ABSTRACT: Rebound hardness (RHN) has recently gained considerable interest as a rock mechanical parameter in the petroleum industry. However, few published studies target a comprehensive integration among RHN and reservoir parameters, which can be highly valuable in reservoir characterization and production. This study focuses on the integration among RHN and facies, mineralogy, natural fractures, reservoir quality, and rock mechanical properties of the unconventional “Mississippian Limestone” play in north-central Oklahoma, USA. In 2415 feet (736 m) of core, RHN correlates with porosity, permeability, and critical rock mechanical properties, suggesting its potential value as a quick and inexpensive tool in reservoir characterization and production design. RHN exhibits varying patterns in relation to porosity and permeability in different play areas, likely related to different depositional settings and sampling bias. Therefore, the prediction of reservoir quality from RHN should be tailored among different play areas with a well-defined sampling protocol. Results also indicate that RHN correlates well with mineralogy but exhibits limited variability among many fractured and non-fractured zones. This suggests that the present-day rock mechanics are likely a combination of earlier “unaltered” and later “altered” characteristics, indicating that the temporal evolution of rock mechanical properties should be considered in reservoir characterization and production design.
The “Mississippian Limestone” Play, located primarily in Oklahoma and southern Kansas (Figure 1), has been developed using conventional vertical drilling techniques for over half a century and has recently become one of the most active unconventional resource plays in North America. The associated strata - the “Mississippian Limestone” (MISS) - is an informal stratigraphic nomenclature which includes the Mississippian (Early Carboniferous)-aged strata present across the U.S. Southern Mid-Continent, including parts of Kansas, Missouri, Arkansas, and Oklahoma (Figure 1). As opposed to the historic “Mississippian Limestone” play, there are several recently discovered play areas nearby that target the Mississippian section, such as the “STACK” play southwest of the “Mississippian Limestone” play area (Figure 1).
The “Mississippian Limestone” play in north-central Oklahoma is characterized by low porosity (largely around 5%) and low permeability (fraction of milli-darcies) with compartmentalized reservoir distribution. Utilizing around 660 digital well logs and five cores with complete Mississippian sections, this study aims to characterize the regional stratal geometry and reservoir distribution using sequence stratigraphic approach for enhancing reservoir characterization and prediction.
Four “third-order” sequences (Sequence 1 to 4), which are generally present throughout the study area, are defined in cores with mud-rich siltstone at base which corresponds to regionally correlative gamma-ray (GR) peaks and silty limestone near the top. On top of them, three sequences (Sequence 5 to 7) are represented by generally higher GR values with a cleaning-upward pattern. The vertical stacking of these seven sequences allows the construction of a regional sequence stratigraphic framework which is possibly originated from eustatic sea-level changes. The “thirdorder” sequences exhibit clinoformal geometry and different distribution patterns. Sequence 1 and 4 are constrained around regional structural features (e.g., Nemaha Uplift) which possibly provide antecedent/syndepositional topography via active uplifting. In contrast, such structural impact on deposition is not observed in Sequence 2 and 3, suggesting the episodic nature of structural activity. Sequence 5 to 7 are only present directly basinward of the Nehama Uplift and prograde basinward through time, possibly attributed to the active uplifting which produced deeper water condition at its basinward side.
Reservoir zones are heterogeneous vertically and laterally in both strike and dip directions. Vertically, these zones are commonly located in the regressive phase of certain “third-order” sequences. Although exceptions are present, such pattern implies the potential value of sequence stratigraphy in predicting reservoir distribution among closely spaced wells. Laterally, these zones are compartmentalized, even for correlative sequences among closely spaced wells (e.g., 1 km), suggesting complex reservoir distribution and the caveat of “correlating” reservoir zones. Detailed understanding of regional stratal distribution in a sequence stratigraphic framework constrained by decent core data is crucial for tailoring exploration and production strategies among different areas of the play.