Research paperKarst dissolution along fracture corridors in an anticline hinge, Jandaíra Formation, Brazil: Implications for reservoir quality
Introduction
Understanding the time-space evolution of fracture patterns can help in modeling and predicting fluid flow in carbonate aquifers and oil reservoirs (Xu et al., 2017) since these fractures influence porosity and permeability (e.g., Antonellini and Mollema, 2000; Zhiqian et al., 2016). Fractures can act as either seals or conduits (Bourne et al., 2001; Agosta, 2008) and can connect or compartmentalize reservoirs at different stratigraphic levels (Ozkaya et al., 2007; Questiaux et al., 2010). The 3D geometry, orientation, size and density of fractures are key parameters in fluid flow modeling and in strategies for reservoir development (Gholipour et al., 2016).
Fractures are very common in carbonate reservoirs (Zhiqian et al., 2016). However, even with recent methodological advances in acquisition and processing of subsurface data, identification and characterization of subseismic fractures is still a major challenge (e.g., Gholipour et al., 2016). This is because fractures can be directly investigated only with borehole data in the subsurface. For example, a few wells have been cored to allow the complex quantification of several fracture systems in reservoirs (e.g., Gholipour et al., 2016), whereas others have integrated structural and stratigraphic analyses of outcrop and borehole data and used them as input parameters in the development of a static geological model of the reservoir (e.g., Ogata et al., 2014b). Second, there is insufficient quantitative characterization of fracture geometries, spacing, and size in folded carbonate strata to allow 3D modeling of fracture at a reservoir scale (Gholipour et al., 2016). Third, few techniques can be used to determine fracture spacing in fractured reservoirs, such as image logs, production data, and well tests, which provide only estimates of fracture density (e.g., Moody and Esser, 1975; Gholipour, 1998; Gholipour et al., 2016). In addition, horizontal wells can provide in a few cases quite accurate data on fracture density if oriented correctly (Ogata et al., 2014b). Fourth, fracture density in folded strata may depend on bed thickness, lithology and petrophysical properties, but these parameters must be derived from wells, which may not be available in several parts of a reservoir (Jadoon et al., 2006).
In the last two decades, fracture corridors, defined as closely spaced fracture sets (e.g., Laubach et al., 1998; Olson, 2004; Belayneh et al., 2007; Ogata et al., 2014a), have been described to form pathways for flow and to greatly influence top seal integrity at different stratigraphic levels (Ogata et al., 2014a; Watkins et al., 2015). The fractures along the corridors are generally arranged in the same direction as the maximum horizontal stress (σ1) at the moment of deformation and usually occur at damage zones. Fracture corridors are often found where fault displacement dies out laterally and extends into a fault-tip damage zone, as well as at fold hinges (Bockel-Rebelle et al., 2004; Singh et al., 2009; Ozkaya, 2013; Ogata et al., 2014a; Lamarche et al., 2018). In the subsurface, fracture corridors are one of the main structural heterogeneity inside fractured reservoirs and can form very high-permeability pathways, such as super-k zones (Bockel-Rebelle et al., 2004; Questiaux et al., 2010). These high-permeability structures can control the fluid movements in such reservoirs and have a strong effect on fluid flow, which increases hydrocarbon production but may connect the water table with overlying reservoirs and rapidly lead to water breakthrough (Singh et al., 2009; Lamarche et al., 2018). Mapping vertical and lateral extent and physical properties of fracture corridors is essential for understanding and forecasting sub-surface fluid flow, in particular in faulted and/or folded reservoirs. As a result, fracture corridors have been recognized as important fracture patterns in oil reservoirs and aquifers (e.g., Braathen and Gabrielsen, 1998, 2000; Singurindy and Berkowitz, 2005; Ogata et al., 2014a; Bauer et al., 2016; Li et al., 2018). However, more case studies are needed, specially showing the association between fracture corridors and dissolution in karstified carbonates.
