The Relationship of Basic Geological Concepts to Petroleum Geology
Geology is the study of the earth, its internal and external composition, structure and the processes by which it develops and changes. The earth is constantly changing; thus, its processes and the history of its processes are important in the formation and preservation of economic mineral and petroleum deposits.
As the earth changes, clues are formed that are important in the exploration and recovery of hydrocarbons. The history of the earth, and particularly the sedimentary record, is replete with clues, some obvious, some subtle, that provide important information in the search for natural resources and raw materials. Geologists, geophysicists and engineers who explore for, find and produce petroleum must be able to interpret the geologic record. Additionally, they must evaluate physical, chemical and biological parameters as part of a successful exploration program. In this manner, the characteristics of the earth and its history tell the explorer and producer how petroleum can be found profitably and eventually recovered and brought to market.
This article will discuss how certain basic geologic principles, concepts and parameters apply to the much narrower scope of petroleum. Some geologic phenomena and processes relate directly to petroleum generation, migration, accumulation and production. Others are not intrinsically petroleum-related- but can become so by geologic happenstance. Petroleum geology relates a general rationale based on foundational concepts to the specific, which is not possible without without geology as the supporting framework. A student cannot know about petroleum accumulations in river channel or desert deposits unless he knows about how river sands and dunes are formed and how substantially different, they are. It is not enough to encounter a sand of unknown origin and call it a reservoir. Nor is it sufficient to drill a geologic structure without understanding its deformation character. The sand deposit and the structure have orderly parameters that require specific production treatment different from that needed to produce other types of sands and structures. Only based on an understanding of foundation geological concepts can we recognize a reservoir, for example, as a beach sand deformed into a block-faulted anticline. The beach designates certain geometric and reservoir characteristics, and the block faulting designates certain fracture and deformational factors. Hydrocarbon accumulations are defined and produced according to the best related techniques, based on their reservoir and structural characteristics. A comprehensive understanding of basic geological concepts and models is essential. Without it, we cannot hope to be other than random in our approach to the efficient and safe development of petroleum resources.
This article will discuss:
- Historical Geology and Stratigraphy
- Minerals and Rocks
- Sedimentary Processes of Weathering, Erosion and Deposition
Historical Geology and Stratigraphy
Historical Geology
For most of us, the enormity of geologic time is almost impossible to grasp. Data suggest that the earth is about 4.1 billion years old, and, accordingly, geologic time is referred to in increments of millions, tens of millions and even billions of years; much greater than any time increment that we could ever experience.
Geologic time is represented by the presence rock strata, for instance as seen in roadside rock outcrops, and it is also represented by periods of erosion, deformation, organic evolution, or floral and faunal extinction. In adjacent or distant locales, the same geologic time interval can be represented by different rock types or by the absence of rocks, for example, due to erosion. Elapsed geologic time encompasses all of the events that affect the earth and, for the observer, the clues that represent those events. Petroleum forms, migrates and accumulates as a direct result of geologic processes and events, which are represented by clues to be found in the rock record. The petroleum geologist seeks out and interprets these clues as significant data that provide the rationale for hydrocarbon exploration, based on basic geological concepts and models.
The evolution of the earth and its processes of construction and destruction, as well as the resulting continual change, bear directly upon the earth’s features at any particular time. Part of the expression of change is manifested in the processes that relate to the generation, migration and accumulation of hydrocarbons. Largely through our understanding of basic geologic concepts, we have been able to find and produce large quantities of oil and gas that have accumulated in relation to the life-bearing parts of geologic time.
Stratigraphy
Strata or layers of sedimentary rock vary in distribution, thickness and character with changes in depositional environment and sediment supply (Fig. 1). Changes in characteristics of sedimentary rocks are important since they control parameters important to the discrimination of potential petroleum source and reservoir rocks. Stratigraphy is the study and classification of layered rocks, their depositional succession and geographic distribution. In its name is specific allusion to study of the origin and character of layered sedimentary rocks.
Figure 1 Layered rock sequence illustrating strata of sandstone, shale, and limestone.
