文|Grant Cox Research Fellow,University of Adelaide
文|Alan Collins Professor ofGeology, University of Adelaide
翻译：赵晗；审校：潘燕婷
导语：澳大利亚发现大量天然气资源，揭示古海洋特性。
所谓的“非传统”油气资源存储于页岩中，这是一种具有层次的、纹理清晰的岩石。
在澳大利亚北部地下两到三千米埋藏着能源。近期这些名为Velkerri Shale的层状岩石被估计蕴藏有超过118万亿立方英尺的天然气。
尽管这些天然气储备量确实很大，但更值得注意的是其年代。这些岩石沉积于14亿年前的罗珀海道。这些岩层及其包裹的油气比通常发现油气资源的岩层早十亿年。
那些构成油气的碳氢化合物（因其由氢原子与碳原子构成）是古海洋中细菌长期分解后的残留物。
它们是迄今为止发现的最不同寻常的碳氢化合物。不寻常是指包含它们的岩层类型以及它们的年代。这些古物给予我们罕有的机会去通过细菌残留来研究1400百万年前古海洋的化学构成、古代大气的构成以及生命的特性。
包含Velkerri页岩的盆地的剖面图。剖面图为南北方向，位于澳大利亚北部Tennant Creek和Katherine之间。
古海洋
近期通过研究在Velkerri Shale所提取的有机物中的稀有元素，尤其是铈（Ce）和钼（Mo），我们对古海洋环境有了许多新的认识。铈和钼可以作为指示物指出14亿年前大海中有多少氧气。研究展示出一个甚至在表面都极度缺氧的海洋。在海洋的更深处则是富含硫化氢的有毒环境。
这些研究表明地球大气在那时是缺氧的，含氧量可能低于3%。而现在大气含氧量为21%，并且海洋也富含氧气。
我们的同事对从同一岩石中提取生物产生的分子进行了进一步研究。这些“生物标记物”展现出一个被细菌主导的海洋。
但为什么Velkerri岩层的年代如此不寻常？这很大程度上与油气的产生和保存的一系列特殊事件有关。
油和气如何在地下产生
油气产生于黑色页岩。这些岩层富含有机物（曾经是有生命的物质）。在这种情况下，有机物是细菌分解的残骸。
将有机物转化为油或气需要热量，但不能太多。岩层需要在数百万年间小心翼翼地被持续加温至60度与160度之间，这通过覆盖岩层来实现。
Velkerri黑页岩 这种岩石含有大量油气以及关于曾经环境的地球化学线索。
地球越往深处越热，这点矿工们很清楚。世界上最深的矿井（在南非）在地下3.4千米处，那里岩石温度达到了惊人的60度。
油和气产生于稍微更深处。这些温度被称为油气“窗口”。如果有机物没有达到这个温度窗口，油气则不会产生。
躲避碰撞的板块
板块碰撞和地表移动的岩层是油气贮存的敌人。
许多地表形态都与地质板块有关。在超大陆聚集时期，大型板块碰撞的发生十分偶然。大规模的板块融合发生周期为6亿年。
但如果存储有油气的黑页岩在板块边缘，通常会由于埋得太深而加热过多并遭到破坏。
Velkerri页岩存储有大量油气意味着这些岩石在14亿年间没有经受过160度以上的高温。这非常值得关注，因为澳洲那时处于罗迪尼亚超大陆和盘古大陆聚合处。
我们使用的大多数油气能源产生于5亿年内的岩石，而其中大多数少于3亿年。较新的岩石还没有那么长时间，因此不太可能被夹在碰撞的板块之间。
这些年轻的油气资源形成于寒武纪大爆发之后——一个动植物资源极其丰富的时代。
重新考虑“石油峰值”
就目前的消耗率来看，Velkerri页岩可以为澳大利亚提供90年的天然气资源。虽然只有一部分天然气可开采，但其估值为118万亿立方英尺的陆上非传统页岩气资源将使得这个国家成为世界上第一大天然气出口国。
油气资源为非可再生资源。“石油峰值”或者“碳氢化合物峰值”是指当达到最大的油气开采率的时间。但鉴于非传统油气的技术限制以及人们意识到天然气可以从十分古老的岩石（比如Velkerri页岩）中开采，这意味着碳氢化合物峰值的到来可能被推迟。
非传统油气开发技术仍旧充满争议。博学者的公开辩论将最终决定是否开发这种页岩。
如果Velkerri页岩从勘探转入生产，我们将使用产生于一个“黏液世界”中的天然气，那是一个近十亿年前地球上还没有进化出复杂生物的世界，那时细菌主导着海洋，大气还缺乏氧气。
此工作与后续工作由以下机构联合完成：Adelaide大学，South Australia大学，Wollongong大学，捷克科学院，北方领土地质调查，能源起源和Santos。本文观点仅代表作者，不代表《科学美国人》。
原文
https://theconversation.com/a-time-capsule-containing-118-trillion-cubic-feet-of-gas-is-buried-in-northern-australia-80268
A time capsule containing 118trillion cubic feet of gas is buried in northern Australia
Two to threekilometres beneath the surface of Australia’s Northern Territory sits buriedenergy. The layered rock formations known as the Velkerri Shale were recentlyestimated to contain over 118 trillion cubic feet of gas.
