Mudstone can store formation water when acting as a caprock, which is a part of water sources in early oil exploitation. To improve crude oil quality, it is necessary to understand readsorption behavior of water. The behavior of water is closely related to the physicochemical structure of mudstone. Interactions between water and chemical structures are hard to detect because of the overlap of O-H infrared vibration bands (IR) in H2O and that in mudstone, thus D2O was innovatively used to interact with mudstone. In this study, mineral compositions and pore structures of mudstone were identified by X-ray diffraction, Xray fluorescence, and low-temperature N-2 adsorption/desorption, respectively. Furthermore, water retention contents at different relative humidities (RH) were investigated by comparing hysteresis loop areas at low (S-1) and high (S-2) RH based on water adsorption/desorption isotherms. Interactions between water and mudstone were investigated by studying the IR spectra changes and release of D, HD, OD, and HOD during drying of mudstones containing adsorbed D2O using in situ diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy and thermogravimetric coupled with mass spectrometry (TG-MS), respectively. The results showed that the specific surface area (S-BET) and mineral compositions controlled S-1, while S-2 was related to capillary pore volume. The detection of OD and HOD by in situ DRIFT at 300 degrees C and by TG-MS at 450 degrees C manifested that hydrogen bonds were formed between water and mudstone. Based on this study, we suggested that production wells should be constructed as far as possible on top of mudstone caprocks with relatively lower pore volumes and higher kaolinite contents to improve crude oil quality in early exploitation.