Origin and significance of monomethylalkanes from Yanchang Formation source rocks in Ordos Basin

Monomethylalkanes are common branched alkanes in the saturated hydrocarbon fraction of lacustrine source rocks. However, there is no agreement about their original source. The abundance of monomethylalkanes, from the Triassic Yanchang Formation source rocks in the Ordos Basin is relatively high. Determination of TOC, Rock-Eval, compound specific carbon isotopic composition, and GC-MS analysis of saturated hydrocarbons show that the organic matter of source rocks from Chang7-Chang10 have a mixed source, and that the type of organic matter is Ⅱ-Ⅲ. The steroid hydrocarbon distributions of source rocks showed nearly equal abundances of C₂₇ and C₂₉ steranes, suggesting mixed organic matter. The monomethylalkanes have 12 to 32 carbons, having a similar distribution to the n-alkanes, and the 2-MMAs and 3-MMAs are predominant. The δ¹³C values of Pr and Ph are 1‰ to 2‰ heavier than corresponding n-alkanes. 2-MMAs and 3-MMAs with the same carbon number have almost the same δ¹³C value, indicating they have the same source. The δ¹³C values of monomethylalkanes range from -29.3‰ to -32.8‰, which are higher than n-alkanes. Moreover, combining with the sedimentary environment information of the Ordos Basin, it appears that the monomethylalkanes might come from phytoplankton and heterotrophic bacteria, and they can be used as a biomarker to judge the predecessor of sediment and petroleum.