Machine learning-based classification of petrofacies in fine laminated limestones.

Characterization and development of hydrocarbon reservoirs depends on the classification of lithological patterns from well log data. In thin reservoir units, limited vertical data impedes the efficient classification of lithologies. We present a test case of petrofacies classification using machine learning models in a thin interval of finely laminated limestones using pseudo-well data created over outcrops (radiometric and unconfined compressive strength logs). We tested Gaussian naïve Bayes (GNB) and support vector machine (SVM) techniques to classify eight petrofacies types, divided into two groups. The objective was to observe the capacity of some well-known models to classify petrofacies with a high-frequency vertical variation of diagenetic heterogeneities in an extreme scenario within a thin sedimentary interval. The GNB was less effective (F 1 score of 0.29), and the SVM achieved the best results in classifying the main facies patterns (F 1 = 0.47). However, the GNB performed better when the analysis was focused on distinguishing the two main groups of petrofacies. The results demonstrate that high-frequency facies variations present a challenge to the automatic identification of lithofacies, mainly due to local variations in horizontal heterogeneities (on the mm- to cm-scale) created by depositional and diagenetic processes, which impact the flow in porous media.

Stratigraphic Relations of the Ipubi Formation: Siliciclastic-Evaporitic Succession of the Araripe Basin.

The Ipubi Formation represents the Aptian-Albian siliciclastic-evaporitic succession of Araripe Basin, NE Brazil. This succession comprises siliciclastic rocks (bituminous shales and claystones) and evaporites (gypsum and secondary anhydrite) and represents part of the lacustrine-shallow marine post-rift phase I. This study used sequence stratigraphy concepts to define the relations between changes in the relative lake level and the formation of Ipubi deposits. Results show that the organic-rich shales of the Ipubi Formation formed during a transgressive pulse that covered large areas of the proximal domains. These deposits overlie a regional unconformity that marks the end of the deposition of the underlying Crato Formation. A High Stand stage that followed the transgression influenced the formation of evaporitic deposits. Climate conditions played a major role in influencing the triggering and stopping of evaporite deposition. Thus, a new relative lake level fall event caused the exposure of the Ipubi Formation deposits, and created another regional subaerial unconformity accompanied by widespread karstification of evaporite beds. A posterior transgression caused the deposition of siliciclastic rocks of the Romualdo Formation over the Ipubi Formation strata, and also promoted a new event of karstification of the Ipubi upper evaporite beds.