ABSTRACT

Maps at scales of 1:12 to 1:48 of portions of quarry floors and scanlines along vertical to near vertical outcrop faces in the Austin Chalk reveal the change in fracture intensiry, connectivity, length, and ordering with respect to orientation along the trend from Del Rio to Dallas. Outcrops with more marl beds and/or a larger total percent smectite clay (Del Rio, Waco, Dallas) display, in general, more random, more closely spaced, and more interconnected fractures. Fractures in the vicinity of San Antonio where smectite clay content is low, by contrast, are, 1) developed into well oriented sets and, 2) more widely spaced, 3) longer and, 4) less interconnected than other areas studied. Several of the fracture patterns mapped indicate that the concept of "fracture set" has only local applicability in the Austin Chalk. Highly curved fractures outside of the San Antonio area and the occurrence of en passant fracture geometry suggest low differential stress at the time of fracturing. Random orientation of all but the longest fractures suggests uniform extension of the Austin Chalk in the horizontal (bedding) plane.