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Many explorationists employ the spontaneous-potential (SP) curve shape as an aid in interpretation of sandstone depositional environments and prediction of subsurface sand-body occurrence. The bell, cylinder, and funnel-shaped SP profiles are among the most widely used. The basic assumption of users of these curve shapes is that decreasing deflection of the SP curve from the baseline is due to decreasing quartz grain size and/or increasing clay content in a reservoir sandstone. However, theoretical, experimental, and actual field data indicate that quartz grain size bears no relation to the amount of SP deflection. Clay content does show a relation, but is often overshadowed by a number of variables which affect SP.
Hydrocarbons can also influence SP, often yielding a false bell profile. In addition, borehole or formation-pressure differentials, variations in mud-filtrate resistivity, and regional differences in formation-water salinity can greatly alter the SP curve shape. Bed thickness, especially when less than 3 ft (1 m), also exerts some control over the SP response.
Field examples in which these factors influence SP in Gulf Coast sandstones demonstrate that the SP curve shape is often misleading. Paleoenvironmental reconstructions and predictions of subsurface sand-body occurrence based on such shapes would therefore be in error.
Curve shapes derived from the micro-resistivity tool (expanded dipmeter curve) are suggested as alternatives to SP curve shapes. Unlike the SP, the micro-resistivity tool is immune to the effects of hydrocarbons, variations in mud-filtrate resistivity and formation-water salinity, pressure differentials, and bed thickness.
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