Petroleum or oilis a naturally occurring, yellow to black liquid found in geological formations mostly beneath Earth’s surface. Crude oil is composed of carbon (83%
e85%), hydrogen (10%e14%), nitrogen (0.1%e2%), ox- ygen (0.05%e1.5%), sulfur (0.05%e6.0%), and metals (<0.01%). The mother liquid splits into several constituents by fractional distillation, and refined components are used as various types of fuels.
Natural gasis a naturally forming complex hydrocar- bon comprised primarily of methane associated with minor higher alkanes, carbon dioxide, nitrogen, hydrogen sulfide, or helium. It occurs as an independent gas reservoir or in close proximity as layers on top of petroleum pools/coal beds (Fig. 4.17). Natural gas is a clean fossil fuel used as a source of energy for heating, cooking, electricity genera- tion, and in vehicles.
A fossil fuel, petroleum/natural gas is formed when large quantities of dead organisms, usually zooplankton and algae, are buried underneath sedimentary rocks, and subjected to decomposition under both intense heat and high pressure. The liquid is generally moved away from the place of origin and trapped in suitable highly porous geological structures/reservoirs between impervious beds.
Hydrocarbon fossil fuels (coal, petroleum, and natural gas)
FIGURE 4.17 Conceptual diagram showing oil, gas and water accumulation in an anticline structure as interpreted through geological-geochemical- geophysical studies and supported by drill-holes (bottom).•/•are location of positive/negative drill holes (top).
are formed from dead and decaying plankton, plants, and organic matter (zooplankton and algae) that lived<360 million years ago (Carboniferous period). These organic lifeforms originated from ancient oceans, primordial swamps, lagoons, rivers, and fan-shaped deltas. The decaying organic matter decomposed under high pressure and temperature caused by sediments being piled on top of organic matter over time. Eventually, the dead organic matter heated and compressed sufficiently to form coal, petroleum, and natural gas over hundreds of millions of years.
Oil exploration is an expensive, high-risk operation, and is conducted primarily by national governments and/or large exploration companies. The following steps are custom-made for the exploration of oil and natural gas:
1. A high degree of knowledge is required of stratigraphy and source rock package from Carboniferous or younger ages when organic-rich rock such as oil shale or coal was subjected to high pressure and temperature over an extended period of time to form hydrocarbons.
Sedimentary rocks are the most significant medium/
source for oil and gas generation and accumulation in large quantities forming hydrocarbon basins/
reservoirs. The primary components of the collector sedimentary rocks are originated from clastic and car- bonate environment. The clastic forms include alluvial fans, tidal plains, deltas, sandbanks, underwater dunes, sand ridges, barrier islands in coastal marine environ- ments (marine beaches and offshore), debrites, and turbidity fans. The carbonate environments entail carbonate platforms, high-energy shallows, restricted shoals, lagoons, inner shelves, bodies of organogenic reefs, perireef limestone, debrites and turbidites or allodapic limestone, and reef and perireef bioclastic limestone outside the carbonate platforms (Haldar and Tisljar, 2014).
2. The ideal sedimentary process includes building, destruction, accumulation, sedimentation, and cementa- tion. Coarse-grained texture, graded bedding and struc- ture, excellent porosity and permeability, the large presence paleoorganisms in the form of skeletal debris, shells of Corallinaceae algae, bryozoans, corals, sessile foraminifera (Nubecularia) and thick shells, bivalves, oysters, foraminifera, skeletons of cyanobacteria (blue- green algae), oncoids, and peloids are the outstanding indicators of hydrocarbon reservoirs. The cover pelites (silt, clay, and mud) and marls (evaporates) are signifi- cant insulator rocks that stop oil/gas from moving away (Haldar and Tisljar, 2014).
3. Hydrocarbons are squished from source rock by three density-related mechanisms: newly matured hydrocar- bons are less dense than their precursors, which causes
overpressure; hydrocarbons are lighter and so migrate upward due to buoyancy; andfluids expand as further burial causes increased heating. Most hydrocarbons migrate to the surface as oil seeps, but some will be trap- ped. Look for significant visible surface features such as oil and natural gas seeps (Fig. 5.19) and underwater shallow/deep seabed craters caused by escaping gas (Fig. 5.18).
