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The most abundant kind of deformation of rocks is by fracturing. There are three principal classes of fractures--(1) joints, (2) faults, and (3) small, irregular breaks (including shatter and breccia zones). In general, joints may be defined as more or less regular groups of relatively long fractures that are paralleled by little or no displacement or orientation of rock components.
Joints occur in sets that may be parallel, radiate, or concentric. Sets occur singly or severally and with no universality of systems. The angular relations of intersecting sets range from sharply acute
to normal. In a given area two or three sets may be about equally developed in length and frequency, or one set may predominate. Joint sets may be confined to single beds or transect several formations and extend for more than a hundred miles. Strong and possibly thick beds tend to be less jointed, but existing joints are relatively long. Thin, relatively brittle beds are almost always jointed normal to their bedding. Relatively weak rocks are generally disrupted by small, irregular breaks.
Shale, mudstone, claystone, and siltstone probably are much more fractured than any other sedimentary type, but the breaks are generally tight, inconsistently oriented, short, irregular, and commonly oblique to the bedding. Although individual joints are comparatively short in shale or claystone they are probably concentrated into zones which may have marked vertical or horizontal extent. The most persistent and apparent jointing in regions of flat beds is nearly vertical.
Very many if not most joints appear to have formed soon after deposition during compaction, irregular settling, and broad, although irregular downwarping of the area of accumulation. Such early formed joints may be local and irregular, or they may be widely developed and display certain marked trends because of secular deep-seated wrenching, to broad coupling stresses acting mildly upon the entire area of deposition, or because of persistent depositional lineations. Composition, degree of consolidation, and bed-to-bed variation in brittleness or ductility strongly affect the amount of early fracturing. Early formed joints as well as some late ones have been thought to be caused by earthquakes and earth tides, but wind and glacial movement may more easily joint some types of near-surfa e rocks.
Despite the evidence of early jointing during little or no deformation, undoubtedly new jointing develops during moderate and strong crustal deformation. During this operation early formed joints may be extended and the angular relations between sets may be changed.
Under dynamic compression the stress pattern becomes more intricate, refracted, and variable. The local stress patterns change repeatedly with the progression of the deformation and the development of principal flexes and shear zones. Fracture thresholds are reached at various times and places for the various rock components, and withal the fracture pattern in its culmination is likely to be a complex mixture of reoriented non-diastrophic and multiple-stage tectonic joints, some of small and some of great dimensions. In general, however, the fracturing is accentuated in the parts that are otherwise most deformed and in the beds least supported by adjoining incompetent members. Fracture systems of uplifts and fold belts commonly have such a complex derivation that they bear little obvi us or regular relation to them.
Fracture reservoirs for petroleum result where relatively brittle rocks are broken in an irregular manner and on a scale and frequency that may develop in a network in otherwise effectively impervious rocks. Sandstone, for example, may have many large fractures, but interconnection and imperviousness are not generally great. Shale or claystones may contain many fractures, but they are usually tight, short, and not so effectively connected. Fracture reservoirs may occur in any common lithologic type with the possible exception of salt. The requisites for effective development of fractured reservoirs in claystone or impervious siltstone appear to be (1) increased brittleness with respect to adjacent similar beds and (2) proximity of source petroleum. Shatter and breccia zones of several origins may also serve as tabular reservoirs or channels between reservoirs. These form principally in association with faulting or with folding where brittle beds are in maximum stress imbalance or reach a threshold of fracturing.
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