The impact of the allochtonous salt and overpressure development on the petroleum system evolution in the thunder horse mini-basin, Gulf of Mexico
Keywords: Geological Sciences.

The Northern Gulf of Mexico Basin is part of an ocean basin characterized by a complex structural framework. The complex structural framework is shaped by the dynamic interaction between salt tectonics and sedimentation. Salt withdrawal mini-basins are among the structural features produced by this interaction and are of particular interest to hydrocarbon exploration. The mini-basins provide significant accommodation in which thick packages of sediments can accumulate. The accumulation of sediments can be very rapid during episodes of high sedimentation, such as occurred in the middle Miocene episodes in the northeastern Gulf of Mexico. Rapid accumulation of sediment in turn changes the local topography of the mini-basin and also leads to significant buildup of overpressure. In such settings, the structure and stratigraphy in the vicinity of a mini-basin are influenced by salt movement. Moreover, the rock properties can be altered due to salt movement and overpressure development. Therefore, the factors of salt movement and overpressure development are crucial in understanding the evolution of the petroleum system in mini-basins. Insights on the roles of these factors in defining the evolution of the petroleum system are beneficial for hydrocarbon exploration. Such insights are of use in addressing practical problems in reservoir characterization, pore pressure prediction, and basin and petroleum system modeling (BPSM). This dissertation establishes insights on the roles of these factors through meticulous quantification of related geologic processes to address some of the stated practical problems above. Chapter 1 quantifies the spatial variations in sediment compaction and clay diagenesis to define spatial trends of elastic properties, which are used for seismic reservoir characterization. We demonstrate the advantages of using this integrated method in a frontier area. Chapter 2 studies the impact of high sedimentation and salt movement on the thermal history of a mini-basin to propose a workflow for predicting the effects of smectite to illite diagenesis on overpressure. Chapter 3 investigates the implications of lateral slip along salt-related faults to pressure and thermal history to address the proper application of BPSM techniques in constructing paleo-geometry when modeling these faults. All three chapters of this dissertation focus on the Thunder Horse mini-basin in the Mississippi Canyon area by integrating 3D seismic data with well logs, biostratigraphic data, and borehole measurements of pore pressure and temperature. Chapter 1 evaluates spatial changes in effective stress and smectite to illite diagenesis across Thunder Horse mini-basin using a 2D basin model that accounts for salt movement and properly calibrated with a single well. The results from the 2D model indicate that the central part of the mini-basin known as Thunder Horse field is associated with higher effective stress and shallower zone of smectite to illite transformation than the northwestern part known as Thunder Horse North field. Proper rock physics models are subsequently built to link the basin modeling results to seismic impedances and use them for quantitative seismic interpretation with spatially limited well control. The rock physics models are designed to account for the effects of sediment compaction and smectite to illite diagenesis on seismic impedances. The results from the quantitative seismic interpretation with a single well and basin modeling extrapolations of seismic impedance (extrapolation workflow) are comparable in their quality with those results obtained through the quantitative seismic interpretation with multiple wells scattered across the area (reference workflow). The training data sets of the seismic impedances of lithofacies corresponding to both of these workflows are similar in terms of the distribution of values and the pronounced spatial trends. In addition, the seismic inversion results of both of them are similar in terms of the quality of inverted impedances. Ultimately, these two workflows are close to each other in estimating the net pay volume of the reservoir and show the same degree of uncertainty in mapping reservoir lithofacies. This chapter was published first in AAPG Bulletin in May, 2017 'Ahead of Print' and officially appeared in the April, 2018 Bulletin. The publication focused on showcasing the workflow of combining basin modeling with seismic reservoir characterization. A refined version of this workflow that rigorously addresses spatial variability of training data of seismic impedances of lithofacies and uncertainty was accepted for publication in Geophysics in March, 2018. Both publications are co-authored with Tapan Mukerji, Allegra Hosford Scheirer and Stephan A. Graham. Dr. Tapan Mukerji contributed to the conception and design of the study. Dr. Allegra Hosford Scheirer provided guidance on building the basin model. Dr. Stephan A. Graham provided guidance on relating seismic impedances to geologic processes in the subsurface. Chapter 2 simulates thermal history of the mini-basin to quantify the impact of high sedimentation and salt movement. Then, the chapter integrates the modeled thermal history with rock physics models to predict the generation of overpressure due to smectite to illite diagenesis. A time-dependent solution of thermal history, simulated with a 2D basin model across Thunder Horse mini-basin, shows calibration to corrected bottom hole temperatures and illitic content of XRD data when combined with the proper kinetics. The time-dependent solution indicates fluctuations of the high heat flux by the middle Miocene due to high sedimentation. In addition, the solution suggests mitigation of the transient effects of high sedimentation by the high conductivity of the extruded salt sheet that completely covers Thunder Horse North field. Comparing the time-dependent solution with a steady state solution, the steady state solution overestimates temperature and illitic content through time and the differences between the two solutions are more significant in Thunder Horse field. After building rock physics models that account for thermal history to define a relationship between effective stress and both P-wave velocity and density on the basis of illite content, the rock physics models show a predictive power of pore pressure that is sensitive to the incorporated solution of thermal history. On one hand, incorporating a solution of thermal history that addresses the geologic factors of high sedimentation and salt movement (i.e., time-dependent solution) yields accurate prediction of pore pressure from seismic P-wave velocity based on the rock physics models. On the other hand, oversimplification of the solution of thermal history with a steady state solution leads to inaccurate estimation of pore pressure by the rock physics models. Therefore, addressing the geologic factors controlling the thermal history is essential to accurately predict pore pressure from seismic velocity using rock physics. This chapter is submitted into Marine and Petroleum Geology and co-authored with Tapan Mukerji, Nader C. Dutta and Allegra Hosford Scheirer. Dr. Tapan Mukerji contributed to the design of the study, rock physics modeling, pore pressure prediction, and thermal history modeling. Dr. Nader C. Dutta advised on the design of the study and the rock physics modeling and pore pressure prediction. Dr. Allegra Hosford Scheirer guided on constructing the basin model. Chapter 3 compares the techniques for constructing paleo-geometry in BPSM (i.e., pure porosity controlled backstripping vs imposing structural restorations on paleo-geometry) in terms of the simulated pore pressure and thermal history across a salt related structure of lateral slip. This chapter focuses on an expulsion rollover fault to the northeast of Thunder Horse mini-basin (i.e., listric fault that soles in a salt decollement). The two techniques of constructing paleo-geometry differ exclusively in the thickness of stratigraphic layers and stratigraphic contacts with salt through geologic time. These differences in paleo-geometry cause differences in the simulated pore pressure and thermal history. The technique of imposing structural restorations on paleo-geometry results in higher pore pressure build up over time and higher temperatures earlier in the history of the mini-basin when compared to the technique of porosity-controlled backstripping. These differences between the two techniques are spatially concentrated in the vicinity of the expulsion rollover fault. Therefore, the lateral slip impacts pressure and thermal history across the structure and the spatial extent of this impact depends on the amount of lateral slip. This chapter is submitted to Basin Research and co-authored with Kristian E. Meisling, Tapan Mukerji and Allegra Hosford Scheirer. Dr. Kristian E. Meisling provided guidance on seismic interpretation across the salt structures and on the sequential structural restoration. Dr. Tapan Mukeri helped with the initial design of the study and with some of the interpretations of the basin models. Dr. Allegra Hosford Scheirer helped with some of the interpretations of the basin models.