Download MA-1-6 The role of extrusive igneous rocks in exploration Andrew

MA-1-6
The role of extrusive igneous rocks in exploration
Andrew Racey
Andy Racey Geoscience, Hampshire, UK
Extrusive volcanic rocks occur in most geological settings and over the entire geological column
and can host hydrocarbons in a broad range of volcanic rock types. They are especially common
in rifts, back-arcs, fore-arcs and even foreland basins forming an average of 25% of basin fill.
Commercially producing volcanic reservoirs are geographically and stratigraphically widespread
and include: Upper Carboniferous to Palaeogene of China ( Songliao, Junggar, Erlian, Sichuan,
Santanghu and Bohai Bay basins); Cretaceous Campos Basin of Brazil; Permo-Triassic and
Jurassic of the Neuquen and Austral basins of Argentina; Palaeocene of the Cambay Basin, India;
Miocene-Pliocene of the Nigata and Akita basins of Japan; Miocene Petchabun Basin of Thailand;
Kura Basin of Azerbaijan and Georgia and also in the USA (Pliocene of Utah and Oligocene of
Nevada). To date there are over 300 global records of hydrocarbon discoveries and significant
shows in volcanic rocks of which around 170 have proven reserves. Currently around 60% of the
worlds conventional hydrocarbon resources are in sandstones, 40% in carbonates and <1% in
volcanics.
All aspects of the petroleum system can be affected by volcanism. Volcanic activity is often
associated with thermal doming and uplift which can trigger erosion and possible deposition of
sandstones if the surrounding uplifted geology is of suitable mineralogical composition. Such
activity is common in the older portion of many rift basins. These uplift events can occur several
times during a basins evolution and lead to the development sandstone reservoirs whilst volcanic
reservoirs may also develop where prolonged exposure, weathering and tectonism have caused
dissolution and porosity creation. The emplacement of thick lava sequences can affect drainage
patterns (sediment input points, access to different hinterland compositions etc) especially during
periods of volcanic quiescence within volcanic sequences. The presence of volcanic rocks can
also downgrade the reservoir quality of associated clastic reservoirs if eroded volcanic material is
incorporated into such sandstones since burial diagenesis and the associated alteration of
volcanic fragments to various clays such as chlorite can result in an overall reduction of reservoir
quality. This can happen even when the volcanic component of such sandstones is as low as 10%.
In interbedded volcanic/ sedimentary sequences it is often assumed that interbedded sediments
will be rich in eroded and weathered volcanic material and thus have poor reservoir quality.
However, such clastic sediments may be derived from extrabasinal sources of favourable
provenance (e.g. quartz-rich basement such as granites, acidic metamorphic rocks or older
sandstones) and may therefore contain little or no volcanic material thus improving the potential
reservoir quality of such sandstones. Understanding the regional topographic gradient and
composition (provenance) of the river catchment areas are critical in exploring for such sandstone
reservoirs within volcanic (lava) sequences. Basalts are more likely to deflect fluvial systems than
become be eroded and redeposited by them. The presence of palaeosols within lava sequences
may represent a large amount of time .
Volcanic lithologies can also form top, base and lateral seals to conventional (sandstone and
carbonate reservoirs) and to volcanic reservoirs.
Subaerial volcanism may create depressions in which lakes and swamps can form and where
organic material can accumulate to form a source rock. If such lake waters are warmed by
volcanism this can further encourage organic productivity (algal blooms etc) further enhancing
their source potential. Lacustrine intervals with source potential are noted for example in the
Deccan basalts of India (pers obs). Uplift and doming caused by deeper associated intrusions can
potentially isolate areas of water in both marine and lacustrine settings and thus also create areas
in which source rocks may develop especially where water circulation becomes restricted allowing
the development of anoxia and the preservation of organic material. Volcanic ashes can cover
large areas causing extinctions of plants/animals whose organic remains may accumulate and
during subsequent burial can potentially mature to form viable source rocks.
Migration of hydrocarbons into volcanic reservoirs is complex migration distances are generally
short. Consequently for volcanic reservoirs to be charged they need to be located close to a
mature source rock. CO2 is commonly associated with volcanic activity and may also play a role in
hydrocarbon migration in that it can displace lighter hydrocarbons. Volcanism can add heat and
thus accelerate source rock maturation and enhance fluid migration and may also destroy or
degrade earlier existing hydrocarbon accumulations.
Most producing volcanic reservoirs occur onshore and at a broad range of depths from a few
hundred metres to 5000 m with the deeper examples (those >3000 m tending to be located in
China and Japan). These fields are often located close to a potential source rock (i.e. there is a
short migration distance from the mature source to the volcanic reservoir). Many examples occur
in rift systems, presumably because this is where volcanism and potential rift source rocks (be
they lacustrine or marine) are most commonly juxtaposed e.g. in China. However, other
extensional settings such as back-arc and fore-arc basins also appear to be important where the
source rock tends to be dominantly deep marine e.g. in Japan.
The silica content of volcanic rocks affects their viscosity with more viscous, silica-rich lavas such
as rhyolites having a more limited lateral extent and often showing greater thicknesses than less
viscous (silica-poor) basalts which can extend laterally over much larger areas (assuming a flat
underlying topography) and tend to create thinner individual flows. In subaerially erupted
sequences the higher viscosity of the rhyolites make it more difficult to exsolve gas and this limits
the degree of vesicle development 9a major source of primary porosity). Less viscous rocks such
as basalts can exsolve more gas creating more vesicles and therefore tend to possess more
primary vesicular porosity. Moreover, the vesicular porosity in these basalts can be developed
over a much larger area with the best vesicular porosity developed in the lava flow tops. These
vesicular flow tops may be connected vertically by joints and/or fractures to form a potentially
extensive “layered” reservoir.
Volcanic rocks are less affected by compactional porosity loss than typical sandstone and
carbonate reservoirs due to their greater mechanical strength and, can therefore form potential
reservoirs in the deeper parts of basins where more conventional sandstone and carbonate
reservoirs are unproductive. Hydrocarbons are currently produced from acidic, andesitic and basic
(basaltic) lavas and volcaniclastics with most of the production coming from onshore fields. These
reservoirs often show a high degree of variation in porosity (1 - 35%) and permeability (0.1 - 250
mD and rarely > 1 Darcy). Volcanic rocks are often highly heterogeneous in terms of their rock
fabric and commonly possess complex pore architectures which are controlled by a range of
primary and secondary processes. The dominant controls on igneous porosity are primary
degassing structures (such as lava tunnels, caves, vesicles and pipes) which are typically
concentrated towards the flow tops; fractures (both primary i.e. cooling joints or quench fractures
and, secondary tectonic fractures) and subsequent secondary dissolution and alteration during
weathering and/or burial. Reservoir development is complex and also depends on the original
silica and volatile contents of the magma which affect viscosity and degree of subsequent
alteration/fracturing especially if rapidly quenched by water and the degree of primary vesicle and
vug development respectively.
It is clear that the presence of volcanic rocks in a region should not always condemn an area from
hydrocarbon exploration but it does require applying different play concepts and exploration
methods. Many volcanic sequences in sedimentary basins are either unexplored or underexplored
and thus they can afford a significant upside exploration potential in their own right as well as
providing other elements to the petroleum system.