Characteristics of Branched Alkanes in the Source Rocks of the Ordovian from the Ordos Basin and Their Geological Significance

Key words: 
• branched alkanes /
• Ordos Basin /
• Ordovician /
• source rock

Abstract: Taking source rock core samples collected from 6 wells of Majiagou Formation of Ordovician in Ordos Basin as the target, branched alkanes in the saturated hydrocarbons were investigated by GC/MS. The results show that there is relatively high abundance of medium-chain branched alkanes of which carbon number range from 15 to 21, including 2-methyl (iso-alkanes), 3-methyl (anteiso-alkanes), high-methyl branched alkanes, dual-methyl branched alkanes and acyclic isoprenoid alkanes, in which the carbon number of iso-alkanes and anteiso-alkanes has a wide distribution, including short, medium and long-chain. According to sedimentary characteristics and other organic geochemical indexs in the study area, methyl-branched-chain alkanes with medium-chain length of Majiagou Formation of Ordovician in Ordos Basin are considered deriving from some special bacteria and algae, and the cyanobacteria is the most likely direct input parent material. Meanwhile, some heterotrophic bacteria's transformation to the original sediments is one of the important reasons that the accumulation of medium-chain branched alkanes. The long-chains iso-alkanes and anteiso-alkanes may have much to do with thermal catalytic rearrangement.