Petrological record of hydrocarbon accumulation in the Kela-2 gas field, Kuqa Depression, Tarim Basin

The reservoir diagenetic process and the petroleum charge histories of the Cretaceous Bashijiqike Formation (K1bs) in the Kuqa Depression, Tarim Basin were studied using an integrated petrographic, fluid inclusion, reservoir diagenesis and basin modeling approach. A suite of techniques were used in the investigation including optical microscopy, scanning electron microscope (SEM), X-ray diffraction (XRD), fluorescence spectroscopy, microthermometry, and Laser Raman microspectroscopy. The workflow comprises petrographic characterization of the reservoir sandstone, identification of fluid inclusion assemblages, and construction of the diagenetic history of the reservoir sandstone with reference to the hydrocarbon charge and emplacement. Key diagenetic events identified include quartz overgrowths, early calcite cementation, dolomite cementation, alteration of kaolinite, bitumen emplacement, micro-quartz and ankerite cementation. The first diagenetic event was characterized by quartz overgrowths and followed by calcite and dolomite cementation. Kaolinite and ankerite cementation were the last diagenetic events, coincided with hydrocarbon charge and emplacement. Three episodes of hydrocarbon charge and emplacement have been recorded by the diagenetic products and fluid inclusions in the K1bs Formations: (1) the first episode of oil charge was recorded in the early calcite cement, and were trapped around at 18 Ma; (2) the second and third episodes of hydrocarbon charge were recorded in the telo-diagenetic ankerite, and were trapped around at 4-6 Ma. The timing of oil emplacement determined from fluid inclusion homogenization temperature and basin modeling is consistent with the formation order of the diagenetic minerals. This study has thus demonstrated that integrated analysis of fluid inclusion and reservoir diagenesis can be an effective tool for determining hydrocarbon charge and emplacement history.