Austin Chalk in south-central Texas was deposited in an unique environment that consisted of a relatively shallow-water platform and adjacent deeper water. Shallow water promoted deposition of highly fossiliferous chalks that contained appreciable quantities of aragonitic constituents. These anomalous sediments periodically were transported into the adjacent basin, a site of more typical chalk deposition.

Porosity and geochemical trends support petrographic evidence that the Austin Chalk underwent a greater degree of diagenesis than did European and North Sea chalks of similar age. Porosity reduction occurred more quickly and earlier in the Austin Chalk's burial history, and at shallower burial depths than in these other chalk sequences.

Exposed, relatively shallow-water Austin Chalk sediments now average 20% porosity but were never deeply buried. Porosity reduction resulted from early physical compaction followed by freshwater dissolution of aragonitic grains and associated cementation by non-ferroan calcite. Relatively low bulk iron and strontium concentrations resulted from this diagenesis.

More basinal and oil-productive Austin Chalk averages 5% porosity. After early physical compaction, most porosity loss resulted from pervasive pressure solution and concomitant cementation. Some cementation occurred when aragonite (where present) was stabilized under higher burial temperatures prior to pressure solution. Cements are mostly ferroan calcite. Progressive burial diagenesis further obliterated primary matrix fabrics and gradually depleted bulk oxygen-18. Relatively higher bulk iron and strontium concentrations reflect this diagenetic history.

Austin Chalk is capable of producing solely from its preserved matrix porosities and permeabilities although late-stage fracturing does enhance production.

End_of_Article - Last_Page 922------------