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It is said that most of the fossil fuels in existence today (coal, oil, gas) came from the Carboniferous period. A unique set of circumstances lead to the creation of fossil fuels, which depends on anaerobic decomposition:

  • Plants evolved bark as an effective defense against insect herbivores
  • Very few organisms at the time could digest or decompose the bark fiber lignin

Suppose that the Carboniferous period never happened, or for whatever reason the period never produced fossil fuels. How much fossil fuel would we have today instead? How much of today's fossil fuel deposits were created during the Carboniferous?

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  • $\begingroup$ This might be true for coal (though probably overstated) but oil does not derive from land plants, it has a marine origin (hence the offshore oil rig). $\endgroup$
    – plannapus
    Commented Sep 18, 2014 at 6:44
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    $\begingroup$ @plannapus: Offshore rigs are offshore because that's where the oil and gas are today. The source rocks that produced the hydrocarbons are often continental (lacustrine, say), as they are often from a relatively early phase of the basin's history, such as early rifting in a rift basin or passive margin's case. $\endgroup$
    – Matt Hall
    Commented Sep 18, 2014 at 12:07
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    $\begingroup$ Just as an example, most of the oil from the northern North Sea is Jurassic and was deposited in marine mudstones. The point of my previous comment was that the phenomenon the OP was describing is the one producing specifically coal, not all fossil fuel. $\endgroup$
    – plannapus
    Commented Sep 18, 2014 at 13:08

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The process you're describing in the question is the one at the origin of coal. Although coal deposits are known from the Devonian to the Quaternary, they have been 3 major periods of depositions (see Thomas 2013): the Carboniferous-Early Permian, the Jurassic-Cretaceous and during the Cenozoic. So, indeed the Carboniferous is one of the main period of coal deposit but I couldn't find any numerical evidence in the literature that this is the "main" period of deposit. The idea that it is is most likely due to the fact that the western Europe and eastern USA coal mines (hence historically the most noteworthy) all happen to be Carboniferous.
But as far as oil (petroleum) is concerned, according to Klemme & Ulmishek 1991 (straight from the abstract):

Six stratigraphic intervals, representing one-third of Phanerozoic time, contain petroleum source rocks that have provided more than 90% of the world's discovered original reserves of oil and gas (in barrels of oil equivalent). The six intervals are (1) Silurian (generated 9% of the world's reserves), (2) Upper Devonian-Tournaisian (8% of reserves), (3) Pennsylvanian-Lower Permian (8% of reserves), (4) Upper Jurassic (25% of reserves), (5) middle Cretaceous (29% of reserves), and (6) Oligocene-Miocene (12.5% of reserves)

See Klemme 1994 for a detailed explanation on the origins of the Jurassic source rocks (i. e. 25% of the world's reserve at the time): most of them are marine, but some are fluvial or from lagoonal inland seas (table 3.2).

References:
Klemme, 1994. Petroleum systems of the world involving Upper Jurassic source rocks. In Maggon & Dow (Eds), The petroleum system - from source to trap, AAPG Memoir, 60: 51-72.
Klemme & Ulmishek, 1991. Effective petroleum source rocks of the world; stratigraphic distribution and controlling depositional factors AAPG Bulletin, 75: 1809-1851.
Thomas, 2013. Coal Geology, 2nd Edition. Wiley-Blackwell, 444pp.

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