--> Origin and Significance of Thick Carbonate Grainstone Packages in Non-Marine Successions: A Case Study From the Barra Velha Formation, Santos Basin

AAPG ACE 2018

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Origin and Significance of Thick Carbonate Grainstone Packages in Non-Marine Successions: A Case Study From the Barra Velha Formation, Santos Basin

Abstract

Potential reservoir facies represented by lacustrine shoreline grainstones and rudstones are typically relatively thin compared to those from marine basins because of limited fetch, reducing wave action and producing a shallow wave base. This is especially the case in low gradient endorheic lakes, in which rapid lake level oscillations preclude the development of a stable shoreline. However, the closed lake deposits from the Cretaceous Barra Velha Formation, Santos Basin, locally have thick (decametre-scale), continuous packages of grainstone and rudstone comprising fragments of crystal shrubs, spherulites, intraclasts and, in some cases, peloids and volcanic fragments.

Grainstones and rudstones of this type occur in two distinct settings: (1) the escarpment and dip slopes of tilted fault blocks along which a marked thinning of the Barra Velha Fm is evident; (2) apparent debris aprons adjacent to seismically resolvable mounds that developed on narrow elongate fault blocks. Grainstones and rudstones of the former are sharp-based packages, with fining-upwards trends, of well sorted and well-rounded grains, with mainly planar and low-angle stratification, but cross-lamination and planar cross-bedding is locally present. Paleocurrents are normal to paleoslope. These are interpreted as most likely carbonate fan delta shoreline deposits with wave reworked sheet flood or terminal splay deposits of alluvial fans. Those associated with dip slopes of half grabens are generally finer than those associated with scarp slopes which are also less mature in texture. Core plug measurements indicate porosities of up to 25% but moderate permeability ranging from 1 mD to ca. 100 mD. The grainstones associated with mound-like features are more coarsely textured, less mature and likely represent colluvial wedges as a result of the sub-aerial disaggregation of mounds formed largely of crystal shrubs. Core plug measurements indicate good reservoir quality with up to 25% porosity within predominantly intergranular pore space and permeabilities ranging from 10 mD to several Darcies. While much effort has focused on the origin of the in-situ constituents of the Barra Velha Formation, such as crystal shrub facies, numerous reservoirs are dominantly composed of more conventional re-worked, detrital deposits and constitute significant potential targets.