Diagenetic Influences on Reservoir Characteristics of Terry Sandstone, Spindle Field, Colorado

Phillip D. Hays, Thomas T. Tieh

The Upper Cretaceous Terry sandstones of Spindle field, Weld County, Colorado, were deposited as offshore bars in a shallow-marine environment. The Terry Sandstone Member of the Pierre Shale is presently buried at depths of 4,500-5,000 ft (1,375-1,525 m).

The Terry sandstone reservoir was diagenetically created by selective dissolution of calcite cement and detrital material in downdip sands. Hydrocarbon entrapment occurred as a diagenetic/stratigraphic trap through preservation of cement updip and by shale boundaries laterally and vertically. The arkosic litharenites of the Terry sandstone have experienced four major stages of diagenesis: (1) early compaction and calcite cementation, (2) cement and grain dissolution and renewed compaction, (3) chlorite authigenesis, and (4) late illite and smectite authigenesis. In contrast to the updip Terry sandstones which contain primarily authigenic kaolinite, the downdip sandstones are dominated by authigenic chlorite. The precipitation of chlorite as the stable mineral phase in the downdip sand tones may have resulted from relatively high Fe²⁺ and Mg²⁺ activities and the decrease in Al³⁺ activity by the formation of Al³⁺ complexes within the invading pore fluids. As these fluids migrated updip, they became less efficient at complexing Al³⁺, thereby increasing Al³⁺ activity and initiating kaolinite precipitation.

In the study area, samples which have experienced dissolution of calcite exhibit porosities of up to 20%. However, permeabilities are low (0.1-11.0 md) due to the presence of pore-lining chlorite and the development of a pseudomatrix related to postdissolution compaction of glauconite pellets. The authigenic chlorite and glauconite pseudomatrix reduced porosity and blocked pore throats. Semilogarithmic plots indicate a logarithmic decrease in permeability with increased chlorite and glauconite content.

Terry sandstone reservoir quality has been strongly modified by the diagenetic processes of cementation and compaction. These diagenetic processes resulted in the destruction of primary porosity, the formation of secondary porosity, and the subsequent partial occlusion of secondary porosity. Variations in reservoir properties and production potential within the sandstones can be ascribed to both primary lithologic contrasts and subsequent diagenetic modifications.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.