Fracture characterization in oil-based mud boreholes using image logs: example form tight sandstones of Lower Cretaceous Bashijiqike Formation of KS5 well area, Kuqa Depression, Tarim Basin, China

Abstract

The natural fractures are dominated factors for the gas yield in deeply buried tight sandstones of Lower Cretaceous Bashijiqike Formation of KS5 well area, Kuqa Depression. For the high temperature and pressure environment, boreholes were drilled in oil-based mud. To characterize fractures in ultra-deep formation, electrical imagers (High-definition Formation MicroImager (FMI-HD), Earth Imager (EI), and Oil Base MicroImager (OBMI)) and acoustic imagers (Circumferential Borehole Imaging Log (CBIL) and Ultrasonic Borehole Imager (UBI)) were run in five oil-based mud openholes in KS5 well area. In this paper, fractures in different classifications were interpreted and compared in various borehole images. Opened and partially opened fractures were, respectively, continuous and discontinuous bright sinusoidal traces in electrical images. Closed fractures should be recognized by electrical and acoustic images cooperatively. In Bashijiqike Formation, opened and partially opened medium-angle, high-angle, and network fractures were well developed, while low-angle fractures were infrequent. Bashijiqike Formation might be divided into fractures well-developed, no-developed, and less-developed intervals upwards vertically. The strikes of fractures concentrated in nearly NWW-SEE, paralleled the strikes of major faults. This paper might help in understanding the complete vertical distributional characteristics of fractures in tight sandstones of Lower Cretaceous Bashijiqike Formation of KS5 well area and in turn on the exploration potential of petroleum systems.