Dolomite and Palygorskite: An Alternative Pathway for the Formation of Palygorskite Clay

Abstract

Palygorskite is a magnesium-bearing, diagenetic clay mineral that can severely impact reservoir quality. In the rock record, palygorskite is frequently associated with Late Mesozoic and Early Cenozoic dolomites. Although laboratory synthesis under earth surface conditions has been unsuccessful, the presence of palygorskite has been inferred to form in warm, alkaline fluids rich in Si, Al, and Mg. Palygorskite is often associated with peri-tidal environments, where the mineral is interpreted to form as either a replacement of detrital smectite clay via a dissolution-precipitation reaction, or by direct precipitation from solution. Most published case studies claim that dolomite and palygorskite form concurrently under elevated pore fluid Mg concentrations. Published evidence supporting concurrent formation is equivocal, however. Here, we present petrological evidence of an alternative pathway for the palygorskite formation as observed in core recovered from the Umm er Radhuma Formation (Eocene) in central Qatar. In the core, the Umm er Radhuma is comprised of completely dolomitized sub-tidal to peri-tidal carbonates characterized by decreasing crystal size and increasing degree of mimetic replacement up section. Within the carbonates, significantly higher percentages of palygorskite are observed near the cycle caps. Thin section and SEM-EDS analyses show that palygorskite crystals form not only in pore spaces between euhedral dolomite crystals, but also on the outermost surfaces and within the partially dissolved cores of dolomite rhombohedra. These observations clearly indicate that dolomite and palygorskite formed in sequence, and are consistent with a model whereby the release of Mg ions into solution during dolomite dissolution promoted the replacement of detrital smectite by palygorskite. This alternative diagenetic model helps explain the association of palygorskite and dolomite in the rock record, and provides valuable insight into the diagenetic conditions under which these two minerals interact and thus alter reservoir quality.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018