Distribution and source significance of 2-methylalkanes in coal-measure source rocks, northwest China

GC-MS analysis was conducted on 59 coal-measure source rock (CSR) samples in the study area. Between nC(14) and nC(38), a series of 2-methylalkanes were detected. The 2-methylalkanes distribution can be summarized into 3 distribution modes: Front peak-type, Middle peak-type and Behind peak-type distribution. The same distribution mode is distributed at different maturity stages, and there are different distribution modes at the same thermal evolution stages, indicating that the distribution was slightly influenced by thermal evolution. C-27-C-28-C-29 regular sterane triangulation is a template which is commonly used to judge the source of organic matter and sedimentary environment. But it is not applicable to judgment of the source of organic matter of coal-measure source rock (CSR) as higher plants are the basic substance. The organic matter constitution in CSR is still complicated, we need to know secondary organic matter in CSR except for plant so as to analyze the sedimentation and diagenetic environment. Within C-17-C-22-C-28 2-methylalkanes (iC(17)-iC(22)-iC(28)) triangulation, samples with Front peak-type distribution of 2-methylalkanes are distributed near the 2-methylalkane C-17 (iC(17)) end member. Samples with Middle peak-type distribution are distributed near the 2-methylalkane C-22 (iC(22)) end member. Samples with Behind peak-type distribution are distributed near the 2-methylalkane C-28(iC(28)) end member. In combination with distribution characteristics of isoprenoid alkenes and biomarker compounds, different regions in the iC(17)-iC(22)-iC(28) triangulation can be divided into different sources of organic matter and sedimentary environment. Based on this, the iC(17)-iC(22)-iC(28) triangulation used to judge the parent material organic matter of CSR was established. In comparison with C-27-C-28-C-29 regular sterane triangulation, it is more effective to judge the source of organic matter in CSR with iC(17)-iC(22)-iC(28) triangulation.