Delineation of horizons and suites based on quantitative indicators of oil-field geophysical parameters

Abstract

The limits of horizons IX and X and a parting suit in the productive strata are adjusted using the methods of electrical resistivity of clay and the maximal potential of sandy rocks as a case-study of Gunsehli and Bahar fields in the South-Caspian Basin. It is known that in oil-field geophysics, delineation of oil and gas bearing suites and horizons by the existing methodology uses the curves of standard electric logging. However, despite that the standard electric logging reflects apparent resistivities of all rocks across the section, it disalllows evaluating stratigraphic characteristics of rocks. Differentiation ability of the electric logging curves depends on many factors, including variation of borehole diameter, thickness and resistivity of surrounding layers, drilling mud resistivity, equipment parameters, etc. All these have their negative impact while outlining the oil and gas bearing suites and horizons using the standard electric logging. In this respect, in distinction from the existing methodology, this article suggests to correlate borehole logs using not the apparent resistivity curves but the curve of the true resistivity variation in clays with depth and the change in maximal self-potential in sandy layers. It is shown that adjsutment of outlines of lithostratigraphic units, including horizons and suites, and correlation of well sections based on the new approach applied jointly with other data for identification of oil and gas pools opens up new opportunities, which, in turn, can enhance geological efficiency of well logging.