Isotopic evidence on the diagenetic evolution of coastal sabkha reservoirs from the Solimões Basin, northern Brazil Andreia Dias Elias a,1 , Luiz Fernando De Ros a,b, ⁎ , Ana Maria Mizusaki a,b , Koji Kawashita a a Instituto de Geociências, Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves, 9500, 91509-900, Campus do Vale, Porto Alegre (RS), Brazil b CNPq, Brazil Received 26 January 2006; received in revised form 19 April 2006; accepted 9 June 2006 Available online 3 October 2006 Abstract The isotopic composition of diagenetic carbonates, sulfates, illites and associated diagenetic constituents of the Carboniferous Juruá sandstones, Solimões Basin, largest hydrocarbon reservoirs of northern Brazil, was analyzed in conjunction with petrographic characterization and chemical composition. Eolian Juruá sandstones, deposited in a coastal sabkha of a wide cratonic sag, are the best reservoirs, but diagenetic processes introduced strong and complex heterogeneities, which affect hydrocarbon production. Dolomite and anhydrite cementation, together with compaction, exerts the main control on the quality of the reservoirs. Early dolomite cements precipitated under strong evaporation, while late dolomites are related to thermal decarboxylation. Late anhydrite cements are related to the interbedded evaporites and possibly to the oxidation of dissolved sulfide. Authigenic illite K–Ar ages are related to the voluminous Triassic basic magmatism (around 200 Ma), and to the Jurassic– Cretaceous tectonism (around 150 Ma), which may have also affected the late anhydrite and dolomite–ankerite cementation. This isotopic study revealed the major conditions of the diagenetic processes that strongly affected the Juruá reservoirs, which are essential for the development of models that will optimize the production, as well as decrease the risks in the exploration for new Juruá reservoirs. © 2006 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. Keywords: Sandstone diagenesis; Coastal sabkha; Isotopes; Dating; Reservoir quality 1. Introduction The diagenetic evolution of the Carboniferous sandstones of the Juruá Formation, Solimões Basin, northern Brazil, is highly complex, and promoted the introduction of strong heterogeneity in these largest hydrocarbon reservoirs of northern Brazil. The objective of this study is to use the isotopic characteristics of diagenetic constituents of the Juruá sandstones, particularly of the carbonates, sulfates and illites, supported by the petrographic characterization and chemical composition of these and associated constituents, in order to define the paragenetic evolution of the reservoir sandstones and to understand the genetic conditions and distribution of the diagenetic processes. This understanding is important for the development of geochemical models that will improve the efficiency of develop- ment and production of the reservoirs, as well as to decrease the risks of exploration for Juruá reservoirs in the basin. 2. Geological setting The study area is situated in the Solimões Basin. This is a large Paleozoic basin, covering around 600,000 km 2 of the Amazonian Craton (northern Brazil). The Basin is limited to the south by the Brazilian Shield, to the north by the Guyanas Shield, to the west by the Acre Basin across the Iquitos Arch, and to the east by the Amazonas Basin across the Purus Arch (Caputo and Silva, 1990; Eiras et al., 1994; Fig. 1). The Carauari Arch divides the Solimões Basin into the Jandiatuba and the Juruá sub-basins, having exerted an Gondwana Research 11 (2007) 553 – 567 www.elsevier.com/locate/gr ⁎ Corresponding author. Instituto de Geociências, Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves, 9500, 91509-900, Campus do Vale, Porto Alegre (RS), Brazil. Fax: +55 51 3316 7302. E-mail addresses: andreia.elias@petrobras.com.br (A.D. Elias), lfderos@inf.ufrgs.br (L.F. De Ros). 1 Present address: PETROBRAS UN-RIO/ST/CER, Av. General Canabarro, 500, Rio de Janeiro, Brazil. 1342-937X/$ - see front matter © 2006 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.gr.2006.06.007