Peculiarities of seismic data interpretation in Central Asia
The region within intermountain troughs of the Central Asia (Fergana, Afgan-Tadjik, Alayskaya, Atbashinskaya) in territories of Republics Kirghizstan and Tajikistan is one of the most interesting places, from the point of view of a structural geology, and at the same time the most difficult for seismic researches. Characteristic feature of the trough tectonics is presence of large scale thrusts and uplifts which complicated marginal areas of troughs at their joint with a mountain frame.
In the late 80-th, specialists of the JSC Saratovneftegeofizika' reported about reorientation of oil and gas exploration in this area for searching of oil traps under thrust blocks using data of CMP seismic. This idea has been supported by academician V.E. Khain, and doctors D.S. Orudjeva, K.N. Kravchenko and A.N. Obukhov. By the beginning of 90-th, oil traps related to underthrust (autochthonous and parautochthonous) blocks have been distinguished and prepared for drilling at the north and south rims of Ferganian basin and Alanian and Afgan-Tajik troughs. The later drilling had revealed large scaled thrusts with horizontal dislocation of rocks up to 3 km. In some areas oil inflow has been received from autochthonous underthrust blocks (Fig. 1).
Processing and interpretation of the seismic prospecting data from the Central Asia is significantly different from the areas with platform tectonic and horizontally layered sections. The following points should be mentioned:
- Processing and interpretation in this case should be considered as undivided procedures in sense that processing is interpretational process which depends on basic the model considered for processing.
- Choosing parameters of migration mapping is also important. This procedure has interpretational significance during the processing. (Fig. 2)
- Summation window selection.
As an example of optimality of this approach we may consider drilling results from the area of South rim of Fergana basin. A borehole, recently well-drilling, opened autochthonous Neogene rocks beneath thrust block of Palaeogene-Mesozoic deposits. These data supported an idea on uplift/thrust geological model for this area. Reprocessing of an previous seismic prospecting data with application of method of interpretational processing allowed to improve tracing of reflecting bed for autochthonous block (Fig. 3).
Fig. 1. Seismic time section (South edge of Fergana depression)
Fig. 2. Comparison of different variants of migration.
A — migration with CDP velocities (correlation has taken from variant B); B — migration with CDP velocities reduced for 20%
Fig. 3. Comparison of different variants of processing
A — Processing of 1996 (processing complex SDS-3); B — interpretational processing of 2005
Fig. 4. Seismic time section of latitudinal strike