Abstract
This study discusses the formation mechanisms of the delta front reservoir of K1q4 of the Quantou Formation in the central region of the Fuyu oil field and summarizes the types of deposition based on an analysis of facies markers and a comprehensive study of the sedimentary microfacies using well cores, logs, and geologic data. The results show that K1q4 of the Quantou Formation mainly formed through shallow-water sedimentation on a delta front and includes five sedimentary microfacies: underwater distributary channels, underwater natural levees, underwater diversion bays, river mouth bars, and distal bars. Due to the effects of allocyclicity, the composite sand body evolved by vertical overlaying, whereas because of the effects of autocyclicity, the composite sand body evolved by horizontal transport. Three main types of composite sand bodies are located in the study area, including composite underwater distributary channels without levees, composite underwater distributary channels with natural levees, and composites of underwater river mouth bars with distributary channels.
Highlights
Studies of composite sand bodies are the core and basis of reservoir sedimentology
The results show that K1q4 of the Quantou Formation mainly formed through shallow-water sedimentation on a delta front and includes five sedimentary microfacies: underwater distributary channels, underwater natural levees, underwater diversion bays, river mouth bars, and distal bars
Due to the effects of allocyclicity, the composite sand body evolved by vertical overlaying, whereas because of the effects of autocyclicity, the composite sand body evolved by horizontal transport
Summary
Studies of composite sand bodies are the core and basis of reservoir sedimentology. Composite sand bodies can macroscopically control the geometry and distribution of the sand bodies in a reservoir (Zhang et al 2009, 2010; Er et al 2013; Li et al 2013) and microscopically control the internal variations of the physical properties in the reservoir (Zeng et al 2013; Cao et al 2010; Zhu et al 2012; Ji et al 2013). The target stratum for oil exploitation is K1q4 of the Quantou Formation, which is composed of typical lake-basin shallowwater delta sediments that mainly include siltstone and fine-grained sandstone with a constant reservoir distribution. The underwater natural levees contain parallel bedding and lenticular bedding, and the underwater diversion bays include crumpled structures and parallel bedding All of these bedding types form at smaller scales with weak hydrodynamics, and most occur in the delta front sediments. During the different stages of lake filling, the type of sand body structure, the mode of overlaying, and the relative preservation of the underwater distributary channel on the delta front can be affected by the variation of the ratio between the accommodation space and the sedimentary supply (A/S ratio). Based on the study of the sedimentary subfacies, four subenvironments for the sand body can be identified, including underwater distributary channels, underwater natural levees, river mouth bars, and distal bars.
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