Hydrocarbon Versus Microquartz Inhibition of Quartz Cementation in North Sea Sandstones: Empirical and Experimental Evidence

Linda M. Bonnell¹, Richard E. Larese², and Robert H. Lander^1
1 Geocosm LLC, Austin, TX
² Clastic Petrology Consultant, Durango, CO

The occurrence of high porosity zones in the Brae Formation sandstone, Miller Field, UK North Sea has been attributed to the inhibition of quartz cementation by early hydrocarbon charge (Marchand, et al., 2000, 2001, 2002). Alternatively, Aase & Walderhaug (2005) suggest that quartz cementation in this field has been inhibited by the presence of microcrystalline quartz coatings on quartz grains. Our group independently examined data from 4 Miller Field wells that transect the hydrocarbon and water legs. Like Aase & Walderhaug, we found a strong correlation between the occurrence of microcrystalline quartz coatings and low quartz cement abundances.

The inhibition of quartz cementation by microcrystalline quartz coatings has been described by Aase et al. (1996) in Ula Formation sandstones. However, other workers interpret these coatings to be incipient overgrowths that ceased to grow due to hydrocarbon incursion. To evaluate these contrasting interpretations, we used a hydrothermal reactor to induce quartz cementation in Miller Field and Ula Field samples. The samples had both microcrystalline quartz coatings and naked quartz surfaces at grain contact scars. While large quartz overgrowths formed on naked contact scars, surfaces covered by microcrystalline quartz showed only minor growth indicating that these coatings can inhibit the formation of normal quartz overgrowths.

To further test whether microcrystalline quartz in Miller Field samples could account for observed porosity preservation, we modeled quartz cement in all samples using the Touchstone diagenesis model. Quartz cementation kinetics were calibrated to water leg samples and used to predict quartz cement in hydrocarbon leg samples. If cementation was retarded by hydrocarbons, the model should overpredict quartz cement in hydrocarbon leg samples. We found, however, that models that account for grain coatings accurately predict quartz cement in both water and hydrocarbon leg samples.