Abstract
Where primary porosity and permeability of a rock are unfavourable for hydrocarbon production, fractures can improve reservoir potential by enhancing permeability. Higher fracture intensity may create a better connected fracture network, improving fractured reservoir quality. Investigations into the controls on fracture intensity commonly conclude that either structural or lithological factors have the greatest influence on fracture abundance. We use the Swift Reservoir Anticline in north western Montana to investigate how fracture intensity varies throughout the structure, and determine that although structural factors do influence fracture intensity, lithology is the main control at outcrop.
The Swift Reservoir Anticline exposes bedding surfaces of the Mississippian Castle Reef Formation dolomite. Field data indicates that fracture intensity is highest in the fold forelimb, decreasing into the backlimb except in outcrops of coarse dolomite where fracture intensity is low, regardless of structural position. Field fracture intensity correlates with whole rock quartz, kaolinite and porosity percentages. We suggest porosity and composition influence bulk rock mechanical properties, which, in turn, control the fracture intensity at outcrop. Fracture intensity has a stronger relationship with lithological than structural factors, therefore we suggest that the key to predicting fracture intensity in the subsurface here is understanding how lithology varies spatially.
The Swift Reservoir Anticline exposes bedding surfaces of the Mississippian Castle Reef Formation dolomite. Field data indicates that fracture intensity is highest in the fold forelimb, decreasing into the backlimb except in outcrops of coarse dolomite where fracture intensity is low, regardless of structural position. Field fracture intensity correlates with whole rock quartz, kaolinite and porosity percentages. We suggest porosity and composition influence bulk rock mechanical properties, which, in turn, control the fracture intensity at outcrop. Fracture intensity has a stronger relationship with lithological than structural factors, therefore we suggest that the key to predicting fracture intensity in the subsurface here is understanding how lithology varies spatially.
| Original language | English |
|---|---|
| Pages (from-to) | 209-228 |
| Number of pages | 20 |
| Journal | Geological Society Special Publications |
| Volume | 487 |
| Early online date | 16 Jan 2019 |
| DOIs | |
| Publication status | Published - 2020 |
Bibliographical note
Title of special publication: Folding and Fracturing of Rocks: 50 Years of Research since the Seminal Text Book of J. G. RamsayThis research was funded by Oil Search Ltd, Santos Ltd and InterOil, through the University of Aberdeen Fold-Thrust Research Group. Electron Microscopy was performed in the ACEMAC Facility at the University of Aberdeen with assistance from John Still. Joyce Neilson is thanked for advice on the use of ImageJ software. Midland Valley are thanked for the use of their Move software for field data collection and model building. We thank Alfred Lacazette and Stefano Tavani for reviewing the manuscript and providing constructive comments.