Oil and natural gas reservoirs

By the 1870s, the hydrocarbon industry had accepted the concept that a subsurface rock volume with sufficient porosity, permeability, and capping element effectively trapped localized concentrations of crude oil and natural gas. Such a concentration was termed a hydrocarbon (oil and/or natural gas) reservoir. This concept greatly increased early successes in finding hydrocarbon, and it remains a fundamental tool in worldwide exploration for oil and natural gas.

Background

With the advent of the age in the United States, initiated by the drilling of the first oil well in Pennsylvania in 1859, the search began for scientific methods useful in the direct or indirect indication of the presence of accumulations of subsurface oil and gas. Early methodologies included river bottom locations (“creekology”), geographic projection of discoveries (“ruler geology”), and the presence of surface hydrocarbon seeps (“seepology”) or surface mounds (“topography”). While of varying success in establishing new reserves, none of these methods adequately explained the concentration of oil and in subsurface rocks of the Earth.

Subsequently, publications by John F. Carll of the Pennsylvania Geological Survey explained that hydrocarbon concentrations were not present in subsurface caverns, pools, or lakes, but rather were contained in the natural pore space common to the class of rock. By the end of the nineteenth century, consensus suggested that economic hydrocarbon accumulations were associated with subsurface of porosity adequate to contain significant volumes of hydrocarbon, sufficient permeability to allow transfer of the contained hydrocarbon to the surface by way of a borehole, and the presence of a cap or roof rock which effectively holds the oil and gas in place until released through the borehole. This combination of rock porosity, rock permeability, and cap rock defines an oil and natural gas reservoir.

Reservoir Rock Type

Throughout the world, hydrocarbon reservoirs are commonly composed of sandstone or carbonate rock, the latter of which is either (calcium carbonate) or dolomite (calcium and magnesium carbonate). Studies indicate that approximately 57 percent of all reservoirs are composed of varying types of sandstone; conglomerate, greywacke, orthoquartzite, and siltstone are common types. About 40 percent of reservoirs are composed of carbonate rock. The remaining 3 percent of reservoirs are composed of shale, chert, and varieties of and rock.

Rock Porosity and Permeability

Rock porosity refers to the percentage of rock volume that is occupied by interstices or voids, whether connected or isolated. Under normal conditions, subsurface rock porosity is filled with water varying in chemistry from fresh to very saline. In rock provinces favorable to the formation of hydrocarbon, long-term geologic processes cause migrating microvolumes of dissipated oil and natural gas to concentrate into accumulations by replacing water-filled pore space with hydrocarbon-filled pore space. Sandstone reservoir porosities normally range from a low of 10 percent to a high of 35 percent. Carbonate rock reservoir porosity is generally lower than sandstone porosity.

Rock permeability is the measure of ease with which contained gas or liquid under pressure can move freely through interconnected pore space. Reservoir permeability is expressed in terms of millidarcy units, named for Darcy’s law. Sandstone and carbonate reservoir permeabilities generally vary from a low of 5 to more than 4,500 millidarcies.

Porosity and permeability are integral physical characteristics of the reservoir, as they determine, respectively, the amount of oil and gas the reservoir contains and the potential volumetric production rate of the reservoir over time. Under normal conditions, porosity and permeability are primary characteristics—in other words, characteristics that were created at the time of the rock’s formation. Secondary porosity and permeability can be created through postdeposition or fracturing of reservoir rocks. Oil and natural gas reservoirs possessing high porosities and permeabilities, whether primary or secondary in origin, are greatly valued.

Reservoir Cap Rock

While porosity and permeability are essential elements of any reservoir, a relative lack of permeability in the rock forming the reservoir cap is equally essential. The reservoir cap, or roof rock, is an impermeable rock unit that keeps the oil and natural gas in place until that time when reservoir integrity is altered by a borehole. The presence of oil and natural gas seeps throughout the world is indicative of reservoirs that have lost their integrity, allowing the contained hydrocarbon to leak slowly out of the reservoir and rise to the surface of the Earth.

