Diagenesis of a tight gas sand reservoir: Upper Cretaceous Mesaverde Group,Piceance Basin,Colorado |
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| Institution: | 1. Department of Geology and Geological Engineering, Colorado School of Mines, Golden, CO 80401, USA;2. Department of Geosciences, Georgia State University, Atlanta, GA 30302-4105, USA;3. Department of Earth and Atmospheric Sciences, University of Alberta, Canada T6G 2E3;1. Department of Geology, University of Nigeria, Nsukka, Nigeria;2. Nautilus Limited, Ashfields Farm, Hermitage, Berkshire, United Kingdom;1. College of Earth Science and Engineering, Shandong University of Science and Technology, Qingdao 266590, China;2. Laboratory for Marine Mineral Resources, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266590, China;3. Exploration & Production Research Institute, SINOPEC, Beijing 100083, China;1. Research Institute of Unconventional Oil & Gas and Renewable Energy, China University of Petroleum (East China), Qingdao 266580, China;2. Energy & Geoscience Institute, The University of Utah, Salt Lake City, UT 84102, USA;1. Department of Geosciences, University of Oslo, P.O. Box 1047, Blindern, NO–0316, Oslo, Norway;2. Lundin Norway AS, Strandveien 4, NO–1366, Lysaker, Norway |
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| Abstract: | The Upper Cretaceous Mesaverde Group in the Piceance Basin, western Colorado, contains thick sections of low porosity, low permeability sandstones that are reservoirs for large accumulations of hydrocarbon gas. The Mesaverde sandstones are lithic arkoses and feldspathic litharenites, containing quartz, plagioclase, variable K-feldspar, chert, and volcanic rock fragments. Important diagenetic processes that have affected the sandstones include compaction, dissolution of feldspar and/or alteration to clay, precipitation of carbonate and quartz cements, precipitation of illite, and alteration of detrital clays (mixed-layer illite–smectite). Porosity is relatively constant; a decrease in primary porosity downward is approximately balanced by an increase in secondary porosity, with significant microporosity in authigenic and diagenetic clays. K-feldspar is almost completely absent below 5500 feet (1675 m). Fibrous illite is relatively abundant above this depth and variable in abundance below. The K–Ar ages of the clays increase with depth, from 40 m.y. at the top of the sampled section to 55 m.y. at the base, indicating illitization of a precursor smectite at approximately the 100 °C isotherm. Samples with abundant fibrous illite have relatively smaller age values. Mass balance calculations indicate that dissolution of K-feldspar, illitization of smectite and precipitation of fibrous illite were coupled. These reactions suggest the transport of dissolved potassium upward hundreds of feet (100–300 m) in the section, possibly associated with water driven vertically by the migration of gas. |
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