Quantitative characterization and modeling strategies of architectural units in carbonate platforms

Reservoirs within carbonate platforms are governed by complex variations in facies zones, exhibiting a wide range of sedimentary microfacies, including tidal channels and reef shoals. Since the sedimentary microfacies interbed with each other, carbonate platform reservoirs feature diverse architectural styles and complex macroscopic structures. Consequently, conventional individual modeling techniques are often inadequate to effectively characterize the reservoir structures of carbonate platforms that experienced rapid facies transitions under differential sedimentary settings. Therefore, it is necessary to develop quantitative characterization and modeling strategies for the architectural units of reservoirs tailored to different microfacies. By integrating core observations, logs, and seismic data, this study presents a hierarchical analysis of architectural units of carbonate reservoirs in the H oilfield in Iraq. As a result, two types of sedimentary architectural unit assemblages are identified within the carbonate platforms: tidal channel-bioclastic shoal complexes and tidal bioclastic delta complexes. Based on seismic response characteristics, the interface morphologies and spatial distribution patterns of these architectural units are delineated. Accordingly, for microfacies within these architectural units, we systematically analyze their differential overlapping styles in both plane and cross-section views based on their scales and evolution patterns. Furthermore, a layered and segmented modeling strategy is established for carbonate platform reservoirs. The analytical results indicate that tidal channel-bioclastic shoal complexes primarily occur within the MB1-2 layers. These architectural units exhibit roughly uniform thicknesses (average single-stage thickness of approximately 15 m) and high width-to-thickness ratios. Based on variations in architectural style, these complexes can be subdivided into three segments: meandering, aggradational, and migratory segments. Within each segment, distinct interfaces of tidal channel-bioclastic shoal complexes are identified. The primary differences among the three segments arise from tidal channels in different migration directions, which result in varying degrees of erosion and incision of bioclastic shoals under the influence of the meandering of single-stage tidal channels. Consequently, the shoal bodies show mutual truncation and stacking. Based on these findings, a multi-level architectural modeling strategy is proposed for tidal channel-bioclastic shoal complexes. Specifically, the 5th-level architectural model is established using a deterministic modeling method with deterministic interfaces as constraints. Then, the 4th-level architectural model is constructed within the framework of the 5th-level architectural model using the object-based modeling (OBM) method, with modeling parameters set based on the statistical geometric features of architectures. On the other hand, tidal bioclastic delta complexes are predominantly identified within the MB2 layer. These architectural units pinch out from the center toward both sides, featuring relatively large average single-stage thicknesses (approximately 30 m) and low width-to-thickness ratios. Based on variations in architectural style, these complexes can also be further subdivided into three segments: distal progradational delta, incised valley, and proximal retrogradational delta. Under the constraint of the 5th-level deterministic architectural interfaces, the 4th-level architectural models of tidal bioclastic delta complexes are established through multi-point statistics (MPS) simulation. During the modeling process, the progradational pattern of tidal deltas is incorporated for image training and modeling parameter setting. This study systematically reveals the architectural types and their geometric features within carbonate platforms and establishes modeling workflows based on geometric features, providing a reference for geological modeling of similar reservoirs.