Geodynamics and Oil Content of HorizontalStrike-Slip Fault Structures in Western Siberia

摘要

Extensive 3D surveys on the vast territory of northern West Siberia have permitted identificationrnand detailed characteristic of echelon fault zones in the sedimentary cover. Analysis andrncomparison of echelon fault zone details with the physical modeling results allow such zones tornbe confidently interpreted as tectonic structures originating from incipient horizontal faults.rnHorizontal faults in the earth crust are well known and studied in many parts of the world;rnhowever, it is West Siberian seismic data where early stages of horizontal faulting processrnreceive especially clear manifestation. The northern West Siberia section is characterized by therndevelopment of mainly Jurassic-Cretaceous terrigenous sedimentary cover, 3.0-3.5 km thick,rnoverlain by the Paleogene and underlain by the dislocated and partly metamorphic Paleozoicrnbasement.rnWhat echelon fault systems look like on the seismic data? The paper presents a few examples.rnFig. 1 is one of them. The area in Fig. 1 displays the appearance of a system of echelon faults inrnthe sedimentary cover. This configuration is consistently replicated with slight modifications atrnall reflector levels starting with the basement surface upwards the section to the uppermostrnhorizons (Paleocene) and the day surface.rnMost comprehensive description and explanation of echelon fault systems is provided fromrnphysical modeling of slip dislocations in relation to the pliant sedimentary cover and more rigidrnbasement; the modeling was performed by M. V. Gzovskiy and E. I. Chartkova in the 70s of thernlast century at Tectonic Physics Laboratory, Earth Physics Institute, (former) USSR Academy ofrnSciences. Fig. 2 shows different stages of a horizontal slip-strike fault from its inception to fullrndevelopment. At the very origination of the fault, a series of anticlinal and synclinal drag folds,rnecheloned relative to the fault axis, starts appearing in the sedimentary cover along and above thernbasement fault line (Fig. 2a). This is the first plication phase of the incipient horizontal fault.rnNext, as the horizontal slip-strike fault develops, systems of vertical echelon cracks and a firstrnsystem of shear cracks appear (Fig. 2b). Increase in the horizontal fault causes the shear cracks tornwiden, while joint cracks become graping joint fissures. It is precisely at this phase of thernincipient fault development that wide vertical migration channels for formation fluids get formedrn(Fig. 2c). At this same phase, a second system of shear cracks appears in parallel to the majorrnaxis of the basement fault. As the basement fault continues to develop, the second system ofrnshear cracks turns into a single fault, thus completing the horizontal fault development in thernsedimentary cover (Fig. 2d).