The Characteristics Of Inversion Structures In Bongor Basin And Its Relationship With Hydrocarbon Accumulation

The Bongor Basin is a passive rift Basin formed under the effects ofCentral African shear zone with dextral strike–slip. It has experienced two inversionstages. Exploration of practice has confirmed that oil and gas reservoirs in the basinare all connected to inversion structures. So studying inversion structure is veryimportant for oil and gas exploration.In this paper, using seismic and logging data to recovery the amount of erosion atthe end of the Cretaceous and the Paleogene, the methods are mainly strata thicknessratio and acoustic transit-time. Its results show that both of them has the same featureof “Strong in west-weak in east and strong in north-weak in south”, but the amount oferosion is much more at the end of Cretaceous than it at the end of Paleogene.Analysis of subsidence history showed that from the Cretaceous to Paleogene the totalsubsidence and tectonic subsidence volume of the basin is reduced, it explain that thetectonic activity is much stronger in Cretaceous than it in Paleogene. Because thebasin formed under the effects of dextral strike-slip, the former normal faults arealmost with large inclination. So in the process of inversion the faults didn’t haveobvious inverse displacement and the main performance is folding of stratums, so it isbelonged to fold-type inversion structure. According to the interpretation of seismicdata, inversion structures are divided into three categories, including thick folds type、front valley-rear peak type and front peak-back valley type. On this basis, it hasdivided into seven sub-categories. The distribution of its different types in the plane ismainly controlled by the early structural frame, it’s mainly front valley-rear peak typeat steep slopes, front peak-back valley type at gentle slope and thick folds type at deepdepressed. Using balanced profile technology to reproduce the tectonic evolution ofthe basin and determined the formation of inversion structures and intensity of reverse.The relationship between inversion structure and hydrocarbon accumulation mainlyperformance in three areas: First, it provides traps for hydrocarbon accumulation, most of which is inversion anticline. Second, the inversion intent has an importantinfluence on hydrocarbon accumulation. Weak inversion can’t improve trapconditions, so it’s not conducive to formation of large fields while strong inversionmay undermine the former reservoirs, causing leakage and escaping. Somoderate-intensity inversion is only most favorable for hydrocarbon accumulation,not only provide effective traps for hydrocarbon accumulation, but also can avoid thedestruction of the former reservoirs. Third, the formation and distribution of oil andgas reservoirs in different place has a significant difference because of the differenttype of inversion structures. Analysis of inversion structure has become an importantapartment of basin analysis, it has important significance guiding for basin oil and gasexploration. From the analysis of intensity and distribution of inversion structure, themost favorable exploration place is at gentle slope position in central of basin.