Estimation of thermal maturity in the Bakken source rock from a combination of well logs, North Dakota, USA

摘要

Numerous attempts have been conducted to construct a reliable model to relate petrophysical and geomechanical characteristics of shale to geochemical properties. In this study, continuous logs were built to predict the thermal maturity of the Bakken Formation from a set of petrophysical logs. First, dynamic modulus (E-d) was created from shear and compressional sonic and, density logs and then converted to E-i (nanoindentation based Young's modulus) using the existing experimental relationship between E-d and E-i. Next, continuous TOC logs were generated for the Bakken Shales based on an empirical relationship between well log derived E-i and TOC contents that were measured in the lab. There was found an acceptable difference between estimated log-based TOC and measured values in three separate wells. Solid bitumen reflectance as a reliable indicator of thermal maturity was measured on all samples from the same wells in the scarcity/absence of vitrinite maceral. The relationships between GR/NPHI/RHOZ wireline logs versus solid bitumen reflectance (BRO%) as the maturity index was then investigated in twelve wells. Based on the strength or weight of the relationship between each log and BRO%, a new parameter "T" was defined to represent thermal maturity from well logs. A good agreement between parameter T and BRO% values was observed which provided an empirical equation to make the estimation of BRO% values for the whole length of the shale members possible. Ultimately, the continuity of BRO% values, would enable us to establish a relationship between BRo% and measured T-max values to generate continuous T-max logs for the Bakken Shale members. Using these newly developed well logs, it would become possible to make a 3D property models for BRO% and T-max (the thermal maturity property cube) for an accurate petroleum system evaluation based on GR/NPHI/RHOZ logs.