Characterization of Paleogene hydrothermal events and their effects on reservoir properties in the Qikou Sag, eastern China

• Published in: Journal of petroleum science & engineering Vol. 146; pp. 1226 - 1241
• Main Authors: Yu, Zhichao, Liu, Keyu, Liu, Li, Qu, Xiyu, Yu, Miao, Zhao, Shuang, Ming, Xiaoran
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.10.2016
• Subjects: Clay minerals; Emplacement; Hydrothermal events; Intrusions; Qikou Sag; Intrusions; Clay minerals; Hydrothermal events; Emplacement; Qikou Sag
• ISSN: 0920-4105; 1873-4715
• DOI: 10.1016/j.petrol.2016.08.026

Widespread igneous activities occurred in the Qikou Sag, eastern China since the Paleogene with numerous dikes and sills preserved in the sedimentary sequence. Based on an integrated petrographic, fluid inclusion and organic geochemistry investigation, we have documented these hydrothermal fluid events and the effects of the associated thermal fluids on the reservoir properties. The Paleogene volcanic rocks in the Qikou Sag consist mainly of a set of basalt-diabase series of basic volcanic and intrusive rocks. Strong thermal anomalies caused by the hydrothermal fluid flows are present in the Qikou Sag, as evidenced from hydrothermal alteration of the sediments and abnormal depth profiles of vitrinite reflectance values (Ro), Rock-Eval pyrolysis parameters (e.g. Tmax, S1 and S2), fluid-inclusion homogenization-temperatures (Th) and clay-mineral transformations. An abrupt increase in Ro observed in the deep strata of the Qikou Sag is interpreted to be mainly caused by deep-sourced thermal fluids. Magmatic thermal fluids associated with intrusions and deep-sourced thermal fluids had caused different impacts on the transformation of the mixed-layer I/S clays: the magmatic thermal fluids affected the transformation of the clays by causing a potassium-deficient fluid environment after the emplacement of the intrusions, whereas the deep-sourced thermal fluids only provided a heat source. The shallow (<3500m) basic magmatic CO2-rich hydrothermal fluids have significantly enhanced the reservoir porosity and permeability, although the effect is only limited to the surrounding area of about five times of the intrusion body thickness. In contrast, the deep CO2-rich thermal fluids (>3500m) appear to have a sustained effect and exert a much broader thermal impact on the reservoirs due to their continuous nature and prolonged activity. •Igneous intrusions can have positive effects on the porosity-permeability of reservoir.•The deep thermal fluids constitute the major thermal activities in the Qikou Sag.•Five evidences of the hydrothermal fluids in the Qikou Sag were firstly identified.