Maturity Evolution History and Hydrocarbon Generation Evolution History of Hydrocarbon Source Rocks in the Fengcheng Formation, Shawan Sag, Under the Influence of Overpressure

With the continuous advancement of oil and gas exploration technologies and associated theoretical frameworks, deep to ultra-deep oil and gas exploration has emerged as a pivotal component of contemporary petroleum geology research and currently seems as a focal area of interest. But, overpressure is prevalent in deep to ultra-deep formations, and there are substantial discrepancies in studies regarding the impact of overpressure on the maturity of hydrocarbon source rocks and hydrocarbon generation. These discrepancies have hindered a comprehensive understanding of hydrocarbon formation and phase states in ultra-deep settings, thereby constraining deep oil and gas exploration endeavors.

The Shawan Sag in the Junggar Basin is characterized by widespread and high-intensity overpressure development, coupled with huge hydrocarbon resources in the lower stratigraphic assemblage. Among these, The Fengcheng Formation, serving as the primary hydrocarbon source rock, is the main source of oil and gas for the reservoirs within the sag. Consequently, this study focuses on the Permian hydrocarbon source rocks in the Shawan Sag of the Junggar Basin as the primary research target. After ascertaining the overpressure development in the Permian strata, typical low-maturity samples were selected to conduct hydrocarbon generation physical simulation experiments under varying pressure conditions. Based on these experiments, maturity evolution and hydrocarbon generation kinetic models that account for pressure were established, and thermal-maturity-hydrocarbon generation evolutionary history simulations were performed for typical wells and a 2D cross-section.

The results reveal the following: (1) There is a negative correlation between vitrinite reflectance and pressure in the vertical direction, with Ro evolution being lower than the normal trend in overpressure zones. (2) Thermal simulation experiments confirm that, under identical temperature conditions, higher pressure leads to a lower equivalent Ro and a greater proportion of medium-to-heavy components in the generated hydrocarbon products, demonstrating the inhibitory effect of pressure on hydrocarbon source rock maturity and hydrocarbon generation products. (3) Based on the results of physical simulation experiments and measured geological data, a 2D thermal-maturity-hydrocarbon generation evolutionary history for the Shawan Sag was simulated. It is concluded that the hydrocarbon source rocks in the Fengcheng Formation of the Shawan Sag are predominantly Type II highly mature hydrocarbon source rocks. The hydrocarbon generation threshold is suppressed until the end of the Triassic, with significant oil generation commencing in the late Jurassic and entering the highly mature stage by the end of the Cretaceous. Regarding hydrocarbon generation products, the cracking of heavy hydrocarbons to generate gas in the Fengcheng Formation is inhibited by overpressure. This study contributes to enhancing the theoretical understanding of overpressure-inhibited hydrocarbon generation and holds practical significance for hydrocarbon exploration in ultra-deep formations within the Shawan Sag.