Price L C, Schoell M
US Geological Survey, Denver Federal Center, CO 80225, USA.
Nature. 1995 Nov 23;378(6555):368-71. doi: 10.1038/378368a0.
It is widely accepted that natural gas is formed from thermal decomposition of both oil in reservoirs and, to a lesser extent, the organic matter in shales from which the oil was derived. But laboratory pyrolysis experiments on shales do not reproduce the methane-rich composition typical of most gas reservoirs, leading to suggestions that other mechanisms, such as transition-metal catalysis, may be important. The discrepancy might, however, instead arise because gas (and oil) deposits have migrated from their source rocks, so that the reservoir composition might not be representative of the composition in the source rocks where the hydrocarbons were generated. To address this question, we have analysed gas samples coproduced with oils directly from a source rock (the Bakken shales, North Dakota, USA) where the local geology has prevented significant hydrocarbon migration. The methane contents of these Bakken-shale gases are much lower than that of conventional gas reservoirs, but are consistent with that from pyrolysis experiments on these shales. Thus, because these Bakken gases form with (rather than from) oils, we argue that compositional differences between gases from source rocks and conventional gas deposits result from fractionation processes occurring after hydrocarbon expulsion from the source rock.
人们普遍认为，天然气是由储层中的石油以及程度较轻的页岩中的有机物质热分解形成的，而页岩正是石油的来源。但是，对页岩进行的实验室热解实验无法重现大多数气藏典型的富含甲烷的成分，这使得人们认为其他机制（如过渡金属催化）可能很重要。然而，这种差异可能反而源于气体（和石油）矿床已从其源岩中迁移出来，因此储层成分可能无法代表生成碳氢化合物的源岩中的成分。为了解决这个问题，我们分析了直接从源岩（美国北达科他州的巴肯页岩）中与石油共生产出的气体样本，当地地质条件阻止了大量碳氢化合物的迁移。这些巴肯页岩气的甲烷含量远低于常规气藏，但与对这些页岩进行热解实验得到的结果一致。因此，由于这些巴肯气是与石油一起形成的（而非由石油形成），我们认为源岩气体与常规气藏气体之间的成分差异是由源岩排出碳氢化合物后发生的分馏过程导致的。
