Features of the structure and evaluation of oil saturation factor of low-resistivity upperJurassic reservoirs on example of the Vat’egan field of Western Siberia

The article discusses the features of low-resistivity reservoirs of Vasyugan suite of the Vat’egan field of Western Siberia and the problem of estimating of their oil saturation factor. Low-resistivity reservoir is a productive reservoir, according to the perforation and production data, which has low resistivity, and thus underestimated values of oil saturation factors obtained using this resistivity and traditional petrophysical relations. The factors affecting the decrease in the resistance of rocks are discussed. It is concluded that the fractional composition has a major impact on the reduction of the resistivity in the Vasyugan sediments of the Vat’egan field. An algorithm for the separation of UV1 layer into lithotypes according to the logging data, corresponding to traditional and low-resistivity parts of the section is shown.
Materials and methods
Methods of separation of sediments of the Vasyugan suite into lithotypes according to the logging data
Results
The method of separation of the deposits of the Vasyugan suite on the example of the Vat’egan field is shown. The technique of estimation of oil saturation factor, differentiated by litotypes is given.
Сonclusions
1. In a number of Nizhnevartovsk arch oilfields in Vasyugan suite there are low-resistivity productive reservoirs along with traditional sandstone reservoirs in the same section of UV1 layer.
2. The main feature of low-resistivity reservoirsis in reservoir bottom resistivity reduction due to high resistance on the formation top, which is interpretated in GIS aswater-bearing reservoir. During the test on forecasted water-saturated reservoir, waterless oil flow is gained.
3. At the moment, the only criteria for low-resistivity reservoirs detection on new oilfields is high quality test of the UV1 bottom layer.
4. Examination of possible natural reasons for low-resistivity reservoir appearance on Vatiegansk oilfield allowed to reveal lithofacies reason for specific electric reduction in the bottom of the UV1 layer and to establish criteria for core samples and GIS transition from lithotype 1 of traditional reservoir at the top to lithotype 2 of low resistivity reservoir at the bottom of the UV1 layer.