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Deep Burial Dolomitization Driven by Plate Collision: Evidence from Strontium-Isotopes of Jurassic Arab IV Dolomites from Offshore Qatar

VAHRENKAMP, VOLKER C., and STEVE R. TAYLOR, Shell Research, Rijswijk, The Netherlands

The use of strontium-isotope ratios of dolomites to constrain timing and mechanism of diagenesis has been investigated on Jurassic Arab IV dolomites from offshore Qatar. Reservoir quality is determined by two types of dolomites, which were differentiated geochemically (cathodoluminescence, fluid inclusions, and carbon and oxygen stable isotopes): (1) stratigraphically concordant sucrosic dolomites with high porosity formed during early near-surface diagenesis (Jurassic) and (2) stratigraphically discordant cylindrical bodies of massive, porosity-destroying dolomites formed late during deep burial diagenesis (Eocene-Pliocene).

Detailed Sr-isotope analysis of dolomites from the Arab IV confirms an Early Jurassic age of the sucrosic, high porosity dolomites [(87)Sr/(86)SR = 0.70707 for NBS 987 = 0.71024] with magnesium and strontium being derived from Jurassic seawater.

Sr-isotope ratios of tight massive dolomites are significantly higher (up to 0.70845), corresponding to Paleozoic or Cenozoic seawater ratios. Based on fluid inclusion and isotope data, a deep burial origin (<2 km depth) of the tight massive dolomites had been proposed basically eliminating the possibility of Cenozoic seawater being the source of magnesium and strontium. Instead, magnesium (and strontium) is proposed to have been scavenged from infra-Cambrian evaporites. Strontium from infra-Cambrian evaporities [(87)Sr/(86)SR = 0.70817 - 0.70838] superimposed a Sr-isotope signature typical of Paleozoic seawater onto the deep burial dolomites.

Late Tertiary compressional orogeny of the Zagros belt to the north is proposed to have caused large-scale squeezing of fluids from the pore system of sedimentary rocks. A regional deep fluid flow system developed dissolving infra-Cambrian evaporites upflow and causing large-scale deep burial dolomitization downflow.

 

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)