"GEOLOGICAL CONFIGURATION"STREANES IN IMMATURED SOURCE ROCKS

Abstract: Occupied by "biological configuration' steranes first, then down to a certain depth, the relative richness of "geological configuration' increases obviously, its C₂₉ааа一20S/ааа一(20S+20R) and авв- (20R+20S)-C₂₉/∑C₂₉will be as high as 24.6-47.7% and 23,0-51.1。respectively. But beyond a cerlain depth, again the "biological configuration' steranes occupy the absolute majority. It causes the "geo- logical configuration' steranes showing the distribution feature of "from low to high to low’in whole immatured source rocks. It is more interesting that above mentioned distribution feature has an obvious change law in various sections which could not be explained by "reworking': (1) The depth where high "geological configuration" steranes appeared is closely related with the geological age. The older the age is，the shallower the maturity threshold is and the later the high value appears cnamely, much shallower buried depth;in the other hand, the younger the age is，the deeper the maturity threshold is and the later the high value appears correspondingly (namely, much deeper buried depth.For instance, in Erlian Basin, the maturity threshold of the Bazhong group (K₁b₂) and the Bashang group K₁b₃ in Mesozoic is 1500m and 1800m, respectively, their high value appears in 285-530m and 635-1260m, respectively; while the maturity threshold is in the range of 2500-3100m For several sedimentary sections 1n the Palaeogene Shahcjie group 3 Formation (E_s3) of the Bohai Bay Basin the high value appears laterly m 1215-1908m. as for Neogene (N₂) of Qinghai Qaidam Basin with maturity threshold of 3300m, the high value appears much laterly about 2600m. (2) The low value after the high value of "geological configuration' steranes always corresponds to signiricant deoxidation of kerogen. That is，the high value always appears baCore significant deoxidation of kerogen. Once kerogen enters the significant dcoxidation period, the relative richness of“geological configuration" sterane is obviously decreased correspondingly, then causing another low ratio region appears between C₂₉ааа一20S/ааа一(20S+20R) and авв- (20R+20S)-C₂₉/∑C₂₉.