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

The PhD project has been ongoing since January 2016 and is scheduled to end January 2020. So far all ECTS credits and compulsory dissemination credits are completed and approved. Magmatic intrusions are formed by warm molten rock that makes its way upwards in the crust and solidifies before it reaches the surface. During intrusion, magma of temperatures exceeding 1000 oC heats up the much colder surrounding rocks. The aim of the project is to quantify the effects of sill intrusions on potential petroleum systems. The first paper in the project "The importance of sill thickness and timing of sill emplacement on hydrocarbon maturation" (Marine and Petroleum Geology, 2018) studies the effect of the thickness of magmatic sill intrusions on the temperatures and hydrocarbon maturation in sedimentary basins. The results show large differences on the effect of 50 meter versus 100 meter thick sills. Some areas show that all organic material is transformed into hydrocarbons with 100 meter thick sills present, and show no matured organic material for 50 meter thick sills. Also the timing of sill emplacement can make a difference. This is especially true when the source rock lies between two or more sills intruded with a time lapse. These are all important aspects to consider when interpreting sill thickness and timing of magmatic intrusion on seismic data. Midway evaluation at the University of Bergen was completed and approved in March 2018. A popular scientific paper in Norwegian was published at geoforskning.no in 2018 under the title "Fremtidig energikilde?". This was submitted to the website`s yearly popular scientific dissemination competition. An oral presentation was held in January 2019, at the annual Tectonic Studies Group international conference with the title "Transient thermal effects in sedimentary basins with normal faults and magmatic intrusions - A sensitivity study". The second paper "Transient thermal effects in sedimentary basins with normal faults and magmatic sill intrusions - A sensitivity study" (Geosciences, 2019) focuses on the thermal effects of normal faulting in basins with magmatic sill intrusions. As fault movement occurs, the basin momentarily experiences thermal instability in the vicinity of the fault zone. How long this thermal instability lasts, is dependent on the physical properties of the rocks in the basin and the time lapse between fault movement and the emplacement of magmatic intrusions. The results show that the largest thermal differences are found in the part called the hanging wall. This is the part of the basin that moves downward along the fault zone during faulting. If sills intrude in the hanging wall shortly after fault movement, the rocks in the hanging wall will be colder compared to the rocks at the same depth on the opposite side of the fault. However, if the sills intrude with a time lapse in relation to the fault movement, the sedimentary rocks have had the opportunity to heat and the effect of the intruding sills are larger. This is due to the higher temperatures in the basin when the sills intrude. Other elements that influence the thermal effects of sill intrusions in sedimentary basins are; fault displacement, time span of faulting and deposition, fault angle, the rocks thermal conductivity and heat capacity, basal heat flow and how the faults are restored. Article number 3 "The influence of magmatic intrusions on diagenetic processes and stress accumulation" (Geosciences, 2019) addresses the effect of magmatic intrusions on diagenetic processes in sedimentary basins. Diagenesis is the term for all processes, physical, chemical and biological, that occur with sediments during burial going from sediments to solid rocks. The focus is on the conversion of non-crystalline silica in the sediments via micro-crystalline to crystalline silica, a process which is temperature dependent. It also deals with the conversion of the clay mineral smectite into illite, and the dissolution and deposition of quartz minerals. Both of these processes are also temperature dependent and it is therefore possible to model where such transformations may have occurred in areas of sill intrusions. Diagenesis can contribute to changes in the physical properties of the rocks. These changes can cause rocks to respond differently to local stress conditions, thus contributing to increased fluid flow in areas of magmatic intrusions. The study looks at how magmatic intrusions can affect reservoir quality, and emphasizes the need to include magmatic sills and diagenetic changes in basin modeling. The latter part of the project will be used to summarize the theory and the results from the project in the doctoral dissertation.

Doktorgradsprosjektet omhandler magmatiske intrusjoners påvirkning på petroleumssystemet, noe som reflekterer letevirksomhetens økte fokus på leting i geologisk komplekse områder. Prosjektet faller naturlig inn i kjernevirksomheten til Tectonor AS, og vil kunne bidra til at selskapet får utvidet kompetanse på de komplekse prosessene som er forbundet med magmatisme i leteområdene i Norskehavet og Barentshavet.

Magmatiske intrusjoner er dannet av varm flytende stein som presser seg oppover i jordskorpen og blir liggende i lagrekken uten å nå overflaten. Ved intrusjonstidspunktet kan magmaen ha en temperatur på over 1000 oC, mens vertsbergartene den intruderer i har betydelig lavere temperatur. I et sedimentært basseng vil derfor intrusjoner føre til økt temperatur, noe som kan være både godt og dårlig nytt for potensielle petroleumssystemer. For at organisk materiale skal kunne omdannes til olje eller gass, må kildebergarten være i olje- eller gassvinduet (hhv. 80-120 oC og 120-150 oC) over en lengre periode. I et område med magmatiske intrusjoner, vil temperaturøkningen relatert til intrusjonene gjøre at omdanningen til hydrokarboner kan foregå på et grunnere dyp enn normalt. Temperaturøkningen gjør også at bergartene i området rundt intrusjonene endrer egenskaper på grunn av kjemisk omdannelse. Dette kan gjøre at porene i bergartene tettes igjen, slik at oljen og gassen ikke klarer å strømme til, og akkumuleres i, potensielle feller. Når egenskapene til bergartene endres, kan også måten de reagerer på spenningsendringer forandre seg. I ytterste konsekvens kan dette føre til at forkastninger kan reaktiveres, noe som gjør det mulig for olje og gass å lekke ut fra potensielle feller. Denne studien vil fokusere på å oppnå en bedre forståelse av prosessene relatert til magmatiske intrusjoner, slik at vi kan forbedre beslutningsgrunnlaget i jakten på olje og gass i disse områdene.