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Clastic
diagenesis and carbonate cement vs. fracture carbonate
Research
Collaboratores:
Dr. Peter Eichhubl, Dr. Andras Fall, Dr. Stephen E. Laubach
Bureau of Economic Geology, Jackson School of Geosciences, The University of Texas at Austin
Natural
fractures observed in core from producing tight-gas sandstone
reservoirs are either partially or completely sealed by cements. While
partially cemented fractures retain permeability, completely cemented
fractures impede flow. Thus, whether fractures are open or sealed can
significantly influence exploration and productivity of tightly
cemented reservoirs. This study focuses on the geochemical aspects of
fracture calcite cement, which frequently seals (degrades) porosity of
large fractures completely where it is present. In Cretaceous Mesaverde
Group sandstones in the Piceance Basin, Colorado, large fractures are
either open but lined by quartz, or they may be lined with quartz and
sealed with calcite. Based on petrographic observations on fracture
cement and host rock of, carbonate pore cement precipitated in the
sequence of (1) ferroan-dolomite, (2) ankerite, and (3) calcite.
Textural relations indicate that carbonate pore cement formed as a
result of breakdown of detrital Ca-bearing feldspar. Based on stable
isotope and fluid inclusion analyses results indicate that calcite
precipitated from a diagenetically modified pore fluid with a low
water/rock ratio. Sr-isotope ratios are distinctly above Cretaceous to
Tertiary seawater composition, indicative that the Sr, and by analogy
Ca, source is dominated by radiogenic strontium. Based on the textural
evidence for feldspar dissolution the likely source of radiogenic Sr is
K-feldspar, with clays as a possible secondary source. Feldspar
dissolution as the primary source of fracture calcite, and the
inference of low water/rock ratios suggest that fracture calcite cement
precipitation is controlled by the detrital composition of the
surrounding host rock. |
