Delta fronts are often characterized by high rates of sediment supply that result in unstable slopes and a wide variety of soft‐sediment deformation, including the formation of overpressured and mobile muds that may flow plastically during early burial, potentially forming mud diapirs. The coastal cliffs of County Clare, western Ireland, expose Pennsylvanian (Namurian) delta‐front deposits of the Shannon Basin at large scale and in three dimensions. These deposits include decametre‐scale, internally chaotic mudstone masses that clearly impact the surrounding sedimentary strata. Evidence indicates that these were true mud (unlithified sediment) diapirs that pierced overlying strata. This study documents a well‐exposed ca 20 m tall mud diapir and its impact on the surrounding mouth‐bar deposits of the Tullig Cyclothem. A synsedimentary fault and associated rollover dome, evident from stratal thicknesses and the dip of the beds, define one edge of the diapir. These features are interpreted as recording the reactive rise of the mud diapir in response to extensional faulting along its margin. Above the diapir, heterolithic sandstones and siltstones contain evidence for the creation of localized accommodation, suggesting synsedimentary filling, tilting and erosion of a shallow sag basin accommodated by the progressive collapse of the diapir. Two other diapirs are investigated using three‐dimensional models built from ‘structure from motion’ drone imagery. Both diapirs are interpreted to have grown predominantly through passive rise (downbuilding). Stratal relationships for all three diapirs indicate that they were uncompacted and fluid‐rich mud beds that became mobilized through soft‐sediment deformation during early burial (i.e. <50 m, likely <10 m depth). Each diapir locally controlled the stratigraphic architecture in the shallow subsurface and potentially influenced local palaeocurrents on the delta. The mud diapirs studied herein are distinct from deeper ‘shale diapirs’ that have been inferred from seismic sections worldwide, now largely disputed.

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David G. Morse (ISGS) led the effort to find a suitable injection site and all of the well drilling work to the casing point including the coring and open hole logging operations. Morse coordinated the Coal Characterization program and was the primary author for the coal characterization section of this report. Contributors to the coal characterization are as follows: ISGS contributors were Keith Hackley, gas chromatographic and isotopic characterization; and Christopher Korose, Geographic Information Systems and volumetrics. Laboratory experiments on coal adsorption, shrinkage, and swelling were conducted by Satya Harpalani, Southern Illinois University at Carbondale. Maria Mastalerz, Agnieszka Drobniak, and John Rupp (Indiana Geological Survey) were involved with planning, performed some desorption and gas chemistry and all maceral characterization analyses, and studied the roles of various coal macerals and coal pore size distribution in sequestration. Ivan G. Krapac (ISGS) coordinated the monitoring, verification, and accounting (MVA) program at the Tanquary site. He designed and developed the general MVA requirements of the site and worked with individual contributors towards the synergistic and collaborative effort of this program. Krapac coordinated, compiled, and edited the MVA strategy and MVA observations and discussions sections. ISGS contributors to both field efforts and data analysis and interpretation included Peter Berger (geochemical modeling), Gayathri Gopalakrishnan (GFLOW modeling), Keith Hackley (gas characterization), Donald Luman (color infrared imagery), Ed Mehnert (groundwater modeling, water well installation and development), and William Roy (geochemical modeling). Abbas Iranmanesh and Bracken Wimmer assisted with monitoring well installation and development, conducted the monthly water and gas sampling of the domestic and injection site ground water and the injection coal seam monitoring wells, and contributed to documentation of sampling methodology. ISGS well drilling crew led by Jack Aud installed groundwater monitoring wells. Bob Butsch (Schlumberger Carbon Services) completed the cased hole log analyses. Scott Frailey led the pilot planning, scheduling, and logistics of implementing the plan through post-carbon dioxide (CO 2) injection testing and ultimate abandonment of wells and reclamation of the site. Frailey designed the pressure transient tests and performed real time analyses of the data. He coordinated the numerical modeling and pressure transient analyses for analyzing the pressure and injection rate from the CO 2 and water injection periods. Frailey was the primary author for the introduction and sections on site selection; pilot site design modification and well arrangement; drilling operations, open hole tests, and casing; completion; coal seam (in situ) characterization (pre-CO 2); field observations during active CO 2 injection; interpretation, analyses, and modeling of pilot results; and conclusions. Text for these sections was also contributed by