While these gasreserves are clearly large, what is really remarkable is their age. These rockswere deposited 1,400 million years ago, in an ocean known as the Roper Seaway.These rocks, and their encased oil and gas, are about a billion years older thanrocks where oil and gas are usually found.
The molecules thatmake up the oil and gas – called hydrocarbons because they consist of hydrogenand carbon atoms – are the long-decomposed remains of dead bacteria thatinhabited ancient oceans.
These are the mostunconventional hydrocarbons yet discovered; “unconventional” because of thetype of rock they’re contained in, and their age. This antiquity gives us arare chance to use the remains of the bacteria to examine the chemistry of theancient oceans, the composition of the ancient atmosphere, and the nature oflife 1,400 million years ago.
Ancientoceans
Recently we havelearned a lot about Earth’s ancient marine environment.
This has been achievedby analysing rare elements, particularly cerium (Ce) and molybdenum (Mo)extracted from the once-living organic matter within the Velkerri Shale. Ce andMo act as indicators of how much oxygen was available in the oceans 1,400million years ago. The studies reveal an ocean starved of oxygen, even insurface waters. At deeper depths this ocean was completely toxic, rich inhydrogen sulfide.
These results indicatethat Earth’s atmosphere at this time was oxygen-poor. In fact, it is likelythat it contained less than 3% oxygen. Currently we enjoy 21% oxygen in Earth’satmosphere and a largely oxygenated ocean.
Further work by ourcolleagues has focused on extracting molecules of biological origin fromthe same rocks. These “biomarkers” have revealed an ocean dominated bybacteria.
But why is the age ofthe Velkerri Shale so unusual? It’s got a lot to do with the unique sequence ofevents that need to occur to produce and preserve oil and gas.
Howoil and gas forms underground
Oil and gas are producedfrom rocks known as black shales. These rocks contain high levels of organicmatter (materials that were once living); in this case, the organic matter isthe decomposed remains of bacteria.
To convert organicmatter to oil or gas requires heat, but not too much: you need to delicatelycook the rocks to temperatures between 60℃ and 160℃ for a few million years.You do this by burying the rocks.
Earth gets hotter asyou go deeper, something miners know well. In fact, the deepest mine in theworld (in South Africa) goes down 3.4 kilometres, and the rocktemperatures down there are an incredible 60℃.
Oil and gas form whenthe rocks get a little deeper than this. These temperatures are often referredto as the oil and gas “window”. If the organic matter does not reach thistemperature window, oil and gas are not produced.
Avoidingcolliding plates
The enemy ofpreserving ancient oil and gas is plate tectonics, or moving layers ofrock at Earth’s surface.
Much of the planet issculpted by plate tectonics. In fact, major continental collisions happen quiteepisodically, during the assembly of supercontinents. These massive continentalamalgamations occur in a regular 600-million-year cycle.
But if oil- andgas-bearing black shales find themselves at a plate margin, they often getburied too deeply, heating them too much and destroying them.
The fact that theVelkerri Shale contains vast quantities of oil and gas means that these rockshave not experienced temperatures above 160℃ for 1,400 million years. This isquite remarkable considering that Australia has been involved in major tectonicupheavals associated with the assembly of two supercontinents during thistime, Rodinia and Pangaea.
Most of the oil andgas we use as an energy source comes from rocks less than 500 million yearsold, and the vast majority are less than 300 million years old. Young rockshaven’t been around that long, so are less likely to have been sandwichedbetween colliding plates.
These young oil andgas reserves formed after the Cambrian explosion, at a time when plant andanimal life was abundant on Earth.
Rethinking“Peak Oil”
The Velkerri Shalecontains enough gas to power Australia for more than 90 years, at currentconsumption rates. While only a proportion of this gas would be recoverable, aresource estimate of 118 trillion cubic feet forms a large onshoreunconventional shale gas reserve in a country soon to be the largestgas exporter in the world.
Oil and gas arenon-renewable resources. The concept of “Peak Oil” or more broadly “PeakHydrocarbons” refers to the point in time when the maximum rate of oil and gasextraction is reached. However, the technological development of unconventionaloil and gas, and now the realisation that gas can be sourced from extremely oldrocks, such as the Velkerri Shale, means that the arrival of “PeakHydrocarbons” may be further delayed.
The development ofunconventional oil and gas remains contentious, and well-informed publicdebate will ultimately decide whether such shale gas resources are developed.
If the Velkerri Shalemoves from exploration to production, we will be making use of gas produced ina “slime world” that existed nearly a billion years before the first complexlife on Earth evolved, where bacteria ruled the seas and the atmospherewas largely devoid of oxygen.
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