Petroleum Geological Characteristics Of Central Graben Carbonates In North Sea Basin

The North Sea Basin is the largest oil and gas basin in Europe,and the Central Graben is one of the main oil and gas areas in the North Sea Basin.Its Chalk reservoir containing oil and gas in Upper Cretaceous to Paleocene is a special carbonate reservoir,which is significantly different from that of general reef or shoal facies carbonate reservoirs.In this paper,the theory and method of petroleum geology are used to study the sedimentary characteristics of the Chalk reservoir,the pattern of sedimentary facies,the controlling factors of the reservoir physical property,and the control factors of the trap,and so on.This paper based on the study of the distribution of the Chalk oil and gas reservoirs having been found and the reservoir forming patterns of typical reservoirs,to analyze the main controlling factors of Chalk reservoirs.The study shows that Chalk accumulation is controlled by many factors such as sedimentation,diagenesis and tectonism.The main component of Chalk is coccolithophorids and planktonic/benthic foraminifera skeleton.It has the characteristics of high porosity and low permeability,the main pore types are intergranular pores.The Central Graben in the Cretaceous was continuously thermally subsided,and the rapid rise of sea level in the Middle Albian made the input of terrigenous debris reduced,as well as the massive development of the coccolithophorids and foraminifera,which were the main reasons for the formation of Chalk.There are two main sedimentary facies models in Chalk Reservoir: 1.Rain of Plankton falls directly into the basin to form Pelagic Chalk;2.Chalk is transported by gravity flow to form Redeposited Chalk.The redeposition of Chalk is better reservoir quality than Pelagic Chalk because of lack of bioturbation and early cementation and so on.Mechanical compaction and pore-filling cementation are the main factors of the deterioration of the physical property of the Chalk reservoir,and the key to maintain good physical property is overpressure and hydrocarbon filling.The trap types of Chalk oil and gas fields are relatively single,mainly structural and lithologic structural traps.It is believed that some traps are also related to hydrodynamic conditions.The Zechstein salt diapir continued to Neogene.It is very important for the formation and high production of Chalk oil and gas fields.On the one hand,salt diapir formed structural traps.On the other hand,salt diapir made the reservoirs develop faults and natural fractures,which are favorable for the migration and high production of oil and gas.Three types of hydrocarbon pooling patterns of Chalk reservoirs are summed up.Tectonic action controls the formation of traps and controls the distribution of oil and gas fields.The Upper Jurassic high quality source rocks,which are widely distributed,a good combination of lower source rocks and upper reservoirs,and the reasonable match period of hydrocarbon accumulation are favorable for the formation of Chalk oil and gas fields.