摘要:
沉积有机相是具有相似沉积环境、生物组合、成岩环境、氧化—还原条件以及相近有机质特征的地层单元,其分布受沉积盆地中层序地层格架所控制。因此,层序沉积有机相分析可成为确定和预测烃源岩分布的一种有效工具。本文采用层序地层分析与沉积有机相分析相结合的方法,将陕甘宁盆地下奥陶统马家沟组划分为三个层序,并探讨了每个层序中沉积有机相的分布特征。研究表明,沉积有机相在层序地层格架中的分布呈显出一定的规律性,在盆地西部及南部的较深水层序中,沉积有利相类型主要为开阔台地C相,在垂向上变化不大,有机碳含量为 0.0 8%～ 0.1 8%。在盆地中部浅水层序中,海侵体系域的沉积有机相类型主要为开阔台地C相及局限台地D相,有机碳含量为 0.1 2 %～ 0.2 5 %,而高位体系域的沉积有机相类型主要为局限台地D相和蒸发台地E相,有机碳含量为 0.1 7%～ 0.49%,反映出盆地中部浅水层序中的高位体系域是烃源岩分布的最有利地带。
Abstract:
A sedimentary organic facies is defined as a stratigraphic unit, in which there are the similar characteristics of depositional environment, biological assemblage, diagenetic environment, reduction-oxidation condition, and the genetic type of organic matter. As the distribution of sedimentary organic facies is controlled by sequence stratigraphic framework in sedimentary basin, sequence sedimentary organic facies analysis can become a very useful tool to determine and predict the distribution of source rocks. Based on the analysis of sequence stratigraphy and sedimentary organic facies, Majiagou Formation of Lower Ordovician in Ordos basin is divided into three sequences and the distribution of sedimentary organic facies in each sequence is discussed in this paper. The results indicate that the distribution of sedimentary organic facies in sequence stratigraphic framework of Majiagou Formation present a certain regularity. In the western and southern part of the basin, the sedimentary organic facies of deeper- water sequence are mainly composed of open platform C facies from bottom to top, in which the content of organic carbon is 0.08% ～ 0.18 %. In the middle of the basin, the sedimentary organic facies of transgressive system tract of shallow- water sequence are mainly composed of open platform C facies and restricted platform D facies, in which the content of organic carbon is 0.12% ～ 0.25 %, whereas the sedimentary organic facies of highstand system tract are composed of restricted platform D facies and evaporation platform E facies, in which the content of organic carbon is 0.17% ～ 0.49 %.The best source rocks are mostly distributed over highstand system tract of shallow- water sequence in the middle of the basin
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