Forward analysis of origin of mixed hydrocarbons and relative contribution of source rocks in Tarim Basin: A case study from Tazhong area

Due to blend of multi-source oil and gas, as well as multi-period tectonic movements in the Tarim Basin, the origin of marine oil and gas and relative contribution of each set of source rocks that generated them is not clear.... [ view full abstract ]

Due to blend of multi-source oil and gas, as well as multi-period tectonic movements in the Tarim Basin, the origin of marine oil and gas and relative contribution of each set of source rocks that generated them is not clear. Based on oil/gas-source correlation and because of similar biomarkers, many geochemists identified the upper-middle Ordovician source rocks as the primary origin of oil and gas in the Tarim Basin. However, the fact that distribution of source rocks with high organic matter (TOC >0.5%) in this unit is quite limited, does not explain the discovery of large-scale reserves of oil and gas. Moreover, proven gas reserves in Tarim Basin have exceeded the third round of hydrocarbon resources assessment in the basin, conducted by Strategic Research Center of Oil and Gas Resources, Ministry of Land and Resources, Peoples’ Republic of China. The above contradictions show limitations of geochemical inversion such as oil/gas-source correlation used to analyze the origin and quantitative source assessment in superimposed basins. In this study, the process of hydrocarbon generation, and expulsion from source rocks, and migration, accumulation and destruction of oil and gas were simulated using forward modeling analysis. The amount of prospective resources can be obtained by deducting the amount of residual hydrocarbon, hydrocarbon in invalid phases, hydrocarbon loss during migration, and hydrocarbon destroyed by tectonic movements, from total amount of hydrocarbon generated, which can be used to assess the relative contribution of each set of source rocks to the origin of oil and gas. Results show that oil and gas discovered in the Tazhong area are mainly from two sets of source rocks with different amounts of organic content. The Cambrian-Lower Ordovician source rocks provide more resources than the Upper-Middle Ordovician source rocks, the ratio being 63% to 37%; source rocks with low organic matter provide lesser amount of resources than source rocks with high organic matter, the ratio being 32% to 68%. Ordovician probably has more reserves than the estimated amount because a part of it originates from the deeper Cambrian source rocks or source rocks with low organic matter (TOC ≤0.5%)