Abstract

Hydrocarbon-bearing zoned calcite cements occur widely in Jurassic–Cretaceous fault-zone cores and sandstone outcrops of the northwestern Junggar Basin (northwest China). Hydrocarbon-bearing bands alternate with nearly hydrocarbon-free bands at a micron scale. Analytical results from biomarker organic geochemistry, Fourier transform infrared microspectroscopy, and trace-element geochemistry on these zoned cements suggest that at least three different types of fluids have participated in their formation. The first fluid type is probably primary, unmodified lacustrine formation water, from which the hydrocarbon-poor bands are formed and are characterized by Mg-rich calcite. The other two types of fluids include basinal fluids (e.g., hot hydrocarbon-bearing fluids) and meteoric water. The hydrocarbon-rich bands in which the hydrocarbons have been biodegraded and the Mn content is relatively high suggest a mixture of hydrocarbon-bearing basinal fluid and meteoric water. The alternating growth of hydrocarbon-bearing and hydrocarbon-free bands of calcite cements implies that the cement formation is episodic; it is related to alternating episodes of mixed petroleum-bearing fluid and unmodified primary formation waters, respectively. The fault appears to have been a mixing zone where seismic pumping during the movement of associated regional faults occurred. Thus, in the northwestern Junggar Basin, the micron-scale hydrocarbon-bearing zoned structure of the calcite cements is likely a reflection of episodic petroleum fluid migration in fault zones.

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