Abstract:

The deep coarse-grained siliciclastic rocks of the 4^th member of the Paleogene Shahejie Formation （Es⁴） in Minfeng-Yanjia area of the eastern section of the steep slope zone to the north of Dongying Sag， Bohai Bay Basin， are taken to identify markers and time of deep hydrothermalism and make a preliminary discussion on its geological significance. A variety of data are applied in the study including the core， scanning electron microscopy （SEM）， thin section， fluid inclusion， X-ray diffraction （XRD） of clay minerals， vitrinite reflectance （R_o）， physical properties of core sample and hydrocarbon source Rock-Eval parameters； besides， the research results of tectonic evolution， magmatic activity， stratigraphic burial and hydrocarbon accumulation history are also taken into consideration. The results show that， there are explosive breccia， cubic pyrite and some mineral assemblages， such as mixed layer illite/montmorillonite-ankerite-apatite， mixed layer illite/montmorillonite-apatite-pyrite， chlorite-apatite-pyrite and apatite-pyrite， which just reflect the deep hydrothermalism in the Es⁴ coarse-grained siliciclastic rocks of the study area. The vertical evolution of clay minerals is characterized by an abnormal increase of illite/montmorillonite ratio and transient dramatic increase of kaolinite content. The contents of free hydrocarbon （S₁） and pyrolysis hydrocarbon （S₂） are abnormally high at a burial depth ranging from 3 400 m to 4 800 m， while they are relatively low at a burial depth of 5 500 m when the maximum pyrolysis temperature （T_max） is abnormally high. At a burial depth of 4 269 m to 4 800 m， there are abnormally high-temperature inclusions in quartz fractures. The δ¹³ C{}_{\mathrm{C}{\mathrm{O}}_{\mathrm{2}}} in the carbon stable isotopes of natural gas is greater than -8 ‰， indicating inorganic origin and formation water of NaHCO₃ type as associated with the deep CO₂-rich hydrothermalism. The characteristics mentioned above all indicate deep fluid activity of mantle-derived CO₂-rich origin in the Es⁴ coarse-grained siliciclastic rocks in Minfeng-Yanjia area during the Guantao-Minghuazhen sedimentary period. The deep hydrothermal fluid has played a dual role in reforming the physical properties of the coarse-grained siliciclastic reservoir. On the one hand， it causes fractures and promotes dissolution to improve the reservoir physical properties； on the other hand， the authigenic minerals in precipitation clog pores thereby hinder fluid migration and damage the physical properties of the reservoir. In addition， deep hydrothermal fluid has got involved in the thermal evolution and hydrocarbon generation of source rocks in the lower section of the 4^th member of Shahejie Formation （Es^4L）， which has a potential impact on hydrocarbon expulsion and crude oil cracking of the source rocks.

Key words: identification marker, hydrocarbon generation, reservoir modification, deep hydrothermal fluid, deep coarse-grained siliciclastic rock, the 4^th member of Shahejie Formation （Es⁴）, Dongying Sag, Bohai Bay Basin