黔北坳陷过成熟烃源岩和固体沥青中正构烷烃系列的双峰态分布

对黔北坳陷几个露头和钻井地质剖面上前寒武系-下古生界烃源岩和金沙岩孔古油藏储层沥青地球化学特征的分析结果表明这些已处于高、过成熟阶段的地质样品中正构烷烃系列普遍存在双峰态分布现象,且构成每个峰群中的正构烷烃均无碳数优势,这一现象在其他地区的相应层位地质样品和热模拟实验产物中均得到了印证,并提出了利用FCPR和LCPR两个参数来表征前、后两峰群的相互关系。当两个参数均大于1.0时,指示正构烷烃系列具双峰态分布。鉴于研究区烃源岩的时代古老,当时沉积有机质的来源为富氢的菌藻类,富含长链脂族结构则是这些有机质形成高碳数正构烷烃的重要原因。此外,所研究地质样品中有机质附存形式的多样性和复杂性可使它们发生差异成熟作用,结果可能导致不同分布特征的正构烷烃发生叠置而出现双峰态分布现象。因此,这些古老地质样品中正构烷烃系列的双峰态分布可能是指示来源低等生物如藻类的有机质经历强烈热演化作用的潜在标志。

The Bimodal Distributions of n-alkanes in the Post-mature Marine Source Rocks and Solid Bitumen from the Northern Guizhou Depression

The distributions and compositions of normal alkanes were analyzed in the Precambrian- Lower Paleozoic post-mature marine source rocks from seven outcrop sections and two drilling sections and solid bitumen from Yankong paleo-reservoir in the northern Guizhou depression. It was found that the bimodal distributions of normal alkanes are very common, and nC₁₇ and nC₂₅ is main peak for the former and later peak groups, respectively. However, no odd or even carbon predominance can be observed in every peak group. This phenomenon occurs also in the similar post-mature geological samples from other areas and the products of thermal simulation from the Proterozoic source rocks. Therefore, the bimodal distributions of n-alkanes should be an objective geochemical phenomenon in the related post-mature source rocks. Two parameters such as FCPR and LCPR can be used to describe the distributions of normal alkanes in these source rocks and solid bitumen. When they are more than 1.0, normal alkanes have a bimodal distribution. Considering that these geological samples are very old, in which sedimentary organic matter should be mainly derived from various algae and bacteria rich in hydrogen and long chain aliphatic structures, they are an important sources of long chain alkanes in these related source rocks. In addition, Organic matter in a geological sample exists in very complicate and diverse forms such as free and bound, differential maturation for these different organic matter will occur. At this time, the mixture of normal alkanes derived from different organic matter may happen and result in the bimodal distribution of normal alkanes. Therefore, it may be used as a potential indicator to indicate strongly thermal maturation for organic matter from lower organisms such as algae.