Abstract: Validity of the Use of the Spontaneous Potential Curve Shape in the Interpretation of Sandstone Depositional Environments

By

John W. Snedden

The spontaneous potential curve shape is frequently used in the interpretation of sandstone depositional environments. The "cylinder-", "funnel-", and "bell-shaped" SP profiles are among the most frequently employed. According to the advocates of this technique, the "cylinder" represents a clean sandstone, the "bell" a sandstone with a fining upward texture, and the "funnel" a sandstone which displays a coarsening upward texture. However, the validity of this commonplace practice has never been thoroughly established.

Theoretical and experimental work and actural field examples suggest that the vertical trend of the SP deflection does not display a direct relationship with the vertical trend of variables known to be controlled by the sandstone paleoenvironment. The vertical trend of quartz grain size shows a low linear correlation with the vertical trend of SP deflection. The vertical trend of clay content shows a higher correlatlon. but changes in clay type and cation exchange capacity can have more impact upon the SP than the simple volume of clay.

Field examples drawn from the Upper Cretaceous and Tertiary of the Gulf Coast show that hydrocarbons, local variations in the mud filtrate salinity, and regional differences in formation water salinity can greatly alter the SP curve shape. This can result in erroneous interpretations of sandstone origin.

Curve shapes derived from the microresistivity measurements of the dipmeter tool are suggested as an alternative to the SP curve shape. The greater sensitivity of the dipmeter tool, its immunity to the problems of hydrocarbons and Rmf/Rw contrasts, and the relationship of microresistivity to primary rock properties are factors favoring the use of microresistivity curve shapes for the interpretation of sandstone depositional environments from subsurface data.

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