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Factors Affecting the Precipitation of Quartz Under Hydrothermal Conditions

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Factors Affecting the Precipitation of Quartz Under Hydrothermal Conditions FACTORS AFFECTING THE PRECIPITATION OF QUARTZ UNDER HYDROTHERMAL CONDITIONS Chris Pepple A Thesis Submitted to the Graduate College of Bowling Green State University in partial fulfillment of The requirements for the degree of MASTER OF SCIENCE August 2007 Committee: John Farver, Advisor Charles Onasch, Co-Advisor Kurt Panter ii ABSTRACT John R. Farver, Advisor Charles M. Onasch, Co-Advisor Natural Pocono Sandstone and synthetic quartz crystals, crushed and sieved (125- 250um), have been used to experimentally evaluate changes in the rate and nature of quartz cementation under hydrothermal conditions. Experiments were carried out from 1 hour to 5 weeks at temperatures of 300-600°C and at 150 MPa confining pressure to simulate cementation conditions analogous to a quartz reservoir at depth. Experimental charges consisted of AlCl3, amorphous silica, NaCl brine, and one of the quartz sample materials weld-sealed in a gold tube. Interactions of quartz systems with and without the addition of iron oxides (goethite) were conducted to determine the influence of iron oxides on cementation. After the experiments, samples were impregnated with epoxy and then analyzed using cathodoluminescence (CL), plain polarized light (PPL), cross polarized light (XPL), and scanning electron microscopy (SEM). Mosaic images were constructed for node counting of grains, cement, and porosity for synthetic quartz samples, and grains and porosity measures for Pocono Sandstone experiments. The mosaic images illustrate precipitation and dissolution occurred at sample bases and tops due to a saturation gradient formed by the interactions of AlCl3 and amorphous silica. Measured cement and porosity values of synthetic quartz samples were used to calculate precipitation and transport rates detailing differences between pure and goethite system experiments. Consistently, synthetic quartz goethite experiments showed greater cementation in response to increased silica solubility and goethite’s pervasive and adsorptive nature towards quartz. In addition, it is shown that significant amounts of quartz cementation iii can occur due to saturation gradients between quartz and amorphous silica even in the absence of a temperature or pressure gradient. iv AKNOWLEDGMENTS Through out these last two years I’ve been given much support and encouragement from family, friends, and colleagues. I owe my advisor Dr. John Farver a great deal of thanks for his persistence, patience, and unfaltering support of myself and this project through times when direction and end seemed no where in sight. He provided the necessary assistance and encouragement for success of which I cannot thank enough. A great thanks also to Dr. Charlie Onasch whose guidance, persistence, and “what if” questions proved a great resource in shaping these experiments and this project which I am indebted to. Many thanks also to Dr. Kurt Panter for being on my committee and asking those fundamental and application questions that helped me keep perspective and direction on this project. Special thanks and recognition for financial support go out to Francis Furman, Katzner Bookstore Grant Committee, Geological Society of American Grant (8370-06), and to the Department of Geology. Additionally, thanks goes to Shaun Wallace for his assistance in the laboratory and photo editing. I also would like to thank my family and friends for their support, advice, and good times which helped me keep perspective. Thanks to my siblings Dave and Ellen for patience, and an open ear when I really needed it. Finally I would like to thank most my parents. Without their consistent encouragement and support, none of this would be possible. v TABLE OF CONTENTS Page 1. INTRODUCTION………………….…………………………………………………….. 1 1.1 Previous research……………………………………………………………….. 2 1.2 Nature and rate of cement formation………………………………………….... 6 1.2.1 Silica dissolution……………………………………………………… 7 1.2.2 Silica transport………………………………………………………... 7 1.2.3 Silica precipitation…………………………………………………….. 9 1.2.4 Changes in porosity and permeability…………………………………. 10 1.3 Questions to be answered……………………………………………………….. 10 2. METHODS……………………………………………………………………………….. 12 2.1. Starting materials……………………………………………………………….. 12 2.1.1. Pocono sandstone……………………………………………………. 12 2.1.2. Synthetic quartz sandstone…………………………………………... 13 2.1.3. Iron oxides…………………………………………………………… 13 2.2. Experimental methods………………………………………………………..... 14 2.2.1. Preparation of experimental charges ………………………………… 14 2.2.2. Method of post experimental preparation……………………………. 15 2.3. Analytical methods……………………………………………………………… 16 2.3.1. Cathodoluminescence imaging……………………………………….. 16 2.3.2. Scanning electron microscopy……………………………………….. 