Deep geologic structure of the South Caspian megabasin and regularities of geophysical and geochemical fields variation within hydrocarbon-bearing zones
Humbat O. Veliyev
In order to achieve more efficient exploration for hydrocarbon deposits within the South Caspian megabasin all available vintage and recently acquired seismic and gravity data has been analyzed with applied advanced technologies taking into account geodynamic conditions.
A series of structural maps have been compiled over the key stratigraphic interfaces: Conrad and Mohorovicic discontinuities, scale 1:500000, basement top, Jurassic and Cretaceous deposit tops scale 1:200000.
Based on refractor velocity data 7,9-8,2 km/sec M discontinuity within the South Caspian meqabasin is observed within the range of 40-50 km depth, including offshore area south off Absheron peninsula 40-50 km, Neftchala-deniz-Kur dashy 40 km, Gizil-aghach-deniz area 40-50 km, Saatly-Kurdamir-Aghsu-Shamahy 40-50 km, Ismailly-Caramaryiam-Goychay-Shaki and Gazah-Ganja zone 40-50 km.
Occurrence depth of Conrad discontinuity in the region, based on the refractor velocity data 6,7-7,7 km/sec is estimated in the range from 10 to 25 km depth: and particularly in the Lower Kur trough 25-30 km, Talish-Vandam gravity maxima in the range of 10-25 km, Shamahy-Ismailly-Shaki and northern margins of Middle and Lower Kur troughs varies in the range of 17-25 km. Granite layer thickness variation across the region amounts to 4-17 km. In South Kur basin its thickness within transition zone drops down to 4-5 km. Seismic wave reflector velocity the Shouth-Caspian megabasins basement varies in four intervals: 5,8-6,0; 6,1-6,3; 6,4-6,5 and 6,6-6,8 km/sec which occurrence depth ranges 18-28 km within South Caspian basin 12-18 km in Lower Kur trough, 14-16 km in Evlah-Aghjabedi; 4-7 km in Ismailly-Oghuz; and 9-14 km in Gabirry-Alazan trough. These troughs are subdivided by uplift zones.
NW-SE trending Mingechevir-Saatly-Talish uplift is restricted by Udabno-Goychay, West-Caspian and Mingechevir-Lankaran buried faults.
Talish-Vandam uplift separates Middle-Kur and lower Kur basins. West Caspian fault is suggested to branch off its right margin based on typical surface features observed in the area.
Sedimentary fill increases its thickness up to 25-30 km towards lower Kur and west Caspian offshore areas. Conducted study made it possible to compile isopach sketches for Jurassic and Cretaceous deposits at 1:200000 scale. Petrophysical properties and lithology were identified based on well data. Paleogene-Neogene deposit lithofacies and petrophysical properties have been studied in detail from the view of their hydrocarbon potential.
We have also analyzed maps designed taking into account thermal and pressure conditions at -2000, -4000 and -6000 m depth slices. As a result of this analyses it was identified that starting from Kur trough towards the South Caspian Basin and according to said above depth slices thermal gradient diminishes 1,5-2,0 times maximum temperature ranges from 45 to 110îC within the depth interval from -1000 to -6000 m, whereas at -7000-10000 m depth range its value exceeds 100-150îC. Oil generation and its migration were substantiated given respective thermal and pressure conditions had been proven in subsurface application. First time geodynamic factor (geophysical and geodynamic fields spatial and temporal variation), was taken into account while geologic setting analysis, that causes mistie between identical stratigraphic interfaces on seismic profile intersection knots, that were shot at various times. These geodynamic effects should be considered in future investigations. This factor impact first was taken into account while earthquake analysis, which seismodynamic conditions was evaluated as K=9-14. These studies made it possible to evaluate earthquake initiated seismic wave impact when driving mechanism of seismo-tectonic motions is properly substantiated. It is indicated that earthquake focuses are generally clustered at depth interval rangins from 3 to 20 km (65 %) comprising Meso-Cenozoic stratigraphic section and geodynamic processes take a significant part in faulting pattern initiation.
It is also shown that after geodynamic processes activation geophysical field intensity increases by 5-10 % or 30-50 % and more and continues for some 27-28 and even 449-392 days and more within the vast areas.
The developed technology taking into account geodynamic effects on geophysical fields has been proven robust while overlooked pay formations within Calamatdin, Small Harami, Neftchala, Kurovdagh and Michovdagh was identified. As a result of this endeavors there first time have been identified about 100 commercial pay formations taking info account geologic geodynamic phenomena along with thermal and pressure conditions and so proved its reliability to evaluate pay formation.