Fig 1.
Geological structural background of the study area. (a) Global paleogeography and the location of SCB during Middle Permian (~255.7 Ma) [48].
(b) The distribution map of ELIP basalts and the location of the Sichuan Basin [37,49,50]. (c) The location of study sections, wells, the deep basement faults, and Rd3 and Sd distributions of the Middle Permian in the SCB [51,52]. (d) Stratigraphic column of the Middle Permian Formation in the SCB.
Fig 2.
Photographs showing the petrographic characteristics of replacement dolomite (Rd1 – Rd3).
(a) Very finely crystalline Rd1 with original residual fabric (XJG; P2m). (b) Rd2 dolomite crystals were recrystallized from Rd1 dolomite (XJG; P2q). (c) Euhedral to subhedral crystalline Rd2 have a foggy core surrounded by a clear rim (XJG; P2q). (d) Field photo showing grey-dark grey sucrosic Rd3 dolomite with vugs (CJG; P2q). (e) Field photo showing grey sucrosic Rd3 dolomite (XJG; P2q). (f) Rd3 with medium to coarse non-planar crystals (XJG; P2q). (g) Coarse crystalline Rd3 showing mosaic contact (EY; P2m). (h) Fine crystalline Rd2 recrystallized to Rd3 with coarse and curved crystals (XJG; P2m). (i) Coarse crystalline Rd3 display undulous extinction (SDZ; P2q).
Fig 3.
Photographs showing the petrographic characteristics of saddle dolomite cement (Sd).
(a) Zebra pattern composed of Rd3 and Sd (ZC; P2m). (b) Milky white Sd fill in the fractures (TL6; P2m). (c) Milky white Sd fill in the dissolved pores (MX42; P2q). (d) Rd3 and Sd are cut by stylolites (XJG; P2m). (e) Quartz filled in the pores of Rd and Sd (XJG; P2q). (f) Sd exhibits strong undulatory extinction under cross-polarized light (CJG; P2q).
Fig 4.
Photomicrographs showing fluid inclusions petrography. a-b.
Irregular distribution of isolated FIs in dolomite crystals (XJG; P2q); c. FIs outline the growth zone of Sd dolomite (TL6; P2m); d. FIs outline the growth zone of Sd dolomite (XJG; P2q).
Fig 5.
Homogenization temperature and salinity of dolomite fluid inclusions. (a) The Th profile of Rd3 and Sd from three different areas in the Sichuan Basin.
(b) The salinity profile of Rd3 and Sd from three different areas in the Sichuan Basin.
Fig 6.
δ13C versus δ18O values of different dolomites and hist rocks in the three regions.
Fig 7.
δ13C versus δ18O values of the dolomitization fluids in the three regions.
Fig 8.
87Sr/86Sr ratios of limestones, Rd3, and Sd in the three regions.
87Sr/86Sr ratios of Permian seawater range from 0.70680 to 0.70807 [62,63].
Fig 9.
Diagenetic sequence of the dolomites in SCB.
Fig 10.
Photographs of SEMS showing the hydrothermal minerals in Sd.
(a) Fluorite filled in Rd3 under SEM observation (EY; P2m). (b) Siderite filled in Rd3 under SEM observation (TL6; P2m). (c) Quartz filled in Rd3 under SEM observation (XJG; P2q).
Fig 11.
A cartoon showing the mode of the dolomitization mechanism in this study is related to ELIP.