Origin of carbonate cements and reservoir evolution of tight sandstone in the Upper Triassic Yanchang Formation, Ordos Basin, China

The Upper Triassic Chang 8 Member, the eighth member of the Yanchang Formation, is a key reservoir interval in the Jiyuan area of the Ordos Basin. The reservoir quality of the Chang 8 Member tight sandstones is extremely heterogeneous owing to the widespread distribution of carbonate cements. The carbonate cements commonly develop near sandstone-mudstone interfaces and gradually decrease away from the interfaces to the centres of the sand bodies. However, the content of carbonate cements (<= 6%) has a positive correlation with the visual porosity in the Chang 8 Member sandstone, revealing that the carbonate cements contribute to the compaction resistance and the residual primary pores of reservoirs during the diagenetic process. Three main types of carbonate cement are identified: type I (calcite), type II (calcite and ferrocalcite), and type III (dolomite and ankerite). The type I calcite is characterised by enriched delta C-13 (mean -3.41 parts per thousand) and delta O-18 (mean -15.17 parts per thousand) values compared with the type II (mean delta C-13 = -7.33 parts per thousand, delta O-18 = -18.90 parts per thousand) and type III (mean delta C-13 = -10.0 parts per thousand, delta O-18 = -20.2 parts per thousand) cements. Furthermore, the mean delta O-18 value (-4.7 parts per thousand) of the type I pore fluids is 1.5 parts per thousand and 0.9 parts per thousand lower than the type II (mean -3.2 parts per thousand) and type III (mean -3.8 parts per thousand) pore fluids, respectively. This indicates that the evolving pore fluids experienced some relative strong water-rock interactions that provided the original materials (e.g. Ca2+, Fe3+, and Mg2+) for the carbonate cements during the diagenetic process. The highly saline lake water directly provided the primary material for the type I calcite precipitation, which also provided the material necessary for the precipitation of the type II and type III carbonate cements, causing enriched delta O-18 values of the pore fluids during the precipitation of the type II and type III carbonate cements. Although the earlier dissolved pores were filled with ferrocalcite, dolomite and ankerite in the middle-late diagenetic stages, some residual pores and fractures remained to become the potential reservoir storage spaces for the oil and gas exploration in the Jiyuan area.