Summary

Within Norphlet Fm. gas reservoirs in Mobile Bay (offshore Alabama, USA), quartz cement is concentrated at the top of the section forming a low porosity “tight zone” of widely variable thickness. Detailed petrographic analysis and fluid inclusion microthermometry indicate that quartz cement formed late in the paragenetic sequence under deep burial conditions.

Thermal convection of formation water represents a potentially attractive model to explain the observed distribution of quartz cement. To test this model, we calculated a time-dependent heat flow model for a single large gas field using finite element modeling techniques and applied the results to calculate the rate of quartz precipitation that could result from this process. These calculations indicate that, given the timing of quartz cementation and reasonable assumptions of permeability, the process of thermal convection is probably not sufficient to account for the observed volume of cement.

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