Key words: Gribnoye field, Upper Jurassic deposits, well log correlation, chlorites, water-retentive capacity, OWC tilt, types of well log, block structure of the horizon.
The results of a detailed study of oil deposit in the YuS1 are presented. The results are based on the automated well log correlation regarding thin rock section analysis, formation tests and hydrodynamic tests.
According to the well log interpretation results the section under study was divided into 16 lithological members. It is established that sedimentation was occurring sequentially in the lower and upper parts of the section. Parallel bedding of the layers and sufficient continuity of the overall thicknesses confirm sequential sedimentation. Several types of the section were identified due to the changes in the thicknesses of separate members.
Each type of the section has block extension over the deposit area which is characterized by the distinct thicknesses of individual members. Sedimentation of these members with maximum thicknesses is identified by greater rate of structure dipping. Conversely, the minimum thickness values of the members are fixed under the conditions of slight structure dipping.
Non-conformities are observed in the middle part of the section in case of structure increasing. These non-conformities are recognized in the reducing thicknesses of separate members up to their complete disappearance. Boundaries of the section type change can be regarded as tectonic block boundaries.
On the basis of hydrodynamic studies the researchers of Lukoil established that the wells with similar development properties are grouped in the distinct blocks. When comparing hydrodynamic test data with the results of the detailed well log correlation almost complete similarity of the fault configuration was determined.
According to earlier researches it was considered that the main reservoir YuS1a was productive and oil-saturated in almost all the wells. The lower reservoirs were water-saturated due to high induction log readings.
The thin rock section analysis showed that the lower reservoirs YuS1b and YuS1c contain chlorites in significant amount in the cementing material. Chlorites as known to be characterized by high-water-retentive capacity that essentially affects not only the high induction log readings, but also influences the sharp deterioration of reservoir properties, making reservoirs YuS1b and YuS1c nonproductive. The results of well tests confirm this fact.
It was established that OWC change occurs simultaneously with the detected block dipping on both structures.
Neglecting of these important features resulted in erroneous picture of OWC tilt on both structures of the Gribnoye field.
