关键词: 开平凹陷, 断拆裂陷, 半深湖亚相, 油气运移, 断阶带, 洼槽区

Abstract: Kaiping sag has been proved to be an oil generating sag, where no significant oil and gas discovery has been made in more than 20 years of exploration. The complex formation mechanism of sag, unclear oil formation conditions, and unclear oil and gas migration and accumulation directions are the key problems that restrict major breakthroughs in exploration. Focusing on these three key problem, a study was performed on tectonic analysis, sedimentary analysis, oil-source rock correlation, and oil-gas migration. The findings show that Kaiping sag is a small- to medium-scaled detachment-type sag, and has typical detachment characteristics involving low angle normal fault system, multi-stage magmatism, and separated superimposed subsags. The main subsag of Kaiping sag has undergone fault depression and detachment in the first and second episodes of rifting, respectively, forming a depression structure of being "deep in the middle and gentle on both sides". The rifting unloaded the deltaic megaclast from uplifts on the south and north sides of the sag to the step-fault zones and depressions on both sides of the main subsag, so that the main subsag developed large-scale semi-deep lacustrine source rocks. The relationship between TOC contents and C₃₀ 4-methyl steranes of source rocks indicate that high-quality source rocks are developed in the main subsag with 570 million cubic meters of oil resources. The pumping effect of the southern step-fault zone can increase the oil migration amount in the southern depression by about 30 % . This overturns the former traditional idea proposed based on the separation trough holding that the migration to the southern step-fault zone is limited. The analysis of brittleness and ductility of the fault-related mudstone shows that below 3 600 m, brittle mudstone is developed and faults in the brittle regime are easy for oil and gas transportation. Above 3 600 m, ductile mudstone is developed and faults in the ductile regime are easy to be sealed. The roof of vertical hydrocarbon migration along the faults in the main subsag of Kaiping sag is the Lower Member of Enping Formation and the Upper Member of Wenchang Formation. Comprehensively based on oil and gas migration and accumulation, trap conditions, and reservoir-cap conditions, it is concluded that the southern step-fault zone of the main subsag of Kaiping sag is the most favorable exploration area, and the near-trough area in the northern slope of main subsag is an important exploration field. These findings guide drilling in the southern step-fault zone to discover oil layers of more than 100 meters, and has broken new ground in exploration of medium- and large-sized oilfields in the deep water area of the South China Sea.

Key words: Kaiping sag, detachment-type faulted rift, semi-deep lacustrine subfacies, oil and gas migration, step-fault zone, trough area