Bayan - it depends on the environment (deep water, shallow water, onshore) and what stage you are looking at - licensing/exploration phase/production/post production/remediation. I would recommend this article as a pointer:
It is difficult to be specific without knowing more details about your research area, however observations are always key to a successful EIA. Look for baseline studies and data if they exist, then follow up with a carefully planned environmental sampling program. Please message me if you would like to talk about specific aspects of your study.