The shape of spontaneous potential curves is frequently used in the interpretation of sandstone depositional environments. The "cylinder-," "funnel-," and "bell-shaped" SP profiles are among the most frequently employed. However, the validity of this commonplace practice has never been thoroughly established.

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

Field examples 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 shape of the SP curve. This can result in erroneous interpretations of sandstone origin.

Curve shapes derived from the microresistivity measurements of the dipmeter tool are an alternative to those of SP curves. 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.

End_of_Article - Last_Page 1220------------