Sedimentary environment and genesis of the Early Cretaceous microbial carbonates in Santos Basin, Brazil.

The pre-salt microbial carbonate has become one of the most important targets for global oil and gas exploration in the deep water Santos Basin in recent years. The pre-salt microbial carbonate was mainly deposited in the Barra Velha Formation (BITE) of the Early Cretaceous (Aptian). However, its genesis and sedimentary evolution need to be further explored. In this study, the lithologic characteristics of microbial carbonate were described by systematically observing 102m cores, 533 sidewall cores and 917 thin sections from the BITE Formation. A total of 42 core samples with different depths were selected and tested for strontium isotope, stable carbon and oxygen isotope, major and trace element. Integrated the lithologic characteristics with geochemical tested results, the following conclusions can be drawn: (1) The lithofacies of the BITE Formation mainly includes three types of microbial carbonate of stromatolite, spherulite and laminate, which are distributed regularly in the upper, middle and lower members of the BITE Formation. (2) The general sedimentary environment of the BITE Formation was characterized by a semi-closed and closed lacustrine environment with high salinity in an arid and evaporative paleoclimate. The variation of the lacustrine connectivity and paleoclimate led to formation of different lithofacies of microbial carbonate in the BITE Formation. (3) A sedimentary evolution model of microbial carbonates controlled by paleoclimate is established. In the early sedimentary stage of the BITE Formation, sedimentary environment was mainly a semi-closed to closed deep water lacustrine reducing environment with a characteristic of high salinity in the paleoclimate of semi drought and evaporation, which was favorable for microbial bonding and a large amount of laminate deposition. In the middle stage, the sedimentary environment changed to a semi-closed lacustrine deposition with relatively low salinity in a relative humid paleoclimate accompanied with water and a small amount of terrigenous detritus injection, which resulted in the weakening of microbial effects and the deposition of spherulite. In the later stage, the sedimentary environment became a closed shallow water reducing lacustrine deposition with ultra-high salinity in extreme arid and evaporative paleoclimate, which was conducive to the proliferation of anaerobic halophilic microorganisms and the extensive deposition of stromatolite.