Abstract
The formation conditions and time sequences for various types of wrench-related fractures are not clear. Based on a parabola-type failure criterion, this paper has gotten new insights on those questions. In a simple shear, the occurrence of either tensional fractures or Riedel shears is controlled by the ratio (Rtc) of tensile strength to cohesion. In a pure shear, the occurrence of either second order tensional fractures or second order Riedel shears is controlled by the ratio (Rtci) of tensile strength to cohesion, given a constant inner frictional coefficient. Where the Rtc or the Rtci is less than a certain value, the en echelon tensional fractures will occur first. Where the Rtc or the Rtci is bigger than the certain value, the Riedel shears will occur first. Where the Rtc or the Rtci is equal to the certain value, the en echelon tensional fractures and the Riedel shears will occur simultaneously. The understandings will enhance the research on wrench related fractures and will be of significance in petroleum exploration and development, because fractures are both important accumulation spaces and key migration paths for oil and gas.
Highlights
Wrench zones and their related structures were common both in outcrops and in oil-bearing areas [1]-[10]
The occurrence of either second order tensional fractures or second order Riedel shears is controlled by the ratio (Rtci) of tensile strength to cohesion, given a constant inner frictional coefficient
Physical modeling conducted by Naylor et al [22] showed that en echelon R-shears would occur in the early stage of a wrench belt development, T-tensional fractures in the middle stage and R'-shears with cease of the T fractures in the late stage and further, netted faults and lens would form in this stage as well
Summary
Wrench zones and their related structures were common both in outcrops and in oil-bearing areas [1]-[10]. Physical modeling conducted by Naylor et al [22] showed that en echelon R-shears would occur in the early stage of a wrench belt development, T-tensional fractures in the middle stage and R'-shears with cease of the T fractures in the late stage and further, netted faults and lens would form in this stage as well. All kinds of structures may occur in an outcrop wrench belt or in a subsurface oil-bearing area [1] [4] [25], including R-shears, R'-shears, P-shears, Y-shears, T-tensional fractures and principal displacement zone. The time sequence of the wrench related structures was addressed analytically neither in physical models nor in natural examples. Based on parabola-type fracture or failure criterion, this paper discusses the stress status and rock mechanics for the occurrence of T-tensional fractures and Riedel shears. The time sequence of their occurrence is addressed as well
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