Identification methods and the main factors controlling water washing in secondary oil reservoirs: A case study of Qingshuihe formation of lower Cretaceous in Mosuowan area of Junggar Basin

Abstract

Secondary oil reservoirs have undergone multi-stage structural evolution, the hydrocarbon forming process is complex, and the main controlling factors of reservoir formation are unclear, and it is difficult to effectively identify, which seriously restricts the next exploration and extraction. Secondary oil reservoirs of the Qingshuihe Formation in the Mosuowan area of Junggar Basin were used as a case. Using experimental analyses of water washing simulation, group composition, rock pyrolysis, resistivity, high-pressure mercury injection, and homogenisation temperature of inclusion, this study carried out research on the identification methods of secondary oil reservoirs, causes of water production of oil-bearing cores and main controlling factors. The results show that water washing in secondary oil reservoirs is characterised by low saturated hydrocarbon, low aromatic hydrocarbon, high colloid, high asphaltene, a low ratio of S1/S2, low oil saturation, and low resistivity. Based on this feature, a comprehensive identification method combining geochemistry and logging of secondary reservoirs is established. The oil deposits of the Qingshuihe Formation in the Mosuowan area have two stages of hydrocarbon filling, the first stage is Late Cretaceous, forming primary oil reservoirs. The second stage is Neogene, and oil reservoirs are adjusted to form secondary oil reservoirs. Under the influence of Himalayan movement, the oil adjustment and dispersion were caused by structural tilting movement, which led to the water production of oil-bearing cores with favourable physical properties and large thickness in the research region. The adjusted residual oil reservoirs are major governed by paleogeomorphology, sedimentary microfacies, reservoir physical properties, and low amplitude structure. Four secondary hydrocarbon accumulation models are established: thin interbedded lithologic oil reservoir, sedimentary filling lithologic oil reservoir, low amplitude structural oil reservoir, and stratigraphic pinch-out lithologic oil reservoir. The research results have important instructive meanings for the identification and prospecting of secondary hydrocarbon reservoirs.