Enrichment based isolation methods and molecular identification were used to investigate the microbial communities in produced water obtained from Periquito (PQO) and Galo de Campinas (GC) onshore oilfields in Brazil. Produced water was enriched with Postgate B, Postgate C and Baars media and incubated. DNA was extracted, PCR amplified and16S rDNA fragments were sequenced using Illumina TruSEq. 4.2 million reads were analysed and deposited at the Sequence Read Archive at NCBI. No significative

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... ifferences in microbial community composition could be attributed to the enrichment but significant differences were observed from two oil fields. The dominant Bacterial Orders detected from both oilfields were Desulfovibrionales, Pseudomonadales and Enterobacteriales. Pseudomonas were found predominantly in Periquito oilfield (19.8%) with only 2,4% at Galo de Campinas. Pleomorphominas (3.76%) and Shewanella (4,69%) were exclusive to and possible biomarkers for the Periquito Oilfield. 11.05% and 7.62% of the sequences were not classified at genus level and detected at GC and P. Abundances changed for Desulfovibrio from P, 29.8% at PQO and 16.08% at GC. The Clostridium_sensu_stricto varied from 2.8% at PQO and 2.4% at GC). Pseudomonas were found predominantly in Periquito oilfield (19.8%) with only 2,4% at Galo de Campinas. Pleomorphominas (3.76%) and Shewanella (4,69%) were almost exclusive to and possible biomarkers for the Periquito Oilfield. These data provide a bacterial biodiversity benchmark for future produced water treatment and microbially enhanced oil recovery (MEOR).