Evidence for a palaeo-oil column and alteration of residual oil in a gas-condensate field: Integrated oil inclusion and experimental results

Abstract

In the Phuong Dong gas condensate field, Cuu Long Basin, Vietnam, hydrocarbon inclusions in quartz trapped a variety of petroleum fluids in the gas zone. Based on the attributes of the oil inclusion assemblages (fluorescence colour of the oil, bubble size, presence of bitumen), the presence of a palaeo-oil column is inferred prior to migration of gas into the reservoir. When a palaeo-oil column is displaced by gas, a residual volume fraction of oil remains in pores. If the gas does not completely mix with the oil, molecular partitioning between the residual oil and the new gas charge may change the composition and properties of the residual oil (gas stripping or gas washing). To simulate this phenomenon in the laboratory, we sealed small amounts of crude oil (42 and 30°API) and excess pure gas (methane, ethane, or propane) in fused silica capillary capsules (FSCCs), with and without water. These mixtures were characterized with the same methods used to characterize the fluid inclusions, heating and cooling stage microscopy, fluorescence spectroscopy, synchrotron FT-IR, and Raman spectroscopy. At room temperature, mixtures of ethane and propane with the 30°API oil formed a new immiscible fluorescent liquid phase with colour that is visually more blue than the initial oil. The fluorescence of the original oil phase shifted to yellow or disappeared with formation of semi-solid residues. The blue-shift of the fluorescence of the immiscible phases and strong CH stretching bands in FT-IR spectra are consistent with stripping of hydrocarbon molecules from the oil. In experiments in FSCCs with water solid residues are common. At elevated temperature, reproducing geologic reservoir conditions, the fluorescence changes and therefore the molecular fractionation are enhanced. However, the precipitation of solid residues is responsible of more complex changes. Mixing experiments with the 42°API oil do not form a new immiscible hydrocarbon liquid although the fluorescence displays a similar yellow shift when gas is added. Solid residues rarely form in mixtures with 42°API oil. FT-IR spectra suggest that the decrease of fluorescence intensity of the original oil at short wavelengths to be due to the partitioning of low molecular weight aromatic molecules into the vapour phase and/or the new immiscible liquid phase. The decrease of fluorescence intensity at long wavelengths appears to be due to loss of high molecular weight aromatics during precipitation of solid residues by desorption of aromatics and resins from asphaltenes. Desorption of low molecular weight aromatics and resins from asphaltenes during precipitation can also increase the fluorescence intensity at short wavelengths of the residual oil. Water clearly affects the precipitation of semi-solid residues from the oil phase of the lowest API gravity oil.The change of hydrocarbon phase(s) in UV–visible fluorescence and FT-IR enclosed within the FSCCs were compared with the fluorescence patterns of natural fluid inclusions at Phuong Dong gas condensate field. The experimental results support the concept of gas-washing of residual oil and are consistent with the oil inclusion attributes from the current gas zone at Phuong Dong field. The hydrocarbon charge history of the fractured granite reservoir is interpreted to result from the trapping of residual oil after drainage of a palaeo-oil column by gas.