The offshore oil and gas geochemical indices generally include hydrocarbons and their alteration products in submarine sediments as well as hydrocarbons in seawater media. The oil and gas geochemical exploration with concentrations of hydrocarbons and their alteration products as the main objects is one of the few methods for evaluating offshore oil and gas prospects. In present, bottom bed water sampling and analysis and coring analysis of submarine sediments have constituted an effective and relatively economical technical combination, and the coring measurement of submarine sediments and in-situ sniffer analysis are main means for offshore oil and gas geochemical exploration. Numerous offshore oil and gas geochemical exploration researches conducted both in China and abroad prove that offshore oil and gas geochemical exploration can provide important information for the recognition of oil-and gas-bearing prospect blocks and the judgement of oil and gas attributes in oil- and gas-bearing prospect structures.