Analysis Of Fault Development And Formation Stress Characteristics In Post-rift Stage Of Shulu Sag

In the period of Guantao Formation and Minghuazhen Formation,the active faults developed in the post-rifting period control the process of oil and gas migration,accumulation and preservation.Therefore,it is of great significance to clarify the characteristics of fault development and formation stress in the post-rifting period for oil and gas exploration and development in the Shulu Sag.In this paper,the geometric characteristics of faults are studied from the aspects of fault dip angle,tendency,extension length,fault distance and fault combination mode.Then,combined with the fault distance-depth curve,growth index,displacement-distance curve,fault distance contour map,etc.,the kinematics analysis of the fault is carried out.Finally,the paleo-stress of the post-rift period was inverted by the fault slip direction data method,and the formation stress characteristics of the period were clarified.According to the nature of activity,the faults in the post-rifting period can be divided into four categories.The first category is a new fault(type Ⅰ),the second category is a continuous growth fault(type Ⅱ),and the third category is a re-active fault(type Ⅲ),which is mainly propagated upward by the pre-existing fault.The fourth type is the dip-connected fault(type Ⅳ),that is,the fault formed in the later stage is connected with the fault formed in the early stage in the tendency to re-activate the original fault.The main difference between the re-active fault and the dip-connected fault is that the fault-distance-depth curve of the dip-connected fault has multiple fault-distance maximum points.From the perspective of geometry,the new faults are mainly NEE-near EW-trending,and the strikes of continuous growth faults,re-active faults and dip-connected faults are mostly NE-trending faults.The scale of re-active and dip-connected faults is larger than the other two.Most of the new active faults are straight faults,and usually the extended horizon only reaches the Dongying Formation,while the faults affected by the activities of the rifting period are mostly shovel-type normal faults and slope-flat faults,and the faults extend multiple horizons.From the perspective of kinematics,the type Ⅰ fault nucleates and moves in the post-rift period,and the fault throw-depth curve shows obvious isolated fault growth characteristics.The type Ⅱ fracture nucleates and grows from the deep part of the rift period,and is active in each period.The fault displacement-buried depth curve increases linearly to the deep fault displacement.The type Ⅲ fault nucleated and moved during the rifting period.After a period of burial,it propagated upward again during the post-rift period.The fault disⅠplacement-buried depth curve showed an obvious inflection point upward,and the corresponding fault displacement gradient decreased rapidly.The type Ⅳ fault is nucleated during the rifting period.After a period of stationary period,it grows and connects with the newly formed fault in the post-rift period and re-acts.The fault distance-buried depth curve shows that there are two maximum points of fault distance separated by a minimum value of fault distance,and this position is the location where the fault grows and connects.In terms of the formation stress characteristics,this paper inverts the paleostress state in the post-rifting period on the basis of using seismic data to restore the slip direction of the fault.The inversion result is that the maximum principal stress σ1 is upright,the minimum principal stress σ3 orientation is N343°,the intermediate principal stress σ2 orientation is N73°,and the differential stress ratio Φ = 0.411,indicating that this period was subjected to nearly north-south extensional stress.