dike, Oil & Gas Glossary

A dike is a sheet-like body of igneous rock that cuts discordantly across the bedding or structural fabric of the rock it intrudes, typically standing vertically or steeply inclined where the host strata lie flat. The defining feature is discordance: a dike slices across preexisting layers rather than slipping between them, which separates it from a sill, the concordant intrusion that follows bedding planes. Dikes form when magma is driven under pressure into fractures and zones of weakness in the crust, then chills and crystallizes in place. Because the magma exploits planar openings, the resulting body is tabular, often only centimetres to a few metres thick but traceable for hundreds of metres to kilometres along strike and down dip. The original definition that a dike can form from both igneous and sedimentary material is worth keeping in mind: most dikes are igneous, but clastic or sedimentary dikes also occur, created when liquefied sand or mud is forcefully injected upward into overlying beds during rapid loading, seismic shaking, or overpressure. In petroleum geology the igneous dike matters less as a rock curiosity and more as a control on the plumbing of a reservoir. A dike of fine-grained basalt or diabase chills against the cool host rock and develops very low matrix permeability, so it behaves as a flow baffle or a complete seal that can carve a single mapped reservoir into separate pressure compartments. Operators that overlook this risk drilling a development well into a fault block that looks continuous on a structure map yet is hydraulically isolated, leaving stranded reserves on the far side of the intrusion. Dikes also rewrite the thermal history of the rocks they touch. Magma can exceed 1,000 degrees C (about 1,830 degrees F), and the narrow aureole of baked rock flanking the intrusion experiences a short, intense pulse of heat that accelerates thermal maturity in any nearby source rock, sometimes overcooking it to dead carbon or, conversely, locally generating hydrocarbons in an otherwise immature interval. The same heat can bake coal seams into natural coke and convert mudstone into hornfels. Reading a dike correctly therefore means treating it as three problems at once: a geometric barrier to flow, a localized thermal anomaly, and a potential conduit that the same fracture network may once have provided for migrating fluids before the magma froze it shut.

Key Takeaways

Discordant by definition: A dike cuts across bedding and structural layering, which distinguishes it from a concordant sill that intrudes parallel to strata. The cross-cutting relationship is also a relative-age tool: the dike must be younger than every layer it slices, a principle exploration geologists use to date intrusive events against the sedimentary section in seismic and well data.
Compartmentalizes reservoirs: Chilled fine-grained dike margins develop near-zero matrix permeability and act as lateral flow barriers. A network of interconnected dikes and sills can fragment a contiguous reservoir into isolated pressure compartments, reducing recovery efficiency and leaving stranded hydrocarbons. This is a recognized subsurface risk wherever intrusive complexes cut prospective sedimentary basins.
Resets local thermal maturity: Magma temperatures above 1,000 degrees C (1,830 degrees F) bake a contact aureole, spiking source rock maturity over a zone roughly one to two dike-widths on either side. The effect can overmature kerogen to inert carbon or, in immature sections, drive a localized burst of hydrocarbon generation. Vitrinite reflectance profiles across the contact quantify the disturbance.
Forms from both rock types: Most dikes are igneous, crystallized from injected magma, but clastic (sedimentary) dikes also exist where overpressured, liquefied sand or mud is injected upward into fractures during seismic or loading events. Clastic dikes can be reservoir quality themselves or signal paleo-overpressure and seal failure in the host section.
Seismic and aeromagnetic signatures: Diabase and basalt dikes carry strong magnetic susceptibility, so they often appear as linear highs on aeromagnetic surveys before any well confirms them. On seismic, dikes are harder to image than sills because their steep, thin geometry scatters energy rather than producing a bright concordant reflector, making integrated potential-field and seismic interpretation essential.

How a Dike Compartmentalizes a Producing Reservoir

When a vertical diabase dike cuts a sandstone reservoir, the chilled contact zone seals tightly against the host sand and reduces cross-dike permeability to a fraction of a millidarcy. Production from a well on one side draws the pressure down locally, but the dike prevents pressure communication with sand on the far side. Field engineers detect this through pressure-transient analysis: a sealing-boundary response on a buildup test, mismatched initial reservoir pressures between offset wells, or fluid contacts that sit at different depths across the mapped structure. Once the barrier is confirmed, the development plan must place a producer in each compartment rather than relying on a single well to drain the whole pool, which directly changes well count, capital cost, and booked reserves.

Contact Metamorphism and Source Rock Effects

The heat carried by an intruding dike does its work fast and close. Thermal modelling shows the aureole width scales with dike thickness, so a two-metre dike bakes roughly two to four metres of host rock on each flank before the magma chills below reaction temperature. In a source rock, that pulse can push thermal maturity from the oil window straight into the gas or overmature zone, measurable as a sharp spike in vitrinite reflectance against the contact. Where the regional section is immature, the same heat can locally generate and expel small volumes of hydrocarbons, creating an anomaly that an unwary interpreter might mistake for a basin-wide charge event rather than a metres-wide intrusive overprint.

Fast Facts

The Mackenzie dike swarm of the Canadian Shield, emplaced about 1.27 billion years ago, fans out across more than 2.7 million square kilometres of northern Canada and ranks among the largest dike swarms on Earth, with individual dikes traceable for hundreds of kilometres. By contrast, the Western Canadian Sedimentary Basin is remarkably free of major igneous intrusion, which gives the basin a clean, burial-driven thermal history. That absence is precisely why the kimberlite dikes feeding the Fort a la Corne diamond field in central Saskatchewan stand out as such notable exceptions.

A dike is best understood alongside the sill, its concordant counterpart that intrudes along bedding rather than cutting across it, since the two often occur together in a single intrusive network. Both create permeability barriers, the property that governs whether fluids can cross the intrusion at all. The heat a dike carries drives thermal maturity in adjacent rock, the process that converts kerogen in a source rock into producible oil and gas. Reading these four terms together explains both the trap-bounding and charge-altering roles a single intrusion can play.

Real-World WCSB Scenario: A Kimberlite Dike in the Saskatchewan Section

An operator drilling a Mannville gas prospect near the Fort a la Corne kimberlite field northeast of Saskatoon encounters an unexpected altered, magnetic interval the aeromagnetic survey had flagged as a linear high. Coring confirms a steeply dipping kimberlite dike roughly 3 m wide cutting the Cretaceous section, with a baked, low-permeability contact aureole on each side. The dike isolates the targeted Mannville sand from the offsetting pool mapped 400 m to the east, explaining a 1,100 kPa (about 160 psi) pressure difference between the two. Rather than the single development well budgeted at roughly 2.8 million CAD, the team must drill a second well across the barrier to access the eastern compartment.

The added well raises development cost by close to 2.6 million CAD but recovers an estimated 280 e3m3 (about 9.9 MMcf) of otherwise stranded gas, turning a geological surprise into a defensible reserves addition once the compartmentalization is mapped and booked.