Characteristics and Origin of Calcareous Interbeds of Chang 6 Member in Jiyuan Oilfield

Abstract

Calcite cementation is one main cause for reduction of porosity and permeability. In the case of Chang 6 Member of Jiyuan Oilfield, calcareous interbeds have been investigated for the characteristics and genetic mechanisms within delta-front Chang 6 oil reservoirs so as to reveal their spatial distribution. As shown by research, carbonate content in this area more than 10% will significantly reduce reservoir porosity and permeability, as well as its oil-bearing property. In vertical, calcareous interbeds are commonly developed at interfaces, where mudstone contacts with either top or bottom of underwater distributary channels of delta front, or within channel sand bodies. Besides, underwater levees with intercalation of sandstone and mudstone and the delta-front channel bars at the top or bottom have found calcareous interbeds. Meanwhile, calcareous interbeds in plane mostly distribute in the under-Water distributary channels and also partly underwater levees. In research area, calcite cementation predominantly formed during late diagenesis, In the acidic water for under the action of carbonate cementation, provides Ca2+ 、Mg2+ and HCO3- etc material base，Carbonate cementation content prong to accumulating at the top or bottom of neighboring mudstone and sandstone producing “calcite top” and “calcite bottom”.