The Wildest Plays On The Planet! Huge Exploration Potential
The vast majority of the world’s hydrocarbons have historically been discovered in what are termed conventional settings. These are often structural closures into which oil or gas has migrated, becoming trapped in layers of porous, reservoir sandstone or limestone beneath an impervious cap rock. Stratigraphic traps in which similar reservoirs are encased in shales make up a smaller proportion of successful hydrocarbon plays, and in recent years we have seen the rise of a variety of unconventional plays.
But beyond these accumulations lies a subset of the truly “unconventional”, simply wacky plays, where the geologist needs an open mind to appreciate the “wildest plays on the planet”. They range from hydrocarbons hosted in meteor craters, in granites and in gigantic subterranean caves; reservoirs sealed by tar, by lava; and often unexploited subsurface resources like gas hydrates, nitrogen, helium and CO2. Finally there are some plays too “out there” even to take seriously.
Why look at them? Believe it or not, many of these play elements contribute to the world’s largest oil fields. Some have wildcat success rates exceeding 20%, and if nothing else, by examining these types of plays our explorationists may gain new insights into ways to think outside of the box. Just be careful when you float these ideas to your boss, as he or she may laugh you out of the room!
Meteor craters – making an impact
Around 250 impact craters have been identified around the world. Around a fifth of these have associated hydrocarbons. Typically the bedrock is fractured, and source rocks may build up in the crater lake, making this a working play. The largest such structure is Chicxulub, in the GOM. The shattered carbonate breccia associated with the impact on the K-T boundary is complimented by the sealing ejecta layer, which enabled the aging supergiant Cantarell Field of Mexico to produce more than 2.1 Million barrels per day at peak production.
A further nine astroblemes are known to be producing in North America alone. One of these, the Avak Crater, is located in Barrow in Alaska, and was drilled by the army to fuel the military base there. The concussion that made the crater created folds in surrounding Silurian rocks that trapped natural gas beneath impermeable cap rocks. Another such feature is the Ames Crater, which was discovered in the Sooner Trend in Oklahoma in 1991, based on some exceptionally productive wells. When examined, cuttings included brecciated granite with good shows, as well as shattered quartz and feldspar with cleavage faces. The crater is more than 10 km across, and is believed to have been caused by a meteorite impact. The Viewfield Field in Saskatchewan is considered to represent an impact crater, as is Tookoonooka in Queensland, Australia.
Fractured basement – broken dreams
More than 30 countries have fields that produce from igneous, volcanic or metamorphosed basement. Incredible to imagine oil and gas being produced from granite, yet the Suban Field in Indonesia holds more than 8 Tcf of gas in fractured granites, while fields in Thailand and Libya also have extensive production from granitic basement. Production may exceed 20,000 bbl/d from extensive fracture networks. Hurricane Energy have recently drilled successful wells into tonalite basement in the West Shetlands. These plays have contributed to dubious theories of abiotic (no organic source rock) oil.
Typically these reservoirs are more difficult to evaluate than conventional reservoirs, and are often discovered by chance rather than by design. However in Russia and Thailand drilling into crystalline basements has been carried out intentionally, and I strongly suggest deepening any wells where the reservoir is in contact with the source rock. Generally a high proportion of deep seated fractures are subvertical, so horizontal wells may be key to unlocking these resources.
Caves filled with oil
I think that we have all been in that conversation where it turns out that laymen imagine that oil sits in giant caves underground, and all you need to do is poke a “straw” into them. Well, in several offshore fields in southern Italy, that is correct. The reservoir is a heavily karstified, carbonate platform i.e. the limestone platform was deposited in the Jurassic, and then much later (end Cretaceous) was exposed at surface, leading to the development of an extensive subterranean cave system, into which the oil migrated. The offshore Rospo Mare Field has a UR of 94 MMbbl. Exposures of the same rocks onshore show similar cave systems.
