Thermal Anomalies Near Salt: A Porosity Preservation Window

The large contrast in thermal conductivity of salt relative to sand and shale influences the thermal history of strata associated with thick salt sequences. As a consequence, temperatures above large salt bodies are typically elevated and those beneath salt suppressed, compared to laterally depth-equivalent strata located away from salt. Factors affecting the magnitude and shape of thermal anomalies include salt thickness, geometry, and timing of emplacement, as well as burial depth and basement heat flow.

Porosity loss related to quartz cementation is known to be highly dependent on thermal history. Quartz precipitation rate increases exponentially with temperature, rising by a factor of approximately ten between 70° and 130°C. The potential for porosity preservation due to suppression of quartz cementation rate in sandstones located beneath allochthonous salt bodies, can be quantitatively evaluated using a combination of 3-D basin modeling and numerical modeling of compaction and quartz cementation (Touchstone). This approach can also be used to assess reservoir quality risk prior to drilling and estimate porosity and permeability in undrilled downdip aquifer sands. Sandstone porosity also impacts acoustic properties and lithology/pay detection from seismic. As a result of suppressed cementation, seismic indicators of reservoir and pay (e.g. amplitude anomalies, flat spots, etc.) may persist to greater than expected depths.

Our modeling results strongly suggest that the potential for preservation of sandstone porosity is greatest in relatively young sedimentary deposits. Under favorable circumstances, good reservoir quality could extend to depths in excess of 30,000 feet. The results of 3-D thermal modeling indicate that the effects of salt on the underlying clastic strata are transient and thermal anomalies will dissipate as the sediment volume heats up over geologic time. As subsalt temperatures rise, precipitation rates for authigenic quartz will increase. Sandstones that lack significant grain coats are especially prone to porosity loss due to quartz cementation under such conditions.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California