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Magmatism in Sedimentary Basin – Application in Oil Exploration

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.

In this article we will show how tools like Hardledge® and Strataledge®, helps in the routine work of exploration and production of complex hydrocarbon reservoirs

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).

Example of occurrence of magmatic rocks
Figure 1. Example of occurrence of magmatic rocks associated with sedimentary rocks in the Argentinian Neuquén basin. This reservoir of fractured igneous intrusion holds 25 million barrels of recoverable oil per field, and are characterized by rapid initial production rates of up to 10,000 barrels/day. (Source: Senger et al., 2017)

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 discoveries of reservoir with related volcanic rocks around the world
Figure 2. In green, the discoveries of reservoir with related volcanic rocks around the world. (Source: Senger et al., 2017).

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

Cross section through a volcanic basin
Figure 3. Cross section through a volcanic basin highlighting some of the key terminology and relationships of the igneous rocks with the host basin (link)

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).

Examples of sills and dikes infiltrated among the oldest layers
Figure 4. Examples of sills and dikes infiltrated among the oldest layers (link).

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.

 Vesicular porosity
Figure 5. (A) Vesicular porosity (B) Vesicular porosity with microfractures (red arrows) Reis et al., 2014.

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

Genesis, Transport and Deposition of Clastic Sediments – An Overview

What are clastic sediments?

Clastic sediments are formed by loose particles of various sizes, which can evolve into sedimentary rocks such as sandstones, shales and conglomerates. They are essentially made up of sand, mud and gravel, formed from the weathering and erosion of pre-existing rocks.

  • Weathering comprises physical, chemical and biological processes that alter, disintegrate, or dissolve rocks. Physical weathering occurs when a rock is fragmented through purely mechanical processes, which do not significantly alter its chemical or mineralogical composition. Chemical weathering, in turn, involves chemical and mineralogical changes of the pre-existing rocks, which may result in hydration, oxidation, reduction or dissolution of their mineral components, and formation of new mineral phases stable under surface conditions.
  • Erosion is responsible for moving materials derived from weathering, to be carried by the action of water, gravity, wind or ice, and deposited in various marine or continental environments.
Figure 1: Natural arch produced by wind erosion on rocks subjected to a differential weathering in Jebel Kharaz, Jordan. – Source: link

It is possible to characterize the sediments that compose a clastic sedimentary rock with the help of a magnifying glass on a hand sample, or through the analysis of a thin section with a polarized microscope. Composition, textural, structural and fabric aspects can be described through the use of Petroledge® software. The program automatically generates textural and compositional classifications, recognizes the tectonic provenience of the sediments, and also interprets diagenetic environments, aspects which will be further addressed in incoming texts.

After erosion, the particles are available for transportation. Water, wind, ice and gravity are the main agents for sediment transport. Gravity can act alone or associated to other agents, such as water, thus constituting the main sediment transport agent in nature. In fact, rivers account for approximately 95% of the sediment flow to the oceans.

The sedimentary particles remain in motion while the flow energy (usually proportional to its velocity) is sufficient to sustain them. This can be represented for the different particle sizes by the Hjulströms Diagram.

Hjulströms (1939) diagram – Erosion, transport and deposition intervals according to flow velocity and particle size. Source: link

Types of sedimentary transport

Sediment transport can be classified according to its competency (related to the transported grain size), its capacity (related to the amount of sediment that the agent can transport) and the load (amount of sediment that the agent effectively carries). A river capable of carrying particles larger than sand sized is a highly competent agent. Also, the greater the volume flow of the river, the greater the load in motion and, therefore, the greater its capacity.

The force that moves the particles during transportation is provided by fluids, and depends mainly on their velocity and viscosity. The flow of fluids can be separated into 2 types: laminar flow and turbulent flow. In laminar flows, the particles immersed in the fluid move parallel to each other, in the direction of transportation. In turbulent flow, however, the particles immersed in the fluid move in all directions, but with a displacement parallel to the transportation direction. This type of flow has a much greater erosion power than laminar flow. As velocity increases, the flow tends to become turbulent.