4. Common reservoir rocks are gravels, conglomerates, breccias, porous sand/sandstones (clastic), and lime- stones and dolomites (carbonates). Strongly fractured igneous and metamorphic rocks (e.g., fractured granite) may be appropriate collectors under suitable conditions.
Hydrocarbons are contained in a reservoir rock. Reser- voirs must also be permeable so that the hydrocarbons willflow to the surface during production.
5. Hydrocarbons are buoyant and have to be trapped within a structural (e.g., anticline, fault block) or strati- graphic trap. A hydrocarbon trap has to be covered by an impermeable rock known as a seal or cap rock to pre- vent hydrocarbons escaping to the surface.
6. Exploration tools are highly sophisticated geophysical methods (gravity, magnetic, passing through passive or regional seismic reflection surveys) to detect the ex- istence and determine the extent of these anomalies in large-scale features of subsurface geology. Detailed time domain seismic survey continues to identify the leads to create a profile of the substructure.
7. A prospect is identified and evaluated by exploration well drilling to conclusively confirm the presence/
absence of oil and/or gas.
8. Finally, well drilling, geophysical well logging, anal- ysis, interpretation, resource/reserve estimation, and economic evaluation continue to declare a reservoir for production planning.
World proven petroleum reserves as at early 2017 stand at 1,726,685 million barrels. Thefirst 10 countries in order of rank are (in million barrels): Venezuela (300,878), Saudi Arabia (266,455), Canada (169,709), Iran (158,400), Iraq (142,503), Kuwait (101,500), UAE (97,800), Russia (80,000), Libya (48,363), and United States (35,230).
Source: www.en.wikipedia.org/wiki/List_of_countries_
by_proven_oil_reserves.
Total oil production in 2016 averaged 80,622,000 bar- rels per day (bbl/day). The top 10 countries contribute w68% of the total amount (in bbl/day): Russia (10,551,497), Saudi Arabia (10,460,710), United States (8,875,817), Iraq (4,451,516), Iran (3,990,956), China (3,980,650), Canada (3,662,694), UAE (3,106,077), Kuwait (2,923,825), and Brazil (2,515,459).
Source: www.en.wikipedia.org/wiki/List_of_countries_
by_oil_production.
4.9.1 Use of Oil and Gas
The primary uses of oil and gas include the following:
1. Petroleum products include transportation fuels, fuel oils for heating and electricity generation, asphalt, and road oil, and feedstocks for making chemicals, plastics, and synthetic materials that are in nearly everything we use.
2. Liquefied petroleum gas or liquid petroleum gas (also called propane or butane) areflammable mixtures of hy- drocarbon gases used as fuel in heating appliances, cooking equipment, and vehicles.
REFERENCES
Banerjee, P.K., Ghosh, S., 1997. Elements of Prospecting for Non-Fuel Mineral Deposits. Allied Publishers Ltd., p. 320
Deb, M., Pal, T., 2004. Geology and genesis of the base metal sulphide deposits in the Dariba-Rajpura-Bethumni Belt, Rajasthan, India in the light of basin evolution. In: Deb, M., Goodfellow, W.D. (Eds.), Sediment Hosted Lead-Zinc Sulphide Deposits: Attributes and Models of Some Major Deposits in India, Australia and Canada. Narosa Publishing House, New Delhi, pp. 304e327.
Haldar, S.K., 2007. Exploration Modeling of Base Metal Deposits.
Elsevier Publication, p. 227.
Haldar, S.K., Tisljar, J., 2014. Introduction to Mineralogy and Petrology.
Elsevier Publication, p. 356.
Roy, A.B., Jakhar, S.R., 2002. Geology of Rajasthan, (Northwest India), Precambrian to Recent. Scientific Publishers, India, p. 412.
Roy, A.B., 2010. Fundamentals of Geology. Narosa Publishing House, p. 291.
Sarkar, S.C., Banerjee, S., 2004. Carbonate-hosted lead-zinc deposits of Zawar, Rajasthan, in the context of the world scenario. In: Deb, M., Goodfellow, W.D. (Eds.), Sediment Hosted Lead-Zinc Sulfide De- posits: Attributes and Models of Some Major Deposits in India, Australia and Canada. Narosa Publishing House, New Delhi, pp. 350e361.
Exploration Geochemistry
Chapter Outline