GNGTS 2021 - Atti del 39° Convegno Nazionale

GNGTS 2021 S essione 3.1 392 GEOPHYSICAL METHODS FOR SOURCE ROCKS INVESTIGATION V. De Tomasi, G. Gambacorta Eni S.p.A. Natural Resources, San Donato Milanese, Italy Introduction The estimation of kerogen volume and quality in source rocks is fundamental for estimating the production potential of a basin. Methods for its estimation are usually based upon analysis of rocks coming fromoutcrops or core samples. Recently several authors demonstrated the feasibility of using geophysical methods for estimating kerogen in source rocks (Loseth 2011, Amato et al. 2018). These methods are based upon the fact that the kerogen-rich rocks are characterized by low impedance and high intrinsic attenuation. Several methods to detect kerogen-rich rocks have been tested, and results are here briefly presented and discussed. Theory and Methods Rock physics models for source rocks were calculated using data from core samples to seed the model. Kerogen elastic properties were obtained from literature (Carcione 2000, Yan and Han 2013). The model was used to predict seismic impedance and Amplitude Versus Angle (AVA) response vs. Total Organic Carbon (TOC) abundance in the source rock. As expected, the TOC causes a rapid decrease of rock acoustic impedance for increasing TOC values (fig. 1, left). The impedance model was compared against well data from four locations, and a good agreement between model predictions and experimental data was found. It was observed from data that kerogen with a marine organic matter origin (kerogen II) has a lower impedance when compared against the kerogen mainly terrestrial (kerogen III). Acoustic impedance inversion can then be used to estimate within shales TOC presence from seismic data, and the model allows to directly convert impedance into TOC values. The drawback of impedance inversion is that it requires at least one well to calibrate inversion, so its value as an exploration tool could be limited. We then considered the AVA response for kerogen detection sensitivities. Trial AVA models were computed, changing anisotropy Figg. 1 - Left plot shows measured TOC vs impedance fromwell logs. The red line is the impedance predicted by Backus model. Blue and black dots identify data from wells located in a distal environment, the purple ones are from a proximal well, while green dots refer to data coming from a well located on the coastline. Right plot shows AVA intercept vs. gradient plot for the top interface of the source rock layer. The points are color coded according to their location along the seismic line.