One way to improve the understanding of fractures below seismic resolution in structurally complex reservoirs is the study of field outcrop analogues, where fracture geometry, density and properties can be directly studied (e.g., Antonellini and Mollema, 2000; Lacombe et al., 2011; Healy et al., 2015; Ennes-Silva et al., 2016). This study investigates the relationship between gentle folding and the development of fracture corridors and karst features in carbonate units. We focus on the following topics: (1) investigate the controls that regional folds may exert on the origins, locations, and properties of fracture corridors, (2) describe and quantify how these fractures are karstified and how they then contribute to enhancing porosity and permeability in oil reservoirs, and (3) propose a schematic model for the formation of flow corridors in carbonate units. To this end, we selected the Jandaíra Formation, a 50-600 m-thick Late Cretaceous carbonate platform in the Potiguar Basin, equatorial margin of Brazil, as a reservoir analog (Fig. 1). This formation provides one of the largest exposures of a Cretaceous carbonate platform in Brazil and exhibits a high density of outstanding carbonate exposures ~1–2 km wide, which allow continued fracture-karst quantification. We integrated seismic and borehole data with unmanned aerial vehicle (UAV) imagery and field data (1) to investigate and quantify the architecture of fracture corridors in a 10 km-wide anticline and (2) to define the overall fold geometry using parameterized fracture corridors in the anticline hinge. We present a conceptual model for the development of fracture corridors in folded and karstified carbonates. Our study indicates that fracture corridors concentrate in the anticline hinge, even in very gentle folds, where karstification is pervasive. This study could provide a template to predict zones of higher permeability, which could be used for first-order prediction of highly permeable flow corridors in karstified and fractured carbonate reservoirs.
Section snippets
Stratigraphic units and general tectonics of the Potiguar Basin
The Potiguar Basin (Fig. 1) is on the equatorial Atlantic margin of Brazil. The origin and evolution of the Potiguar Basin are related to the rupture of the Pangea supercontinent from the Triassic-Jurassic (~200 Ma), which culminated in separation between the South American and African plates and gave rise to the equatorial Atlantic Ocean (Matos, 2000).
The first sediments in the Potiguar Basin were deposited during the Neoberriasian-Barremian, with the development of rifts filled with a thick
Methods
This study was conducted by integrating borehole and seismic lines with field data in the southwestern part of the Potiguar Basin in an area of 50 × 30 km with outstanding Jandaíra outcrop exposures located between the cities of Mossoró and Apodí (Fig. 1). First, we performed regional mapping and well correlations based on 106 2D seismic lines and 51 wells from the Brazilian National Agency of Petroleum, Natural gas and Biofuels (ANP) (Fig. 3a) to recognize and assess the fold geometry (i.e.,
Structural framework and fold geometry from seismic and well data
The study area in Fig. 4a encompasses mostly E-W- to NE-SW-trending normal faults that affect both the synrift deposits and the local postrift sequence of the upper Açu Formation and the Jandaíra Formation. This rift structure consists of a series of grabens and horsts. The major faults exhibit throws greater than 1000 m in the rift sequence and small throws (<100 m) in the postrift sequence. The rift-postrift boundary is marked by a clear reflector (orange horizon in Fig. 4b). The postrift
Fold evolution and fracture corridors
Fracture corridors can occur in association with three main structures: (1) fault damage zones, including segment relays; (2) fault-tip zones; and (3) fold-related crest zones (Ogata et al., 2014a). In our study area, the anticline crest (fold hinge) exhibits a 1.5 km-wide and 20 km-long fracture corridor. This structure formed during regional NW-SE-oriented subhorizontal compression most likely during the Miocene-Quaternary (Bezerra et al., 2019).
Several studies have discussed fractures and
Conclusions
This present study aimed to investigate the occurrence of fracture corridors in the anticline hinge of carbonate units and their relationship with karstification. The main conclusions are as follows.
- (a)
The Apodi anticline exhibits a 1.5 km-wide, 20 km-long hinge formed during regional Miocene-Quaternary NW-SE-oriented compression. The folding of depositional sequences and their stratigraphic boundary horizons, especially the maximum flooding surfaces, indicates that the fold is ~10 km wide and
CRediT authorship contribution statement
Fabio Luiz Bagni: Data curation, Formal analysis, Conceptualization, Writing - original draft, Writing - review & editing. Francisco H. Bezerra: Data curation, Conceptualization, Writing - original draft, Writing - review & editing. Fabrizio Balsamo: Data curation, Formal analysis, Writing - review & editing. Rubson P. Maia: Data curation, Formal analysis, Writing - review & editing. Marcello Dall'Aglio: Data curation, Formal analysis.
Acknowledgments
We thank two anonymous reviewers and MPG editor Enrique Gomez-Rivas for their detailed and constructive comments, which significantly improved our work. We thank the Brazilian Agency of Oil, Gas, and Biofuels (Agência Nacional do Petróleo, Gás e Biocombustíveis, ANP). We thank Caroline Cazarin for fruitful discussions and guidance that enhanced the quality of our work and Joanderson Araújo for help with field measurements and scanlines. We also thank João Marinho de Morais Neto for discussions
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