Geochemical Atlas of Oils and Source Rocks and Oil-Source Rock Correlations: A Case Study of Oil and Ggas Fields in the Mangyshlak Basin (Kazakhstan)

Abstract

This paper presents the results of fingerprinting and biomarker analysis of 183 crude oil samples derived from 19 oil and gas fields in the South Mangyshlak basin and Rock -Eval pyrolysis performed on 93 core samples from 12 fields of this basin. Geochemical atlas was made based on the oil fingerprinting studies, according to which, the studied fields of the South- Mangyshlak basin can be divided into 3 groups: same type of oils on the scale of groups of fields. This group includes Permian-Triassic oils of the Karaginskaya Saddle (Northern Karagiye, Atambay, Alatyube, Ashchiagar) and the Peschannomyssko-Rakushechnaya uplift zone (Oymasha), which have very similar oil fingerprinting either within each field or on the scale of these fields. oils of the same type within the same field. The second group includes oils from Pridorozhnoye, Airantakyr, Burmasha and Bekturly fields, each of which has its own unique oil fingerprinting of light aromatic hydrocarbons. different types of oils within the same field. This group includes oils from the rest of the studied Zhetybay- Uzen steps such as Asar, East Zhetybay, South Zhetybay, Aktas, Turkmenoy, Tasbulat, Uzen and Karamandybas fields, since within each of these fields were identified several different groups of oils that require a detailed study of their biomarkers to determine their genetic origin and trace hydrocarbon migration pathways. The obtained results of oil fingerprinting are consistent with their biomarker compositions, according to which the oils of the first group, identified by their fingerprinting, were generated from marine shaly source rocks, while those of the second group were from shaly source rocks, deposited in whether marine or lacustrine environments. The biomarker composition of the third group oils exhibited that the oils within each of these fields have at least two genetic sources: the oils of the lower pay zones have shaly marine OM source, while those of the upper upper pay zones were formed by OM deposited in lacustrine environment. Their biomarkers clearly show a regular change in the contribution of lacustrine OM input to marine one with the depth increase of pay zones and within the field of this group thermally more mature oils of a marine source (23-24 horizons Uzen and Karamandybas, J-10 and J-11 fields of Asar, Zhetybay, Zhetybay East) were mixed with relatively lower mature oils of the upper lacustrine source (13-16 th pay zones in Uzen and Karamandybas, J-J-3a, J-4a in Asar field, J-5 in Zhetybay, J-1 in Zhetybay East) with their upward migration. To characterize source rocks, where the studied oils were generated, Rock-Eval pyrolysis was performed on core samples of different ages, which revealed that in reality the reservoirs of the Asar, Zhetybay, Karamandybas, Uzen and Akkar fields can be considered synclinal, that means, it serves as both reservoir and source rock, which explains the different oil fingerprintings and the vertical and lateral differences in their biomarkers. The main role in the formation of deposits in the stratum played the lateral migration of hydrocarbons from the most submerged parts. Rock -Eval results also enabled to clarify the reasons for the different compositions of oil biomarkers: for example, by pyrolysis, the studied core samples from the Permian- Triassic and Triassic deposits of Oymashaa, Atambay -Sartyube and Pridorozhnoye are so thermally overmature that they are in the zone of gas generation and possess low generative potential, and as shown by biomarker analysis, their oils have such a high thermal maturity that only tiny amount of biomarkers is left. This circumstance is reflected in a sharp reduction in the resources of liquid hydrocarbons in Permian- Triassic deposits compared to Jurassic deposits and in an equally sharp increase in the share of gas formation. Also "oil-source rock correlation" studies were made to discern genetic link between oils and hydrocarbons extracted from potential source rocks.