The presence of igneous rocks within sedimentary basin has already been seen only as a hindrance to the occurrence of oil and oil research. However, is increasing the number of world discoveries of oil where magmatic rocks constitute hydrocarbon reservoirs. The study of these reservoirs known as unconventional has shown the importance of magmatic events in sedimentary basins for the exploration of hydrocarbons.
Igneous-sedimentary oil system
When we talk about oil reservoirs, we soon think in sedimentary rocks, mainly sandstones and carbonates. These rocks are commonly associated with better hydrocarbon reservoirs and are called conventional reservoirs.
However, igneous rock may also constitute a reservoir. The igneous-sedimentary oil systems are unconventional, mixed systems in which one or more essential elements or processes involved are related to magmatic events (Figure 1).
It turns out that for a long time the presence of intrusions and extrusions of magmatic material in the basin was seen only as unfavorable in terms of exploration. The magma was responsible for destroying the organic matter and the oil previously generated, besides obliterating the pores of the rocks-reservoirs.
Recent studies show that in fact volcanic complexes impact the oil systems in a variety of ways, not necessarily destroying or obliterating their viability, but favoring the formation of conventional or unconventional reservoirs.
The importance of knowledge of the magmatic rocks in the sedimentary basins as potential reservoir rocks of hydrocarbons has been strongly discussed in the last years, due to the numerous exploratory discoveries (Figure 2) involving these rocks as carriers of hydrocarbons.
The petroleum system comprises five main elements: (1) a source rock subject, over sufficient time, to conditions leading to hydrocarbon generation; (2) pathways for the generated hydrocarbons to be expelled from the source rock and move to a reservoir rock; (3) a porous and permeable rock to serve as a reservoir for the hydrocarbons, and (4) an enclosing structure for trap the oil with (5) low permeability extremities for seal the reservoir.
The magmatic events may affect any of these five elements, favoring:
1) Hydrocarbon generation
Unlike of conventional oil systems, where the formation of oil and gas is due to the heat supply generated by the subsidence of the basin, in atypical petroleum systems, magma intrusions are responsible for the increase in temperature in the system.
The emanated heat around of the magmatic intrusion causes the vaporization of the water contained in the pores of the embedding rock, resulting in the dehydration and decarbonization and consequent maturation of the organic matter. The intrusions commonly occur as (Figure 3):
(1) layer-parallel and transgressive sills
(2) saucer-shaped intrusions
(3) layer-discordant sub-vertical dykes
(4) localized volcanic centers
The most common types of igneous intrusions in sedimentary basins are dikes and sills. Dikes are discordant structures, usually perpendicular or inclined to intruded bedding. Sills are concordant structures, parallel or subparallel to the sedimentary layers. Both dykes and sills form contact metamorphic aureoles caused by localized heating of the adjacent host rock (Figure 4).
The extent of the thermal effect on the oil system of a sedimentary basin depends on factors such as mineralogy of the embedding rocks, thickness and temperature of the intrusive, depth of intrusion, composition of the available fluids, the time and duration of the magmatic event, among others.
The effect of an intrusion on the embedding rock is equivalent to the thickness of the intrusive body. As we move away from the igneous body there is a progressive decrease in the levels of organic carbon and expansive minerals. Multiple intrusions have this potentiated effect. In addition, the greater the depth the greater the transmitted heat and the larger the effect dimensions.
The distinct geophysical properties (density and resistivity) between the intrusions and the host rocks facilitate the identification of the igneous rock through seismic profiles, for example. Nonetheless, many thin sills fall below the seismic resolution. In addition, the complex and often discordant geometry of igneous bodies presents significant challenges to imaging both in seismic data and in resistivity mapping.
The recognized of igneous bodies and your registration in the fieldwork provides the necessary for link the geophysical measurements to exposed igneous intrusions where critical details. Strataledge® software has a large taxonomy of igneous lithological units that facilitate the process of descriptions of outcrops and cores and allow the integration with geophysical profiles, which help in identification and location of igneous bodies in the basin.
2) Oil migration
The oil migration process can occur in three stages: 1) Primary migration: oil is expelled from the generating rock to the carrier bed; 2) Secondary migration: the oil migrates inside the carrier rock into the trap; 3) Tertiary migration: any movement of the oil after your trapping.
An igneous intrusion can function both as a conduit for hydrocarbon migration and as a barrier to the flow of fluids. If the intrusion present faults and has good permeability, it will act as a migration route. If it is mineralized and impermeable, it will form a structural trap preventing the passage of fluids.
