Material Balance Considerations for the Generation of Secondary Porosity by Organic Acids and Carbonic Acid Derived from Kerogen, Denver Basin, Colorado, USA

Summary

Data on source and reservoir rocks in the Wattenberg area of the Denver Basin were used to evaluate the hypothesis that carboxylic acids and carbon dioxide generated from the thermal maturation of organic matter create secondary porosity in sandstone reservoirs. For the Lower Cretaceous source rocks, information on the type of kerogen, richness, maturity, and thickness was collected for each formation. We used subsidence modeling to determine the temperature and burial history of the source rocks. For the reservoir and potential reservoir sandstones (Fox Hills, Larimer-Rocky Ridge, Terry, Hygiene, Codell, Dakota J, and Lyons) overlying the deepest part of the basin, the composition, total macroporosity, and secondary porosity were determined using 115 thin sections cut from cores from 18 wells. Data on the stratigraphic thickness needed to calculate volume (for 1 km(su2)) of sandstones came from geophysical logs. Porosity in nonreservoir sandstones throughout the stratigraphic column was evaluated using geophysical logs and thin sections of drill cuttings. These nonreservoir sandstones contain only trace amounts of porosity and were not included in the calculations. Also, three carbonate formations were examined in thin section, but disregarded for this study because of lack of macroporosity.

Two types of calculations were made. In the first type, the amount of plagioclase and calcite that could be dissolved by carboxylic and carbonic acids, respectively, generated by decarboxylation of organic matter, is complementary and additive. A second type of calculation is additive, but in our opinion speculative, and involves the generation of organic acids from redox reactions. We used data from hydrous pyrolysis experiments using shale source rocks to establish the conversion efficiency of kerogen oxygen to mono- and dicarboxylic oxygen. To maximize the calculations we considered the ratio of dicarboxylic to total carboxylic to be 0.52, which is a high value reported for the San Joaquin Basin. For these optimum conditions, we calculate that 1.72% secondary porosity can form in the sandstones. By point count, using a conservative approach in identifying secondary porosity, the secondary porosity is 2.49%. The secondary porosity is probably higher than this, but it is impossible to accurately document. These calculations suggest that the supply of acids may be insufficient to explain the observed secondary porosity. The source rocks in the Denver Basin contain almost exclusively Type-II kerogen. If this kerogen had been Type III, which is more oxygen-rich, then presumably more carboxylic acids and carbon dioxide could have been generated to create greater porosity.

This modeling approach to evaluate the material balance considerations for the generation of acids from source rocks and the creation of secondary porosity is different from previous attempts because we use data from a specific basin rather than taking a general model using published average data for kerogen.