Geochemical characteristics of light hydrocarbons in Upper Paleozoic tight gas from Daniudi Gas Field, Ordos Basin

Abstract

The Daniudi Gas Field is one of the typical tight sandstone gas fields in the Ordos Basin. In order to better understand the genetic types and source of natural gas and reveal the migration phase of natural gas, the geochemical analysis of light hydrocarbons in the Upper Paleozoic tight gas from the Daniudi Gas Field has been conducted in this study. The results indicate that, the C5-7 light hydrocarbons in the Upper Paleozoic tight gas are dominated by iso-alkanes, and the C6-7 light hydrocarbons display low contents of aromatics (< 10%), which are even undetectable. The C7 light hydrocarbons are dominated by methylcyclohexane (MCH) with the relative MCH contents exceeding 50%. The K1 and K2 values of natural gas from the Lower Shihezi Formation (P1x) are consistent with those from the Shanxi (P1s) and Taiyuan (C3t) formations, respectively. The δ13C1 values of the P1x gas are consistent with those of the P1s gas and significantly different from those of the C3t gas. Compared with the P1s gas, the P1x gas mainly displays lower benzene/n-hexane, benzene/cyclohexane and toluene/n-heptane ratios, as well as significantly higher n-heptane/methylcyclohexane ratios. Geochemical characteristics of light hydrocarbons and carbon and hydrogen isotopic compositions of alkanes indicate that, the Upper Paleozoic natural gas from the Daniudi Gas Field is typical coal-derived gas. The P1s and C3t gases were in-situ self-generated and self-accumulated, whereas the P1x gas was mainly derived from the underlying P1s source rocks and accumulated via vertical migration in free phase, with insignificant contributions of the C3t source rocks. Due to the effect of migration and water solution of natural gas, the deviation exists in the identification of thermal maturity for tight gas from the Daniudi Gas Field using the light hydrocarbon indexes such as heptane and isoheptane values and benzene/n-hexane ratios.

Keywords

• light hydrocarbon compounds
• thermal maturity
• natural gas origin
• gas-source correlation
• migration phase state
• daniudi gas field
• ordos basin