塔里木盆地寒武纪—奥陶纪优质烃源岩沉积与古环境变化的关系:碳氧同位素新证据

The Relationship Between the Cambrian—Ordovician High-TOC Source Rock Development and Paleoenvironment Variations in the Tariam Basin,Western China:Carbon and Oxygen Isotope Evidence

对塔里木盆地东部塔东2井上震旦统和寒武系—奥陶系样品进行TOC和碳、氧同位素及微量元素分析,发现该井稳定碳、氧同位素值在寒武系—中奥陶统发生明显变化,该层还赋存高有机质丰度烃源岩,其TOC值的高低可反映当时的生物产率和埋藏率。碳和氧同位素值在寒武系/上震旦统分界处正向漂移,说明正好在进入寒武纪前气候明显变冷,海平面下降。接着,下寒武统底部以碳和氧同位素值的快速负向漂移为标志,表明古气候迅速变暖和海平面大幅度上升,烃源岩TOC值也达到全井柱最高值。古气候显著波动、海平面最高及其频繁变化和沉积物高TOC是早寒武世的显著特征;而晚寒武世—早奥陶世,同位素正向漂移,暗示海平面下降,沉积物TOC也随之下降并降至最低;到了早奥陶世末,即早、中奥陶世的过渡时期,同位素又开始负向漂移,TOC又开始升高;中奥陶世,同位素强烈负向漂移,TOC又升至新高。稳定碳、氧同位素值的变化及其所指示的古气候与海平面变化,以及与TOC的响应关系,指示冰期、冰后期之交,古气候迅速变暖和海平面大幅度上升有利于烃源岩发育;同时暗示,δ13C、δ18O与生烃母质生物的有机生产率、有机埋藏率之间存在某种内在联系。即高TOC含量反映高的有机生产率和高的有机埋藏率;而与其同步反方向变化的δ13C、δ18O则暗示海平面较高,沉积环境属远陆海域,表层水中生烃母质生物的光合作用很强。而有机质的高产率和高埋藏率,导致海水中δ12C和δ16O被大量地固定在沉积物中。这样,就使海水中相对富集13C和18O重同位素而使海水变“重”;而这种“重”海水,又导致了同时期浅水区碳酸盐沉积物的δ13C和δ18O值明显增高。

Sediemnatry elemental and isotopic records of seawater and its isotopic composition are the most promising recorders of the history of this planet on geological time scale. The Tarim basin of northwestern China contains thick marine Cambrian-Ordovician sediments. Stable carbon and oxygen isotope values of these sediments from the basin show that dramatic changes occurred in Early Paleozoic. Drastic positive shift of carbon and oxygen isotopes at the boundary of Precambrian and Cambrian suggests a remarkable cooling and drop of sea level occurred right before the transition. Then the beginning of Cambrian or Paleozoic is marked by a rapid negative shift of both carbon and oxygen isotope values, suggesting a rapid warming of climate and large scale rise of sea level. Drastic input of meteric water is perhaps the major cause of the decrease of isotope values in these sediments. Optima climatic conditions of Early Cambrian provided a premier condition for the biological exploration of Early Cambrian. Enhanced bioproduction in Cambrian is shown also by the significantly higher TOC data. Severe fluctuation of climate and sea level change as well as high TOC values marked the Early and Middle Cambrian. Later Cambrian and early Ordovician is probably a rather stable period of time regarding the climate and sea level in the region. By the end of early Ordovician and the transition of Early to Middle Ordovician, isotope signals are characterized by severe negative shift. TOC raised to the new high in the Middle Ordovician （from 〈0. 5% to～5%）, suggesting elevated bioproduction （and preservation）. These lines of evidence all suggest an enhanced climate condition. Large volume of melted water from the nearby ice sheet brought in not only fresh water, but also nutrient thus providing an optimum environment for another bioexplosion and optimum condition of oil source formation.