Gareth Johnson

Structural model creation: the impact of data type and creative space on geological reasoning and interpretation

Abstract Interpretation of sparse or incomplete datasets is a fundamental part of geology, particularly when building models of the subsurface. Available geological data are often remotely sensed (seismic data) or very limited in spatial extent (borehole data). Understanding how different datasets are interpreted and what makes an interpreter effective is critical if accurate geological models are to be created. A comparison of the interpretation outcome and techniques used by two cohorts interpreting different geological datasets of the same model, an inversion structure, was made. The first cohort consists of interpreters of the synthetic seismic image data in Bond et al. (‘What do you think this is?: “Conceptual uncertainty” in geoscience interpretation’, GSA Today, 2007, 17, 4–10, http://dx.doi.org/10.1130/GSAT01711A.1); the second cohort is new and interpreted borehole data. The outcomes of the borehole interpretation dataset support earlier findings that technique use, specifically evidence of geological evolution thought processes, results in more effective interpretation. The results also show that the borehole interpreters were more effective at arriving at the correct interpretation. Analysis of their final interpretations in the context of psychological and medical image analysis research suggests that the clarity of the original dataset, the amount of noise and white space may play a role in interpretation outcome, through enforced geological reasoning during data interpretation.

Impact of seismic image quality on fault interpretation uncertainty

Uncertainty in the geological interpretation of a seismic image is affected by image quality. Using quantitative image analysis techniques, we have mapped differences in image contrast and reflection continuity for two different representations of the same grayscale seismic image, one in two-way-time (TWT) and one in depth. The contrast and reflection continuity of the depth image is lower than that of the TWT image. We compare the results of 196 interpretations of a single fault with the quality of the seismic image. Low contrast and continuity areas correspond to a greater range of interpreted fault geometries, resulting in a broader spread of fault interpretations in the depth image. Subtle differences in interpreted fault geometries introduce changes in fault characteristics (e.g., throw, heave) that are critical for understanding crustal and lithospheric processes. Seismic image quality impacts interpretation certainty, as evidenced by the increased range in fault interpretations. Quantitative assessments of image quality could inform: (1) whether model-based interpretation (e.g., fault geometry prediction at depth) is more robust than a subjective interpretation; and (2) uncertainty assessments of fault interpretations used to predict tectonic processes such as crustal extension.

Estimating geological CO2 storage security to deliver on climate mitigation

Carbon capture and storage (CCS) can help nations meet their Paris CO2 reduction commitments cost-effectively. However, lack of confidence in geologic CO2 storage security remains a barrier to CCS implementation. Here we present a numerical program that calculates CO2 storage security and leakage to the atmosphere over 10,000 years. This combines quantitative estimates of geological subsurface CO2 retention, and of surface CO2 leakage. We calculate that realistically well-regulated storage in regions with moderate well densities has a 50% probability that leakage remains below 0.0008% per year, with over 98% of the injected CO2 retained in the subsurface over 10,000 years. An unrealistic scenario, where CO2 storage is inadequately regulated, estimates that more than 78% will be retained over 10,000 years. Our modelling results suggest that geological storage of CO2 can be a secure climate change mitigation option, but we note that long-term behaviour of CO2 in the subsurface remains a key uncertainty.Carbon capture and storage can help reduce CO2 emissions but the confidence in geologic CO2 storage security is uncertain. Here the authors present a numerical programme to estimate leakage from wells and find that under appropriate regulation 98% of injected CO2 will be retained over 10,000 years.

Fault Seal Analysis of a Natural CO2 Reservoir

For potential CO2 storage sites it is crucial to know if faults will act as flow baffles or if CO2 will be able to migrate out of the reservoir complex. Geomechanical fault seal analysis for a CO2 reservoirs is very similar to hydrocarbon reservoirs. However, fault rock seals have the potential to act very different in a CO2-rock-water system compared to a hydrocarbon-rock-water system. Natural CO2 reservoirs are common in sedimentary basins world-wide and here we present the results of a fault seal analysis, with emphasis on juxtaposition and fault rock seals, of a natural CO2 reservoir from the Colorado Plateau. The reservoir has leaked CO2 for more than 350 ka along faults. Our results show that the existing gas column can overcome the capillary entry pressure of the fault rocks, leading to migration of CO2 to the surface. Additionally, the fault is orientated favourable for reactivation in the current stress field.