The spontaneous potential method is one of the oldest ways of recording rock formation properties. It utilizes small electric potentials between the surfaces of the borehole and the electrode (Glover). The differences between the electronic potentials on the surface of different formations caused by the presence of pores and permeable beds allow determining different layers based on spontaneous potential and resistivity of the fluids. This practice report will provide the analysis of the lithology logs made using the spontaneous potential method.
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Measurements used in this report were taken by the company UNOCAL, Offshore county, Louisiana state. Coordinates and characteristics are as follows: latitude: 28o16’13.7906’’, longitude: 91o19’6.6031’’, well OCS-G-21116 No. 2 ST-1, ship shoal block 295. Elevation: DF=107 feet, GL=-245 feet. Date of excavation and measurement: 14 March 2000. The type of fluid used at the excavation site was Ecoflow 200.
According to the charts of spontaneous potential and resistivity received from the site, different layers and openings present the lithological formation. The area between 9000 and 9100 is permeable sand and saltwater, followed by what looks to be permeable sand with fresh water. Layers of permeable sand, freshwater, and impermeable shale continue up to the 9500 marks, after which there is a large chasm with saltwater. It returns to impermeable shale after the 9600 marks.
As it is possible to see, the distances between the spikes are frequent and high. The example charts represent a much more stable shale-permeable bed pattern. It could be concluded that the number of layers and beds filled with sand and water is very high. Either the ground at the location of the excavation is extremely unstable, or there is something in the clay that affects the results of the research.
Glover, Paul. “The Spontaneous Potential Log.” The University of Leeds. Web.