Porosity Evolution to 7.5 km (25,000 ft) Depth in Miocene Sandstones of Southern Louisiana

D. W. Houseknecht, C. W. Freeman

Miocene strata of the Louisiana Gulf Coast remain important exploration objectives, especially for deep gas resources, even after decades of development. Burial diagenesis has controlled the evolution of sandstone porosity with depth in a systematic way that can be used to anticipate reservoir potential.

Although total porosity of Miocene sandstones generally decreases with depth, diagenetic processes operating over distinct ranges of depth contribute to the amount and types of porosity present Compaction is the most important control of porosity. Mechanical compaction, including reorientation, repacking, and fracture of brittle grains and plastic deformation of ductile grains, is predominant down to 4.5 km (15,000 ft). Chemical compaction (intergranular pressure solution) contributes to porosity loss below 3.0 km (10,000 ft) and mechanical compaction of grain-dissolution and grain-alteration products is important below 4.5 km.

In these Miocene sandstones, cementation is less important to porosity evolution than compaction. Quartz cement contributes to porosity reduction below 3.0 km. Carbonate cement, most of which appears to have precipitated at depths less than 3.5 km (12,000 ft), occludes porosity on a local scale. Grain dissolution (feldspars and lithics) contributes to modest porosity enhancement between 3.0 and 6.0 km, but resultant porosity typically contains clay minerals (by-products of grain dissolution), and dissolution porosity is reduced by compaction below 4.5 km.

Reservoir quality can be anticipated based on understanding the impact of these diagenetic processes with depth. Down to 4.5 km, primary intergranular porosity is predominant and decreases consistently with depth, although 10 to 25 percent porosity is common at 4.5 km depth. Deeper than 4.5 km, both primary intergranular and grain dissolution porosity are important to reservoir viability, and total porosity ranges from 0 to 20 percent.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995