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S U M M A R YAutomatic differentiation (AD) is the technique whereby output variables of a computer code evaluating any complicated function (e.g. the solution to a differential equation) can be differentiated with respect to the input variables. Often AD tools take the form of source to source translators and produce computer code without the need for deriving and hand coding of explicit mathematical formulae by the user. The power of AD lies in the fact that it combines the generality of finite difference techniques and the accuracy and efficiency of analytical derivatives, while at the same time eliminating 'human' coding errors. It also provides the possibility of accurate, efficient derivative calculation from complex 'forward' codes where no analytical derivatives are possible and finite difference techniques are too cumbersome. AD is already having a major impact in areas such as optimization, meteorology and oceanography. Similarly it has considerable potential for use in non-linear inverse problems in geophysics where linearization is desirable, or for sensitivity analysis of large numerical simulation codes, for example, wave propagation and geodynamic modelling. At present, however, AD tools appear to be little used in the geosciences. Here we report on experiments using a state of the art AD tool to perform source to source code translation in a range of geoscience problems. These include calculating derivatives for Gibbs free energy minimization, seismic receiver function inversion, and seismic ray tracing. Issues of accuracy and efficiency are discussed.

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16 17 Geological storage of CO2 generally involves injection of a CO2 stream into a high porosity and 18 permeability reservoir, contained by one or more overlying low permeability formations. 19Sandstone reservoirs and associated cap-rocks of targeted CO2 storage sites therefore have 20 distinct properties such as porosity and mineral contents. Their geochemical response or 21 reactivity to injected supercritical CO2 and associated changes in porosity, and permeability 22 trapping of CO2 as ferroan carbonates such as siderite, ankerite and dolomite over longer time 46 scales when pH is buffered. 47Changes to porosity, mineral content, and water chemistry after pure CO2 reaction observed here 48 and in other published studies were dependent on mineral content and fluid accessibility. These 49 results could be generalized to other sandstone reservoirs where it is expected to inject CO2. The 50 results can also be used to validate geochemical models to build longer term predictions. 51 52 Keywords 53 54 Surat Basin 55 Precipice Sandstone 56 Evergreen Formation 57 Hutton Sandstone 58 CO2 sequestration 59 60 25-60 %) after CO2 core-flooding at 60 -69°C and 19 -25 MPa pore pressure (Saeedi et al., 131 2016). Porosities however remained similar, with the permeability changes attributed to the 132 decreased pH destabilizing kaolinite which migrated plugging pore throats. 133While the above studies at high salinity conditions demonstrate generally higher reactivity of 134 calcite cements and potentially clay-rich cap-rocks that reservoir sandstone, changes in water 135 chemistry, porosity or permeability can be more complex. The different conditions used in each 136 study, especially temperature, and batch or flow conditions complicate direct comparisons 137

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