Basin and petroleum system modeling and global sensitivity analyses of natural gas hydrates
Keywords: Geology.

(I) Topics are broadly defined then followed by chapter highlights: Gas hydrate is a solid, ice-like, form of natural gas that is found in the low temperature, high pressure conditions of shallow sediment in deep marine environments and in permafrost regions. This solid form of natural gas is extensively found offshore every continent on Earth and potentially has a greater amount of energy than all other forms of oil, gas, and coal combined. Therefore, it is of interest for industry, academia, and government sectors, particularly for nations that have limited domestic natural gas resources. Gas hydrates tie in with CO2 sequestration or storage, energy resources, the global carbon cycle, and geohazards. Basin and petroleum system modeling is a quantitative algorithmic approach that utilizes diverse datasets including, but not limited to, well logs, paleontology, stratigraphy, petrophysics, and seismic data to make deterministic, iterative, forward-modeling predictions. It integrates geology, geophysics, geochemistry, engineering, geostatistics, and rock physics to model the sedimentary and tectonic evolution of basins, as well as to model and predict the generation, migration, and accumulation of hydrocarbons in up to three dimensions through geologic time. Though widely used for the modeling of conventional oil and gas systems, basin and petroleum system modeling only recently has been used to study gas hydrate systems, with the first non-proprietary gas hydrate basin and petroleum system model published in 2015. Sensitivity analysis is the study of how variation of uncertain input parameters impacts the response of interest and has great potential application to basin and petroleum system modeling of gas hydrates. A couple of strengths of sensitivity analysis are that it helps determine which data are most important to acquire for reducing uncertainty and it can help simplify a complex problem by identifying less important input parameters. Local sensitivity analysis is a one-at-a-time sensitivity analysis technique that analyzes the effect of one parameter on a function at a time, keeping the other parameters fixed. It explores only a small fraction of the design space, especially when there are many parameters, and is a simple screening method that is widely used across disciplines. Furthermore, the local sensitivity analysis method does not evaluate parameter interactions for non-linear effects. On the other hand, global sensitivity analysis is a powerful tool that has never before been used for gas hydrate basin and petroleum system modeling despite it being effective at evaluating parameter interactions for non-linear effects. Global sensitivity analysis helps understand and simplify the complexity of problems and elucidates what model variables impact data, decisions, and forecasts. (II) Chapter 1 highlights: We built a detailed (more than 25 million cells) quantitative 3D basin and petroleum system model of Terrebonne Basin, Gulf of Mexico, for dynamic gas hydrate studies and to be used to support planning for scientific drilling. Original interpretations of the geology, using seismic imaging and well logs, are presented, including a proposed mechanism for the presence of giant gas mounds. Our model predicts present-day gas and gas hydrate volumes, saturations and distributions of accumulations, marine gas hydrate recycling (by which gas hydrate saturations at the base of the gas hydrate stability zone increase through time due to, for example, sediment burial), and the potential source of gas in the basin (specifically, thermogenic versus biogenic). The source of gas determines whether light or heavy gases likely exist, which have different economic implications, the latter being more valuable. Our model is calibrated to porosity and pressure data and our model-based gas hydrate saturation predictions align with what is observed in well log and seismic data vertically and laterally. We suggest that our 3D model has application to future studies that seek to understand gas hydrates as they relate to faults, fractures, lithologic variations, salt tectonics, erosion, pressures, changing water column conditions, temperature changes, and gas sources, as these Earth system features have all been incorporated into our model. (III) Chapter 2 highlights: By harnessing theoretical 2D basin and petroleum system models and real-world inspired models based on the well-studied salt diapir-associated gas hydrate sites at Green Canyon (Gulf of Mexico) and Blake Ridge (U.S. Atlantic coast), we demonstrate that salt structures provide a heat flow-driven mechanism for marine gas hydrate recycling that results in enhanced saturations. Our work also provides insight into the roles of basal heat flows, salt diapir diameters, and sediment thermal conductivities in controlling optimal gas hydrate accumulations in salt basins. Broadly speaking, we suggest that gas hydrate and associated gas accumulations above salt diapir crests represent attractive targets for hydrocarbon resource exploration and for scientific drilling expeditions aimed at characterizing these systems. It therefore follows that salt basins are compelling localities for studying our newly proposed mechanism of salt diapir heat flow-driven enhanced gas hydrate and gas accumulations. (IV) Chapter 3 highlights: We developed a widely-applicable, novel automated method that results in thousands of unique 2D basin and petroleum system models of gas hydrates and it applies global sensitivity analysis to them. To put this in perspective, only tens of basin and petroleum system models of gas hydrates have been published. Our work is the first time, at least in the public domain, that global sensitivity analysis has been coupled with basin and petroleum system modeling of gas hydrates. This tool improves the efficiency of basin and petroleum system modeling of gas hydrates by ~40 times, as well as eliminating sampling bias by randomly building models using the Monte Carlo approach. We believe our 2D basin and petroleum system model scenarios, as well as their associated organized databases of 10s of thousands of extracted input and output values, can be used as templates and guides for future basin and petroleum system modeling of gas hydrates and of other hydrocarbon systems. Our work provides insight into the relative importance of different geologic properties when assessing gas hydrate stability zone thicknesses, gas hydrate saturations, and gas saturations by utilizing quantitative and objective measures of sensitivity. Furthermore, this powerful tool reveals important geologic input interactions that cannot otherwise be observed using the traditionally used method of local sensitivity analysis. One of our many geologic takeaways or recommendations is that professionals who plan to explore for gas hydrate accumulations should consider shallow to midwater depths more so than deepwater, because our results show that those basin models are more conducive, geologically, for gas hydrate accumulations that have relatively high saturations. Our two distinct sets of models span a wide range of basin scenarios intended to represent: (1) the entire world and (2) the sites where gas hydrates have been found or inferred. We use these results to answer questions about how to improve global map predictions. Our work provides original plots illustrating the relationship between basal heat flow and the gas hydrate stability zone that could be useful in new ventures or other exploration of conventional petroleum systems where a gas hydrate stability zone is observed or inferred. Basal heat flow is among the least known values when gathering information about a basin. Our plots can be used as a guide to determine what the likely range of basal heat flows is acceptable for a basin, which can result in the difference between generation of oil or gas