KGS--Petroleum: a primer for Kansas--Structure

Petroleum: a primer for Kansas, Page 5 of 15
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Structure

Once formed, sedimentary rocks are subject to various kinds of deformation, such as folding and faulting. Three general types of folds are anticlines, synclines, and monoclines.

Folds

Anticlines are upfolds in the layered rocks. They are usually long, high wrinkles in the rocks that may extend for hundreds of feet, but normally they are miles or tens of miles long and may have numerous prominences. Anticlines are much like the "up-wrinkles" produced in a rug or sheet of paper when pushed or squeezed from side to side and are formed in much the same way as the Earth's crust was compressed or shortened by lateral forces. Circular upfolds in the rocks are called "domes." Anticlines are important types of "structural traps" in petroleum geology, as petroleum migrating up the dip along a flank of the fold is trapped at the crest. It can't rise any farther up the tilted strata and can't go back down the other flank, at least until the fold is full of oil and/or gas. A good example in Kansas is the El Dorado anticline that is a major producing oil field. The Central Kansas uplift is a large "antiform" with numerous smaller anticlines that collectively have produced in excess of 2.5 billion barrels of oil.

Synclines are the opposite of anticlines. A syncline is a downfold, usually occurring between two anticlines. Every upfold on our wrinkled rug or sheet of paper has one or two adjacent downfolds. Synclines, like their associated structures the anticlines, are elongate, perhaps extending for many miles. More or less circular depressions in the layered rocks are called "basins," but that term is usually reserved for very large depressions, tens or hundreds of miles wide, in the Earth’s crust. Such large basins are natural centers for thick accumulations of sedimentary rocks. Unlike anticlines, synclines only form structural traps for petroleum when the depressed strata occur above the water table in dry rocks and oil gathers in the bottom by gravity flow. Such synclinal oil fields are rare, although the first oil produced west of the Mississippi River was from a syncline at the Florence-Canyon City field in central Colorado (fig. 5).

Figure 5--Layered rocks of the Earth's crust are often folded when the crust is shortened by intense compressional forces. Upfolds are called "anticlines," downfolds are called "synclines," and broad downfolded area are known as "basins."

folded rocks are created by compression

Faults

Faults are breaks, or fractures, in rocks along which one side has moved relative to the other side. There are many kinds of faults, some of which are important as structural traps for petroleum in Kansas. Faulting, for example, is important to entrapment and migration of oil at Salina-Lindsborg fields in Saline County. The most common type of fault is the "normal fault' (fig. 6a). In this case, rocks along one side of the break simply drop down relative to the other side. An example is the Humboldt fault zone, a series of normal faults which can be traced from Nebraska southwestward across the state of Kansas along the east side of the Nemaha ridge. This happens when the Earth's crust is stretched.

Figure 6a--Fractures in the Earth's crust along which movement has taken place are called "faults." Where one side of the fault is dropped down by gravity relative to the other side, it is called a "normal fault."

block diagram showing normal fault

Another kind of fault occurs when one block of rock is faulted upward and over another. If the plane of the fault is steep, it is called a "reverse fault" (fig. 6b) and if it is low angle, it is called a "thrust fault" (fig. 6c). The fault at Salina-Lindsborg fields is a reverse fault, and the Humboldt fault zone includes some reverse faults. Reverse and thrust faults occur when the Earth's crust is compressed, or shortened. When this occurs, folds usually form first, only to break into thrust faults when the strength of the strata involved is exceeded by the compressional forces.

Figure 6b--Where one side of the fault is pushed up and over the opposite side along a steep fracture, it is called a "reverse fault."

block diagram showing reverse fault

Figure 6c--A "thrust fault" occurs where a sheet of rock is forced up and over the opposite side of the fault along a low-angle break. Rocks are sometimes displaced many miles along large thrust faults.

block diagram showing thrust fault

We are only now learning that the most important kind of faulting is when one block of the Earth's crust is forced to move laterally (horizontally) in relation to the other side. Such faults, known technically as "strike-slip faults," and rather colloquially as "wrench faults," form when lateral stresses (twisting in the horizontal plane) act on large expanses of the Earth (fig. 6d). Wrench faults usually form in broad bands, or swarms of many faults, that may be tens of miles wide and hundreds of miles long. Such extreme friction occurs that the rocks literally are shattered along a wide path and movement occurs along dozens or perhaps hundreds of individual faults and drag folds. The many faults that are active today in southern California, the San Andreas and related faults, constitute a wrench-fault zone. In that case, the western, or seaward, block is moving northward relative to the eastern, or continental side along hundreds of active faults.

Figure 6d--This block diagram of a wrench-fault zone shows that vertical movement is often very complex. Faults may appear to be normal or reverse along the fault zone, and fault reversals and scissoring are common; horizontal displacement is greater than these vertical complications.

block diagram showing wrench fault

Geologists are now finding numerous wrench-fault zones, especially in rocks of younger Precambrian age, on every continent, and the midcontinent of North America is no exception. Structures associated with the Central Kansas uplift occur in patterns and trends which resemble those involved in wrench-fault zones. The Midcontinent Rift System that traverses from northeast to southwest across eastern Kansas is believed by some geologists to be an ancient, billion-year-old, wrench-fault zone.

