Abstract
Sediments with organic matter content (total organic carbon) TOC ≤ 0.5% which can act as effective source rocks are critical and challenging in the field of petroleum geology. A new method is proposed through a case study to identify and evaluate the effective source rocks, which is applied to study the changing characteristic of hydrocarbon-generation potential index with depth. The burial depth corresponding to the beginning of hydrocarbon-generation potential index reduction represents the hydrocarbon expulsion threshold in source rocks. Then, new identification standards are established for discrimination of effective source rocks of Middle–Upper Ordovician in Tarim Basin. The critical value of TOC for effective source rocks change with their burial depth: the TOC > 0.5% with source rock depth > 4000 m, TOC > 0.4% with depth >4500 m, TOC > 0.3% with depth > 5000 m, TOC > 0.2% with depth >5500 m. Based on the new criteria, effective source rocks in the Middle–Upper Ordovician are identified and their total potential hydrocarbon resources is evaluated, reaches 0.68 × 109 t in the Tazhong area, which is 65.4% higher than that of previous studies and consistent with the exploration result. Thus, this new method is of significance to resource evaluation and can be applied in the carbonate source rocks and mudstone source rocks with high degree of exploration.
Highlights
Effective source rock refers to the source rocks which have expelled hydrocarbons and made contribution to the formation of hydrocarbon reservoirs in a petroliferous basin (Hunt, 1979; Tissot et al, 1978). Ronov (1958), the former Soviet scholar, is the pioneer who proposed to identify the effective source rock with the value of total organic carbon (TOC)
Based on the new method, the TOC value for identifying effective source rocks should be more than 0.2% but less than 0.5% at a depth of about 5200 m, and the research result shows that the thickness ratio of the effective source rocks to total source rocks changes from 7.8% to 46.1% in Tazhong area (Figure 7(a))
Simple TOC index of source rocks in petroliferous basin cannot be used as the identification criterion for effective source rocks; it can neither reflect their generated hydrocarbon amount nor their expelled hydrocarbon amount
Summary
Effective source rock refers to the source rocks which have expelled hydrocarbons and made contribution to the formation of hydrocarbon reservoirs in a petroliferous basin (Hunt, 1979; Tissot et al, 1978). Ronov (1958), the former Soviet scholar, is the pioneer who proposed to identify the effective source rock with the value of total organic carbon (TOC) . Ronov (1958), the former Soviet scholar, is the pioneer who proposed to identify the effective source rock with the value of total organic carbon (TOC) . He found that the TOC of the shale source rocks is larger than 0.4% in the petroliferous basins of Siberia platform, and the average organic matter abundance TOC < 0.5% in areas without oils discovered. He believed that it was the difference in organic matter abundance that determined the presence or absence of hydrocarbon reservoirs. This method would be applied in Tazhong area, Tarim Basin, and verify its reliability
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