It is difficult to imagine modern international oil and gas business without constantly transforming integration processes. Their necessity is dictated both by domestic national economic conditions in the hydrocarbon producing countries, and international conditions of the world oil and gas business market, as it becomes more and more difficult to be successful and to withstand the fierce competition in this sphere. Integration processes in international oil and gas business may take a variety of forms from the emergence of long-term and rigid market structures to temporarily create joint structures, depending on the current market conditions and situational goals and objectives. The article argues that the existing literature theoretical distinction between horizontal and vertical forms of integration in real practice is often conditional. In real practice, these forms are rarely implemented according to an outlined and purely classical scenario, more often the integration processes are carried out on a parallel and simultaneous scheme, "overlapping" with each other, that is, on a combined basis, as evidenced by specific examples from global and domestic practice cited in the article. The article also shows that combined schemes of international integration are used in the so-called M&A (mergers and acquisitions) deals, mergers and acquisitions of oil and gas companies. The analysis shows their complex combination and practical implementation in international oil and gas business markets. The analysis showed that the number of small and medium-sized companies in M&A deals is falling. However, the total capitalized value of transactions remains approximately at the same level. This stable financial balance in transactions indicates intensive cooperation of large companies with each other. In this regard, such form of integration as creation of international oil and gas joint ventures acquires special importance. The article shows economic and technological advantages of this cooperation, as well as the solution of the largest problems and achievement of significant practical results in the sphere of oil and gas business. At the same time, in the practice of studying the functioning of joint ventures, it is difficult to define and indicate their feasibility and effectiveness in specific economic conditions. If for large companies it is usually not in doubt, in the case of smaller companies the effectiveness is not so obvious. The authors propose to solve this practical problem through a specific calculation mechanism to determine the synergistic effect and emergence. The authors give a variant of such calculation in this article