This first major project involving two Gulf state oil firms will not only bring them access to India’s large and rapidly expanding fuels and chemicals market, but also secure an outlet for crude exports. The deal advances Adnoc and Aramco’s plans to become major global integrated players.

Abu Dhabi state petroleum firm Adnoc has joined its Saudi counterpart Aramco in a joint venture to take 50% equity in India’s proposed Ratnagiri refining and petrochemicals complex...