Analysis of the tectonic characteristics of the western tectonic complex of N2 area

: The western part of N 2 area is located at the tectonic high point of D oil field, with faults cut into each other, staggered distribution and complex fracture structure. In this paper, based on 3D seismic interpretation, the research on fault geometry, kinematic characteristics, fracture deformation period and grade division are systematically carried out. The quantitative discriminatory criteria of fault plane segmental growth are applied to implement the reliability of the fault plane combination method.


Presentation of the problem
The western part of N2 area is located at the high point of SRT back-slope structure.Its development area is 26.23km2, and the structure is very complex.There are 71 developed faults, accounting for 46.4% of the whole area, and the density of faults is as high as 2.7 faults/km2.The type of fault combination is complex, and the planes are mostly developed with intersecting faults, so it is necessary to systematically carry out the study on the characteristics of fault development, formation mechanism and genesis period, and finely delineate the way of fault planes combination, so as to provide geological basis for the adjustment and tapping of the block.

Analysis of fault development characteristics and genesis mechanism (I) Fault development characteristics
The tectonic complex area in the west of N2 area is cut by north-northwest and north-east faults on the plane, among which 29 north-east faults are developed, mainly with a fault distance less than 5 meters and an average extension length of 802 meters; 42 north-northwest faults are developed, mainly with a fault distance greater than 10 meters and an average extension length of 1272 meters.The north-northwest fault has a large fault distance and a long plane extension.Fracture profiles are mostly nested V-shaped, Y-shaped and anti-Y-shaped.The fault burial depth-distance curve is applied to quantitatively characterize the fault nucleation point, and the maximum distance marks the fault nucleation point; the maximum distance of the north-northwest and northeast faults is in SaⅡ or SaⅢ group, and is commonly nucleated in SaⅡ and SaⅢ group.Combining the contemporaneous fauld, active stage and interbedding, the SII oil layer is divided into five sets of fault systems: the Nen-late-Ming-late Paleogene inversion period active fault, the Yaojia-Nen 1st and 2nd member active fault, the Yaojia-Nen 1st and 2nd member active fault, Jia-Nen 1st and 2nd member inversion period active faults, Quantou Formation-Yaojia-Nen 1st member inversion period active faults, Denglouku Formation-Yaojia-Nen 1st member 2nd member rifting period, inversion period active faults .Reversal period: Type V faults are revived and reactivated, Type I faults (north-east) are formed, Type III and Type IV continue to be active; depressional period (Yaojia Formation-Nen I and II section): Type II and Type III faults are formed, Type IV faults continue to be active; depressional period (Quantou Group depositional period) Type IV faults are formed, Type V faults continue to be active; depressional period: Type V faults begin to form.North-northwest trending faults are active in multiple phases and continue to develop successively.The north-east oriented faults are formed by late extrusion stress.By comparing the results of physical simulation experiments in different orientations with the results of the study area, the stress direction suffered is 45° with the north-northwest oriented fault.

Fault plane connection method
Fault growth is a dynamic process that undergoes a continuous process of destruction-connection-redestruction.Segmental growth is a process in which a large number of smaller displacement positive faults gradually interact with each other to grow and connect in segments to form a small number of larger scale large faults as the remote stress increases.Fault displacementdistance curve is one of the important methods to identify the segmental growth of faults, and the presence of low value zone of displacement is a segmental growth fault, otherwise it is an isolated fault.It is possible that the segmented growth fault is interpreted as two faults due to the limitation of seismic resolution.In order to finely portray the fault plane connection mode, 33 faults with fault distance greater than 5m in the whole area were identified for development type according to the above method, and 10 segmental growth faults were identified.The method of quantitative determination of fault growth stage by applying "conversion displacement (D)/distance (S)" -quantitative criteria of fault segment growth process.
For "soft connection" lateral stacked faults, the conversion displacement (D) is the sum of the displacements of the two faults at the center of the stacked fault section, and the separation distance is the vertical distance between the two faults at the center of the stacked fault section.For hard-connected faults, the conversion displacement is the sum of the two fault displacements at the center of the overlapping section of fault A and extinction fault B. The separation distance is the distance between the two faults at the center of the overlapping section of fault A and extinction fault B. If due to seismic resolution limitation, the extinction fault B may not be developed, the conversion displacement is equal to the displacement in the middle of the abruptly changed section of the fault strike, and the departure distance is equal to half of the distance between the faults in the subparallel section.
Based on the three-dimensional seismic data of SL basin, the data of fault transition displacement (D) and separation distance (S) are counted to improve the quantitative criteria of fault segment growth connection; when D/S is less than 0.27, the fault segment is in the "soft connection" stage of lateral stacking, when D/S is between 0.27 and 1, it is in the stage of starting rupture The "transition" stage, when D/S is greater than 1, the fracture is in the stage of complete rupture "hard connection" stage, that is, divided into three stages: lateral stacking, the beginning of damage and complete destruction.

Figure 5 .Figure 6 .
Figure 5. Fine comparison of small layers in the Saltu formation of the 139-01# fault with connected wells

Figure 7 .
Figure 7.Comparison of the results of physical simulation experiments in different orientations with the results of the study area

( 1 )
The tectonically complex area is cut by northnorthwest and north-east faults on the plane, and the north-northwest faults have large fault distances and long plane extensions.Fracture profiles are mostly nested Vshaped, Y-shaped and anti-Y-shaped.(2)The north-northwest and north-east faults are commonly in the SⅡ and SⅢ component nuclei, and the north-northwest fault is a synclinal fault, while the northeast fault is a non-synclinal fault.(3)The north-northwest fault is active in multiple phases and continues to develop successively.The north-east fault is formed by late extrusion stress, and the stress direction is 45° from the north-northwest fault.(4) There are 10 segmental growth faults developed in the tectonically complex area, and the fault growth quantitative discrimination criteria are applied to finely delineate the fault plane connection mode: 7 in the hard connection stage, 2 in the transition stage and 1 in the soft connection stage.

Table 1
General characteristics of the top surface fault of SaⅡ oil formation in the western tectonic complex area of N2 area

Table 2
Comparison of the thickness of the trans-northwest trending faults From the fine comparison results of the small layer of the over north-east-oriented 1513 fault connecting wells, the fault stratigraphy and fault thickening coefficients do not differ much and show non-synclinal characteristics.The overall comparison of faults in the whole area shows that the north-northwest fault is syngenetic and the north-east fault is non-syngenetic.

Table 3
Comparison of the thickness of the trans-northeastfacing 1513# fault

Table 4
General characteristics of faults of each fracture system on the top surface of SaⅡ oil formation