Stratigraphic and lithofacies control on pore characteristics of Mississippian limestone and chert reservoirs of north-central Oklahoma

Abstract

We have determined how stratigraphy and lithofacies control pore structures in the Mississippian limestone and chert reservoir of north-central Oklahoma. There are 17 lithofacies and 29 high-frequency cycles documented in the Mississippian interval of this study. The high-frequency cycles have thicknesses ranging from 0.3 to 30.5 m (1–100 ft) and are mainly asymmetric regressive phases. The pore characteristics, measured through digital-image analysis (DIA) of thin-sections photomicrographs ([Formula: see text]100), exhibit unique correlations with core porosity, permeability, and lithofacies within a sequence-stratigraphic framework. There are five fundamental correlations observed. First, porosity from DIA and laboratory core measurements has a strong positive relationship ([Formula: see text]). However, some values from DIA porosity yield relatively higher values, specifically in spiculitic mudstone wackestones and argillaceous spiculitic mudstone wackestones. The difference is hypothesized due to the presence of isolated nanopores that are not accessible by helium during measurement of core porosity. Second, the relationship between pore circularity and permeability is indeterminate. The indeterminate relationship is related to a complex internal pore network, intensive diagenetic alteration, an unconnected microfracture network, and isolated pores. Third, positive moderate to strong correlations ([Formula: see text]) between porosity and permeability are observed only in four lithofacies. Fourth, coarse-grained lithofacies within the uppermost depositional sequence of the Mississippian interval have a heterogeneous pore-size distribution, whereas fine-grained lithofacies tend to exhibit a homogeneous pore-size distribution. Fifth, higher reservoir quality is associated with the upper intervals of high-frequency shallowing-upward cycles. This confirms that the sequence-stratigraphic variability of lithofacies is important to predict reservoir quality and its distribution. An alternative graphical method of pore-size distribution is also developed. To be a useful “technique,” examples of the plot are demonstrated using samples in this study. The plot successfully provides simple identification of pore-size classes, quantitative percentage of pore-size class, dominant pore class, and approximate minimum and maximum pore size.

Convolutional neural networks as aid in core lithofacies classification

“…We used cores described using traditional methods published by Suriamin and Pranter (2018), capturing images using modern photographic equipment to generate the set of labeled data to feed our CNN. The total section used for this project consists of approximately 700 ft from one core from the Mississippian limestone and chert reservoirs in the Anadarko Shelf, Grant County, Oklahoma.…”

“…The last column of this table (the test set) comprises the selected images described in Figure 1, and it is the test set used for further analysis in this paper. Classes with fewer than 30 original images were not used in this study (modified from Suriamin and Pranter, 2018). *Classes that were augmented by horizontally flipping the images.…”

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“…Core-based rock-type descriptions are important for understanding the lithology and structure of subsurface geology. Using several hundred feet of labeled core from a Mississippian limestone in Oklahoma (data from Suriamin andPranter, 2018 andPires de Lima et al, 2019), we selected a small sample of 285 images from five distinct lithofacies to be classified by the retrained CNN models. Pires de Lima et al (2019) describes how a sliding window is used to generate CNN input data, cropping small sections from a standard core image.…”