Piceance Basin Mesaverde anomalous self-potential response; identification of capillary seals in a basin-centered gas accumulation

Wells in the Piceance Basin show anomalous large-magnitude (up to 200 mV), large-interval (>2000 ft [610 m]) self-potential (SP) log responses in the Mesaverde gas-producing interval that can be best explained by electrokinetic potential resulting from water flow toward producing Mesaverde wells. Water flow is compartmentalized by capillary seals that are formed when gas generated from coals saturates adjacent thinly bedded sandstones and shales. Capillary seals can be identified by shifts in the SP baseline. The first wells drilled in an area with no previous Mesaverde production have very little SP response, as is expected in tight sandstones with single- to double-digit microdarcy permeability. After Mesaverde production is established in a new area, the SP log begins to show stepwise changes to more negative values beginning in the upper Mesaverde and becoming increasingly more negative with increased depth. The magnitude of the change to more negative values increases with time in an area of active Mesaverde production; some of the more recent SP logs have negative deflections of over 200 mV. This type of SP anomaly has not been reported before, and these anomalies can be used to identify large-scale water movement within a reservoir.