Knowledge of the parameters that control magmatic intrusions generates important information about the paths of fluid migration. For example:
a) Factors such as composition, cooling rate, depths and permeability of h ost rock influence the nature of the fracture network.
b) Structurally complex zones such dike-sills junctions, sill inflection points and intrusion-host rock interfaces are typical of zones with good permeability.
c) The hydrothermal fluids activity, post-emplacement diagenetic processes and tectonism give information if the intrusions are open and interconnected or cemented and closed.
d) Heterogeneities between magmatic intrusion and sedimentary rock can be an important migration route.
e) The geometry of the igneous plumbing system will also influence the migration routes.
3) Storage of hydrocarbons
For a rock to be considered as a reservoir, it must have an appropriate combination of porosity and permeability values that enable the accumulation of hydrocarbons. It’s know that the primary matrix porosity and permeability of igneous rocks is generally very low.
How can they be good oil storage? In igneous rock, these significant values of porosity and permeability may develop owing to fracturing, zones with vesicles, and in hydrothermally altered zones.
Often the igneous body may present these features, but its effectiveness varies according to the lithological facies. The fracture system must be well developed and interconnected, the volume of vesicles must be considerable and the degree of alteration, associated with microfracture.
The vesicles (Figure 5A), for example, act as pores and are concentrated as the top and base of spills and originate from the dissolution of vesicular material. It is common during the cooling of the lavas to form microfracture (Figure 5B) by thermal contraction that form a network joining vesicles that assist in the dissolution of the filling material and allow the entry of oil.
In addition, highly weathering processes in these zones and fluid circulation contribute to the increase of microporosity, creating channels and spaces for hydrocarbon migration and trapping.
Intemperic processes can cause compositional changes and variation in the characteristics of volcanic rocks, increasing their permeability and porosity values. Altered samples develop micropores as a consequence of the predominance of clay minerals and mineral alterations of feldspar and volcanic glass.
On the other hand, solid and impermeable rocks as extrusive manifestations can act as effective lateral sealants or migration barriers, allowing the accumulation of hydrocarbons generated in the adjacent sediments. Sills act as vertical seals, while dikes act as side seals.
Recognize the geometry of the igneous bodies and the structural elements that were induced by magmatic intrusions and are present in the embedding is of paramount importance for the understanding of the reservoir.
Intrusions of igneous rocks in sedimentary basins may be useful as stratigraphic landmarks and indicators of turbidity sedimentation, as is the case of layers of bentonite derived from volcanic ash. They can also generate secondary tension fields that can deform the embedding sedimentary rock and generate traps for imprisoning oil. Or contribute as an extra source of heat for oil generation in shallow and cold basins.
Exploration of hydrocarbons in unconventional basins
We saw that the magmatic events may affect the basin and favor the formation of reservoir oil. On the whole, volcanism, tectonic movements, weathering, leaching and fluids are key factors and geological actions for the formation and development of reservoir spaces in volcanic rocks.
The presence of rock types derived from volcanism and/or affected by post-volcanic re-deposition may lead to complex lithologies, with complex diagenetic overprints at the reservoir level. Diagenesis and diagenetic evolution of altered volcanic materials have a profound effect on the pore evolution of hydrocarbon reservoirs. Hardledge® is an essential software for the systematic petrographic analysis of igneous and metamorphic rocks. It enables to do a detailed petrographic description and interpretation. The descriptions easy import in Strataledge® for integrated visualization and analysis with multiple data sources.
Most of the time the reservoirs are offshore, at great depths, making it difficult to understand and the processes that led to the accumulation of hydrocarbons in the spills. It is necessary to develop similar models that allow the knowledge of the permoporous system and the consequent better exploitation of these reserves.
Senger, K., Millett, J., Planke, S., Ogata, K., Eide, C. H., Festoy, M., Galland, O. and Jerram, D. A. 2017. Effects of igneous intrusions on the petroleum system: a review. First break, Volume 35, p. 1-10.
Reis, G.S., Mizusaki, A.M., Roisenberg, A. and Rubert, R.R., 2014. Formação Serra Geral (Cretáceo da Bacia do Paraná): um análogo para os reservatórios ígneo-básicos da margem continental brasileira. Pesquisas em Geociências, 41 (2): 155-168.
- Sabrina Danni Altenhofen – Endeeper