Depositional and diagenetic characteristics of the Lula's Fingers: Evidences of microbial activity and exposure during final pre-salt depositional in Santos Basin

Abstract

Over the past decade, considerable progress has been made in the understanding of non-marine carbonate rocks following the discovery of substantial oil reserves in the pre-salt region off the eastern coast of Brazil. The Lula's Fingers interval, situated at the top of the Barra Velha Formation, has been identified in gamma-ray log patterns as a crucial stratigraphic marker for the pre-salt deposits in the Santos Basin. Despite its stratigraphic significance, comprehensive sedimentological and petrographic studies on this interval are lacking. This absence of detail studies may be associated with the recurrence of fine-grained non-marine carbonate rocks. This study employs a range of techniques, including classical sedimentology (macro- and microscopic description), Markov Chain analysis, and complementary methods such as quantitative analysis using XRD, SEM-EDS, cathodoluminescence, and epifluorescence. These techniques are used to evaluate the sedimentological and diagenetic characteristics of the fine-grained non-marine carbonate rocks in Lula's Fingers interval. In general terms, the interval consists of in situ rocks formed through abiotic chemical precipitation (such as parallel-laminated calcilutite, laminated spherulestone, and shrubstone), as well as reworked calcarenites and calcirudites formed under high-energy conditions, resembling those described for the Barra Velha Formation. However, it differs primarily from the main Barra Velha deposits in the presence of (i) calcilutite exhibiting crenulated lamination, peloidal/pseudo-peloidal to clotted texture, and microbial lumps, providing substantial evidence of microbial activity, and (ii) nodular calcilutites with a pseudo-particulate texture. The presence of nodular calcilutites suggests that the deposition of the Lula's Fingers interval occurred in shallow to very shallow settings, occasionally experiencing subaerial exposure, in contrast to the alkaline lake settings interpreted for most of the Barra Velha Formation. Dolomitization and silicification within the Lula's Fingers interval have significantly altered the rocks, hindering the identification of primary texture and composition. Silica and dolomite commonly fill pores and function as replacive minerals across various lithofacies and microfacies. Lamellar dolomite emerges as the dominant texture within the study succession, often filling interstices between spherules and shrubs. Its presence is likely associated with a precursor detrital or authigenic material, possibly a magnesium clay matrix, which still influences the final texture of the rock despite complete replacement.