Fracture Characterization Based on Microfracture Surrogates, Pottsville Sandstone, Black Warrior Basin, Alabama

By

Laubach, Stephen E., Reed, Robert M., and Gale, Julia F.W.

Bureau of Economic Geology, John A. and Katherine G. Jackson School of Geosciences, The University of Texas at Austin, Austin, TX,

Ortega, Orlando J.

Department of Geological Sciences, John A. and Katherine G. Jackson School of Geosciences, The University of Texas at Austin, Austin, TX,

DOHERTY, Eloise H.

E&G Designs, Austin, TX

Microfractures in sandstone samples were used as surrogates to infer macrofracture characteristics in two Pennsylvanian Pottsville Formation wells, Black Warrior Basin, Alabama. Sidewall cores from one well were oriented using image logs and drilling- and retrieval-related features visible on cores. SEM-based cathodoluminescence imaging of horizontal thin sections allowed identification of quartz-lined microfractures. Microfracture surrogates provide information on fracture orientation, crosscutting relations, intensity, and likelihood of preserved open fractures.

Samples contain two morphologically different categories of postdepositional, quartz-lined microfractures—those related to compaction and those related to regional tectonics. Only noncompactional microfractures were used to infer attributes of macrofractures. We identified two major and two minor sets. Development of microfracture sets varies from sample to sample. Roughly north-trending microfractures are oldest. Northwest-trending microfractures formed next and are the most prevalent. Northeast-trending microfractures are the second-youngest set, with roughly east-west-trending microfractures being youngest. Sets match, within 10 degrees, trends of macrofractures previously identified in outcrop, and timing relations agree with published determinations of macrofracture timing.

The relationship between microfractures and various cements allows predictions to be made about porosity preservation in large fractures, even where these have not been sampled. Macrofractures in these wells are expected to be lined or bridged with synkinematic quartz. Postkinematic ferroan carbonate cements are predicted to completely or partially eliminate fracture porosity. Different samples show different degrees of postkinematic cement, suggesting that macrofracture quality varies. Good production or gas flares were observed in depth intervals having predicted open fractures and moderate to high fracture intensity.