Gas hydrates – seafloor ice
These are naturally occurring, crystalline, ice like frozen gas molecules, typically found in oceanic continental slopes. They are typically stable 100 to 500 m below the seafloor. It is estimated that an incredible 43,000 Tcf of methane is locked up in these deposits. However this is a Goldilocks play, requiring the right lithology, formation temperature and pressure, salinity, water availability, gas source, concentration and transport in rider for the play to work. Even when everything is working, we have not yet worked out how to produce this gas commercially. Most of the reserves are around the US coast, but Japan leads the way in trying to access these hydrocarbons, but they clearly have a long way to go.
Carbon dioxide and Nitrogen – ice cream, anyone?
There are a surprising number of wells producing CO2 in the US, many of them in Utah. The farnham Dome produces from the Navajo Sandstone, and has produced more than 5 Bcf. The wells are called “ice cream wells”, partly because of the dry ice that builds up around the well heads, and also because the CO2 may sometimes be used to make ice cream. The CO2 from some wells is used to make dry ice.
The Kabir Kuh anticlinal structure in the Zagros Mountains in Iran is around 220 km long, and 85% of the fill is nitrogen, hosted in Ordovician and Permian reservoirs. While not of commercial significance (nitrogen makes up 78% of the air around us), this is a fascinating feature, which may relate to an underlying hot spot or radioactive basement. High nitrogen is often associated with high helium concentrations (see below).
Helium – goodbye balloon animals?
Of all the plays in this article, I believe that helium has the greatest commercial potential. Helium is rare and valuable, with possibly only 25 years of supply remaining worldwide. It is formed from the breakdown of radioactive elements, and trapped in overlying natural gas reservoirs. Anything over 0.3% is commercially viable, with fields like Dineh-bi-Keyah in Arizona hosting a helium content of around 6%. The Federal Helium Reserve near Amarillo, Texas, holds the largest reserves; it was originally set up to fuel airships. It is currently being privatized, and the helium sold off. Many South African gold mines, with high associated uranium content, may have significant concentrations of helium in the groundwater. I like to envisage a helium mine where you can recognize the miners by their very high voices!
Gilsonite and Tar mats – colour me black
You touch gilsonite every day of your life. It is a natural, resinous hydrocarbon left after volatiles have escaped, and is commonly found within the Uinta Formation in Colorado and Utah. It typically occurs as vertical seams, many miles long and up to 500 m deep. It is used as photocopy black and for newsprint, and was also famously used to make the lacquer for the Model T Fords; the marketing slogan for the vehicles ran “you can have any colour you want, as long as it is black”.
Tar mats usually form at the oil:water contact, at the interface between the oil and water beneath. This is extra heavy oil, and is completely immobile. However if hydrocarbon migration is staggered over time, they can form incredible seals, nowhere more striking than in the world’s largest single accumulation of heavy crude, in the Orinoco Belt of Venezuela. The world’s fourth largest field, Bolivar Coastal Field (30 BBbbl), is sealed in this way.
Volcanic seals
Another fascinating seal occurs in the offshore Kudu Field in Namibia. Cretaceous sand dunes of the Twyfelfontein Formation are capped by lava, which outpoured during the opening of the Atlantic. The dunes host around 1.2 Tcf of gas, with a basalt top seal, and similar fields have been discovered in the Parana Basin on the South American side of the Atlantic rifting.
You can’t be serious
We finish with some truly crazy hydrocarbon stories from around the world:
• So called “eternal flames” at various locations in the Middle East are typically gas seeps, along with places like Naft Safid in Iran (“white springs” in Farsi), where light oil is seeping.
• Gateway to Hell, Turkmenistan. A well drilled for oil in 1971, by the Russians, produced copies quantities of methane. For reasons unknown the Russians set it alight, and it has burned for 40 years, creating a crater 70 m across.
• Several mines in South Africa are producing gold from carbon seams (e.g. Carbon Leader). The carbon is thought to represent degraded oil around 2.2 Billion years old. Gold concentrations can reach 30 kg/tonne, that is around $440,000/barrel!
Summary
There is another world out there beyond the conventional, in fact beyond the unconventional. However many of these plays are potentially commercial, and often underexploited. I recommend that all of you keep your eyes open for the “big one” that no one else has thought of, however crazy it may appear at first sight. I hope that you had as much fun reading this as I did while researching it 
石油圈
