Characterizing Deep Buried Basement Metamorphic Condensate Gas Reservoir by Image, Logs, Spectroscopy Logs, and Core Data, Bohai Bay Basin, Eastern China

Abstract

The recently discovered BZ19-6 basement reservoir of Bozhong Sag in Bohai Bay Basin is a proven condensate gas reserves of over 100 billion m3. It is the largest deep-buried condensate gas field discovered in the metamorphic basement, with a gas column over 1,000 m. A clear understanding of the controlling factors of the reservoir quality and reservoir distribution is vital to the timely formulation of field development strategy, which may affect following production and recovery. Basement reservoir quality is mainly determined by weathering, lithology, and fracture network due to very low matrix porosity and permeability. The fractures in the target area are unevenly developed due to the multitectonic movements and stress field rotation. In addition, the mineralogical composition of the basement lithology is complex due to the metamorphism and alteration, which may further affect the reservoir quality. Characterizing effective fractures distribution and minerals of the basement lithology across the area is crucial to classify reservoir quality. Porosity and permeability are analyzed based on 311 core samples, and it shows average porosity of 3.91% and average permeability of 0.33 × 10-3 md. Plenty of thin sections were made to analyze and classify lithology into unfavorable and favorable lithology for reservoir quality. Ultrasonic borehole image was used to identify different types of fracture based on the amplitude image and traveltime image. In total, fractures are categorized into four types: they are close fracture, fully open fracture, partially open fracture, and vuggy open fracture. The fracture orientation and density of different types for 10 wells are summarized across the whole area. Spectroscopy logs, X-ray diffraction (XRD), and X-ray fluorescence (XRF) are combined for vertical mineralogical composition and elemental analysis. Core and thin sections are also used to validate the mineralogical alteration from a microperspective. Then, the spectroscopy logs and ultrasonic borehole image were first integrated to identify the structural transition boundary (STB) across the area. Finally, the connection between the reservoir quality, fracture network, and lithology was established. Based on thin sections and spectroscopy logs, fractured or carbonated granite gneiss is attributed to favorable lithology with high porosity, while altered andesite, clayified granite gneiss, and diorite are attributed to unfavorable lithology with low porosity. Ultrasonic borehole image shows fracture orientation rotated in the STB where main elements (Mg, Si, Al, and Fe) correspondingly change and above which effective (vuggy and open) fractures are mainly developed. The test data shows that high production is in the zonation with more effective fractures and higher porosity. Linking element change from spectroscopy logs to fractures orientation variation from ultrasonic borehole image logs for structural analysis is a novel perspective to zoom in on the reservoir compared to seismic or outcrops. The reservoir quality and fracture characterization result will aid in making efficient decisions on developmental strategies, including optimization of future well planning and placement.