Burial Diagenesis of Lower Ismay Limestones: Paradox Basin

W. C. Dawson

Ismay phylloid algal limestones (upper Paradox Formation, Desmoinesian) are major hydrocarbon reservoirs in the Paradox basin. Lower Ismay carbonates have undergone a complex diagenetic history during progressive burial. All porosity (average 15%) is secondary; mesogenetic dissolution is mainly responsible for porosity distribution. Permeability is enhanced by open stylolites and microfractures.

Petrographic (polarized, cathodoluminescence, and fluorescence) and carbon/oxygen isotopic data were used to resolve lower Ismay paragenesis: (1) marine (cementation and micritization); (2) meteoric (neomorphism; precipitation of equant, bladed, and syntaxial zoned nonferroan calcites; and dissolution I); (3) mixing-zone/shallow burial (silicification and mechanical compaction); (4) mesogenetic (stylolitization, dissolution II; ferroan calcite and anhydrite cementation, dissolution III; saddle dolomite and fluorite cementation); and (5) oil migration. Late ferroan calcites have depleted oxygen isotopic signatures: ^dgr¹⁸O - 6.5 to - 9.1 ^pmil (PDB) and ^dgr¹³C + 1.8 to + 3.2 ^pmil (PDB). Isotopic compositions of Ismay saddle dolomites average ^dgr¹⁸O - .7 ^pmil and ^dgr¹³C + 3.4 ^pmil (PDB). Both ferroan calcite and saddle dolomite cements contain oil-filled inclusions. Post-stylolitization porosity (dissolution II) occurs adjacent to stylolites, indicating that stylolites served as conduits for fluid migration. Post-stylolitization mesogenetic dissolution in phylloid algal limestones has been duplicated experimentally at confining pressures simulating up to 15,000 ft of burial.

Optimum reservoir conditions exist where mesogenetic cements (especially anhydrite) have been removed by dissolution III from stylolitized Ismay carbonates. Because phylloid algal carbonates typically have their porosity occluded during early diagenesis, mesogenetic dissolution is essential to reservoir development therein.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.