The widely recognized relationship between grain coatings and porosity preservation at depth has resulted in a great deal of emphasis on the prediction of the occurrence of grain coats. It is not simply the occurrence of grain coats, however, but the completeness of grain coverage combined with the burial history of the sandstone that are the critical factors for preservation of reservoir quality at depth.

We used the Exemplar model to assess the interplay between grain coat completeness and burial history in reservoir quality preservation. In the model, surface area is dependent on grain size, quartz grain abundance, grain coating completeness and porosity. The accuracy of the model in predicting the effect of grain coats on quartz cementation was tested by using samples from North Sea reservoirs at 3000 m depth (T = 120°C). Percent grain coverage was quantified by measuring the coverage of 50 grains and taking the average value. Model results show good agreement between measured and modeled quartz cement abundances.

Using this approach, we evaluated the dependency between completeness of coverage, grain size and temperature history by using compositional characteristics of the North Sea dataset and changing the burial history to simulate deeper burial. We calculated the grain coatings necessary to preserve greater than 15% porosity for fine medium and coarse grained samples with present day temperatures of 120°C and 155°C. The completeness of grain coating necessary for samples with the lower temperature history is highly dependent on grain size with values of 60%, 20% and 0% for fine, medium and coarse grained samples respectively. When the higher temperature history is used, 15% porosity can be preserved only when the grains are almost completely covered, regardless of grain size (93%, 86% and 72% coverage for fine, medium and coarse grained samples).

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah