Abstract:

Thermal evolution of Lower Cretaceous source rocks in Qingnan sub-sag in Jiuxi depression is studied by using several methods including evolution of soluble organic matter, rock pyrolysis, biomarker parameters and vitrinite reflectance. It is revealed that the depth of oil threshold ranges from 4000 to 4100m in the sub-sag. Most of Zhonggou Formation is in immature stage, the bottom of Zhonggou Formation and the upper Xiagou Formation are in low-mature stage, the lower Xiagou Formation and the upper Chijinpu Formation are in mature stage, and the lower Chijinpu Formation is in wet gas-condensate generating stage. Study of geothermal history in Qingnan sub-sag with the Easy-Ro(%) numerical modeling software shows that the Cretaceous and Tertiary palaeo-geothermal gradients are 30℃/km and 29℃/km,respectively.Hydrocarbon generating history of the source rocks has the following characteristics:(1)the Lower Cretaceous source rocks in Qingnan sub-sag have not experienced secondary hydrocarbon generation and the main oil-generating period is from Neogene to Quaternary;(2)the source rocks in lower Xiagou Formation and the upper Chijinpu Formation are in mature stage with large amount of hydrocarbon generation,i.e.they are the main source rocks;(3)source rocks in the upper Xiagou Formation and Zhonggou Formation make little contribution to hydrocarbon generation, due to the low thermal evolution of organic matter;(4)although rocks at the bottom of Chijinpu Formation have experienced relatively high thermal evolution, they have limited hydrocarbon generating capacity, as they are mainly composed of coarse clastic rocks with low abundance of organic matter.

Key words: Qingnan sub-sag, source rocks, thermal evolution, hydrocarbon-generating history