Reasons for formation of low-resistivity oil saturated reservoirs
Analysis and synthesis of the reasons for low-resistivity oil and gas saturated reservoirs formation are carried out. This work allowed identifying two groups of geological cause-and-effect relations, which determine petrophysical and physical reasons for low-resistivity rocks occurrence. The prime mover is facies environment of sedimentation together with tectonic activity resulting in formation of channels for fluid migration. It is shown that in general the secondary geological reasons will always be a rock lithotype (four conductor types) determining film-bound water and electrically conductive minerals, or extension faults and fractures, which make a basis for imposed-epigenetic process resulting in additional surface conductivity. Both groups of geological relations cause an additional (surface) conductivity determined by the following factors: increase of inner area of pore and fracture surface of film-bound water in the rock and, thus, charge density; double electrical layer of clay fraction; interlaminar conductivity against electric charges in three-layer clay minerals; formation of electrically conductive minerals (pyrite) in the reservoir rock. In the case of anisotropy of electrically conductive clay partings in a bed, rock resistivity decreases with the increase of borehole inclination. Increased fracturing of a rock may cause its increased electric conductivity. Geological attributes of promising low-resistivity reservoirs are determined. The mentioned petrophysical reasons for the formation of rocks with higher electric conductivity (not accounted for in the course of well log data interpretation) are defined by the following physical principles: formation of surface electric circuit; increase of charge density; increase of charge mobility.
Key words: low-resistivity reservoir; hydrocarbons; oil and gas saturation; resistivity; well logging.
For citation: Mel’nik I.A. Reasons for formation of low-resistivity oil saturated reservoirs. Geologiya nefti i gaza = Oil and gas geology. 2018;(6):129–136. DOI: 10.31087/0016-7894-2018-6-129-136.
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I.А. Mel’nik Scopus
School of Earth Sciences & Engineering of Federal Independent Educational Institution "National Research Tomsk Polytechnic University", Tomsk, Russia;