--> Abstract: Reservoir Quality Reduction by Quartz Cementation - Is Fluid Composition or Tectonism More Important?, by K. Juhasz-Bodnar, K. Ramseyer, C. Boker, A. Matter, and P. Hoppe; #90937 (1998).

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Abstract: Reservoir Quality Reduction by Quartz Cementation - Is Fluid Composition or Tectonism More Important ?

JUHASZ-BODNAR, KATALIN, KARL RAMSEYER, CARSTEN BoKER and ALBERT MATTER, Institute of Geology; PETER HOPPE, Institute of Physics, University of Berne, Switzerland

The glaciogenic Al Khlata Formation is the lower member of the Permo-Carboniferous Haushi Group in the Interior Oman Sedimentary Basin. Due to a varied subsidence history, the sediments presently range in depth from outcrop to almost 4000 m, representing maximum burial.

Intense quartz cementation is the most important reservoir-quality-reducing process in the studied sandstones. Authigenic quartz formed as multiphase, zoned syntaxial overgrowths on detrital quartz grains varying from trace amounts in outcrop samples to almost 25 vol. % in sandstones buried to 4000 m.

Fluid inclusion microthermometry and oxygen isotope ratios determined by secondary ion mass spectrometry of authigenic quartz show that the porefluid composition changed dramatically during quartz cementation from a highly saline isotopically heavier brine (22 wt% NaCleq; d18O=+3^pmil) to a less saline and lighter (10 wt% NaCleq; d18O=–3^pmil) fluid.

The timing of the onset of quartz precipitation and the dilution of the porefluid were constrained by burial and temperature history modelling and measured homogenization temperatures. The onset of quartz cementation (between 80-60 Ma) and later fluid dilution (between 50-40 Ma) each began simultaneously in all studied samples, independent of concurrent burial depth. This suggests that these processes are controlled by pore pressure evolution related to well known regional tectonism rather than temperature.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah