The role of organic matter in Brazilian Pre-Salt carbonates

Abstract

Diverse types of organic matter morphology were identified in Brazilian Pre-salt carbonates. Stromatolites and laminated facies contain organic layers, filaments and rounded features inside calcite structures, as shrubs and spherulites. Analyses with optical and fluorescent microscopy, scanning electron microscopy with energy-dispersive X-ray spectroscopy, and ion mass spectrometry indicate the presence of organic compounds in thin sections. Fluorescent microscopy shows a high signal for organic portions in the thin section, mainly inside calcite structures and within the matrix. Scanning electron microscopy with energy-dispersive X-ray spectroscopy elemental mapping indicates the presence of carbon in the stromatolite facies, associated either with calcium and oxygen, indicating carbonate, or isolated from other elements, representing carbon content in the samples. The presence of organic matter along and inside calcite structures indicate a strong microbial influence in Brazilian Pre-Salt carbonates precipitation. The purpose of this research is to show that the presence of organic matter is connected to the process of carbonate precipitation and/or dissolution. The occurrence of these organic compounds in distinguishing facies raises the discussion of biotic versus abiotic genesis of the Pre-salt lacustrine reservoirs.