Abundant hydrocarbons have been produced from the pre‐salt lacustrine carbonates of the Cretaceous Barra Velha Formation in the Santos Basin of offshore Brazil. The highly heterogeneous lithology and pore systems make it difficult to predict the reservoir quality via petrophysical well logs. Core description, computed tomography (CT) scanning, thin sections and wireline well logs are utilized to characterize the lithology, pore spaces are reservoir quality of the Cretaceous Barra Velha Formation pre‐salt lacustrine carbonates in the Santos Basin. The results show that the lithology mainly includes shrubstone, grainstone, laminated micrite and massive micrite. The pore systems are dominated by secondary pores, such as intergranular pores, dissolution pores and intercrystalline pores as well as modified primary pores. Geological well logs (conventional well logs, electrical and sonic image logs and nuclear magnetic resonance [NMR] logs) are integrated to build the predictable models of the carbonate reservoirs. The shrubstone, which is dominated by intergranular pores, has the highest reservoir quality, which can be derived from NMR logs. Grainstone contains both primary intergranular pores and abundant secondary dissolution pores. The laminated and massive micrite contain only micropores and therefore have the lowest reservoir quality. Lastly, conventional well logs integrated with electrical and sonic image logs are utilized to predict the distribution of lithology and reservoir quality in single wells, and the results are testified by the NMR log data. The shrubstone and grainstone are future exploration targets of the Cretaceous Barra Velha Formation in the Santos Basin. The results are hoped to give insights into the petrophysical behaviours of various lithologies, and provide theoretical and technical guidance for reservoir evaluation and prediction.
The Santos Basin in Brazil is a hot area of oil and gas
exploration
in recent years, and its subsalt lacustrine mudstones are the main
source rock of the basin. However, there is a lack of studies on the
source rocks of the subsalt Picarras and Itapema formations, which
is not conducive to the accurate evaluation of the source rock characteristics.
Based on logging data of 51 wells and geochemical data of 16 wells,
this paper makes detailed evaluations of the organic matter abundance,
type, maturity, and distribution characteristics of source rocks of
the subsalt Picarras Formation and Itapema Formation in the Santos
Basin. The results show that the characteristics of source rocks of
the Itapema and Picarras formations are similar, both of which have
a high abundance of organic matter. The types of organic matter are
mainly type I and II
1
, and the maturities are in the low-maturity
to the high-maturity stage, which meets the standard of good source
rocks. The total organic carbon content of the source rocks of the
Picarras Formation ranges from 0.4 to 4.0%, much lower than that of
the source rocks of the Itapema Formation, 0.8–5.6%. In addition,
the hydrogen index average value of the source rocks of the Itapema
Formation is 712.8 mg/g TOC, higher than that of the Picarras Formation,
697.5 mg/g TOC, both revealing a great hydrocarbon potential. The
quality of source rocks of the Itapema Formation is better than that
of the Picarras Formation. The two sets of source rocks have great
hydrocarbon generation potential and are mainly developed in the eastern
and western sags of the central depression. Therefore, the surrounding
uplift areas will be the target for further oil and gas exploration.
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