uFramework

uFigure captions

uTectonic events

uFault subdivision

uReferences

uFramework

uFigure captions

uTectonic events

uFault subdivision

uReferences

uFramework

uFigure captions

uTectonic events

uFault subdivision

uReferences

uFramework

uFigure captions

uTectonic events

uFault subdivision

uReferences

Figure Captions

Return to top.

Tectonic Events 

The principal tectonic events that impact Karachaganak Field are: Paleozoic rifting, Hercynian orogenesis of the Urals, and halokinesis. The following sedimentary and tectonic events are evident (Figure 3):

·      Paleozoic rifting which defined the lineaments that ultimately controlled subsequent tectonic evolution;

·      Devonian deposition in an extensional setting;

·      Northward tilting of the Karachaganak Horst (continued extensional regime), deposition of the Upper Devonian and the Carboniferous-aged ramp-platform sediments (Tournaisian to Bobrikovskian);

·      Tula deposition (middle to upper Visean volcanic ash and shale marker – drowning event) and southward tilting, probable indication of onset of Hercynian orogenesis in this area;

·      Extensional and transtensional tectonics during the deposition of the Visean-Serpukhovian section, related to an Hercynian tectonic influence; this is principally observed in the central area of the field (Figure 4);

·      Bashkirian deposition in the northern area of the field;

·      Deposition of the Moscovian in an extensional setting (peripheral bulge?), mainly in the western part of the field;

·      Possible uplift and inversion of some localized depocenters along pre-existing features in the central portion of the field; this event appears to have migrated westward with time, occurring prior to deposition of the top of the Permian pinnacles in the central part of the field, but after the deposition of the reservoir top (Filipov) in the westernmost area of the field. In the eastern part of the field, these movements can be observed to have taken place after Tula deposition; however, further stratigraphic controls are necessary (Figure 5);

·      Growth of Permian pinnacle reefs, principally overlying areas of tectonic inversion, in the central portion of the field;

·      Tectonic inversion in the western area (post-Filipov);

·      Isolation of the Precaspian Basin from the Uralian Ocean, resulting in the deposition of the Kungurian evaporites (Hercynian tectonic impact);

·      End of the Hercynian Orogeny (post-Kungurian – Triassic) and extension with possible start-up of salt tectonics.

Fault Subdivision 

On the basis of this structural model and history, an attempt as been made to subdivide the faults into a number of different families, for modeling purposes, as illustrated in Figure 6: 

1. NNW-SSE faults, characterized by a probable post-Carboniferous inversion. In correspondence to these features, it is possible to observe the development of localized Permian pinnacle reefs. Observatioons of field fluid behavior suggest that these faults are, in some instances, permeability barriers. This assumption may be valid for all faults with an inversion origin, although as yet no dynamic test information can confirm this hypothesis. It may be that diagenetic effects resulted in a deterioration of rock permeability along faults of this nature. 

2. NE-SW faults of a possible inversion-related origin. It is reasonable to assume a decrease in petrophysical properties (permeability) in relation to inversion and with respect to the orientation to the present σ_hmax. 

3. NW-SE faults of a possible inversion related origin, favorably oriented with respect to the present σ_hmax. 

4. NE-SW faults, with normal geometries. These faults do not seem to have been subject to any inversion. In correspondence to some of these faults, the field fluid behavior observations suggest an improvement of the petrophysical characteristics. 

5. Marginal faults, of ENE-WSW to WNW-ESE orientation. On occasions, these faults have significant throws, juxtaposing different lithologies. Moreover, it should be noted that, having probably existed as escarpments in many locations during the growth of the carbonate platform, these fault lineaments also represent a real sedimentological boundary; this assertion is supported by the variation of petrophysical qualities observed between the central and more marginal parts of the field. These fault lineaments, therefore, have a significant petrophysical impact. 

6. NW-SE faults with limited throw and a normal geometry observed, as far as the top of the reservoir. For these faults, probably related to the end of the Hercynian Orogeny and perhaps related to the halokinetic initial event, it is difficult to postulate whether they represent permeability barriers or avenues of preferential fluid flow.  

It must be underlined that the diagenetic phases have not been considered in this discussion. To this end, it could be interesting to review the relationship between diagenetic history and the type of faults observed.

References 

Giovannelli, A., Viaggi, M., Elliott, S., and O’Hearn T., 2001, Reservoir characteristics and sedimentary evolution of a major Pre-Caspian field: the example of Karachaganak: EAGE, Firenze, oral session.

Brunet, M.F., Volozh, Y.A., Antipov, M.P., and Lobkovsky, L.I., 2001, The geodynamic evolution of Precaspian Basin (Kazakhstan) along a north-south section: Tectonophysics, v. 313, p. 85-106.

Return to top.