Three-dimensional reservoir modeling of a tidally-influenced reservoir system: middle Atoka Formation, Arkansas

Abstract

Tidally-influenced reservoirs (TIRs) are common in prominent hydrocarbon fields throughout the world. Tidal influence commonly contributes to an increased distribution of fine-grained interbeds and thin mudstones. These low-permeability layers increase the architectural complexity of the reservoir by acting as baffles and barriers to flow. The complex internal architecture of these systems commonly contributes to low recovery factors and the underestimation of the potential recoverable resource. Despite the potential productive nature of TIRs, there are few published examples of 3D reservoir models attempting to illustrate the connectivity of productive and nonproductive reservoir facies. The few published 3D reservoir models are predicated on the progradational units, and ignore the aggradational to retrogradational units of the TIR system. This study focuses on the aggradational and retrogradational cycles of the Pennsylvanian middle Atoka Formation, aided by 3D quarry exposures. The target exposures, located within the Webco Quarry of White County, Arkansas, encompass unbroken vertical quarry walls, which provide a 3D dataset critical for accurately characterizing the connectivity of the reservoir. The middle Atoka Formation was deposited in the Arkoma Basin as a series of progradational and retrogradational deltas sourced from the northeast. To model the middle Atoka Formation, multiple data sources (core, measured sections, high-resolution photomosaics, and well logs) were integrated to produce a detailed geologic framework. The geologic framework was synthesized into a conceptual geologic model illustrating the facies relationships, facies association morphologies, stacking patterns, and the evolution of the depositional system. The geologic framework was integrated into Schlumberger’s Petrel modeling software to construct a static 3D reservoir model illustrating the three-dimensional distribution and connectivity of facies. The following modeling methods were used and compared to determine the method that most accurately honors the heterogeneous character and the three-dimensional connectivity of facies: sequential indicator simulation (SIS), truncated gaussian simulation (TGS), and multi-point simulation (MPS). In addition, varying cell sizes were used to illustrate how cell size affects the distribution and connectivity of fine-scale heterogeneities, particularly thin mudstones layers. The multi-point simulation (MPS) method produced the static 3D reservoir model that best captures the geologic complexity observed in the Webco Quarry. The MPS reservoir model was compared and contrasted with previous reservoir models constructed on the progradational Sego Sandstone. The aggradational to retrogradational TIR units have smaller facies association geobody dimensions, a higher distribution of heterolithic and mud draped facies, and a higher distribution of thin and laterally-extensive mudstones. The quantitative and qualitative data from this study can be utilized to better constrain the three-dimensional distribution and connectivity of productive and nonproductive reservoir facies in TIR systems. This has significant ramifications to the petroleum industry because it can be used to guide development plans, maximize recovery factors, and enhance performance predictions.