Abstract: Some Aspects of Sandstone Diagenesis that Influence Reservoir Quality

By

Earle F. McBride

Many sandstones show evidence of multiple stages of cementation and decementation by carbonate minerals and of multiple stages of dissolution of sand grains such as feldspar. Secondary dissolution porosity is recognizable in thin section by a variety of textures including oversize pores, skeletal feldspars, canals in quartz, etc. Secondary porosity is the chief effective porosity in many hydrocarbon reservoir sandstones. The dogma that porosity in sandstone decreases linearly with depth until "porosity basement" is reached must be re-evaluated in light of the new evidence of widespread secondary porosity.

Whereas porosity lost at shallow depths by carbonate cementation can be regenerated during dissolution at depth, porosity lost by compaction cannot be regenerated. Ductile grains such as clay clasts, fecal and glauconite pellets, and micaceous rock fragments deform under load to fill adjacent pore space and reduce sand bed thickness. Glauconitic sands and lithic sands can lose all porosity entirely by compaction. These sands will not develop secondary porosity.

Predicting the diagenetic events that control reservoir quality at depth is aided by knowing the original composition of the sand

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