摘要
作者根据几年来所取得的资料.对R_0<0.5％的未成熟生油岩中出现的相对高丰度的“地质型”甾烷,提出了新见解.这种现象似乎是一种客观规律,并且可能是以游离状态分解于地质体中的甾类分子化石异构化作用的结果.这一发现,揭示了干酪根在通常所说的“成熟门槛”以上就已开始降解,释放烃类,从而为“未成熟原油”的存在提供了依据.
Based on a vast amount of data obtained in recent years, the writers put forward a couple of new different from the predecessors for the relative high richness 'geological configuration' ( namely ααα- 20S, αββ-20R and αββ- 20S) appeared in immalurcd source rocks with
It was once a common view on stcrancs in immatrucd source rodks that the 'biological configuration' steranes normally occupy the absolute majority while the appearance of a great amount of 'geological con- stcrancs is viewed as and abnormal phenomenon and thought that is result of mixing with organic matters, or disseminated by deeper crude oil ( A. S. Mackenzie el al., 1980; N.S Mackenzie, 1984; J.D.King et al, 1983) .
However, with study on nine crude oil-undisscminated sedimentary sections proved by gcochcmical anal) sis such as organic carbon, total hydrocarbon in soluble organic matters from Lower Cretaceous of F.r.ian Basin Inner-Mongolia Palaeogene of Bohai Bay Basin and Negcne of Qaidam Basin Qinghai, it is that a high 'geological configuration' Sterane value commonly appears in the immaturcd soarce That is , in the source rock of immaturcd period above the maturity threshold, it is not case that 'biological configuration' stcrancs occupy the absolute majority. Exactly, it is main- ly occupied by 'biological configuration' steranes first, then down to a certain depth, the relative richness of 'geological configuration' increases obviously, its C29ααα-20S / ααα- (20S+20R) and αββ-(20R+20S) -C29/ ΣC29will be as high as 24.6-47.7% and 23.0-51.1%, respectively. But beyond a certain depth, again the 'biological configuration' steranes occupy the absolute majority. It causes the 'geological configuration' steranes showing the distribution feature of 'from low to high to low ' in whole im-matured 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 (namely, 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 K1b3in Mesozoic is 1500m and 1800m, respectively, their high value appears i 285-530m and 635-1260m, respectively; while the maturity threshold is in the range of 2500-3100m for several sedimentary sections fn the Palaeogene Shahejie group 3 Formation (Es3) of the Bohai Bay Basin the high value appears laterly in 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 significant deoxidation of kerogen. That is , the high value always appears before significant deoxidation of kerogen. Once kerogen enters the significant deoxidation period, the relative richness of 'geological configuration' sterane is obviously decreased correspondingly, then causing another low ratio region appears between C29ααα-20S / ααα- (20S+20R) and αββ- (20S+20R) -C29/ΣC29.
There fore, the writers think:
1. It seems to be an objective law of appearance of high value of 'geological configuration' sterane, and possibly it is the result of isomcrization of sterane molecular fossil distributed in geological body as free molecules.
2. The low value appears after high value of 'gcologicla configuration' steranes is probably released by 'biological configuration' steranes which are linked into kerogen network with chemical bond (i.e. ester bond ) during early lithogcncsis associated with significant deoxidation of kerogen, leading distribution feature of 'from low to high to low ' of 'geolo
出处
《沉积学报》
CAS
CSCD
北大核心
1989年第3期91-98,共8页
Acta Sedimentologica Sinica
