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A THREE-TIMES INCREASE OF THE GAS PROSPECT AREA AS A RESULT OF REFINING THE SHAPE OF BUDYSHCHANY SALT DOME


Dnieper-Donets Basin, Ukraine, 2012

 

 

Figure 1. Salt dome and HC traps by 3D seismic and 3D gravity

GEOLOGICAL PROBLEM

The central part of Dnieper-Donets Basin is characterized by active salt
tectonics. Hydrocarbon accumulations are confined to Carboniferous sediments, which are pierced by Devonian salt. Budyshchany salt dome is located within the investigated area.

The problem is caused by inability to map salt dome walls by 3D seismic due to absence of seismic reflections in proximity to the salt dome as well as difficult terrain. High-precision gravity data were used for delineating the shape of Budyshchany salt dome.
Figure 2. Salt dome by 3D seismic data interpretation (a) and 3D gravity inversion (b). The new hydrocarbon trap is adjacent to the south-western wall of the salt dome, in 3D density model it was delineated by density values 2.28-2.37 g/ccm

GEOLOGICAL RESULTS

The results of joint inversion of gravimetric and seismic data showed that the lateral size of Budyshchany salt dome is three times smaller than it was initially interpreted by 3D seismic data (Figure 2).

Low-density areas near the salt wall identify presence of gas saturated reservoirs. New hydrocarbon trap occupies the area where the salt dome had been mapped by 3D seismic (Figure 1).

3D GRAVITY INVERSION WORKFLOW

The initial 3D structural model of target horizons in Carboniferous was built using 3D seismic interpretation results. Underlaying Devonian sequence (including mother salt) and the basement were built using regional 2D seismic lines. The initial 3D density model of target Carboniferous interval was built using log data from wells of nearby Matviivka field. Structural features of the salt dome and mother-salt were refined by 3D non-linear inversion of gravity data for 17 structural surfaces.

3D density model was refined by full-depth 3D linear inversion of gravity data for 11.2 million cells (cell dimension 100x100x50 m).

Standartd deviation (SD) between observed gravity and gravity calculated from the initial 3D density model was 4.639 mGal (Figure 1, d); between observed gravity and gravity calculated from the resulting 3D density model was 0.048  mGal (Figure 1, e) (relative to the gravity field, the initial 3D density model was improved by 97 times).

 

Figure 3. Observed (а) and calculated gravity fields in case of initial (b) and resulting (с) 3D density models and gravity misfit in case of initial (d) and resulting (e) 3D density models

  

 

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