STRUCTURE various deformations of the strata that are considered as favorable mediums for the accumulation of oil and gas are known as structures by the oil man and geologist. The various structures that have more or less bearing on the accumulation and location of wells are the anticlines, synclines, noses, terraces, embay ments and homoclines. These are the most familiar ones encoun tered in producing fields; however, faults, intrusions, extrusions and unconformities may also bring about or affect production in various ways.
The study of these attitudes or structures belong to strati graphic and structural geology, and are based on the determina tion of the various dips.
Dip and Strike. Rocks that have been tilted in various ways are said to be dipping, the dip being the degree of inclination that a rock makes with the horizontal. The course of a stratum in a horizontal plane is known as the strike, and it will be at right angles with the dip. (Fig. xi.) The deviation from the hori zontal may be from a fraction of a degree to the vertical. Moun tains are generally formed by large dips; the smaller undulating dips may or may not find expression in the surface.
Large dips are determined with a clinometer placed along the line of the dip, which is the direction in which water would flow down along the exposed surface of a true dip; the reading is taken in degrees and the direction noted by magnetic bearings. Such readings are represented on a map by the use of an arrow or dip sign. The arrow points in the direction of the dip, and the angle of dip written along the line of the arrow. The direction of the strike is shown by the line drawn perpendicular at the end of the arrow. In regions of small dips or few exposures it is necessary to find the elevation of the outcrops of a key horizon above a datum plane; this may be done with an aneroid barometer, engineer's spirit level, transit stadia or plane table. The elevations so obtained enable the preparation of a structure contour or isobath map, which will be a graphical representation of the dips and strikes. Regional dip may be figured if the eleva tion of a stratum is determined at three points. The points should be preferably the points of a triangle, and not in the same straight line.
The dip of the strata may remain the same for great distances or may change from place to place, forming various folds. A dip in one general direction is known as a homocline. When two homoclines dip from a common line or axis, they form an anticline. In other words, an anticline is an upward fold of the strata forming an arch (convex side up).
Two homoclines dipping TOWARDS an axis form a syncline or trough (convex side down).
Anticlines may be various kinds, such as a dome, level top or level axis and plunging anticlines.
Dome. When the strata dip away in all directions from a central point they form a dome.
Level Top or Level Axis Anticline. If the axis, or the line from which the strata dip away, is level for a distance, it is known as a level top or level axis anticline.
Plunging Anticline. When the anticlinal axis itself dips in a certain the anticline is known as a plunging one.
Anticlinal Nose. A smaller and generally a plunging anti cline along the flank of a main anticline (or along a homocline), is called an anticlinal nose.
Terrace. A sudden flattening of the dip for a short distance, forming a step like structure is a terrace.
Syncline. As previously explained a syncline is the opposite or complement of an anticline; similarly the complements of a dome is a basin; of a level top anticline it is a level axis syncline; and thus we have the plunging syncline, as well as the embay ment which bears the same relation to a syncline that a nose does to an anticline.
Asymmetry. Anticlines and synclines are said to be asym metrical (un-symmetrical) when the dip on one side of the axis is greater than the dip on the other side. (Fig. i6.) Faults. The different stresses at work affect seriously not only the shape of the strata, but sometimes exert such pressure or force upon them as will cause a fracture or break, accom panied by displacement of one side of the strata with respect to the other, along a line, or a shear zone, thus forming a slip or fault. Faults may be due to either lateral pressure (crushing force) or may be produced by tension (pulling apart). The resulting faults may have different characteristics; the former causing thrust and reverse faults, the latter normal faults. The side which has been pushed up over the other is the hanging wall, or as most commonly called, the up-throw, the under side being the foot wall or down-throw. The distance of such throws may be anywhere from a few inches to several hundred feet.
The heave of a fault is the horizontal distance of displacement; the bade is the angle of the fault line or shear zone with respect to a vertical plane.
Intrusives. Igneous rocks forced up into sedimentary rocks are known as intrusives. They may, when cooling, cause a dragging of the broken edges of the sedimentary beds, forming a sort of an anticlinal structure, in which oil or gas may accumu late.* (Fig. r 7.)