A dimensionless parameter called Reynolds Number (Re), named after Osborne Reynolds, who studied fluid dynamics during the nineteenth century, indicates the intervals at which a flow of fluid is laminar or turbulent. The Reynolds number is proportional to the velocity of the flow (V) and to the depth of the channel or diameter of a pipe (L), as well as to the ratio between the density (d) and the viscosity of the fluid (u), according to the equation:

For values of Re < 500, the flow is laminar, whereas for values of Re> 2000, it is turbulent. Laminar flows are common in high viscosity or low velocity fluids. Wind flow is, therefore, essentially turbulent due to the low viscosity of air. Water flows can be laminar when their velocity is very low. However, significant volumes of sediment are usually transported by turbulent water flows.

Representation of flow types in a pipe.

In addition to the effects of fluid viscosity and inertial forces, gravity also influences the way a fluid transfers waves or moves sediment dunes. The Froude Number (Fr), which can be considered the ratio between the average velocity of the flow and the velocity of a wave contained therein, is also a useful dimensionless value for sedimentology studies, expressed as:

where U represents the average velocity of the flow, L the depth of water and g the acceleration due to gravity.

When Fr value is less than 1, the flow is considered subcritical or quiet and a wave can move upstream (against flow). If the value is greater than 1, the waves cannot propagate upstream and the flow is considered supercritical or fast. In addition to being used to define the critical velocity from which a flow is considered subcritical or supercritical at a given depth, the Froude Number is also related to different flow regimes, which are related to characteristic bedforms.

Two flow regimes are recognized: lower and higher. The lower flow regime comprises a subcritical flow in which ripples, dunes and plane-parallel type stratifications are formed. The upper flow regime, in turn, comprises a supercritical flow, in which antidunes and plane-parallel stratifications are stable. This can be understood from a bedform stability diagram (Figure 4). The use of this diagram allows obtaining velocity estimates or identifying changes in velocity or type of flow that resulted in deposition of sediments according to their depositional structures.

Figure 4: Bedform stability diagram for particles under a given water flow (depths between 25 and 40 centimeters and temperature of 10 ° C) (Nichols, 2009).

Fluid-flow dynamics


Under the action of water flowing over a moving background condition, the solid particles that make up the substrate tend to come into motion. In a simplified way, coarse particles such as sand and gravel will be transported if the energy of the stream overcomes the weight of the particles. In the case of finer particles, such as silt and clay, the cohesive force is the main force of resistance that flow must overcome. The forces acting on a particle submerged in a water stream are: the submerged weight of the particle, the lift force that causes the rise of the particle, and the drag force, which drives the particle in the direction of flow. From the relation between these three forces, we can classify the particles transportation according to four different modalities:

  • Suspension: When the lift force balances that of gravity, the particles move floating in the liquid mass.
  • Sliding: occurs when particles slide on the bed.
  • Rolling: occurs with coarser particles, which shapes allow them to rotate.
  • Saltation: occurs through the interaction between the lift and drag forces, a situation in which particles move through jumps (such as a suspension transport in which the particle sometimes comes into direct contact with the bottom).
Figure 5: Particles transported by rolling, saltation and suspension (adapted from Nichols, 2009).Sliding, rolling and saltation are types of transport by dragging. Dragging occurs where the weight of a particle exceeds the lift force.


The viscosity of ice is so high that it is not directly recognized as a fluid. However, ice may transport large amounts of sediment. The ice flow is very slow and laminar, and carries suspended sediments of all sizes.


Air is a fluid of very low density and viscosity. The principles involved in wind transport are similar to those present in water transport; however, the low density and viscosity of air reflect at different thresholds for particle transportation. Usually, air is capable of transporting in suspension only particles below the fine sand size, while coarser sediments are transported by rolling and saltation. Transport occurs at relatively high speeds, in a turbulent way. Like water, wind transport results in the deposition of strata that occur in ripple size up to dunes many meters high.

Gravitational flows

The action of gravity can transport sediments in subaerial or underwater conditions. In gravitational flows, it is common that 20 to 70 percent of the sediment is transported in suspension.

