Molecular microbiological methods have become essential for detecting and identifying microorganisms in complex environmental samples. They provide crucial information about total microbial communities through DNA analysis and metabolically active communities through RNA analysis from various environments, including oilfields. In studies focusing on microbial communities, samples are often preserved using preservatives or reagents to maintain the quality and integrity of nucleic acids. However, there is a scarcity of studies that have evaluated the most effective preservation methods. Therefore, the present study aims to test different methods and describe their impacts on the diversity of total and metabolically active microbial communities in oilfield produced water.