Figure 6e--"Wrench faults" occur where one side of the fault moves slong several, often sinuous, faults, such as shown here along the Nemaha or Humboldt fault zone in eastern Kansas.

Nemaha/Humboldt fault zone in eastern Kansas shows wrench faulting

Basically, faulting of any kind makes structural traps for petroleum. Porous layers are commonly faulted against rocks with low porosity and permeability, stopping the updip migration of oil or natural gas; huge accumulations may result. The inherent complexities of wrench-fault zones may localize dozens or hundreds of oil or gas fields in large linear trends. Southern California is a well-known example; some geologists think that the Central Kansas uplift may be another.

Fractures in otherwise tight, nonporous rocks can themselves be open, providing "fracture porosity" that may contain petroleum. Thus a shale or dense limestone may be fractured and contain oil, forming another type of structural trap. Fractures commonly break through porous rocks and increase the permeability greatly by providing pathways that interconnect rock pores. Fracturing may be much more important in forming reservoirs in Kansas than we realize. Recent research on porosity and permeability development in shallow Pennsylvanian sandstones of southeastern Kansas and northeastern Oklahoma indicates a clear link between production and fracture patterns as mapped at the surface. Fracture porosity is also a component of the pore space in reservoirs in the Hugoton gas area. Ancient weathering along fractures has enhanced reservoir development in fields producing from the Arbuckle Group in the midcontinent.

An important characteristic of all structures, but of wrench-fault zones in particular, is that once a fold or fault begins to move, it is a zone of weakness for all remaining geologic time. Movement will recur each time properly oriented stresses are generated in the crust of the Earth for whatever reason. Consequently, structures originating in Precambrian basement rocks may have been rejuvenated repeatedly throughout geologic history. Each time there is renewed movement on a structure, be it ever so slight, overlying rocks are shuffled up or down, perhaps only by inches or a few feet. Every foot of topographic relief on the seafloor affects the nature of the sediments being deposited, especially in the case of carbonate sediments. Consequently, shoals may develop on the seafloor on and around high structures, and structural depressions become sedimentary basins. Either situation changes the environment of deposition to a degree.

As we have seen, different types of sediments eventually produce different porosity/permeability relationships in the rocks. If the rock type varies due to water depth that is controlled by structural movement during deposition, local variations in porosity/permeability will result. Thus a definite relationship exists between geologic structure and the kind of sedimentary rock that is produced, and stratigraphic traps result.

Petroleum reservoirs produced by structural growth during deposition of sediments are common in Kansas. The myriad oil/gas reservoirs in Pennsylvanian rocks of the Central Kansas uplift are examples, and more are being discovered by drilling every day. A prominent anticline, formed over deep-seated faulting in Haskell County, exhibits recurrent growth which helped develop a series of major oil and gas fields along this structure. The Cahoj field in Rawlins County in northwest Kansas was a topographic high during deposition of the Lansing-Kansas City reservoirs leading, in part, to the stacking of 11 reservoir units in that field. It is still high today. McClain field in Nemaha County produces from Simpson and Viola reservoirs. Both were influenced by ancient topography similar to present-day structure. Other Viola fields along the Pratt anticline in south-central Kansas exhibit early structural growth of this anticline which extends southward from the Central Kansas uplift.

Unconformities

Whenever the sea withdraws and the landscape is exposed to the elements, erosional processes take command. Physical weathering, such as rainfall, the resulting runoff as streams, wind action, and perhaps glacial scouring, work to lower and level the lands. Chemical processes form soils and dissolve soluble rocks such as limestones and evaporates. Structurally formed topography can be beveled to a near plain, exposing the eroded edges of formations that may have been tilted in the process. When the sea returns and new sediments are deposited on the old eroded land surface, the surface of contact is called an "unconformity" (fig. 7). If the old rocks have been tilted and beveled before burial by near-horizontal beds, the surface is called an "angular unconformity."

Figure 7--Uncomformity is a surface between rock layers that shows the effects of a period of erosion or nondeposition. In this case, the rocks below the wavy line (uncomformity) were deposited, folded to form an anticline, and then eroded to a plain before the rocks above the unconformity were deposited. Petroleum migrating along any permeable bed may be trapped when it rises to the eroded surface.

anticline is eroded away, new rocks are deposited on top, which may trap migrating oil

The presence of an unconformity in the layered sequence of rocks indicates that some increment of time is missing in the rock record. It is estimated that the sedimentary rocks deposited in Kansas represent only a small part of geologic time. The remaining time is incorporated in the unconformities. These gaps in the record are at least as important as the rocks and markedly affect their distribution.

Oil and gas migrating upward along inclined rock layers may be trapped at an unconformity if the beds overlying the eroded surface are impermeable. Oil and gas may also have migrated along the unconformity. The major uplifts of Kansas, and some smaller folds and faults, have been elevated, and the resulting tilted strata beveled by erosion at different times during the geologic past. Excellent unconformity traps have resulted, notably along the flanks of the Central Kansas and Nemaha uplifts. Mississippian limestones along the western flank of the Central Kansas uplift contain vast amounts of petroleum beneath the Pennsylvanian/Mississippian unconformity.

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Kansas Geological Survey, Education
Placed online April 2001
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