Sedimentary rocks are not the only rock type, however, because igneous and metamorphic rocks can be formed and emplaced before, after, or coincident with, them in the same or contiguous areas. Therefore, stratigraphic relations include the spatial and temporal formation or deposition of all rock types relative to each other. Understanding rock units and sequences is paramount to classification of geographically and geologically significant stratigraphic increments that comprise paleogeographic and paleogeologic units in the geologic past. In this context the objective of stratigraphy is to describe and classify rock sequences and interpret them toward understanding geologic events.
Minerals and Rocks
Minerals
Minerals are the fundamental building blocks of rock materials in the earth. They are defined as naturally occurring inorganic substances with a definite chemical composition and specific crystal structure. Over 2,000 minerals have been identified in the earth. Minerals are comprised of chemical elements, and although there are over 100 elements represented in the earth’s crust, eight elements predominate and the remaining elements account for less than 1% of the crust, which therefore is actually characterized by a relatively simple composition.
Average composition of the earth’s continental crust includes larger amounts of the most common minerals and much smaller percentages of the rare constituents (Table 1). The first eight elements (Table 2) are the most common and constitute the basic ingredients for most continental crystal rocks, which are predominantly silicates and to a lesser extent oxides. Even less common are sulfides, chlorides, carbonates, sulfates and phosphates.
Table 1 Average Composition of Continental Rocks
Table 2 Main Elements in the Continental Crust
Rocks
Rocks are important to the petroleum industry because they provide source beds and reservoirs for petroleum. They consist of aggregates of minerals (Table 3) in various proportions and are identified by their origin and composition. Differentiation of rock types is based on their origin, which can be igneous, metamorphic or sedimentary. Since the earth is considered to have derived from a molten source, all rock materials on the earth ultimately came from an igneous origin.
Hydrocarbons are normally associated with sedimentary rocks. Source beds for hydrocarbons are always sedimentary rocks. However, although petroleum reservoirs are most commonly comprised of sedimentary rocks, igneous and metamorphic rocks also produce hydrocarbons as reservoirs in some places around the world. Reservoir rocks need porosity, permeability and access to source beds to be productive. It is not essential that they be exclusively sedimentary to meet these criteria.
Table 3 Rock Forming Minerals
Igneous Rocks: Igneous rocks are formed beneath the surface of the earth or on the surface. If they are emitted upon the surface they are extrusive rocks and are said to be volcanic or have a volcanic origin. Igneous rocks that form beneath the surface are called intrusive or plutonic rocks. They cool slowly, differentiate their minerals and form large crystals that result in phaneritic texture, in which mineral crystals can be seen without a microscope.
Igneous rocks have no hydrocarbon source potential. Fractured igneous rocks or porous and permeable weathered igneous rocks can provide adequate hydrocarbon reservoirs, and do so in many locations, including Vietnam, Indonesia, Venezuela, Argentina, Russia, and China. In the USA, examples occur in California, Nevada and Utah. The volcanic Garrett Ranch Formation (Fig. 2), which formed as a cooling incandescent cloud of airborne extrusive igneous material, covered much of central Nevada, USA, less than 50 million years ago. It is an important oil reservoir in the Eagle Springs and Trap Springs fields of east-central Nevada. Oil produced from the Garrett Ranch Formation was generated in underlying lake sediments and older marine beds, and then migrated into the volcanic reservoir rock.
Figure 2 Trap Springs Field map and sections illustrating Garrett Ranch igneous rocks as reservoirs. From Dolly, 1979. Permission to publish by RMAG.
Sedimentary Rocks: Sedimentary rocks are formed from materials derived from pre-existing sources that are transported and deposited. They may result from the accumulation of particles weathered from rock exposures, transported and ultimately deposited; they may accumulate by precipitation from solution; or they may develop as buildups formed from skeletons or life processes of marine animals and plants. Sedimentary rocks can be classified according to environment of deposition, rock type or by origin.
All petroleum source rocks are sedimentary. Black shale, rich in preserved organic materials, is considered to have the best source-bed characteristics, and some carbonate rocks have excellent source-bed potential. Other data suggests that organic-rich sediments preserved in highly saline evaporitic environments can also act as source beds in some cases.
