The Effects of Diagenesis on the Petrophysical Properties of Tight Gas Sands

Peigui Yin and Ronald C. Surdam

The Cretaceous Almond Formation sandstones in the Washakie Basin, Wyoming, were deposited in a marginal marine environment, and are generally rich in ductile lithic fragments. These sandstones were modified extensively during post-depositional diagenesis, and have become tight below 8000 ± 1000 ft depth. Porosity and permeability in these tight sands is damaged by the deformation of lithic grains, precipitation of clays and quartz overgrowths, and late dolomite cementation. Remnant porosity generally takes the form of secondary dissolution pores. Dissolution of carbonate cement enhances both porosity and permeability in a few sands. Partial dissolution of feldspar and lithic fragments leads to the creation of micropores and precipitation of clay minerals, thereby enh ncing porosity, but not permeability.

Porosity and permeability vary significantly in the Upper Almond Formation sandstones. Below ~8000 ft, porosity ranges from 0 to 15% and permeability ranges from less than 0.01 to 10 md. In this study, the sandstones with porosity greater than 8% are denoted porosity "sweet" sands, and sandstones with porosity greater than 8% and permeability greater than 0.1 md are denoted permeability "sweet" sands. These "sweet" zones are characterized by relatively coarse grain size, few lithic fragments, feldspar and carbonate dissolution, less quartz overgrowth and late dolomite cementation, and abundant intergranular slots. These "sweet" sands develop primarily in a high-energy depositional environments. In some cases, fracture porosity/permeability results in the enhancement of sweet zones in he Almond Formation sandstones.

This research was supported by the Gas Research Institute contract #5091-221-2146.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California