The Lower Devonian production deposits of the north part of the Khoreyver depression
|Location||International Geological Congress,oslo 2008|
|Holding Date||20 September 2008|
Devonian deposits in the northern part of the Khoreyver depression are Lochkovian stage. They were preserved only in the onboard zone of the depression. In these deposits the Trebs and the Titov oil fields have been discovered. The oil fields are multi-stratified. Both fields have the incomplete sections of the Lochkovian stage. The Lochkovian lies on the washed surface of the Upper Silurian. The rocks of the Lower-Frasnian substage of the Upper Devonian overlap it. The section of the Lower Devon is reduced on the Titov field. In the sections of the Trebs and the Titov oil fields the thicknesses, the geological unit, the layers are separated, which have similar lithologic and geophysical characteristics.
The Lower Devonian lithologically is divided into three thicknesses. The section begins with the carbonate thickness (Ovinparma regional stage), above will lie terrigenous-carbonate (Sotchemkyrta regional stage), at the upper section - anhydrite- dolomite thickness (Sotchemkyrta regional stage). The terrigenous-carbonate thickness is developed in the limited territory of the Trebs field and is absent in the section of the Titov field.
Carbonate thickness has various substance composition. This is evident on the well logs, and is confirmed by cores. Therefore, the thickness is divided into two lithological units from below upwards where the lithological unit A is terrigenous-carbonate, the lithological unit B is carbonate. The terrigenous-carbonate bundle of the Sotchemkyrta regional stage is separated as lithological unit V. Lithological units A and B are oil-bearing over the Titov and the Trebs fields.
The lithologic unit A of the Trebs field is divided conditionally into two parts: the lower - more carbonate, and the upper - more terrigenous. Both parts are clearly outlined in all wells of field. The lower part of the lithological unit is represeted by dolomitic limestones, by secondary dolomites and their clay varieties. It contains the streaks of marls and mudstones. Carbonates predominate. The thickness of the lower lithological unit increases from the southwest to the northeast. The upper part of the lithological unit containes more clay. It is represeted by secondary dolomites and by limestones, with a significant admixture of clay material and the substitution of carbonates by marls and by mudstones. Thickness varies from 30 m to 40 m.
The lithological unit B is of a more carbonate composition than the lithological unit A. Minimum thicknesses (to 30 m) are observed in the southern part of the Titov field (wells 10, 35, 39, 41, 42, 43, 70) and are connected with the thinning out and with the subsequent wash-out. Thicknesses to 70 m are fixed in the zone of the North Western strike. In this zone the Olenya wells 3, 9, 16, 47, 71, 72, 74 and the Varknavtskaya wells 1, 4, 5, 10, 11, 14, 17 are situated.