Abstract: In 2021, a great breakthrough has been made in discovery of crude oil in the risk exploration well, i.e., Well Gulashou-1, in deep water area of Santos Basin in South America; a daily production of thousands of tons of high-yield oil flows was obtained in the drill stem testing, thus determining a super large oil reservoir in the Alam block in Brazil and confirming its potential for large-scale commercial development. To better understand this major breakthrough, the tectonic-sedimentary environment, accumulation conditions and reservoir characteristics of Well Gulashao-1 were analyzed in detail based on the analysis of regional geological background, in combination with the exploration history. The results show as follows. (1) Alam block is located in the Alam-Uirapuru uplift belt of Santos Basin, facing with Lula oilfield in the Lula-Sugar uplift belt across a lake, and the both has a similar tectonic-sedimentary background. The large paleo-uplift in the center of the basin provides favorable conditions for carbonate sedimentation. (2) Alam block is adjacent to the main hydrocarbon generating sag with abundant oil sources. (3) The inherited paleo-uplift controls the contiguous distribution of lacustrine carbonate reservoirs. (4) Salt rocks, post-salt calcilutites and mudstones have formed multiple effective seals on the oil reservoir. (5) The pre-salt anticlinal trap is developed on a large scale, demonstrating a favorable migration direction of oil and gas. (6) The pool in Well Gulashao-1 is an overpressure reservoir, which produces medium crude oil with low CO₂ and H₂S contents. The successful drilling of Well Gulashao-1 is a major breakthrough of oil exploration expanding from the pre-salt core area to the periphery in Santos Basin of Brazil. It is a major success achieved by PetroChina Company Limited (PetroChina) in adhering to overseas risk exploration and also a successful practice of overseas cooperation for deepwater oil and gas exploration in PetroChina. It is of guiding significance to the pre-salt oil and gas exploration in Santos Basin of Brazil and the development strategy of deepwater oil and gas business in PetroChina.