Facies and diagenesis of early Cambrian patch-reefs in the inner Siberian Craton (Nepa-Botuoba anteclise)

Early Cambrian strata of the Siberian Craton accumulated within several relatively isolated basins of a tropical epeiric sea. The larger central part of the craton was occupied by the Turukhansk-Irkutsk-Olekma epicratonic carbonate platform. During Cambrian Stage 2 (Siberian Tommotian Stage, Osa horizon), the inner part of this platform was a mosaic of vast local uplifts characterized by a variety of shallow-water facies and separated by inner shelf depressions with condensed argillaceous carbonates. These shallow-water uplifts, such as the Nepa-Botuoba anteclise, yielded patch reefs having a distinct ring-shaped structure according to the common depth point 3-D seismic exploration method. Detailed studies of drill cores penetrating these buildups revealed an complicated facies complex within patch reefs and inter-reef depressions. The patch reefs themselves were differentiated into facies comprising a reef core, an inner reef lagoon, reef flanks and talus. These facies differed by assemblages and morphologies of reef-builders, relative abundance of mud and debris as well as by the composition and primary porosity patterns of early marine cements. Petrographic analyses show that different diagenetic pathways and porosity patterns relate to the primary composition of the patch reef. The significant diagenetic development of intercrystalline and intergranular porosity, vuggy and moldic caverns, intense dolomitization, and halite precipitation converts the reef flank and talus facies into high quality reservoirs. The most striking aspect of these early Cambrian reefs was the absence of metazoans and the dominance of microbial reef-builders although stromatolites were not formed and ooidal shoals were not accumulated.