These hydrocarbons were formed when marine animals and plants (zooplankton and phytoplankton) died and sank to the bottom of ancient seas and were covered with sediments in an anoxic environment and converted over many millions of years at high temperatures and high pressure to their current form.
ðiːz hˈaɪdɹəkˌɑːɹbənz wɜː fˈɔːɹmd wɛn mɚɹˈiːn ˈænɪməlz ænd plˈænts zˈuːplæŋktən ænd fˈaɪɾəplˌæŋktən dˈaɪd ænd sˈæŋk tə ðə bˈɑːɾəm ʌv ˈeɪntʃənt sˈiːz ænd wɜː kˈʌvɚd wɪð sˈɛdɪmənts ɪn ɐn ænˈɑːksɪk ɛnvˈaɪɹənmənt ænd kənvˈɜːɾᵻd ˌoʊvɚ mˈɛni mˈɪliənz ʌv jˈɪɹz æt hˈaɪ tˈɛmpɹɪtʃɚz ænd hˈaɪ pɹˈɛʃɚ tə ðɛɹ kˈɜːɹənt fˈɔːɹm.