Fracture and flow characterization of ICDP-DIVE Borehole 5071_1_A from geophysical well logging data

Borehole 5071_1_A of the ICDP-DIVE project intersects the lower part of the lower continental crust and is drilled to a depth of 909.5 m. Several fracture zones are encountered, which not only exert control on the hydrological characteristics of the very low permeability formations in the presence of strong topographic relief, but also affect their mechanical properties. In this study we characterize the fracture network in borehole 5071_1_A with respect to its flow characteristics using a suite of geophysical borehole data. Acoustic and optical televiewer data, as well as normal resistivity logs, are utilized to locate fluid-bearing fracture zones and delineate their geometrical characteristics. Most natural fractures have azimuthal orientations between NNW to NE and exhibit a wide range of dips between 10° – 80°. According to their appearance in the televiewer data they can be divided into three classes, whereby Class 1 consists of the largest aperture fractures and Class 2 and 3 of smaller aperture fractures. Class 1, and clusters of Class 2 and 3 fractures correlate with resistivity anomalies suggesting open fluid-bearing natural fractures, which are encountered along the length of the borehole. To gain further insights into the flow characteristics, a combined analysis of self potential, temperature and mud parameters (conductivity and pressure), as well as flow meter logs, is on-going to locate in- and out-flow zones and to identify water of different compositions and temperature in the borehole. Preliminary results show that in-flow and out-flow zones can be correlated with fractures along the borehole track, whereby three strong in-flow zones around 700 m and 850 m depths correlate with changes in the fluid conductivity. This suggests that different types of water may enter the system, hinting at a compartmentalized complex system with distinct hydraulic zones. The strong in-flow zone at a depth around 850 m is also picked up in passive borehole fiber optic data. Identification of these different flow paths and their correlation to fracture zones provide important information for understanding potential diversity in microbiology in these lower crustal rocks and, support the interpretation of mud gas logging results, allowing a better understanding of the nature and origin of these geofluids.