| The microbial carbonates of the Lower Cretaceous Barra Velha Formation in the Santos Basin, Brazil,
primarily formed in a high-salinity alkaline depositional environment, and have recently become a hotspot for
hydrocarbon exploration and development in deep-water areas. However, research on the characteristics of microbial
carbonate reservoirs formed in such unique environment is relatively limited and controlling factors of reservoir formation
remains poorly understood. Based on integrated core samples, thin sections, well logs, and petrophysical test data, this
study systematically investigates the lithofacies, reservoir space types, and physical properties of microbial carbonates in
the basin. It clarifies the diagenetic sequence and pore evolution of the reservoirs and explores the main controlling factors
and models for the development of high-quality reservoirs. The research results show that: (1) The main rock types of
microbial carbonate reservoirs of the Lower Cretaceous Barra Velha Formation in the Santos Basin include stromatolite,
spherulitite, laminite, rudstone, grainstone and breccia. The formation can be divided into two third-order sequences,
primarily consisting of four microfacies types: microbial reef, grain shoal, microbial spherulitic shoal, and inter-shoal
deposits. (2) The reservoir space is mainly composed of biological framework pore, framework dissolution pore,
intergranular pore, intergranular dissolution pore, intragranular dissolution pore, intercrystalline pore, and dissolution
fracture. Porosity and permeability generally exhibit a positive correlation, indicating the dominance of pore-type
reservoirs. Statistics show that the microbial reef and grain shoal microfacies have better reservoir properties, while the
microbial spherulitic shoal and inter-shoal microfacies show relatively poorer reservoir quality. (3) The diagenetic
sequence and pore evolution of microbial carbonate reservoirs have been clarified. In the early diagenetic stage, meteoric
water dissolution and dolomitization played constructive roles in reservoir evolution. In contrast, mid-to-late
hydrothermal activity led to silica filling of reservoir pores particularly in areas adjacent to faults, which not only damaged
the reservoir but also increased reservoir heterogeneity. (4) Paleoclimate, paleo-water condition, sequence stratigraphy,
and sedimentary microfacies types are the main factors controlling the development and distribution of high-quality
microbial carbonate reservoirs. Combined with diagenetic evolution, an evolution model of microbial carbonate reservoirs
has been established in this study. |
