--> Abstract: Petrographic Recognition of Artificially Induced Microfractures Versus Natural Microfractures in Oil/Gas Shales, by Paul W. Grover; #90124 (2011)

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AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA

Petrographic Recognition of Artificially Induced Microfractures Versus Natural Microfractures in Oil/Gas Shales

Paul W. Grover1

(1) Reservoir Geology, Core Lab, Houston, TX.

Thin section observations of most open microfractures in organic-rich shales/limestones indicate they are artificially created by a variety of processes which are listed below along with their general morphology. Bit-created petal fractures are high angle, straight and slightly diffuse. Microfractures created during coring and gas expansion as the core is lifted to the surface are bedding parallel, occasionally discontinuous, branching and have even walls. Core removal and handling creates continuous, large jagged fractures that are generally bedding parallel but can be higher angle. Microfractures created during plugging and rotary sidewall removal are very high angle to the length of the plug, meandering, diffuse and tend to parallel the outer wall of the plug. Natural microfractures can be recognized by various features including straight, generally bedding inclined morphology and their occurrence in parallel sets. Natural fractures can be cemented, bitumen-stained or have greater precipitation of pyrite or small euhedral ferroan dolomite rhombs along the fracture wall. Secondary grain dissolution and increased micropores along the microfracture wall are also typical of natural microfractures. Stylotized margins, secondary pores along the stylolite and micropores can be found in near-vertical natural microfractures. Greater amounts of hydrocarbon inclusions and isolated pores are also noted in natural microfractures. Ultraviolet petrographic observation with UV doped impregnation epoxy greatly increases the ability to recognize natural microfractures.