REVEALING DEEP CONDENSATE GAS RESERVOIR IN A ARCHEOZOIC METAMORPHIC BURIED HILL RESERVOIR BY ULTRASONIC IMAGE LOGS, SPECTROSCOPY LOGS AND CORE DATA, BOHAI BAY BASIN, EASTERN CHINA

Abstract

The Archeozoic metamorphic buried hill reservoir of Bozhong Sag in Bohai Bay Basin is a proven condensate gas reserves over 100 billion cubic meters. Clear understanding on buried hill vertical zones variation is vital to characterize reservoir quality and reservoir distribution. Conventional core samples analysis is far than enough to characterize the complexity and diversity of basement reservoir. Weathering, lithofacies, and fracture network also play important role in controlling the reservoir property. The fractures in the target area are unevenly developed due to the multi tectonic activities and stress field rotation during the initial period of the basement evolution. In addition, the mineral’s composition of the basement lithofacies is complex due to the metamorphism, alteration and weathering which further affect the reservoir quality. Characterizing effective fracture distribution of the basement lithofacies across the area is crucial to classify the good reservoir zonation. However, identifying fracture by electrical resistivity borehole image endured high uncertainty as conductive fracture are possibly filled by clay, iron minerals (pyrite or siderite) or mud filtrate. Ultrasonic image logs can identify different fracture based on both the amplitude image and transit time image, which can clearly identify effective or non-effective 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, volumetric minerals from spectroscopy logs and conventional logs, the archeozoic metamorphic buried hill was revealed zone by zone. Strongly weathered zone, sub weathered zone and insider buried hill zone were proposed. The reservoir quality, lithofacies identification, multi well fracture development analysis by three buried hill zones and production controlling factors were fully demonstrated in this study.