Diagenesis of Eolian and Fluvial Feldspathic Sandstones, Norphlet Formation (Upper Jurassic), Rankin County, Mississippi, and Mobile County, Alabama

Abstract

Norphlet sandstones in seven cores from Mississippi and Alabama are arkoses and subarkoses deposited in eolian-dune, interdune, and fluvial environments. Similar to the deeply buried (> 5 km) Tertiary feldspathic sandstones of the Gulf basin, all detrital plagioclase that survived dissolution has been albitized. However, some K-feldspar, instead of dissolving, was stabilized by K+-rich brines derived from adjacent evaporites. This K-feldspar then developed extensive overgrowths. Fluvial red sandstone lost all initial porosity by the introduction of preburial pedogenic calcite and compaction. Initial porosity of eolian sands was reduced by compaction to an average of 29%; and later by cementation by quartz, carbonates, anhydrite, halite, K-feldspar, and illite. Quartz and anhydrite cements precipitated between 90° and 100°C (approximately 2.3 km deep), carbonates and halite cements formed below 120°C (< 3 km), and late-stage illite cement formed between 130° and 150°C (4-5 km deep). Cements are patchy, and some, especially quartz and anhydrite, are texture-selective, being more abundant in coarser laminae. Secondary porosity, which makes up approximately half the porosity in thin sections, formed by dissolution of detrital grains (f ldspar, rock fragments) and cements (anhydrite, carbonate, halite). Reservoir bitumen records an early phase of oil entrapment. Reservoir quality is influenced by the abundance of reservoir bitumen and thread-like illite, both of which bridge pores. The radiometric age of illite (approximately 55 Ma) probably reflects its cementation age, but may also record a later recrystallization event. Depleted oxygen and carbon isotopic values for ferroan dolomite reflect the progressive replacement of earlier, more enriched carbonate phases by higher temperature phases. Isotopic data suggest that during the first 30 to 40 m.y. of burial, subsurface diagenesis of the Norphlet Formation was dominated by deep-circulating, hot, meteoric water. This phenomenon may be characteristic of the early diagene ic history of rifted basins.