Quantitative Analysis of Natural Fractures in Chattanooga Shale in Selected Outcrops in Northern Alabama with Terrestrial LIDAR Images

Abstract

Devonian age black shale deposition across foreland basins along the Appalachians has recently become very important economically because of their gas-shale potential. Understanding the natural fracture patterns is important for unconventional gas-shale reservoirs because natural fractures play a vital role in the movement of fluids such as oil and gas, and are also a critical controlling factor in the adequate distribution of hydraulic fracture treatments of gas-shale reservoirs. In Alabama, Shale Gas targets including the Chattanooga are being explored for their gas potential. We have collected LIDAR images from a Chattanooga shale outcrop along the backlimb of the Wills Valley Anticline. LIDAR image acquisition and analysis advance the ability to characterize outcrops in terms of accuracy across the exposure as well as fracture relationships within the bedding. LIDAR imaging of Chattanooga Shale allows for quantitative capture of fracture geometry, orientation, and density. We are using these images to determine the relationship between bedding thickness and fracture density in the Cherty beds of the Chattanooga Shale. The chert content in the Chattanooga outcrop increases upsection. We have obtained three 1 m² close up scan images from three distinctive lithologies of the outcrop. The first one is close to the base of the unit with high organic content and very low chert content. The second one is in middle of the section with intermediate organic content and intermediate chert content. The third one is higher in the section with a very low organic content and high chert content. Our preliminary interpretation suggest that the shale unit tends to be better fractured in the chert rich beds of than the organic rich beds. The chert rich beds contain well developed orthogonal fracture pattern. We are in the process of quantifying a) the chert and organic content of the Chattanooga shale; and b) fracture density and fracture length and their relationship to the bed thickness in three different scanned images. Fractures within the Chattanooga Shale also offer potential insight into subsurface natural fracturing as well as local and regional stress environments when the Wills Valley Anticline developed during the Alleghanian orogeny.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015