Wstęp do geofizycznej oceny własności zbiornikowych osadów górnej jury niecki mogileńskiej

Janusz Frydecki




The generally used quantitative interpretation methods of geophysical materials in drillings do not allow the reservoir properties of rocks to be evaluated satisfactorily, mainly due to an insufficient representation of geological models of rocks on which these methods are based. Searching for a more effective model, by correlating the results of core examinations with geophysical data, was not possible, mainly on account of a considerably low authoritativeness of laboratory information.
On the basis of the results obtained from resistivity logging (resistivity and specific conductivity of rocks), gamma-ray logging (intensity of radiation and its relative differential parameter), and neutron-gamma-ray logging (as above), a regression analysis has been made of these parameters, with a simultaneous introduction of functional dependences between these parameters and the reservoir properties of rocks. By means of a large amount of observations, some conclusions have been drawn from the trends of the directional arrangement of the regression.
On the basis of the directional coefficient of regression there are determined, for the Upper Jurassic of the Mogilno trough, the provenance of the material in study, and the values of the following coefficients of the analytical forms that bind the geophysical parameters with the reservoir ones:

1) cementation coefficient “m” for Archie's formula,

2) exponent “a” of the function binding the clay content in rocks with the intensity of natural gamma radiation,

3) ratio (quotient) of extremal “neutron carbonate porosities” making the base number of the function that connects the gamma radiation intensity, induced by neutrons, with porosity.

The regressions obtained in this way are characterized by high values of correlation coefficients, and the probability of correlation considerably exceeds the value 0.999. The works carried on by means of the least square method have been based on electronic computers.
The method of searching for approximation models of changes in physical properties of rocks in function of changes of reservoir parametres of rocks, worked out and presented by the author, allows us to use for methodical purposes a lot of logging materials accumulated for many years in various regions, without necessity of using not always certain information from the laboratory examinations of drill cores.
The results obtained are burdened with very low errors: 4.46%, 1.681%, 3.071%, respectively. This is due to using in calculations only the direction trends of regressions for considerable amount of observations. This method can successfully be adapted in current interpretations when using digital computers.


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