University of Texas at El Paso DigitalCommons@UTEP Open Access Theses & Dissertations 2017-01-01 Gypsum, Calcite, And Dolomite Caprock Fabrics And Geochemistry From The yG psum Valley Salt Diapir, Paradox Basin, Southwestern Colorado Kevin Lerer University of Texas at El Paso, [email protected] Follow this and additional works at: https://digitalcommons.utep.edu/open_etd Part of the Geochemistry Commons, Geology Commons, and the Oil, Gas, and Energy Commons Recommended Citation Lerer, Kevin, "Gypsum, Calcite, And Dolomite Caprock Fabrics And Geochemistry From The yG psum Valley Salt Diapir, Paradox Basin, Southwestern Colorado" (2017). Open Access Theses & Dissertations. 479. https://digitalcommons.utep.edu/open_etd/479 This is brought to you for free and open access by DigitalCommons@UTEP. It has been accepted for inclusion in Open Access Theses & Dissertations by an authorized administrator of DigitalCommons@UTEP. For more information, please contact [email protected]. GYPSUM, CALCITE, AND DOLOMITE CAPROCK FABRICS AND GEOCHEMISTRY FROM THE GYPSUM VALLEY SALT DIAPIR, PARADOX BASIN, SOUTHWESTERN COLORADO KEVIN LERER Master’s Program in Geology APPROVED: Benjamin Brunner, Ph.D., Chair Katherine Giles, Ph.D. Jennie McLaren, Ph.D. Charles Ambler, Ph.D. Dean of the Graduate School Copyright © by Kevin Lerer 2017 DEDICATION To Julia Mae Lerer, my daughter, who has been a constant source of inspiration to me. Her endless enthusiasm motivated me to pursue my passion for geology. GYPSUM, CALCITE, AND DOLOMITE CAPROCK FABRICS AND GEOCHEMISTRY FROM THE GYPSUM VALLEY SALT DIAPIR, PARADOX BASIN, SOUTHWESTERN COLORADO by KEVIN LERER, B.A. THESIS Presented to the Faculty of the Graduate School of The University of Texas at El Paso in Partial Fulfillment of the Requirements for the Degree of MASTER OF SCIENCE Department of Geological Sciences THE UNIVERSITY OF TEXAS AT EL PASO December 2017 ACKNOWLEDGEMENTS First, I would like to thank my parents, Sheila and Richard Lerer and my brother, Joshua Lerer for their encouragement and moral support during my time at the University of Texas at El Paso. Second, I would like to thank my daughter, Julia Mae Lerer, who I found to be a constant source of motivation. She provided me with the perseverance necessary to successfully earn a MSc degree. The completion of this thesis would not have been possible without the guidance, instruction and support of my graduate advisor, Dr. Benjamin Brunner. Ben encouraged me to exceed my own expectations in the field of geology. Whether involved with field work, lab work, class work or behind a computer screen, Ben has always been willing and able to help me in the pursuit of excellence. I extend a special thank you to Ben for his unmatched patience and understanding in dealing with my occasional hard-headed nature. Dr. Kate Giles, also played an integral role in the completion of this thesis. Kate’s instruction and willingness to indulge my many questions was much appreciated. I also want to thank Kate for the financial support provided to me by the Institute of Tectonic Studies. I would also like to thank the many exceptional instructors, I was lucky enough to study under at the University of Texas at El Paso. They provided me with the knowledge and understanding necessary to successfully navigate the pursuit of my MSc degree. Most notably, I would like to thank Dr. Gail Arnold, Dr. Richard Langford, Dr. Nicholas Pingitore, Dr. Terry Pavlis, Dr. Jason Rickets, Dr. Jennie McLaren and Dr. Jasper Konter. v ABSTRACT Caprocks are found on top of or in a lateral position relative to salt diapirs. The caprocks predominantly comprise sulfate minerals such as anhydrite or gypsum, which presumably accreted during the dissolution of salt from evaporite sediments. In some cases, the sulfate minerals are replaced by carbonate minerals, referred to as ‘carbonate caprock’. Carbonate caprocks associated with salt diapirs are important because they can act as reservoirs or conduits for hydrocarbons, and because they can be easily misidentified as carbonate lithologies belonging to the stratigraphy of the sedimentary sequences adjacent to the diapir, which can jeopardize the accurate interpretation of seismic profiles. Today’s understanding of caprocks is based on observations from salt diapirs found along the US Gulf Coast salt diapir province, home to the famous Spindletop oil field; whereas carbonate caprocks from another salt diapir province in the United States, the Paradox Basin, have been hardly recognized, and in many cases misinterpreted as part of the ‘normal stratigraphy’. At one of the salt walls exposed in Gypsum Valley, Paradox Basin in Southwestern Colorado, abundant and well-exposed carbonate caprock was found in an area referred to as Mary Jane