Porosity reduction of 3 compositionally mature Miocene sandstones from Louisiana was determined petrographically. These sandstones were the S Sand at Weeks Island field, R_Z Sand at East Bayou Postillion field, and Planulina 6 Sand at Jeanerette field. The S Sand attained a maximum burial depth of 15,800 ft (4,800 m), the R_Z Sand 15,700 ft (4,800 m), and the Planulina 6 Sand 14,710 ft (4,485 m), all prior to structural uplift.

Porosity reduction caused by mechanical compaction (^Dgr^phgr) was determined by: ^Dgr^phgr = 40 - (C + ^phgr), where 40 is porosity prior to mechanical compaction, C is cement (petrographically determined), and ^phgr is porosity (petrographically determined), all in volume percent. The term (C + ^phgr) is the porosity remaining after reduction caused solely by mechanical compaction. The S Sand (with the greatest maximum burial depth) has an average ^Dgr^phgr of 11.5%, the R_Z Sand 16.0%, and the Planulina 6 Sand 17.5%. The difference in ^Dgr^phgr between these sandstones is due to differences in the depths at which calcite cementation began to hinder compaction.

By comparing the term (C + ^phgr) of each sandstone to the porosity expected from the Atwater and Miller porosity reduction rate, it appears compaction-arresting cementation occurred at approximately 9,000 ft (2,750 m) in the S Sand, 12,500 ft (3,800 m) in the R_Z Sand, and 14,000 ft (4,270 m) in the Planulina 6 Sand. At greater depths, this calcite cement dissolved, resulting in secondary porosities up to 35 vol. %. Therefore, many deep hydrocarbon reservoirs may exist in the Gulf Coast with porosities greater than expected.

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