Two generations of calcite are recognized: calcite I fill large intergranular pores as poikilotopic crystals (>100um) between uncompacted detrital grains.
Authigenic carbonates in Chang7 sandstones occur as micro to coarse crystalline poikilotopic, pore-filling blocky and isolated pore-filling, indicating precipitation in different stages.
The late generation of poikilotopic calcite is interpreted as a result of plagioclase and early calcite dissolution, which releases cations into pore water and may also be responsible for the precipitation of clay minerals and the silica cements .
Textural data suggest that the late poikilotopic calcite, near the top overpressure surface, is rich in Fe and high porosity is developed in the same depth interval.
The common diagenetic features within the facies are micritized grains dolomitization dissolution neomorphism cementation (poikilotopic
cement isopachous cement syntaxial overgrowths equant spar drusy spar) pressure solution and fracturing (Plates 7-8).
Pores are filled with sparry and poikilotopic
gypsum cement dominates the samples analyzed.
Lucia, F.J., Jones, R.H., and Jennings, J.W., 2004, Poikilotopic
anhydrite enhances reservoir quality: AAPG 2004 Annual Convention, Abstracts Volume, p.
At greater depth, euhedral overgrowths formed on quartz grains, and poikilotopic
calcite cement occluded most primary porosity.
They observed that poikilotopic
anhydrite is common in lagoonal dolomite sediments, but they suggested that anhydrite did not form until after the dolomite was lithified.
The calcite cements, which are composed mostly of anhedral calcite crystals (Figure 9(a)), represent a poikilotopic
texture and local replacement of framework grains (Figures 6(f) and 6(g)).