Patterns of intercalation in alluvial fan reservoirs—A case study of Lower Karamay Formation, Yizhong Area, Karamay Oilfield, NW China

Abstract

This paper discusses the overall distribution characteristics of alluvial fan in the Triassic Lower Karamay Formation, Yizhong Area, the Karamay Oilfield, with core, outcrop, logging and seismic data. On this basis, the shapes, sizes and stacking patterns of intercalations in different levels and sub-facies are analyzed using the hierarchy bounding surfaces method. In the study area, the Lower Karamay Formation contains (from bottom to top) root fan, middle fan and outer fan deposits, with size-reducing grains, as a complete retrograding sedimentary sequence. The intercalation in the alluvial fan reservoir can be divided into two types: lithological intercalation (fine sediments) and physical intercalation (argillaceous sandstone/conglomerate). Origin, pattern and size of intercalations vary significantly in different sub-facies zones. In particular, the intercalations in root fan are “scattered generally and interlaced locally”, presenting as partially discontinuous screen in a high-permeable network; the intercalations in middle fan are relatively stable intercalations and lateral intercalations, with a higher capacity to shade or screen the seepage; the intercalations in outer fan present as a layered cake, with reservoir bodies mosaic-like along the source and lenticular in perpendicularity to the source that are separated by thick barrier beds, showing the highest capacity of screen. Key words: Karamay Oilfield, Lower Karamay Formation, alluvial fan, intercalation, root fan, middle fan, outer fan