Under the heat (above 50 °C) and pressure of burial deep in the earth, the remains were transformed into materials such as bitumen, kerogen, or petroleum.
ˌʌndɚ ðə hˈiːt əbˌʌv fˈɪfti dᵻɡɹˈiːz sˈiː ænd pɹˈɛʃɚɹ ʌv bˈɛɹɪəl dˈiːp ɪnðɪ ˈɜːθ, ðə ɹᵻmˈeɪnz wɜː tɹænsfˈɔːɹmd ˌɪntʊ mətˈɪɹiəlz sˈʌtʃ ɐz bɪtˈuːmɛn, kˈɛɹədʒən, ɔːɹ pətɹˈoʊliːəm.