A Petrophysical Model to Quantify Pyrite Volumes and to Adjust Resistivity Responses to Account for Pyrite Conductivity
Holmes, Michael; Holmes, Antony; Holmes, Dominic
In previous publications by the Authors (AAPG 2011, 2012) a petrophysical methodology was introduced to examine the constituents of unconventional gas and oil reservoirs. Using triple-combo open-hole logs, components in the clean formations are examined separately from shale components. A refinement to the publication is the subject of this paper.
In many reservoirs associated with organic-rich shales, pyrite is present. Sulfur is generated during the thermal maturation process, and if iron is present the result is the formation of pyrite. Petrophysical properties of pyrite are unique - a very high grain density of 5.0 g/cc, and a very high electrical conductivity of 2.70 mho/m. The presence of even small volumes of pyrite can be inferred from anomalously high grain density. By comparing grain density with conductivity, it can be verified if the high grain density readings are a consequence of the presence of pyrite. If pyrite is the culprit, a cross plot of grain density vs. conductivity will show a linear correlation of increasing grain density with increasing conductivity. Interpretation of the data involves the choice of minimum grain density and minimum conductivity for the cloud of data representing pyrite-free formation. By assigning values to both minima, levels containing pyrite can be examined. Volumes of pyrite can be quantified by comparing grain density and conductivity with minimum values. Correct choices of minimum values should yield closely comparable volumes as determined from grain density and from conductivity. Mismatches may be a consequence of how the pyrite is distributed. Disseminated pyrite has a greater influence on conductivity than does nodular pyrite. When pyrite volumes have been determined, conductivity of pyrite can be subtracted from total conductivity, to yield pyrite-free conductivity. This will yield a more accurate assessment of the fluid components - more hydrocarbons, less water - than using original conductivity measurements. The methodology has important implications in the understanding of geochemical data. Quantification of pyrite volumes can be related to the thermal and migration history of the reservoir. There are also relations between sulfur content of crude oil and pyrite content, and this methodology allows calculation in wells with no geochemical data available.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013