Integration of Core Fracture and Lithofacies Descriptions in the Wolfcamp Shale: Implications for Mechanical Stratigraphy and Deformation History

Abstract

Here we present findings from an extensive survey of natural fractures from Wolfcamp Shale cores from the Delaware Basin in west Texas, one of the most active unconventional plays in North America. Understanding natural fractures in the Wolfcamp is important for reservoir characterization, optimizing reservoir stimulation and improving well performance. Fractures were described and measured over more than 6500 ft. of core from 16 wells. Fracture intensity was quantified by measuring frequency and areal density. Fracture types include subvertical veins, horizontal veins, joints and faults. Fracture characterization was integrated with detailed lithofacies descriptions to establish the mechanical stratigraphy. Constraints on the kinematics and timing of fracture formation are discussed.

Subvertical veins are the most common fracture type. These veins are typically less than 1 ft. long with apertures of less than 1 mm and are typically filled with calcite cement with some dolomite and quartz. Fractures terminate at lithology changes or against horizontal veins. Horizontal veins typically have apertures of 1-2 mm. Elongate crystals propagating from medial lines suggest continuous “crack and seal” growth of the veins. Slickenlines on vein walls indicating some shear failure during formation. Of lesser importance in cores are joints and faults. Joints have similar dimensions to inclined veins but are observed far less frequently. Faults are mostly syndepositional with secondary faulting observed only in a few wells.

Lithofacies are the primary control on fracture intensity and height. Carbonates and mudstones contain far more inclined veins than sandstones, siltstones and mass transport deposits (MTD). Horizontal veins are observed exclusively in organic-rich mudstones. Considerable overlap in the range of fracture intensity values was observed for the Beta, Delta and Foxtrot intervals, with significantly less intensity observed in the Alpha interval due to higher sand and silt content.

Horizontal veins are interpreted to pre-date vertical veins and likely formed as the result of pore fluid pressure exceeding lithostatic pressure during peak hydrocarbon generation. Inclined veins are primarily oriented NW-SE with a secondary fracture set trending NE-SW and are interpreted to have formed as tensile cracks with σ₃ oriented NE-SW, perpendicular to the primary fracture set. This orientation suggests these fractures formed during Basin and Range uplift and extension.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018