Structure-Sedimentary Model and Subtle Reservoirs in Fault-Depression Basins:An Example of the Paleogene System in the Lunpola Basin,Tibet

The Lunpola basin,a continental residual basin of the Paleogene fault-depression type,is the only in Tibet with discovered industrial-oil.Early exploration proved that the insufficient understanding of the relationship between the structure-sedimentary model and hydrocarbon accumulation has restricted the exploration to some degree. To clarify this relationship and find out the favorable exploration direction,this work made the comprehensive analysis of tectonic-sedimentary characteristics and reservoir conditions of this basin. The results suggest that in north-south direction,this basin can be divided into a gentle slope zone,deep sag zone,and abrupt slope zone,three tectonic units. Four sedimentary systems are recognized,which are river-alluvial fan,fan delta,lake,and subaqurous fan,of which the river-alluvial fan and fan delta sedimentary systems are located in gentle slope zones,lacustrine depositional system in a deep sag zone,and subaqurous fan in the abrupt slope zone. There are also two integrated patterns of sedimentary systems:alluvial fan-fan delta-shore shallow lake sedimentary system in the gentle slope zone,and underwater fan-half deep lake( deep lake)-shore shallow lake in the steep slope zone. Three structure-deposition models can characterize this basin,which are the bidirectional provenance,fan delta in gentle slope zone,and subaqurous fan in the abrupt slope zone. In the gentle slopes,lithological subtle reservoirs in the fan delta front sand bodies should be the main targets. While it is possible to find structural-lithologic complex subtle reservoirs in the abrupt slope zones. The Lunpola basin may have diversified favorable exploration targets,indicative of a good exploration prospect.