Deep-Learning-Based Automated Stratigraphic Correlation
Authors: Tokpanov Yury , Smith Johanna , Ma Zheren , Deng Lichi , Benhallam Wassim , Salehi Amir , Zhai Xiang , Darabi Hamed , Castineira David 
Published: October 2020

Abstract
Stratigraphic correlation is essential in field evaluation as it provides the necessary tops to compartmentalize the reservoir. It further contributes to other parts of the field development planning cycle such as reservoir modeling, volumetric assessment, production allocation, etc. Traditional approach of manual pairwise correlation is labor-intensive and time-consuming. This research presents a novel automated stratigraphic correlator to create well top and zonation interpretations using supervised machine learning algorithms of Convolutional- and Recurrent-Neural-Networks (CNNs and RNNs).
An automated stratigraphic correlator is created that enables stratigraphic well top and zonation interpretations learned from the well logs of a subset of wells with zonation information manually provided by human experts. The method can efficiently learn the patterns and hidden information from the well logs’ sequential data, implicitly capture the domain expertise, and streamline and automate the traditional manual repetitive work. Our method supersedes existing approaches like Multiple Sequence Alignment (MSA) by incorporating domain expertise through tops/zones picked by geologists. A Bidirectional Long Short-Term Memory (BiLSTM) is used to interpret the log data, since deposition by nature is a sequential process and RNNs can intrinsically capture such series. An Inception autoencoder CNN is also applied in this workflow for stratigraphic interpretation. Reliable post-processing is also included using the predicted zone probability logs to quantify the overall confidence score of well zonation, and to correct misinterpretation when necessary using transition frequencies in log data through a linear chain graphical probabilistic model. The methodology is tested on one of the major Middle East oilfields with around 1,500 wells to prove its efficiency and capability.
The overall methodology involves data pre-processing, deep learning model training and prediction, and the post-processing of model-predicted results. In this specific workflow, the machine learning targets include both the prediction of zones (multi-class classification/segmentation problem) and the prediction of well tops (edge-detection problem). Thus, a supervised multi-task learning on a single field using CNNs and RNNs is implemented to be able to perform different tasks with the same model. The inputs to the training module include trajectory logs and other measured logs such as gamma-ray, resistivity, neutron density, etc. All inputs are normalized to zero mean and unit standard deviation. For wells with missing log values, the approach can either discard it or perform data imputation to reconstruct the data using different automated algorithms. The machine learning engine uses two different algorithms (BiLSTM and Inception autoencoder CNN), with many other deep learning models tested. The training loss function includes zone categorical cross entropy loss, tops edge detection binary cross entropy loss and L2-norm regularization term. The learning rate is dynamically adjusted during training so that it is reduced when the loss is stalled. The post-processing uses the machine learning predicted zone probability logs to select the zoning sequence that maximizes overall zonation probability and treats it as the confidence score of well zonation. This dramatically helps in constraining the outcome stratigraphic interpretation by geological succession and minimizing the correlation error. The entire workflow has been applied to one major Middle East oilfield with a large number of pre-interpreted well logs, with 60% of the wells used to train the deep learning models, 20% used for validation and the rest are for blind test. Both BiLSTM and Inception autoencoder CNN show close to human-level performance in the blind test dataset. The mean absolute error of well tops interpretation after post-processing is around 3 m throughout all analyzed wells, which provided an accuracy of nearly 90% for the blind test dataset. The classification precision and accuracy also demonstrate close-to-human-level performance in the major zones with sufficient data. It has been noticed that for cases without missing data, Inception autoencoder CNN achieves best performance, while BiLSTM benefits a lot from imputation when missing data exists.
The methodology automates and streamlines the originally time-consuming stratigraphic correlation process. It performs better than existing approaches through a well-developed machine learning framework with comprehensive data pre- and post-processing. The resulting stratigraphic correlation proves to be extremely reliable even with a small number of seed wells, and it requires minimal user intervention during the process. Through deep learning techniques such as transfer learning, the proposed methodology can be readily applied to other fields even with limited training data.