General Properties of Joints and Joint Networks in Subhorizontally Layered Reservoir Rocks

Abstract

Properties of individual joints and joint networks in flat-lying, nearly underformed rocks are consistently and predictably related to lithology and to stratal sequence in the following ways. (1) Orientations: Strike dispersions of 15-30[degrees] within joint sets approximate normal distributions. Smaller dispersions are known for early formed sets within fine-grained, well-cemented, brittle rocks; dispersions increase with increasing layer thickness, grain size, and friability. Dip dispersions increase with changes in rock properties. (2) Dimensions: Joint heights are influenced by thickness of depositional units and degree of lithologic contrast between them. The smallest joints form in thinly bedded sequences of contrasting rock type; the largest are in thick, internally homogeneous units and sequences of low lithologic contrast. A correspondence exists between joint height and length for early joints, but lengths of later ones are increasingly influenced by preexisting joints and tend to become progressively smaller for successive sets. (3) Spacing: Joint spacings are influenced by lithology and thickness of the jointed layer through a relation of the form log S = m log T + c, where S = median joint spacing, T = mechanical layer thickness, and m and c are constants dependent on lithology. Mechanical layer thickness may be greater ormore » less than bed thickness depending on lithologic contrast between beds and degree of lithologic homogeneity within beds. (4) Interconnections: The relative proportions of [open quotes]blind[close quotes] endings, terminations, and intersections among joints differ from set to set as a function of the mineralization history and spacings of preexisting joints. (5) Apertures: For early, isolated cracks, joint shapes in cross section are biconvex, and maximum aperture at joint midpoint is proportional to joint length; thus, all joints have approximately the same shape regardless of size. No such relation exists for joints of later sets.« less