Shunshan Xu

Origin of superimposed and curved slickenlines in San Miguelito range, Central México

Interactions between intersecting faults cause local perturbations of the stress field in the vicinity of their intersections. Fault intersections are places of stress accumulation, stress relief and refraction of the stress trajectories; the slip vectors near these intersections are deviated from the maximum shear stress resolved by the far-field stress. In an intersecting fault system, superimposed, arc-shaped and zigzag slickenlines can be formed due to interaction between intersecting faults. We propose some mechanisms in which it is possible to recognize that the superimposed and curved slickenlines are produced from curvilinear translational fault motion. The geometrical models presented in this contribution are consistent with the slickenlines distribution observed in the vicinity of intersection lines, measured in the San Miguelito range, Mesa Central, Mexico. Two tectonic phases have been inferred from our slip vector models near the intersection lines, which is consistent with observations of previously published work.

Tilting mechanisms in domino faults of the Sierra de San Miguelito, central Mexico

A system of normal faults with similar strike that bound rotated blocks in the Sierra de San Miguelito, central Mexico, was studied to determine the genesis of rotation and to estimate the extensional strain. We show that rigid-body rotation was not the main deformation mechanism of the domino faults in this region. We propose vertical or inclined shear accommodated by slip on minor faults as the mechanism for strain in the blocks. In order to test quantitatively the amount of strain, we calculated the extension assuming vertical shear obtaining ca. ev ~0.20. This value is in good agreement with extensions previously reported for the Mesa Central of Mexico. The bed extension required in this model reaches ca. 33% of the total horizontal extension (i. e. ebed =0.34 ev). Assuming self-similar geometry for fault displacements, it is shown that bed strain required in shear models can be liberated by the small faults. If the strain is calculated using the rigid-body rotation model, the lengthening is underestimated by up to 9%. This case study shows that shear models could be applied in volcanic zones.

Determination of true bed thickness using folded bed model and borehole data

The true bed thickness (t) is the actual thickness of a given formation perpendicular to the bedding plane. The value of t depends on the angle and the direction of the dip of the measured formation, as well as the drift angle and azimuth of the borehole. The traditional methods to calculate the parameter t consider only the case of monoclinal beds but not the case of a folded bed, which will cause deviations when the bed dip on the top is different from that on the bottom. To avoid these deviations, this paper shows an approach to calculate the values of t using a folded bed model. The deviations for the monoclinal bed model are positively related to the bed dip, the dip difference and the deviated angle of the wells. A case study from the Cantarell oil field complex in the southern Gulf of Mexico (offshore Campeche) is used to test the folded bed method. The results indicate that this model can yield more uniform spatial change of the values of t, whereas the monoclinal bed model will overestimate the average value of t. Compared to the folded bed model, the maximum relative deviation of t from the monoclinal bed model reaches 22.3% and the maximum absolute deviation of t reaches 34.5 m.

Slicken 1.0

The slip vector on a fault is an important parameter in the study of the movement history of a fault and its faulting mechanism. Although there exist many graphical programs to represent the shear stress (or slickenline) orientations on faults, programs to quantitatively calculate the orientation of fault slip based on a given stress field are scarce. In consequence, we develop Slicken 1.0, a software to rapidly calculate the orientation of maximum shear stress on any fault plane. For this direct method of calculating the resolved shear stress on a planar surface, the input data are the unit vector normal to the involved plane, the unit vectors of the three principal stress axes, and the stress ratio. The advantage of this program is that the vertical or horizontal principal stresses are not necessarily required. Due to its nimble design using Java SE 8.0, it runs on most operating systems with the corresponding Java VM. The software program will be practical for geoscience students, geologists and engineers and will help resolve a deficiency in field geology, and structural and engineering geology. Software designed using Java SE 8.0.Non-Andersonian stress state.Direction of resolved shear on fault planes.

Effect of block rotation on the pitch of slickenlines

Rotation of faults or pre-existing weakness planes produce two effects on the slickenlines of fault planes. First, the rotation leads to changes in the pitch of slickenlines. As a result, the aspect of the pre-existing fault may change. For example, after rotation, a normal fault may show features of an oblique fault, a strike-slip fault, or a thrust fault. Second, due to rotation, stress states on the fault planes are different from those before the rotation. As a consequence some previous planes may be reactivated. For an isolated plane, the reactivation due to rotation can produce new sets of slickenlines. With block rotation, superimposed slickenlines can be generated in the same tectonic phase. Thus, it is not appropriate to use fault-slip data from slickenlines to analyze the stress tensor in a region where there is evidence of block rotation. As an example, we present the data of slickenlines from core samples in the Tunich area of the Gulf of Mexico. The results wrongly indicate that the calculated stress tensor deviates from the far-field stress tensor.

Software for determining the direction of movement, shear and normal stresses of a fault under a determined stress state

In the oil, gas and geothermal industry, the extraction or the input of fluids induces changes in the stress field of the reservoir, if the in-situ stress state of a fault plane is sufficiently disturbed, a fault may slip and can trigger fluid leakage or the reservoir might fracture and become damaged. The goal of the SSLIPO 1.0 software is to obtain data that can reduce the risk of affecting the stability of wellbores. The input data are the magnitudes of the three principal stresses and their orientation in geographic coordinates. The output data are the slip direction of a fracture in geographic coordinates, and its normal (n) and shear () stresses resolved on a single or multiple fracture planes. With this information, it is possible to calculate the slip tendency (/n) and the propensity to open a fracture that is inversely proportional to n. This software could analyze any compressional stress system, even non-Andersonian. An example is given from an oilfield in southern Mexico, in a region that contains fractures formed in three events of deformation. In the example SSLIPO 1.0 was used to determine in which deformation event the oil migrated. SSLIPO 1.0 is an open code application developed in MATLAB. The URL to obtain the source code and to download SSLIPO 1.0 are: http://www.geociencias.unam.mx/~alaniz/main_code.txt, http://www.geociencias.unam.mx/~alaniz/ SSLIPO_pkg.exe. SSLIPO calculates the stress tensor after the rotation of the principal stresses.Calculation of slip on multiple faults under non-Andersonian stresses.SSLIPO obtains magnitude and orientation of normal and shear vectors.Useful tool to model fluid migration and slip tendency on fractures.Fluid migration in the Reforma-Akal oil province modeled using SSLIPO.