Sequence-stratigraphic model of Middle Jurua subformation of the Solimoes Basin (Brazil)
The object of the study is the Middle Jurua oil and gas subformation (Carboniferous) in the Solimoes Basin (Amazonas, Brazil). The subformation is formed by the alternation of sand reservoirs, carbonate rocks (limestones and dolomites) and anhydrites (fluid seals). The objective of the study was to build a sequence-stratigraphic model of the object as a geological basis to estimate its oil and gas potential. The lithological-facies analysis of cores from four wells showed that the deposits of the Middle Jurua subformation were formed in continental, transitional, and marine environments of the arid climate zone. Thirteen facies were identified and characterized. The resulting data as well as the general concept on the structure and genesis of sedimentary series helped to build a conceptual model of a standard subformation sequence and describe its typical cross-section, including LST (continental aeolian and alluvial deposits), TST (marine mudstones, limestones/dolomites and their coastal analogues), and HST (lagoon anhydrites and their coastal analogues). The model became the basis of a detailed stratification of the subformation: 12 sedimentary third-order sequences were identified and traced over the area and were combined into three T-R second-order sequences. Facies maps were built for second-order sequencers as the geological basis for estimating oil and gas potential. It has been established that the sandstones of aeolian complexes in the eastern and central parts of the area of interest are characterized by the best reservoir properties.