The correspondence between the aragonite content of Palaeogene shale and the thermal evolution of source rocks in Jiyang depression

Aragonite is a common mineral in Palaeogene lacustrine sediments of Jiyang depression, mostly developed in carbonate-rich shale, and characterized with chemical genesis. Statistics reveal that the change envelop curve of aragonite content in Palaeogene lacustrine shalehas an obvious correspondence with the burial depth. In the strata above the buried depth of 2300 m, the aragonite content changes little. When the buried depth is greater than 2300 m, the envelop curve of aragonite content is declining quickly with the increase of buried depth. When the buried depth reaches about 3000 m, the envelop curve of aragonite content shows the maximum decline rate. When the buried depth exceeds 3500 m, a majority of aragonite in shale disappears. This change trend of the envelop curve of aragonite contentis basically consistent with that in the thermal evolution stage of source rocks, indicating that the aragonite content change in shale formation is obviously controlled by thermal evolution of organic matters. To further validate such an explanation, multiple groups of dissolution experiment are carried out on the aragonite-rich and calcite-rich shale samples withthe addition of organic acids (acetic acid) and reservoir water under high temperature and high pressure, so as to test thedissolution degree of carbonate minerals prior and posterior to the experiment and analyze the change trends of aragonite content based onfluid characteristics in the thermal evolution of source rocks at different buried depths. The analysis shows that the aragonite content is negatively correlated to concentrationof the organic acid discharged in the maturity evolution process of organic matters. The more thegenerated organic acid is, the more intensified the dissolution of aragonite will be. When the buried depth is more than 3500 m, source rocks enter the high mature-over mature stage; when the formation of organic acidis gradually terminated, the disappearance of aragonite also tends to stagnate, and the recrystalization of shale calcite occurs in shale pore water. © 2017, Editorial Office of ACTA PETROLEI SINICA. All right reserved.