They were covered by mud, buried deeply over time, and gently cooked into oil by geothermal heat at a temperature of .
ðeɪ wɜː kˈʌvɚd baɪ mˈʌd, bˈɛɹid dˈiːpli ˌoʊvɚ tˈaɪm, ænd dʒˈɛntli kˈʊkt ˌɪntʊ ˈɔɪl baɪ dʒˌiːoʊθˈɜːməl hˈiːt æɾə tˈɛmpɹɪtʃɚɹ ʌv .