Feature, origin and distribution of calcareous interlayers: A case of Carboniferous Donghe sandstone in Hade Oil Field, Tarim Basin, NW China

The petrologic characteristics, well-logging, genetic models and distribution patterns of calcareous inter-layers in Carboniferous Donghe sandstone, Hade Oil Field were investigated systematically by using data such as outcrop, cores, thin sections, stable carbon and oxygen isotopic compositions, conventional logging curves, and dip logging curves. Calcareous inter-layers in the study area are made up of calcareous fine sandstone or calcareous siltstone of high compositional and textural maturity, in which calcareous components include calcite, ferro-calcite, dolomite, ferro-dolomite and anhydrate. Although the calcareous inter-layers are usually thin, they can be identified by combining conventional logging curves with dip logging curves. Three genetic models of calcareous inter-layers are identified, including shallow burial fresh water, evaporation and deep burial. According to the development of sedimentary facies, it is inferred that calcareous interlayers are well-developed in shore face and backshore sandstones, and likely to join together and show continuity between wells; based on features of high-resolution sequence stratigraphy, it is concluded calcareous inter-layers in Donghe sandstone have five types of occurrence: “upper calcareous-lower argillaceous” type, “upper argillaceous-lower calcareous” type, “calcareous-argillaceous-calcareous” type, purely calcareous type and “calcareous-argillaceous complex” type.