Revealing a deep condensate gas reservoir in an Archeozoic metamorphic buried hill by ultrasonic image logs, spectroscopy logs, and core data, Bohai Bay Basin, Eastern China

The Archeozoic metamorphic buried hill reservoir of the Bozhong Sag in the Bohai Bay Basin has a proven condensate gas reserves of over 100 billion cubic meters. A clear understanding of buried hill vertical zone variation is vital to characterize reservoir quality and reservoir distribution. Conventional core sample analysis is far more sufficient to characterize the complexity and diversity of basement reservoirs. Weathering, lithofacies, and fracture networks also play important roles in controlling the reservoir property. The fractures in the target area are unevenly developed due to multitectonic activities and stress field rotation during the initial period of basement evolution. In addition, the mineral composition of the lithofacies is complex due to metamorphism, alteration, and weathering, which further affect the reservoir quality. Characterizing the effective fracture distribution of the buried hill formation across the area is crucial to classifying good reservoir zonation. However, identifying fractures by an electrical resistivity borehole image endures high uncertainty as conductive fractures are possibly filled by clay, iron minerals (pyrite or siderite), or mud filtrate. Ultrasonic image logs are used to identify fractures in combination with amplitude and transit time images, which can clearly identify the effective or noneffective fractures. In total, four types of fractures are identified: closed fractures, fully open fractures, partially open fractures, and vuggy open fractures. Based on the lithofacies, fracture development, and volumetric minerals from spectroscopy logs and conventional logs, the Archeozoic metamorphic buried hill is revealed zone by zone: the strongly weathered zone, subweathered zone, and insider buried hill zone. The reservoir quality, lithofacies identification, multiwell fracture development analysis by three buried hill zones, and production controlling factors are fully demonstrated.