Brief Remarks on the Structure of the Karachaganak Field (Kazakhstan)*
Tony Birse1, Arrigo Francesconi2, and Claudio Toscano2
*Modified from presentation at AAPG AAPG International Conference, Paris, France, September 11-14, 2005
2Eni S.P.A. E&P Division ([email protected])
Karachaganak Field is located at the northern margin of the Precaspian Basin. The field overlies a Devonian-Visean-aged horst and is subdivided into a number of distinct fault blocks, upon which Upper Visean to Upper Serpukhovian carbonate-platform lithologies were deposited. Following the development of a significant Carboniferous-Permian unconformity, the Asselian to Artinskian stages witnessed the growth of a number of pinnacle reefs. The subsequent deposition of the Filipov Anhydrite formation marked the onset of a period of evaporite (Kungurian) deposition, implying the complete isolation of the Precaspian Basin from the Uralian Ocean (Giovannelli et al., 2001; Brunet et al., 2001) (Figure 1).
Based on the analysis of the 3D seismic data, the reservoir section of the field can be subdivided into six principal blocks (Figure 2), separated by a number of tentative lineaments, the orientation of which are still not entirely clear. The differentiation into separate blocks became apparent from the Devonian onward, presumably dictated by the pre-existence of ancient basement fault lineaments. These WNW-ESE oriented basement faults and secondary NE-SW and NNW-SSE oriented lineaments controlled the distribution of Devonian sedimentation, including deposition of the source rock. The delineation into these six blocks became particularly significant during the Carboniferous. These lineaments determined the subsequent tectonic evolution of the field.
A further, more local structural-stratigraphic subdivision is also presented (Figures 1 and 3): Devonian sedimentation was followed by pre-Tula carbonate-platform sedimentation (Tournaisian-Bobrikovskian), post-Tula carbonate-platform sedimentation (Upper Visean-Serpukhovian) and then Permian pinnacle reef development (Asselian-Artinskian). This section is overlain by thick Kungurian evaporites, including the Filipov Anhydrite formation.
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.
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 σhmax.
3. NW-SE faults of a possible inversion related origin, favorably oriented with respect to the present σhmax.
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.
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.