Carbonate cemented zones are normally adjacent to the top overpressured surface in the central Junggar Basin, NW China. Stable carbon and oxygen isotopic compositions and petrological investigations of carbonate cements in the carbonate cemented zones indicate that: (1) carbonate cements are composed dominantly of ferrocalcite, ferroan dolomite, and ankerite; (2) carbonate cements are formed under a high temperature circumstance in the subsurface, and organic fluid migration has an important effect on the formation of them; and (3) carbon and oxygen ions in the carbonate cements migrate from the underlying overpressured system. This suggests that the occurrence of carbonate cemented zones in this region results from multiple phases of organic fluid expulsion out of the overpressure compartment through geological time. This study provides a plausible mechanism of the formation of carbonate cemented zones adjacent to the top overpressured surface in the clastic sedimentary basins, and has an important implication for understanding the internal correlation between the formation of carbonate cemented zones adjacent to top overpressured surface and geofluids expulsion out of overpressured system. carbonate cemented zones, formation mechanism, stable carbon and oxygen isotopic, top overpressured surface, central Junggar Basin, coal-bearing strata Citation:Yang Z, Zou C N, He S, et al. Formation mechanism of carbonate cemented zones adjacent to the top overpressured surface in the central JunggarAs one of the most familiar diagenetic authigenous minerals in clastic reservoirs, carbonate cements, have variable mineralogy, texture and chemical compositions, and can be formed under various geochemical conditions [1, 2]. Previous work on carbonate cements focuses mainly on their influences on reservoir quality, tracing the sources and migration-pathways of geofluids and the sealing effects of geofluids [2][3][4][5][6][7]. Most studies hold that carbonate cementation is a destructive diagenetic process, because reservoir quality becomes much worse than before due to the cements' filling into the pores and thus decreasing the porosity and permeability of the reservoir, and improving heterogeneity of the reservoir by dividing thick sandstone bodies into thinner units [4,[8][9][10][11][12][13][14]. Carbonate cements formed in different phases record variations in fluid compositions during the course of diagenesis, which helps decipher the processes of fluid-rock interaction and fluid composition evolution at geological time scale [2,5,[15][16][17][18][19][20][21][22][23][24]. Particularly,