Extrapolation of Fracture Data From Outcrops of the Austin Chalk In Texas to Corresponding Petroleum Reservoirs At Depth

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Abstract Horizontal drilling into naturally fractured reservoirs requires information on the orientation, spacing and connectivity of the fractures relative to the structure and stratigraphy so that drilling depths and directions can be set for maximum productivity. Prior to drilling, this information must come from outcrops, but these data may be suspect, as stress relaxation and weathering may have enhanced the fracture development. To what degree can outcrop data be extrapolated to rocks at depth even if the outcrop rocks are lithologically representative of the reservoir rocks, structural development (strain) is the same, and the physical conditions at the time of deformation were the same so that similar relative strengths and ductilities were obtained at the time of fracturing? The Austin Chalk play of South-central Texas is discussed as a case history relative to the extrapolation question. The rocks at outcrop and in the subsurface were deformed under conditions of low effective confining pres-sure and behaved brittley. Previously published outcrop data on orientation and spacing are reviewed and correlated with recently acquired similar data using Formation Microscanner surveys (FMS) from horizontal segments of boreholes. It is shown that only one set of strongly oriented fractures occurs in the subsurface (nearly vertical and striking NE) whereas at least two sets are developed at the outcrop. However, the set in the subsurface also is well developed on outcrop and usually is the longest, oldest, most systematic and best connected. In addition, the distribution of fracture spacing in the subsurface is equal to or approaches that at the outcrop. Introduction The characterization of naturally fractured reservoirs involves an understanding of the mechanical behaviour of the reservoir rock, the origin of the fractures, the orientation (i.e., strike and dip), spacing, and connectivity of the fractures, and the role fractures play in the fluid flow of the reservoir. Naturally fractured reservoirs such as the Austin Chalk (Texas), Bakken Shale (Montana). Prudhoe-Bay clastics (Alaska), North Sea Chalks, and the Niobrara Limestone (Colorado) have received a great deal of attention since the advent of horizontal drilling. Prior to drilling and the acquisition of subsurface data (cores, logs. production information, etc.) studies of the fracture development at outcrops of the reservoir rocks provide the initial data to guide exploration and production. There is always the question as to how valid are extrapolations of these outcrop data to the corresponding reservoir at depth. It is often argued that fractures at the outcrop are "overdeveloped" as they are subject to relaxation and weathering and therefore provide unreliable models of fractures at depth. This paper is part of a larger study of fractures in the Austin Chalk of South-Central Texas. Here we focus only on the extrapolation of fracture orientation and spacing in the Austin Chalk; complete details are given by McKiernan(1). This emphasis does require, however, inclusion of discussions of the origin of the fractures, including discrimination of regional and local tectonic ones as well as the likely physical conditions existent at the time of fracturing for both the fractures now at outcrop and those in the deep subsurface.