The main diagenetic types of the sandstone reservoirs in Chang-3 reservoir in Longdong oilfield

Ordos basin

include compaction

cementation

corrosion and fracturing.The diagenetic authigenic minerals are dominated by chlorite

authigenetic quartz

calcite and gypsum.Two different phases of hydrocarbon inclusions have been identified according to the formation time series of the host diagenetic minerals.The hydrocarbon inclusions formed in the first phase would have been formed during the burial diagenesis in late early diagenetic stage

and are distributed in the dissolution pores and bottom of overgrowth edge of minerals

such as quartz and feldspar

as well as in earlier fractures.They are characterized by irregularity in shape and small in size

gene-(rally) in the range of 3-8 μm

and are dominated by multiple phase hydrocarbon inclusions.The hydrocarbon inclusions formed in the second phase would have been formed during basin uplifting stage in late latediagenetic stage

and are distributed in the late-formed fractures

siliceous cement and sparry calcite cement.They gene-(rally) contain saline water

and the boundaries of oil

gas and water are clear.It is believed that the former represents the process of hydrocarbon migration and charging

while the latter represents the process of large scale oil/gas accumulation and reservoiring.Based on the temperatures of hydrocarbon inclusion formed in various phases and analyses of the sedimentary and thermal evolution history of reservoirs

the time of hydrocarbon migration and charging is determined to be about 122 Ma(Early Cretaceous)

while that of large-scale accumulation and reservoiring is about 80 Ma(late Late Cretaceous).