Influence of Sedimentary Fabric on Fracture Characteristics of Two Thick Shoreface Deposits of the Lower Cretaceous Moosebar Formation, West Central Alberta

Abstract

Reservoir properties, such as preferential flow pathways, can be better understood by studying the connectivity and continuity of natural fracture networks within the Earth's crust. By quantifying the fracture networks observed in outcrops, extrapolation of fracture populations within the subsurface can aid in maximizing fluid extraction. In this study, the influence of sedimentary fabric on the fracture network of two shoreface sandstones was analyzed to determine the effect of internal fabric on fracture characteristics. Two thick shoreface sandstones of the Lower Cretaceous Moosebar Formation (equivalent to the Wilrich/basal-Falher Members of the Spirit River Formation) were chosen because of their lithological similarities but different sedimentary fabric. Collection of outcrop scanlines allows comparison of fracture characteristics and observation of the influence of fabric controlled by bioturbation and cross-stratification. The consecutively deposited shoreface sandstones are comprised of very fine to fine grained coarsening upwards succession with high clay and glauconite content, and similar thicknesses of ∼3.5 meters. The upper sandstone (A) has well developed hummocky cross-stratification; the lower sandstone (B) is more massive due to intense bioturbation, mainly Ophiomorpha. Scanlines were measured on three distinct sedimentary facies within each sandstone: Facies 1 has bed thicknesses of 5-30cm, Facies 2 of 30-60cm, and Facies 3 of 60-250cm. Although individual fracture characteristics of each facies is not directly applicable to the subsurface, relative ratios between corresponding facies can likely be applied. For example, a 50m visual scanline provides fracture intensities of Facies 3A and 3B of 2.8f/m and 1.8f/m, respectively. Facies 2A and 2B have fracture intensities of 3.3f/m and 2.8f/m, while Facies 1A and 1B are 4.4f/m and 3.2f/m. Therefore, the overall ratio of sandstone A to B tends to correlate to an average of 25% higher fracture intensity in the cross-stratified sandstone. Fracture height appears to be inversely proportional to fracture intensity for both sandstones, which might be caused by the presence of mud on bedding planes in sandstone A, where the majority of fractures terminate; sandstone B has taller and fewer fractures due to its more massive character. Results of this analysis suggest that internal fabric, mechanical bed thickness and presence of mud, have an effect on fracture characteristics of shoreface sandstones.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016