16 2.3.3. Thin section petrography ……………………………………………. 17 2.3.4. Image processing……………………………………………………... 17 2.3.5 Rate calculations………………………………………………………. 18 vi 3. RESULTS…………………………………………………………………………………. 19 3.1. Control samples…………………………………………………………………. 19 3.2. 300° to 600° C Pocono sandstone experiments…………………………………. 20 3.3. 300° and 450° C Non-goethite synthetic quartz experiments…………………… 24 3.4. 450° C Variable experiments……………………………………………………. 29 3.5. 450° C Step down temperature experiments……………………………………. 31 3.6. 450° C Goethite experiments …………………………………………………… 32 3.7. Precipitation and transport rates ………………………………………………... 34 3.8. Base and top grain long axis measures in goethite and non-goethite exp………. 36 3.9 450° C Sandwich experiment……………………………………………………. 38 4. DISCUSION………………………………………………………………………………. 39 4.1. Effects of time and temperature on cementation ……………………………….. 39 4.2. Effect of AlCl3 on cement growth ……………………………………………… 39 4.3. Evidence for cement within the Pocono sandstone …………………………….. 41 4.4. Evidence for sources of cement…………………………………………………. 42 4.5. Amorphous silica alteration and trace elements incorporation…………………. 43 4.6. Nature of goethite on silica mobility, solubility, and cementation……………... 45 4.7. Comparison of goethite vs. non-goethite experiments …………………………. 46 4.8. Rates of precipitation and transport…………………………………………….. 48 4.9. Rate limiting and driving forces in goethite and non-goethite experiments……. 49 5. CONCLUSIONS………………………………………………………………………….. 54 6. REFERENCES………………………………………………………………................... 56 7. APPENDICIES…………………………………………………………………………… 62 vii LIST OF FIGURES Figure Page 1 CL image of CH-22 showing the Pocono Sandstone sample/amorphous silica powder interface……………………………………………………………..... 25 2 The change in percent cement over time for both synthetic quartz experiments with and without goethite shown by the pink line, and the goethite system experiments shown by the blue line……………………………….. 27 3 Changes in porosity for both synthetic quartz experiments with and without goethite shown respectively by pink and blue lines………………………. 27 4 SEM photomicrographs of A Micropore in CH-27 (2 weeks at 450° C) illustrating pit dissolution of synthetic quartz grains near charge top……………… 28 5 Charge CH-30 run four weeks at 450°C and 150MPa…………………………….. 30 6 Synthetic quartz and SiO2 powder interface using PPL (a) and CL (b) of CH-35 showing Fe-oxides surrounding synthetic quartz grains (A), and infilling pore space (B)……………………………………………………….... 33 7 The calculated precipitation rates, in mol/sec, for one, two, and four-week experiments for systems with (blue) and without (pink) the addition of goethite…………………………………………………………………………….. 35 8 The calculated transport rates, in m/sec, for one, two, and four week experiments for systems with (blue) and without (pink) the addition of goethite…………………………………………………………………………….. 35 9 Median long axis lengths of top and base grains in system experiments without goethite……………………………………………………………............ 37 10 Median long axis lengths of top and base grains in experiments loaded with goethite……………………………………………………………………….. 37 viii LIST OF TABLES Table Page 1 Results of % grains, cement, and porosity sorted by descending cement in synthetic quartz samples and ascending porosity in Pocono Sandstone samples……………………………………………………………………............. 21 2 Experimental charge run conditions and list of reagents added…………………… 23 ix LIST OF APPENDICIES App. Page A Illustration of Au-tube showing the location of amorphous silica, cementation, and synthetic quartz grains………………………………………….. 62 B (1) Synthetic quartz starting experiments CH-37 (A) and CH-38 (B) cathodoluminescence (CL) transect images………………………………………. 63 B (2) Cathodoluminescence (CL) transect of CH-41 Pocono Sandstone starting experiment…………………………………………………………………............ 64 B (3) Cathodoluminescence (CL) transect of Pocono experiments CH-11 (A) and CH-21 (B) Pocono Sandstone experiments run at 300º C for 4 and 35 days respectively………………………………………………………………………… 65 B (4) Cathodoluminescence (CL) transect of Pocono experiments CH-15 (A) and CH-18 (B) Pocono Sandstone experiments run at 450º C for 7 and 28 days respectively………………………………………………………………………… 66 B (5) Cathodoluminescence (CL) transect of CH-14 (A), CH-20 (B), and CH-22 (C) Pocono Sandstone experiments run at 600º C for 7 days, 1 hour, and 7 days respectively…………………………………………………………………… 67 B (6) Cathodoluminescence (CL) transect of CH-29 synthetic quartz experiment run at 300º C for 34.1 days………………………………………………………… 69 B (7) Cathodoluminescence (CL) transect under 5X (A) and 10X (B) of synthetic quartz experiment CH-26 run one week at 450º C………………………………... 70 B (8) Cathodoluminescence (CL) transect under 5X (A) and 10X (B) of synthetic quartz experiment CH-23 run two weeks at 450º C……………………………..... 71 B (9) Cathodoluminescence
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