Based on the relationship among diagenetic minerals

the diagenetic sequence of the bituminous sandstones of the Silurian Wongxiang Group in Majiang County

Guizhou Province

is compaction-quartz overgrowth-oil emplace-ment-carbonatization-pyritization-hydrothermal silicification.This sequence is essentially conformable to that of the bituminous sandstone gravels in Early Devonian subterranean stream sediments.Since the diagenetic sequence of the gravels had reached the process of pyritization stage

the compaction-pyritiza-tion should have happened prior to the subterranean stream sedimentation

i.e.in late Silurian.The oil emplacement

as a stage of the above processes

was bound to happen in Silurian.The quartz overgrowth indicates that the buried depth of the sandstones before the oil emplacement was about 2

200-2

600m which is much deeper than that of oil generation threshold.This means that clay rock had entered the hydrocarbon generation environment during Silurian period.The lenticular or thin bedded bituminous sandstones of the 3rd memder of the Wongxiang Group are surrounded by clay rocks

there isn't any passage like fault or crack to connect the sandstones with underlying Cambrian sourcerocks

a rational explanation of the oil in these sandstones could only beoriginated from the surrounding clay rocks.Thus the oil source of the 2nd memder of the Wongxiang Group is also in Silurian rocks but not Cambrian rocks because their oil emplacement are in comformity with that of the 3rd member of the Wongxiang Group.