Structural Trap Comprehensive Assessment For Tongfushi Formation And Dalazi Formation In Yanji Basin

Yanji Basin is a small residual Mesozoic rift basin located in Yanbian Korean AutonomousPrefecture, Jilin province.The tectonic framework is two depressions,one uplift and one slop fromwest to east,namely the Western Slope Zone, the Central Depression Zone, the Taiping UpliftZone and the Eastern Depression Zone, with length of50km from north to south and width of40km from west to east, total area of1670km2. The basin mainly developed Cretaceous stratas,Tongfushi Formation and Dalazi Formation were widely distributed in the basin, with thedevelopment of a few hundred meters of sandstone and dark mudstone, which are the major oiland gas exploration target stratas in the basin. Traps are where hydrocarbon accumulates.Based onthe trap identification and under the guidance of petroleum geology theory, this study chosetypical trap assessment parameters, conducted comprehensive assessment for traps by usinganalytical hierarchy process (AHP) and so on, then optimally selected out favorable traps, whichcan lay a guidance for the discovery of new oil and gas reservoirs.Based on the analysis of several core, log and seismic data, under the guidance of sequencestratigraphy and through the seismic interpretation in the basin, basin tectonic framework wasestablished, various types of structural traps were identified. Among them,18structural traps wereidentified in Tongfushi Formation,most of which are fault blocks and fault noses, the area isbetween0.5km2and21.35km2, the closure magnitude is between100m and750m. In addition,18structural traps were identified in Dalazi Formation,most of which are fault blocks and faultnoses, the area is between0.26km2and39.3km2, the closure magnitude is between50m and350m.The organic geochemistry analysis shows that the dark mudstone of Tongfushi Formation isthe main source rock in the basin. Effective source rocks are mainly located in the Chaoyangchuan Depression, Qingchaguan Depression, Dexin Depression, the cumulative thickness is between20and550m with the average of about200m, the distribution area is over400km2, which allsuggest that the set of source rocks have certain resource potential. Source rocks has high organicmatter with organic carbon content of0.26%to5.95%, and with an average of2.18%; organicmatter is mainly type Ⅱ; Source rocks in Chaoyangchuan Depression and Depression QingChaguan is in low maturity-maturity stage,while source rock in Dexin Depression is in lowmaturity stage.The sedimentary environment in Tongfushi Formation and Dalazi Formation is all fan deltasand lacustrines with the relatively development of sandstone. The average accumulative thicknessof sandstone in Tongfushi Formation is190m, the distribution range and thickness of sandstone inDalazi is larger than that in Tongfushi Formation, and the average accumulative thickness ofsandstone in Dalazi Formation is295m. As for reservoir properties, the average porosity inTongfushi Formation is about8.0%, which belongs to the reservoir with the lowest porosity andlowest permeability; the average porosity in Dalazi Formation is about11.5%, which belongs tothe reservoir with low porosity and low permeability. As for the oil and gas information, oil andgas flow has been found to exist mainly in the reservoir of Tongfushi Formation in the basin, sothe reservoir of Tongfushi Formation is the temporary exploration focus.Based on different petroleum geology between Tongfushi Formation and Dalazi Formation,14parameters has been preferred from trap conditions, source rock conditions, reservoirconditions, preservation conditions of Tongfushi Formation Trap,the weight of individualparameter is determined by using AHP; However, for Dalazi Formation Trap,9parameters werepreferred,the weight of individual parameter is determined by using relatively comparisonmethod. Thus each trap assessment parameter can obtain a more reasonable weight value.Taking the actual situation of the area into account, grading the preferred characteristicparameters and giving a reasonable score, the comprehensive assessment for Tongfushi FormationTrap and Dalazi Formation Trap is conducted. According to the results, there are two Class I traps,five Class II traps,eleven Class III traps in the Tongfushi Formation Structural Trap;there are twoClass I traps, nine Class II traps,seven Class III traps in the Dalazi Formation Structural Trap.Class I traps in Tongfushi Formation and Dalazi Formation in Yanji basin are developed in thewell area of Yan4to Yan9in Dexin Depression southeast of the Eastern Depression Zone and the well area of Yan10to Yan12in Qingchaguan Depression northeast of the Eastern DepressionZone.Class II traps are developed in the well area of Yancan1to Yan1in ChaoyangchuanDepression in the Central Depression Zone and in the Taiping Uplift Zone. Class III traps aredeveloped in the well area of Yan31to Yan1in Chaoyangchuan Depressionin，as well scatterbasin margins.