Relationship between tight reservoir diagenesis and hydrocarbon accumulation: An example from the early Cretaceous Fuyu reservoir in the Daqing oil field, Songliao Basin, China

It is crucial to determine the sequence of reservoir tightening and hydrocarbon accumulation for analyzing the accumulation dynamics, oil migration mode, and oil distribution pattern of tight oil reservoirs. The tight sandstone reservoirs in the Fuyu reservoir occur in both the Daqing Anticline (DA) and Sanzhao Sag (SS), which were used as examples for the qualitative and semiquantitative analyses conducted herein. By integrating the porosity and permeability data, thin section imaging, cathode luminescence, scanning electronic microscopy, and energy dispersive spectra, the diagenesis processes including compaction, calcite cementation and replacement, illite precipitation, quartz overgrowth, and dissolution were analyzed. Typical diagenetic processes and successive orders were established to have occurred in the following sequence: (1) compaction, early calcite cementation, and chlorite coating; (2) dissolution, quartz overgrowth, and kaolinite filling pores; (3) late calcite cementation, calcite replacing feldspar, and quartz; and (4) late illite (feldspar illitization). Utilizing fluid inclusion, isotopic analysis of C and O, and burial history, calcite cementation was determined to be related to the thermal decarboxylation of organic matter, showing that the dominant forming-temperature spanned 60°C–100 °C and that a large amount of calcite cement formed before the Middle-Nenjiang stage (the Early Campanian). Illite precipitation was determined to be related to feldspar illitization with a dominant forming temperature spanning 120°C–140 °C. The illite was mainly developed in the Late-Mingshui stage (the Late Campanian) in the DA. In the SS, illite was formed between the Late-Nenjiang stage (the Middle Campanian) and Late-Mingshui stage. The Fuyu reservoir experienced three–four periods of hydrocarbon charging, with the main charging period occurring in the Late Cretaceous (Late-Mingshui stage). The Fuyu reservoir became tight (porosity <12%) in the DA and SS in the Late-Mingshui and Late-Nenjiang stages, respectively. The formation of calcite and illite cementation and the tightening of the Fuyu reservoir occurred before the main hydrocarbon charging period, indicating that tightening occurred before oil accumulation.