--> Unraveling Controls on Fracture Stratigraphy in Carbonates: The Influence of Regional Stress, Mechanical Properties and Diagenesis

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Unraveling Controls on Fracture Stratigraphy in Carbonates: The Influence of Regional Stress, Mechanical Properties and Diagenesis

Abstract

Natural fracture pathways are a critical component of unconventional hydrocarbon plays. A unique challenge is examining these fracture networks to gain key insights into the connectivity and potential migration pathways of a reservoir. Fracture networks should be considered in conjunction with the varying mechanical properties of the stratigraphy and the regional and local stress regimes. The Ozark Plateau is an ideal study area for fracture research, since the stratigraphy of the area is similar to major onshore carbonate reservoirs currently being exploited, notably the Miss. Lime play, where fractures play a large role in reservoir permeability. In addition, Ozark Plateau fracture networks are the product of several fracturing events from the multi-stage Ouachita Orogeny during the late Paleozoic. This study used field observations of lithology, fracture attributes and hardness data over 17 outcrops in the Mississippian (360–323 Ma) carbonate sequence in Missouri and Arkansas. Outcrops were chosen to give extensive data through the St. Joe to Boone Formations and in areas where there were 3D representations of the fracture patterns. Fracture attributes collected included orientation, intensity, length, and abutting relationships. Rock hardness data were collected from in-situ outcrop samples using a rebound hammer. 34 thin sections were prepared for the description and analysis of all major lithologies observed. Initial results indicate 4 main fracture orientations that resulted from at least 3 discrete phases of deformation during the Miss-Penn. Two primary sets strike NE-SW and NW-SE and appear through most of the sequence. Secondary sets are oriented E-W and NNW-SSE and the presence of these sets may be controlled by mechanical or diagenetic factors. However, present-day mechanical stratigraphy apparently differs from that during the deformation phases, as some fracture sets do not show a relationship to either bed boundaries or the differences in mechanical properties. This study suggests that fractures related to the main regional stress direction can overcome variation in mechanical properties, while fractures forming under local stress perturbations are more sensitive to mechanical and diagenetic changes. Fully characterizing naturally fractured reservoirs requires a detailed analysis of the interplay between regional stress regimes and local variation in mechanical properties.