Fault Characteristics And Its Controlling Effect On Shallow-buried Gas Accumulation In Sanhecun Sag,Zhanhua Depression

This thesis aimed at clarifying the fracture characteristics and gas accumulation mechanism in Sanhecun Sag.Based on integrated study of fault formation and evolution,the geological features of faults and shallow-buried gas reservoirs were illuminated.Then the main controlling factors of gas accumulation were determined and the shallow-buried gas accumulation model was constructed.Based on the 3-D seismic survey data,the characteristics of the faults were clarified,including their distribution,classification,and structural style.From the west to the east,the planar combination features of the faults comprised parallel-aligned,skew-type,and echelon.The profile combination features comprised staircase,graben and horst,as well as the shape of Y.These combinations and their changes suggested that the tension eased off from north to south and from west to east.The fault activity characteristics of ten major faults were evaluated by fault moving velocity.The NEE faults were quite violent in Paleogene and decreased sharply in Neogene,and most of the NE faults were active since Neogene.Although the fault activity velocities were different between faults,most of the faults were active since Paleogene,and a good relationship between fault activity and hydrocarbon accumulation was discovered.In the current fault sealing assessment,three main faults were comprehensively analyzed.It suggested that there were differences between different faults,as well as different parts of the same fault.Gunan fault showed the best sealing ability,and the sealing ability of the northern area was significantly higher than that of the southern area.And the fault sealing ability of hanging wall was better than that of the footwall.There are three kinds of the fault-caprock matching relationships.The gas was enriched below the caprock when the relationship between faults and caprocks were good.Based on the analyze of the component,carbon and hydrogen isotope composition of the natural gas,as well as the saturated hydrocarbon chromatography of the associated gas,the geochemical characteristics and formation mechanism of shallow-buried gas were studied.The results suggested that the shallow-buried gas was basically methane with relatively high dryness and low heavy hydrocarbon,and the gas was composed of dissolved gas and biogenetic gas.Based on the petroleum accumulation conditions,the fluid inclusion measurement,the hydrocarbon history,as well as the relationship between fault activities and gas migration,it is addressed that the faults worked mainly on gas migration,with subordinated sealing.Finally,the fault controlling effect on shallow-buried gas accumulation were summarized,the gas accumulation was dominated by fault vertical transmission,with subordinated framework sand.