Reconstruction of the geothermal field of the Xihu Depression in the East China Sea Basin and its controlling effect on hydrocarbon generation and distribution

Abstract

The Xihu Depression, situated in the northeastern East China Sea Basin, represents the most significant natural gas-producing region in Eastern China. An insufficient understanding of reservoir heterogeneity in petroleum geological conditions—particularly within structural zones beyond the well-explored Pinghu Slope and Ningbo Anticline Belt—has hindered comprehensive hydrocarbon exploration across the sag. Critical knowledge gaps persist in characterizing the geothermal field, reconstructing thermal evolution histories, and constraining hydrocarbon generation phases. These limitations directly impede systematic evaluations of basin selection criteria, reservoir delineation, and their dynamic relationships within petroleum systems. This study analyzes the present geothermal gradient at a unified depth (4000–5000 m), the geothermal heat flow, the geothermal temperature at a unified depth (3000–6000 m), and the plan distribution characteristics of the geothermal temperatures of the exploration strata in the key study area in the Xihu Depression—the Western Slope and the Central Anticlinal Belt. The research in this study is based on present bottom-hole temperature measurements and temperature data for testing for oil, using a one-dimensional steady-state heat conduction equation and the Bullard method. The results indicate that the present geothermal gradient in the Xihu Depression, between a unified depth of 4000 m and 5000 m, ranges from 16.7 °C/km to 44.6 °C/km, with an average of 30.6 °C/km. The present geothermal heat flow is between 32.23 mW/m² and 90.13 mW/m², with an average of 52.03 mW/m², indicating a typical cold basin. The formation temperature gradually increases with burial depth, from 3000 m to 6000 m. In the plane, the formation temperature gradually increases from the south to the north and from the edge of the depression to the center of the depression. The burial history and thermal evolution of the key plays of the Xihu Depression were reconstructed using apatite fission tracks and zircon U–Th/He data, combined with vitrinite reflectance, which revealed that the tectonic uplift that occurred during the Late Miocene Longjing Movement was a critical event in trap formation and hydrocarbon filling. The thermal-hydrocarbon generation history indicates that the Xihu Depression has mostly entered a high maturity stage, with gas condensate and condensate charging occurring between 16.4 Ma and 13 Ma and natural gas filling occurring at 5.3 Ma up to now. Hydrocarbon generation and expulsion in the Xihu Depression occurred early in the north and late in the south, with two stages in the north and one stage in the south. A study of the burial history–thermal history–hydrocarbon generation history based on the reconstruction of geothermal fields demonstrates the matching relationship between hydrocarbon generation, distribution, and accumulation in the Xihu Depression—an understanding that is vital for oil and gas exploration in the Xihu Depression.