Difference of hydrocarbon generation between argillaceous dolomite and mudstone and its influence on oil source: A case study of the Es4 in the Leijia area, Liaohe depression

Abstract

Argillaceous dolomite and mudstone are two terrestrial source rocks that commonly occur together, and the difference in hydrocarbon generation characteristics determines the oil accumulation mode. The fourth member of the Shahejie Formation (Es4) in the Leijia area was used as an example to study the hydrocarbon generation characteristics and the overpressure difference caused by oil generation between argillaceous dolomite and mudstone, using hydrocarbon generation kinetics. Furthermore, the source of oil in the argillaceous dolomite reservoir was analyzed by combining the biomarker characteristics. The results indicated that the hydrocarbon generation characteristics of argillaceous dolomite and mudstone are significantly different. Mudstone starts hydrocarbon generation later and has a shorter hydrocarbon generation cycle, while argillaceous dolomite starts hydrocarbon generation earlier and has a longer hydrocarbon generation period. Due to the difference in hydrocarbon generation characteristics, when the burial depth is below 3000 m (EasyRo <0.85%), the oil-generation overpressure in the mudstone is smaller than that in argillaceous dolomite, and the tight oil in the argillaceous dolomite reservoir is mainly generated by and stored in the argillaceous dolomite itself. However, when the burial depth exceeds 3000 m (EasyRo >0.85%), the oil-generation overpressure in the mudstone surpasses that in argillaceous dolomite, and the tight oil has mixed source characteristics. The biomarker characteristics observed in extracts and crude oil confirm the above view. Whether tight oil exploration in argillaceous dolomite reservoir requires attention to the hydrocarbon supply capacity of adjacent mudstone depends on the maturity of organic matter.