Diagenesis Sequence and Hydrocarbon Accumulation Period of the Ordovician Reservoir in Well Tashen-6, Tahe Oilfield, Tarim Basin, NW China

The main types of diagenesis, diagenetic minerals and their formation time sequence in the Ordovician ultradeep (>7000 m total depth) carbonate reservoir represented by the Yingshan and Penglaiba Formations (well Tashen-6, Tahe Oilfied, Tarim Basin), are determined by applying microscopic observations, microscopic fluorescence detection, and cathodic luminescence analysis in petrographic thin sections. The distinct periods of reservoir diagenesis and hydrocarbon-related events are determined by analyzing the development characteristics of hydrocarbon inclusions and their relationship with the host minerals. The charging periods of hydrocarbon inclusions are identified by constraining the homogenization temperatures of inclusions. The obtained results indicate that the Ordovician Yingshan and Penglaiba formations have experienced at least three periods of hydrocarbon charging and one period of structural transformation. Their relative time sequence relationship with diagenesis processes is as follows: The limestone dissolution of the Yingshan Formation developed initially, and the first period of hydrocarbon charging occurred (during the late Caledonian). The second period of hydrocarbon charging occurred due to the continuous modification influence of dissolution (late Hercynian–early Yanshanian). The limestones of the Penglaiba Formation were exposed to strong tectonism during the second period of hydrocarbon charging in the Yingshan Formation; thus, intralayer microfractures were formed. Additionally, the first period of hydrocarbon charging in the Penglaiba Formation occurred together with the dolomite reservoir (late Hercynian–early Yanshanian). During the subsequent period, dissolution occurred again due to the continuous increase in burial depth. The third period of hydrocarbon charging developed concurrently with the early fractures (late Himalayan). Finally, the unceasing deepening of the strata accompanied by tectonic activity led to the early intergranular dissolution pores to be cut by late microfractures, which caused the crude oil to convert into bitumen through secondary modifications.