Effects of magmatic intrusions on temperature history and diagenesis in sedimentary basins – and the impact on petroleum systems

Abstract

For many volcanic basins, the thermal effect of igneous intrusions is decisive for their petroleum potential because such thermal impact may lead to maturation of organic material in areas that otherwise would remain immature. Many factors contribute to the outcome of such intrusions, and in this thesis the influence of a number of parameters, including sill thickness, timing of emplacement, structural changes of sedimentary basins, lithologies and diagenesis, have been modeled to improve the ability to predict the development of the whole petroleum system as a function of its thermal history. By quantifying the effect of several of these factors, the aim of this project has been to estimate the thermal impact of magmatic intrusions on maturation and diagenesis, from the very first temperature increase in the host rock to the long term influence, in terms of permeability and migration. Sill thickness and timing of emplacement is central in the first Paper where the thermal effect of 0 m, 50 m and 100 m thick sills are compared. The results show large differences on the thermal effect of the tested thicknesses, particularly for 0 m versus 100 m, but also 50 m versus 100 m thick sills. Whereas immature areas in the vicinity of sills that are 50 m thick will remain immature, they become fully matured when the sills are twice as thick. Timing of sill emplacement can be essential, particularly if the source rocks are between two or more sills intruded with a time lapse. Transient thermal effects of normal faulting in basins with magmatic intrusions are in focus in the second Paper. As fault movements occur, the basin momentarily experiences thermal instability in the proximity of the fault zone. How long this thermal instability lasts, depends on several factors, such as the physical properties of the rocks and the time lapse of fault movement. The results show that the largest differences between steady state and transient thermal calculations are found in the hanging wall. If sills intrude shortly after fault movement, the rocks in the hanging wall are colder than the rocks at the same depth in the foot wall. As the thermal effect of magmatic intrusions is dependent on the pre-intrusion host-rock temperatures, the thermal effect of the sills is smaller in the hanging wall than the foot wall due to the lower host rock temperatures. However, if the sills intrude with a time lapse in relation to the fault slip, the sedimentary rocks have become warmer and the effect of the intruding sills is larger. Other factors that influence the thermal effects of sill intrusions in sedimentary basins are fault displacement, time span of faulting and deposition, fault angle, the thermal conductivity of the rocks, specific heat capacity and basal heat flow. How the faults are restored in the modeling process also influences the thermal development in the basin after fault slip. Diagenesis/chemical compaction is the focus of the third Paper. The study quantifies the thermal effect of magmatic intrusions on three different diagenetic processes: the transition of opal A to opal CT to quartz; the smectite to illite transition; and the dissolution and re-precipitation of quartz. All these processes are temperature dependent and may induce deterioration of the reservoir quality by reducing the porosity. Diagenetic alterations can contribute to changes in the physical properties of the rocks. These changes can cause rocks to respond differently to stress conditions in the subsurface. Emplacement of magmatic intrusions influences all the studied diagenetic processes and result in porosity loss of rocks in their proximity. Results show that stresses build up in the stiffer rocks, like the sills and diagenetic altered areas. Such stress accumulations may potentially lead to fault slip or opening of fractures and thus increase the permeability and the potential of fluid migration. Overall, this study shows the need for good representation of the subsurface sill thicknesses and structural development, particularly prior to emplacement of magmatic intrusions. Through magmatic intrusion and their impact on the maturation of organic material, diagenetic processes, location of stress concentrations, and the potential effect on permeability and migration pathways, this study highlights how these factors may have long-term effect on the petroleum system. Other crucial variables are sill thickness and clustering of the sills at multiple levels. The thermal conductivity of host rocks is the factor influencing the transient thermal effects the most, after fault slip and the increased temperatures enhance maturation and diagenesis in their vicinity.