hey guys ,
I recently finished my masters in oil and gas engineering , i want to start going for a phd but in a different field or a related one , but i don't know what majors i can switch to , i also did my BSc in petroleum engineering so i am confused in what major my previous degrees allow me to apply.
I tried speaking to professors , one in earth science and one in mathematics both of them said that i dont have the equivalent degree for these majors, what are my options ?

Are there any reasons why you aren't so keen to pursue a PhD in a field related to Oil and Gas Engineering? In any case, most PhD projects in engineering (and science) these days tend to be multi-disciplinary in nature. With your background, for instance, it's quite possible to pursue a PhD in areas such as chemical engineering and mechanical engineering under the sub-fields like CFD, combustion, heat transfer, fire engineering. These fields I have mentioned are multi-disciplinary and research groups in most universities dealing with these areas would typically have researchers from various backgrounds such as engineers, physicists, chemists, mathematicians etc. Both Earth Science and Mathematics are quite diverse fields. For instance, there's Applied Maths and Pure Maths. Someone with an engineering background may pursue a PhD in Applied Maths, but highly unlikely to be accepted in Pure Maths. Equally, there are certain fields within Earth Science which closely align with Oil and Gas engineering, others are diametrically opposed. Professors (especially in the UK system) wouldn't accept someone who would struggle and has no basic knowledge of the field. But I understand in the US, it works differently because PhD students there do some coursework for two years or so. The coursework can bridge the knowledge gaps.