Research on the Calcite Cements in Tight Sand Reservoir: a Case Study of Triassic Yanchang Formation in Honghe Area, Ordos Basin of China

Calcite is one of the most widely spreading cements in tight sandstone reservoirs. Though the origin of calcite cements is studyed widely, its distribution pattern is not recognized thus stunting the prediction of “sweet... [ view full abstract ]

Calcite is one of the most widely spreading cements in tight sandstone reservoirs. Though the origin of calcite cements is studyed widely, its distribution pattern is not recognized thus stunting the prediction of “sweet spot” in tight sandstone reservoir. This paper, taking Triassic Yanchang Formation in Honghe area, Ordos Basin of China as an example, aims at revealing the favorable cementation conditions and distribution pattern of calcite cements in tight sandstone reservoir using thin section, fluid inclusion temperature measurement, C/O isotope analysis and so on.
(1) Thin section anlysis shows that calcite is mainly cemented at the center of the pore. A relatively thicker sandbody is often cemented by calcite at its top and bottom and a relatively thinner sand body is often fully cemented by calcite.
(2)Temperature of fluid inclusion in calcite cements ranges from 70℃ to 90℃. Combined with the burial history and porosity evolution of study area, it is concluded that calcite cements were coming into being in the residual pore of the sandstone after early fast compaction in the early diagenetic stage. Compaction is relatively weaker where the content of plastic components in the sandstone is less than 60% and the grain size of sandstone is more than 100μm, which will provide favorable space for calcite cementation.
(3) C/O isotope analysis shows that calcite cements have similar C/O isotope characteristics as the mudstone nearby,which can be inferred to be the forming material source of calcite cements. Statistical analysis of cored wells indicates that the thicker the mudstone nearby is, the more thickly the sandstone is cemented by calcite.
To conclude, favorable reservoir because of the lack of calcite cements is often located where the sandbody is relatively thicker, the mudstone nearby is relatively thinner, the content of plastic component in sandstone is less than 60% and the grain size of sandstone is more than 100μm in the studying region.