Simultaneous estimation of in situ porosity and thermal structure from core sample measurements and resistivity log data at Nankai accretionary prism

Abstract

The shallow accretionary prism of the Nankai Trough is a location where both large interplate earthquakes and slow earthquakes occur. Since the physical properties of sedimentary materials are important topics for understanding the structure of the prism, numerous ocean drilling expeditions have been conducted in that region to obtain logging data and core samples. Although the physical properties of the obtained samples are normally measured onboard immediately after coring, estimations of in situ physical properties are difficult because of differences in laboratory and in situ physical conditions. Herein, we propose a new method for estimating in situ porosity from downhole electrical resistivity log data that evaluates in situ porosity and thermal structure simultaneously using correlations between the porosity and resistivity, and between the porosity and thermal conductivity that were established based on laboratory measurements. When constructing physical property correlations, X-ray computed tomography data play an important role in estimating the porosity of samples from which resistivity or thermal conductivity were measured. To validate our method, we compared the estimation with density log data collected at Site C0002 and found that the estimated in situ porosity shows good agreement with the in situ porosity converted from density log data. A comparison with porosity measured onboard for core and cutting samples showed that they are consistent with each other. With this new method, continuous distributions of in situ porosity and thermal structure can be estimated simultaneously based on resistivity log data and heat flow, which are basic quantities acquired during ocean drilling science expeditions. which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

A New Method for the Empirical Conversion of Logging Data to Clay Mineral Fraction in the Nankai Accretionary Prism

“…Although a thermally steady state is assumed when calculating the thermal structure, corrections are sometimes applied according to the depositional history of each site using a correction factor α. A detailed description of this process is described in Yabe et al (2019). The iterative estimation process continues until the results converge.…”

“…Yabe et al applied this method to Site C0002 and showed that the estimated structures are consistent with the porosities and thermal conductivities measured in core samples, as well as with porosities calculated from density log data. In this study, we applied the method developed by Yabe et al (2019) to all NanTroSEIZE sites where electrical resistivity logs are available. The bit resistivity was used for the conversions in this study because it usually has the greatest investigation depth among the acquired electrical resistivity logs, and is less in uenced by the drilling mud invasion effect and/or borehole conditions.…”

“…The third dataset is porosities converted from density log data. Because grain density and uid density are estimated from shipboard MAD measurements as 2.7 and 1.0 g/cm 3 , respectively (Yabe et al 2019), density logs can be converted to porosities. Although it is the most reliable dataset measured in situ, density log data are only available for sites C0001, C0002, and C0009.…”

“…We set the thermal structure correction factor α to 0.78 at the sea oor, and it was linearly increased to 1.0 at 830 mbsf, where the unit boundary between Units 2 and 3 is estimated to occur, because the Kumano Basin sediments have a rapid sedimentation rate (Expedition 315 Scientists 2009c). As this site was investigated previously by Yabe et al (2019), refer to their study for details. At site C0009, wireline logging was conducted only in the deeper part of the hole, which prohibited the application of the method developed by Yabe et al (2019) at this site.…”

“…Using logging data and core measurements acquired throughout the NanTroSEIZE project, we established an empirical conversion function from the NGR log data to the clay mineral fraction. Because the number of radioactive elements around the detector in the logging tool is affected by the porosity of the formation, we compensated for this effect by estimating the porosity structure from electrical resistivity logs using the method of Yabe et al (2019).…”