Through gravity, grain flows can occur, in which part of the grains are suspended by contact interactions between the grains. It is common for grain flows to occur on the sliding face of wind dunes. Highly concentrated sediment and water mixtures can generate mud flows or debris flows. In this case, coarse particles are supported by a mud matrix, which has cohesive strength. This characteristic makes the flow less predictable, with non-Newtonian characteristics.

Liquefied or fluidization flows are a type of gravitational flow in which the grains are held in suspension by the upward movement of the interstitial fluid that escapes with the gravitational settlement of the grains, or by forced ejection of the fluid filling the pores. This occurs in shallow-buried sediments, which can behave as fluids after a sudden shock, which causes an instantaneous loss of contact between the grains thereafter suspended in the fluid.

Turbidity currents are a type of density current that occurs at the bottom of a sea or lake, consisting of turbid mixtures of variable density of sediments temporarily suspended in water. They are less dense than the debris flows, with a relatively high Reynolds number. The result of the deposition of sediments transported by this type of current are the turbidites. Turbidity currents slow down over time and as they move away from their source, resulting in deposition. Since the coarser suspended materials are the first to be deposited, the turbidites have a coarsening upward character.

Turbidity currents from low to medium density ideally form a sequence known as the Bouma Sequence, consisting of 5 divisions. This sequence is represented by poorly selected and massive sands at base (a), overlain by laminated sands of upper flow regime (b), sands with cross lamination (c), laminated silt (d) and hemipelagic mud at the top (e).

High-density turbidity currents, in which the density is greater than 1.1g cm-3, form thick deposits of coarse texture and massive structure at their base, as a result of the interaction between particles. The top of the deposit is characterized by fluid escape structures and is more similar to the Bouma Sequence, representing deposition from lower density flows.

Figure 6: Turbidites are deposited from sediments usually mobilized by underwater avalanches.

Deposition of clastic sediments

Sedimentary particles are deposited when the transportation agent loses competence to carry them or when the force that causes the movement is cancelled. Loss of competence for water or air transportation may be related to decreased flow velocity. In the glaciers, the deposition occurs with the stagnation or retention of the glacier, which may occur due to increase in the melting rate, or decrease of snow accumulation rate. Deposition rates in the different environments are very variable, and usually present values from 3 millimeters per year, in abyssal seabeds, to tens of meters per year in deltaic regions, in which the sediment supply is very high.

The deposition of sediments transported by gravity, water, wind or ice can occur along several transportation cycles. Deposition may be temporary (when the sediment moves repeatedly) or permanent (when the deposited sediments remain immobilized and are buried).

Sedimentation may be episodic or continuous. Episodic sedimentation is characterized by processes of great magnitude separated by long periods of non-deposition. It occurs when an instantaneous depositional process is triggered. This makes estimating the deposition rate quite complex, so that an annual estimate cannot be used. Continuous sedimentation is associated with virtually constant depositional conditions over long periods of time. In the abyssal sea floor, the continuous sedimentation results in the slow accumulation of fine particles. However, in this environment, turbidites deposits may accumulate during events of episodic deposition. It is common, therefore, that events of episodic deposition represent an important part of deep marine sequences.

The superposition of depositional events results in the vertical stacking of layers that record the processes involved in sediment deposition. These sequences can be seen in outcrops or cores, and are of extreme importance for the characterization of depositional environments.

Figure 7: Marginal marine sequence of siltstones and limestones of the Virgin Formation, southwestern Utah, USA. Source: link

The description of sedimentary layers recorded in cores and outcrops can be performed efficiently through Strataledge®, a software that allows their complete and systematic description, comprising the types of rock, minerals and other constituents, textures, structures, thickness and types of contact between beds. With this tool, all the data is acquired in a digital and organized way. In addition, the system allows an easy integration with petrographic data, geophysical logs, photos and other media.


  1. Nichols, Gary. Sedimentology and stratigraphy. John Wiley & Sons, 2009.


  • Elias Cembrani da Rocha – Endeeper