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Diagenetic History of Norphlet Formation (Upper Jurassic), Rankin, County, Mississippi
Earle F. McBride (1)
Most of the Norphlet sandstone examined in four cores (-15,770 to -22,500 feet) is eolian in origin as shown by thick, high-angle crossbeds, bimodal texture with well-sorted laminae, and relict hematite grain coatings typical of desert sand. Norphlet sandstones average 77 percent quartz, 16 percent feldspar, and 7 percent rock fragments.
Partial to total loss of halite cement from cores during coring and slabbing operations hampers the interpretation of diagenetic history. However, the inferred sequence of diagenetic events is: cementation by illite, K-feldspar, quartz, calcite, anhydrite, and halite; development of secondary porosity by dissolution of halite, plagioclase, VRF's, and carbonate cement; hydrocarbon migration and pyrite generation; dolomitization; and cementation by late-stage illite and quartz. Halite and anhydrite were derived from the underlying Louann. Pyrite is present in most samples and formed when sour gas passed through the sands and reduced hematite grain coatings to pyrite. The scarcity of quartz cement is attributed to the lack of shale beneath the Norphlet.
Illite cement of two ages is present. Early-formed illite is leaflike flakes that coat quartz grains that inhibited the development of quartz overgrowths. It is not a seriously deleterious mineral. Late illite is thread-like and grows in narrow secondary pores where it seriously reduces permeability. Illite cement and sericitic alteration of plagioclase are more abundant deeper than 19,900 feet.
Halite was introduced after moderate burial because prior to its development the sandstones underwent modest compaction as shown by sutured quartz grains and pre-cement porosity values of 22 to 30 percent (indicating compactional losses of 15 to 23 percent porosity before cementation).
The original texture of the sands strongly influenced diagenetic events.
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