Reservoir Trap

While a combination of porosity, permeability, and cap rock is common in subsurface rock, these reservoir characteristics must be contained within rock geometry of a nature such that oil and natural gas can by concentrated into economic volumes. The overall combination of porosity, permeability, cap rock, and rock geometry is termed the reservoir trap. Reservoir traps are formed under varying conditions of rock attitude (general disposition and relative position of rock masses) and rock lithology (physical characteristics). Two common types of reservoir traps are recognized: structural and stratigraphic.

A basic premise of states that sedimentary rock—that class which forms all but a minor percentage of reservoir rock—was deposited originally in a horizontal or near-horizontal state. Any subsequent deviation from the horizontal is caused by compressive or earthquake forces acting within the crust of the Earth. One of the most common and sought after structural traps is the anticline, a convex upward flexing of rock strata. In an anticline, the inner core of arched rock, if porous and permeable, allows the concentration of migrating microvolumes of hydrocarbon. Such concentration is achieved because oil and gas have a lower than saline or fresh water, the normal fluids found within the pore space of sedimentary rock. Without a proper reservoir cap rock forming the outer surface of the anticlinal flexure, usually an impermeable shale, hydrocarbon concentrations will slowly leak to the surface. An anticline, composed of an inner porous/permeable rock core and outer impermeable cap rock, forms the ideal structural reservoir trap. Of the 250 largest oil fields in the world, approximately 90 percent are classified as anticlinal reservoir traps.

In contrast to the structural trap, the stratigraphic is dependent upon lateral variability of porosity and permeability within a rock layer as caused by changes in grain size, shape, cementation, compaction, and degree of weathering. For example, in a sequence of tilted sedimentary rock, an upward decline in permeability would block the surface migration of oil or gas as effectively as would a structural reservoir trap. Such a loss of permeability may be caused by a combination of a reduction in grain size, an increase in the degree of cementation filling in the space between individual rock grains, or an increase in compaction resulting from rock burial. Approximately 10 percent of all reservoir traps are stratigraphic in classification.

Examples of Oil and Gas Reservoir Traps

Throughout the Middle East (notably Iran, Iraq, Kuwait, and Saudi Arabia), which contains approximately 49 percent of the recoverable oil and at least 27 percent of the total natural gas of the world, the anticline reservoir trap is ubiquitous. The Ghawar oil field, in northeast Saudi Arabia, is formed by the merging of several elongate anticlines, creating a gigantic anticlinal arch extending more than 233 kilometers in length by 21 kilometers in width. The reservoir rock is limestone, which is overlain by an anhydrite (calcium sulphate) cap rock. Variable porosity and permeability, ranging from 9 to 14 percent and from 10 to 20 millidarcies respectively, is responsible for the average well in this field having a high potential production, that is, approximately 5,000 barrels of oil per day.

In contrast to the Ghawar field, the Santa Fe Springs oil and gas field southeast of Los Angeles, California, is formed of an anticline approximately 3 kilometers in length by 1 kilometer in width. Hydrocarbon production here is enhanced by eight vertically superimposed oil reservoirs overlain by a natural gas reservoir. Each reservoir is composed of sandstone, capped by an impermeable shale.

The Hugoton gas field of southwestern Kansas is an excellent example of a reservoir formed by changes in stratigraphy (physical character). The reservoir is formed of porous and permeable carbonate rock, both dolomite and limestone in composition. In a westward direction, the carbonate rock gradually alters to shale, resulting in a decrease in porosity to the point where commercial quantities of gas cannot be obtained. Further north in southern Alberta, Pembina, one of the great oil fields of Canada, contains similar stratigraphic changes. In this case, four separate oil-producing sandstone reservoir rocks gradually change to shale, the latter acting as the cap rock.

The Chapman oil field of Texas is an excellent example of hydrocarbon production from igneous rocks, normally void of porosity and permeability. Originally formed as flows, with minimal porosity associated with gas vesicles, these rocks were subsequently altered and weathered, resulting in an increase in permeability. Overlying shales act as cap strata for the contained oil.

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