Research Interests

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.