High-resolution sediment accumulation rate determined by cyclostratigraphy and its impact on the organic matter abundance of the hydrocarbon source rock in the Yanchang Formation, Ordos Basin, China

Abstract

Abstract The Upper Triassic Chang 7 unit consists of more than 110 m of lacustrine sediments, including mudstone and shale, and it is considered to be the most important hydrocarbon source rock in the Ordos Basin. Power spectral, wavelet, evolutionary fast Fourier transform (FFT), and correlation coefficient analyses of the gamma ray (GR) and density (DEN) logging series from well Y56 in the Chang 7 unit reveal strong astronomical signals of long eccentricity (405 kyr), short eccentricity (100 kyr), and obliquity (34 kyr) cycles. Those provide an excellent opportunity to establish floating astronomical time scales (ATS) and to discuss how the variations in the sediment accumulation rate (SAR) impact organic matter abundance. Based on the 405 kyr tuning ATS results, the Chang 7 unit in well Y56 covers a 7.26 Ma period from 239.50 Ma to 232.24 Ma. The ages of each subunit in the Chang 7 unit are estimated to be 232.24–234.25 Ma, 234.25–236.90 Ma and 236.90–239.50 Ma, respectively. The high-resolution SARs of the Chang 7 unit vary from 1.18 cm/kyr to 2.08 cm/kyr, with an average of 1.57 cm/kyr. According to the chemical index of alteration (CIA) and the correlation between the SAR and the Al2O3/SiO2 or Al2O3/TiO2 ratio, the main factor affecting the SAR of the Chang 7 unit is the amount of terrestrial debris input, which is the result of the warm, humid paleoclimate and the intense volcanic activity during phase I of the Indosinian. In addition, the total organic carbon per unit of paleoproductivity (TOCpup) was applied to investigate the impact of the SAR on the organic matter enrichment in order to eliminate the effect of paleoproductivity on the organic matter enrichment. We conclude that an SAR is sufficient to protect the organic matter and prevent oxidation but not too high to dilute the organic matter, which reduces the organic matter abundance of the source rock.