Draw. The carbonate and adjacent gypsum caprock display an amazing richness in lithologies, including micritic calcitic and dolomitic caprocks that are silicified to various degrees, and display an astonishing diversity in caprock fabrics, including massive, brecciated, zebraic, and finely laminated benches. In order to enable caprock identification, and to facilitate communication between caprock researchers, this richness in fabrics calls for a comprehensive caprock classification scheme, and first steps towards such unified nomenclature are reported in this study. vi Light carbon isotope signatures of dolomitic and calcitic caprock indicate that hydrocarbon oxidation likely contributed to the formation of either carbonate caprock lithology; whereas the oxygen isotope signature did not provide conclusive evidence for calcite or dolomite as primary or secondary mineral phase. However, the relatively higher abundance of micritic dolomite compared to calcite indicates that dolomite, or a dolomite-precursor, must have been an early mineral phase. The oxidation of the hydrocarbons was likely tied to microbial sulfate reduction. However, all of the classical signatures for this process, such as pyrite, native sulfur or heavy sulfur and oxygen isotope signatures of sulfate associated with carbonate caprock are absent. The only geochemical fingerprint for microbial sulfate reduction is a high content of isotopically light sulfur in organic matter extracted from the carbonate caprock. These observations can be explained by a carbonate caprock formation at the Gypsum Valley salt diapir as the result of a multi-stage process involving oxidation of hydrocarbons coupled to microbial sulfate reduction in a system that was open to fluid flow. The fluid flow provided magnesium and silica, enabling the precipitation of primary dolomite and silicification, while removing sulfide and sulfate. Waxing and waning supply with fluids may also have triggered phase transitions between anhydrite and gypsum, causing rock deformation, which explains the amazing richness and diversity in caprock fabrics found in Gypsum Valley. As such, the carbonate caprock at Gypsum Valley constitutes the ‘open-system’ end member of geochemical settings that are conducive to carbonate caprock formation. vii TABLE OF CONTENTS ACKNOWLEDGEMENTS ............................................................................................... V ABSTRACT ................................................................................................................. VI TABLE OF CONTENTS .............................................................................................. VIII LIST OF TABLES ........................................................................................................ XII LIST OF FIGURES ..................................................................................................... XIII 1. INTRODUCTION ........................................................................................................ 1 1.1 OVERVIEW ....................................................................................................................... 1 1.1.1 Significance................................................................................................................. 1 1.1.2 Carbonate Caprock Settings and Genesis – the Classic Perspective .......................... 2 1.1.3 Carbonate Caprock Settings and Genesis – Emerging New Views ............................ 7 1.2 UNRESOLVED QUESTIONS IN REGARD TO CARBONATE CAPROCK FORMATION ............... 9 1.2.1 Role of Sulfate Reducing Microbes in the Formation of Carbonate Caprock and Associated Sulfur-Bearing Minerals ....................................................................................... 9 1.2.2 Microcrystalline (Primary?) vs. Secondary Dolomite as Major Constituent of Carbonate Caprock................................................................................................................ 11 1.2.3 Open vs. Closed System: Salt Dome Kinematics and Timing of Hydrocarbon Migration – Impact on Formation and Diagenesis of Carbonate Caprock ........................... 12 viii 1.3 QUESTIONS ADDRESSED IN THIS STUDY......................................................................... 14 1.4 TASKS AND APPROACHES .............................................................................................. 15 2. STUDY AREA AND GEOLOGIC FRAMEWORK ..........................................................18 3. METHODS ..............................................................................................................20 3.1 FIELD WORK .................................................................................................................. 20 3.2