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Equant calcite is the most common cement in limestones. It occludes more pore space, and hence has more control over reservoir quality than any other carbonate cement. It is important, therefore, to be able to delineate the environments of precipitation of equant calcite cements. The Lower Cretaceous Pearsall formation in south Texas has a history of equant calcite cementation that began early in the shallow-subsurface meteoric environment and is continuing today in the deep-subsurface basinal environment. Trace-element analysis of Mg+2 and Fe+2 along with morphological characteristics of the Pearsall equant calcite cements delineate environments of precipitation.
Cements precipitated in the early, shallow-subsurface meteoric environment are generally very fine to medium-crystalline equant (less commonly bladed) calcites. The very fine to fine-crystalline calcite cement generally rims grains and is gradational into the medium-crystalline equant calcite cement that fills intergranular and moldic porosity. Early cements commonly have irregular crystal boundaries as seen in thin section.
Late, deep-subsurface precipitated cements are generally coarse to very coarse-crystalline equant calcites. Commonly they have straight crystal boundaries and form one or several large crystals in a pore space. There is generally a sharp change in crystal size with the fine to medium-crystalline equant calcites precipitated in the shallow-subsurface meteoric zone.
Trace-element analysis (electron microprobe) shows a statistically valid difference of Mg+2 and Fe+2 content between the early and late equant calcite cements. The early calcite cements have a higher Mg+2 content (1.8 ± 0.3 mole % MgCO3) and a lower Fe+2 content (785 ± 184 ppm) than the late calcite cements (1.3 ± 0.3 mole % MgCO3 and 2.618 ± 1,952 ppm Fe+2). Also, semiquantitative probe analysis, due to low count rate, indicates that the early calcite cement is richer in Sr+2 (738 ± 286 ppm) than the late calcite cement 472 ± 189 ppm).
Magnesium in the fine-crystalline rim cement shows a pronounced trace-element distribution pattern. In the incipient crystal growth next to the grain, a high Mg+2 peak usually occurs, followed by a decrease in Mg+2 in the late rim cement and coarser equant calcite cement. This initial high Mg+2 peak is attributed to early meteoric diagenesis as described by Benson in 1974 in meteoric cements from Barbados. The Fe+2 trace-element distribution pattern shows an opposite trend from that of the magnesium. The early cements show low Fe+2, whereas the late cements are Fe+2 rich, indicating a reducing environment of precipitation for the late cements.
Early, shallow-subsurface equant calcite cements can be distinguished from late, deep-subsurface equant calcite cements by relative position in pores, crystal size, straightness of crystal boundaries, gradation between crystal sizes, and by trace-element content and distribution patterns. These parameters may be valid environmental indicators of equant calcite precipitation in other limestone formations.
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