Reservoir rocks include many types of sedimentary rocks. Sedimentary rocks containing intrinsic porosity and permeability such as sandstone, conglomerate and reef limestone are considered good reservoirs. Even sedimentary rocks with limited porosity can become good reservoirs if they are naturally fractured. Also, as we now know from the recent successful development of “resource plays”, sedimentary rocks with limited porosity, such as shale or siltstone can become good reservoirs if they are hydraulically fractured. Finally, impermeable sedimentary rocks can and do act as reservoir seals.
Metamorphic Rocks: Pressure, heat and catalytic action related to chemical agents are important factors in changing pre-existing rocks into rocks with different textures and compositions. The process which involves these agents is called metamorphism and results in the formation of metamorphic rocks. Crystal structure, bedding, texture, and other characteristics are changed by metamorphism, which occurs locally or regionally depending upon conditions.
Metamorphic rocks (Table 4) have no petroleum source potential. Fractured and weathered metamorphic rocks can be porous and permeable, and as such can form locally significantly productive reservoirs. The La Paz Field, Venezuela (Fig. 3) is an example of a productive metamorphic reservoir. Unfractured and unweathered metamorphic rocks have no permeability and can provide effective reservoir seals.
Table 4 Development of common metamorphic rocks
Figure 3 Cross section of La Paz field, western Venezuela, metamorphic reservoir. After Smith, 1956. Permission to publish by AAPG.
Rock Cycle
The molten origin of the earth indicates that igneous rocks are the fundamental and original rock type, prior to any chemical and physical degradation. Development of the earth’s hydrosphere and atmosphere resulted in the breakdown of igneous rocks into sedimentary rocks. Continuing dynamic earth processes created metamorphic rocks from igneous and sedimentary rocks, as well as from other metamorphic rocks. Subsequently, all three of these basic rock types also became source materials for other sedimentary rocks.
New igneous, sedimentary and metamorphic rocks are continually being formed from one another by currently active earth processes. This involves a continuous cycle of rock production, alteration and consumption to produce new rock types. The changing of rock types comprises the Rock Cycle (Fig. 4), which characterizes the inter-relationship of rock types, and demonstrates that virtually any rock type can result from alteration of a pre-existing rock type whether by remelting, metamorphism or erosion.
Figure 4 The Rock Cycle
Sedimentary rocks, which are crucial to working petroleum systems as source, reservoir and seal beds, represent important components of the Rock Cycle. Sedimentary rocks are the products of the breakdown, transportation and deposition of all three basic rock types comprising the Rock Cycle. They develop in many different environments, some of which are more prone to generate and accumulate petroleum.
The Rock Cycle forms, changes and destroys rocks as a continuum. Petroleum experiences a similar kinetic history during which time it is being formed, changed and destroyed with its host rocks. At any given time, oil is being generated in some rocks, changing in others, migrating in still others and being eliminated by geologic processes locally elsewhere. Oil is found where physical, chemical and temporal conditions are appropriate; where source beds are properly mature, migration has occurred, an adequate reservoir rock exists and the trap has integrity.
Igneous and metamorphic rocks do not generate petroleum, but under proper circumstances can provide reservoirs and seals for petroleum accumulations. They may also be additionally significant because igneous activity and metamorphism are not generally compatible with petroleum generation and accumulation, and when present, can possibly destroy petroleum.
Sedimentary Processes of Weathering, Erosion and Deposition
Weathering
Weathering is the mechanism by which rocks are broken up and supplied to the agents (e.g. wind or rivers) that transport the fragments to their new depositional sites. The mechanism of breakdown is accomplished by mechanical, or physical, and chemical means, which often work together and are called mechanical and chemical weathering (Fig. 5). Rock exposures undergo constant change when subjected to the elements. Wind, rain, temperature and chemical agents combine to degrade the landscape. In the process, they produce sedimentary materials that are transported and subsequently deposited to form new rocks.
Figure 5 Weathering and its products
Erosion
Rock materials are moved during weathering by various processes and transported to a site of deposition. Removal of rock materials is called erosion, which is responsible for the degradation of the landscape. Along the way, the rock materials are further broken down, rounded and sorted to the extent of their transportation.
