中非裂谷系Bongor盆地强反转裂谷构造特征及其对油气成藏的影响

Structural characteristics of intensively inversed Bongor Basin in CARS and their impacts on hydrocarbon accumulation

近年来，强反转裂谷盆地在油气勘探中引起越来越多的关注，中非裂谷系是强反转裂谷盆地的重要发育区。文中以Bongor盆地为例，分析了盆地的构造特征及其对油气成藏的影响，在此基础上指出勘探方向。研究表明，反转是Bongor盆地最显著的构造特征，受区域挤压应力场作用，Bongor盆地在晚白垩世和中新世发生了两期反转，特别是发生在晚白垩世桑托期（Santonian）的反转造成盆地整体大幅度抬升，地层遭受强烈剥蚀，剥蚀量达600～1 600 m，盆地剥蚀呈现出西强东弱、北强南弱的特点。反转导致残余地层发生明显的褶皱变形，并最终控制了盆地构造圈闭的成型；同时造成上成藏组合油藏的改造、破坏，使得Bongor盆地成藏模式以源内成藏为主。因此，围绕初陷期凹陷、立足下成藏组合是下步勘探的主要方向。

The inversed basins attract more and more attention in the practical petroleum exploration industry for its great resource potential. Regional structure research demonstrated tens of inversed basins gathered in the Central African Rift System （CARS） and this district was becoming an important and representative inversed basin accumulation area. Taking the Bongor Basin as an example, this article analyzes its special structure characteristics, its influences on the hydrocarbon accumulation and then expects to collect some useful exploration suggestion for the basin. The results revealed that the Bongor Basin experienced two inversion stages, i. e. , Late Cretaceous inversion and inversion at the end of Paleogene, so that the inversion structure is becoming the most important characteristic of the basin. Under the two stages of regional inversed compressing stress, the basin was intensively rifted and strongly eroded, among which the I.ate Cretaceous inversion induced the basin to be eroded notablely. Quantitative analysis reveals that the eroded thickness of west and the north of the basin was much larger than the east and the south parts and the total thickness is about 600- 1600 m. It is the inversion structure that controls the reservoir geometry and the hydrocarbon aeeumulation pattern of the basin. The inversion process induced the breakup of the upper enriched hydrocarbon unit and the biodegradation of hydrocarbon. Attributed to these factors, the oil and gas were inclined to accumulate in the lower reservoirs near the source rocks. Therefore, the lower reservoir of the enriched hydrocarbon depression will be the focal points in the basin＇s future exploration.