Source rock evaluation and prediction of effective hydrocarbon kitchen distribution of Upper Triassic in Junggar Basin

Abstract

The Junggar Basin is known for its high potential of petroleum resources, showcasing significant oil and gas discoveries across various layers. However, the potential of utilizing the thick mudstone of the Upper Triassic as a viable source rock in the basin remains uncertain. Therefore, an urgent need exists to assess the Upper Triassic source rock across the basin and predict the distribution of effective source rocks. This evaluation is crucial for informing future exploration strategies in this layer. To shed light on the spatiotemporal distribution characteristics and hydrocarbon generation potential of the Upper Triassic source rocks in the Junggar Basin, as well as to determine the hydrocarbon generation intensity within the effective hydrocarbon kitchen's distribution range, a comprehensive evaluation was conducted. This evaluation involved assessing organic matter abundance, type, and maturity of the Upper Triassic source rock across the basin using various geochemical parameters derived from data collection, profile measurement, drill cuttings observation and sampling, and organic geochemistry testing. Key parameters were then selected, and a multi-level fuzzy mathematical evaluation method was employed to map out the development range of the effective hydrocarbon kitchen. The findings indicate that the Upper Triassic source rock exhibits a broad distribution range spanning the entire basin, with considerable thickness (reaching up to 350 m) and significant burial depth in the central region (up to 11 000 m). The total organic carbon (TOC) content of source rocks varies from 0.75% to 8.30%, with an average value of 1.74%. Additionally, the average values of S1+S2 and chloroform asphalt "A" are 2.78 mg/g and 0.354%, respectively. The predominant organic matter types in the area are type Ⅲ and type Ⅱ2, indicating a dominance of terrestrial higher plant inputs. The favorable kerogen types are primarily located from the western sag to the Fukang Sag. The thermal evolution of the organic matter ranges from low-maturity to mature stages, with some areas reaching high to over-maturity levels. As per the evaluation criteria for hydrocarbon generation potential of coal-bearing mudstone in China, the Upper Triassic mudstone generally represents poor to medium-quality source rocks with significant hydrocarbon generation potential, particularly for gas. The key hydrocarbon source kitchens are situated from the western part of the Fukang Sag to the eastern part of the Shawan Sag in the central depression, and in the Huomatu Anticline of the southern margin thrust belt. Within these areas, the Ro value of the source rock exceeds 1.3%, with oil and gas generation intensities reaching 50×104 t/km2 and 3×108 m3/km2, respectively.

Keywords

• source rock evaluation
• effective hydrocarbon kitchen
• upper triassic
• junggar basin