Erosion occurs in several ways, all of which are involved in reducing the landscape to equilibrium, where erosion and deposition are offsetting processes. The process of erosion is important to the creation of working petroleum systems because, via erosion, sedimentary materials are derived, transported, concentrated, and deposited to form source beds, reservoir rocks and reservoir seals.
Gravity forces all weathered rock material to move downhill with or without the aid of flowing water. Except in very arid areas, downslope movement is routinely affected by the presence of water, which may act as a lubricant for downwardly mobile rock and soil masses. Running water is an important eroding agent because it carries abrasive particles that degrade the bedrock. It also removes weathered and eroded rock materials, revealing fresh rock surfaces to subsequent abrasion.
Stream erosion is a function of a number of factors, including climate, runoff and vegetation. Geologic factors are also significant because structure and rock type underlying the landscape have direct effects on erosion rates and drainage patterns.
Figure 6 Example of natural stream erosion over time.
Deposition
After rock materials are eroded and transported, they are deposited. Sometime after deposition they can be reworked, removed and redeposited elsewhere. Ultimately, however, they are deposited in a place where they are eventually buried and transformed into sedimentary rocks.
Deposition of sediments occurs in many ways, one of which is deposition by streams. Stream deposition is fundamental to the progression of the stream cycles of erosion. Environments of stream deposition are important because they can provide source, reservoir and seal rocks for petroleum.
Stream depsosits include channel and floodplain sediments, which are closely interrelated. The coarsest deposits are characteristic of channel deposition and are subject to highest stream velocities. Floodplain deposits are variable from coarse-grained, poorly sorted natural levees to fine-grained swamp deposits. Channel deposits include channel sands and point bars (Figs. 7, 8). These deposits are coarse and permeable at the base and become finer and less permeable upward.
Figure 7 Thickness Map of Pennsylvanian stream channel sandstone reservoir, Kansas. From Walters, Gutru, and James, 1979. Permission to publish Tulsa Geological Society.
Figure 8 Northeast-Southwest cross-section across Figure 7. From Walters, Gutru, and James, 1979. Permission to publish by the Tulsa Geological Society.
Conclusion
A foundational understanding of geology is crucial for accurately determining petroleum deposits and making drilling decisions. In summary:
- Understanding rock units and sequences is paramount to the classification of significant stratigraphic intervals that comprise important keys to interpreting the geologic past.
- The Rock Cycle forms, changes and destroys rocks as a continuum. In a similar way, petroleum also experiences a similar kinetic history, through which it is formed, changed and ultimately destroyed with its host rocks.
- Oil is found where physical, chemical and temporal conditions are appropriate; where source beds are properly mature, migration has occurred to a porous and permeable reservoir rock, and a viable trap exists.
- The sedimentary processes of weathering, erosion and deposition are very important to the petroleum industry, because it is through these processes that rock materials are derived, transported, concentrated and deposited to form source beds, reservoir rocks and reservoir seals, which are essential elements for working petroleum systems.
To learn more about petroleum geology, we recommend enrolling in an online Geology skill module or a future session of Basic Petroleum Geology.
References
· Link, Peter K.; 2001; Basic Petroleum Geology; 3rd edition; OGCI and PetroSkills Publications; Tulsa, Oklahoma.
Note: A copy of this book is provided to participants in the PetroSkills Basic Petroelum Geology Course.
· Foster, R. J.; 1979; Physical Geology; 3rd edition; C. E. Merrill; Columbus, Ohio.
· Dolly, E. D.; 1979; Geological techniques utilized in Trap Springs Field discovery, Railroad Valley, Nye County, Nevada; in Basin and Range Symposium and Great Basin Field Conference; Rocky Mountain Association of Geologists (RMAG) and Utah Geological Association; pp 455-467.
· Smith, J. E.; 1956; Basement reservoir of La Paz-Mara oil fields; AAPG Bull.; v. 40, pp 380-385.
Walters, R. F., Gutru, R. J. & James, A.; 1979; Channel sandstone oil reservoirs of Pennsylvanian Age in Northwestern Hess County, Kansas; in Pennsylvanian Sandstones of the Mid-Continent; Tulsa Geological Society Special Publication No. 1; pp 313-326.
