranular pores, whereas these sandstones on the flank develop a small amount of primary intergranular pores. The contact types between quartz grains on the anticline crest were mainly tangential, long, and concavo-convex contacts, whereas the contact types between quartz grains on the flank were primarily long, concavo-convex, and sutured contacts. The duration age of the mechanical compaction (MEC) porosity loss of sandy conglomerate (SC), fine-grained sandstone (FS), and medium-grained sandstone (MS) in the E3h4 and E3h5 members ranged from 27 Ma to 7 Ma. The onset of the chemical compaction (CHC) of the E3h4 and E3h5 sandstones were between 10.2 Ma and 10.6 Ma, respectively. Based on similar grain size, mineral composition, and sorting, the degree of the MEC of the E3h4 and E3h5 sandstones on the crest of the anticline is less than that of the flank due to burial depth. The distribution ranges of the MEC porosity loss were 21.92%-26.02% and 22.31%-26.43% for the E3h4 on the anticline crest and flank and 26.74%-27.26% for the E3h5 on the flank, respectively. By integrating similar burial time and clay minerals, the degree of the CHC of the E3h4 and E3h5 sandstones on the crest of the anticline is less than that of the flank due to reservoir temperature. The distribution ranges of the CHC porosity loss were 0.41%-1.05% and 0.44%-1.02% for the E3h4 on the anticline crest and flank and 0.48%-0.91% for the E3h5 on the flank, respectively. During the Late Miocene Longjing movement, the CHC porosity loss ranged from 0.15% to 0.4%. The study results show that the sandstone with porosity between 9% and 11% is distributed sporadically in E3h4 and E3h5, whereas the sandstone with porosity between 6% and 9% is distributed in contiguous pieces. The formation of high-value porosity areas is mainly due to the coarser grain size, shallower burial depth, chlorite coat, underdeveloped illite, lower reservoir temperature, and slight overpressure.