Part 1.1 Petroleum Geology

Objectives After reading the chapter and reviewing the materials presented the students will be able to: Understand the basic concepts of geology Identify the flow through a supply chain. Describe the origins of petroleum. Identify the importance of porosity and permeability. Explain how reservoir pressure influences flow.

Petroleum Geology The petroleum geologist is primarily concerned with rocks that contain oil and gas, particularly rocks that contain enough petroleum to be commercially valuable. A petroleum reservoir is a rock formation that holds oil and gas, somewhat like a sponge holds water. The East Texas oilfield covers thousands of acres. The largest oilfield is Ghawar in Saudi Arabia. It is 1.8 million acres in size, 150 miles long and 19 miles wide, hundreds of feet thick and currently produces 5 million barrels of oil per day. The largest oilfield in the US is Prudhoe Bay oilfield on the northern Alaska slope which covers over 213,000 acres and produces 285,000 barrels of oil per day.

Basic Concepts of Geology Astronomers and physicists estimate that the earth was formed 4.5 billion years ago from a cloud of cosmic dust. The heaviest elements, iron and nickel, sank to the earth’s center and became the core. Lighter minerals formed a thick solid mantle around the outer core. Minerals rich in aluminum, silicon, magnesium, and other light elements solidified into a thin rocky crust above the mantle. Most geologists think that the crust is made up of huge plates that continue to move and change shape. The theory that explains this is called plate tectonics. Oceanic crust is thin, about 5 to 7 miles, and made up primarily of heavy rock that formed when molten rock called magna cooled. The rock of the continental crust is much thicker, 10 to 30 miles, and relatively light. The continuous movement of the earth’s crust has changed the shapes of land masses and oceans.

Geologic Structures Geologists describe two basic structures that occur when rocks deform, or change shape, due to tectonic movement: folds and faults. Folds are rock strata that have crumpled and buckled into wavelike structures. Folds are the most common structures in mountain chains. When rocks near the surface break, the two halves might move in relation to each other, producing a fracture known as a fault. Faults are important in geology because their movement creates areas that accumulate oil and gas.

Life on Earth In the Devonian period, 350 million years ago, vegetation became widespread on the Earth. The presence of life is considered essential to the petroleum story, because according to the prevailing theory, organic matter is necessary for the formation of oil. Geologists group the rocks of the earth’s crust into three types depending on how they were formed into: igneous, sedimentary, and metamorphic. Igneous rocks are formed when molten magma cools and solidifies. Sedimentary rocks are formed from sediments in horizontal layers called strata. Limestone, sandstone, and shale are typical sedimentary rocks. Metamorphic rocks were originally igneous, sedimentary, or other metamorphic rocks that were buried deep in the earth and subjected to high temperatures and pressures. For example, limestone can be metamorphosed into marble, and sandstone into quartzite. Petroleum geologists are mainly interested in sedimentary rocks because most oil and gas formations occur within sedimentary formations. Most of the world’s oil lie in sedimentary rock formations from marine sediments deposited along the edge of continents.

Accumulation of Petroleum The accumulation of hydrocarbons in commercial quantities is a result of millions of years of geologic activity occurring in the right order. Hydrocarbons must be generated in the subsurface, trapped within reservoir rocks, and remain in place until they are produced. Each of these three steps may take millions of years. The most accepted theory is that oil and gas comes predominantly from the remains of microscopic plant and small animal organisms. As these small organisms died, they fell to the ocean floor, where they mixed with the silt, and, and mud to form a rich organic mixture called kerogen. Kerogen is rich in both carbon and hydrogen. Increasing pressure and temperature cause kerogen and hydrocarbon molecules to recombine to make oil and gas.

Porosity & Permeability of Oil Bearing Rocks Rocks have tiny openings or pores that are visible under magnification. Any oil and gas that exists in the rock is found in these pores, somewhat like the way sponge holds water. The greater the porosity of a formation, the more petroleum it can hold. To be commercially valuable, most reservoir rock must have a porosity of 10% or more. A rock is permeable when its pores are connected allowing oil, gas, and water to move from one pore to the other. The unit of measurement of permeability is known as a darcy. Sandstone and carbonates such as limestone and dolomite are generally the most porous and permeable rocks and are also the most common reservoir rock. Shale oil and tar sands require extensive technological innovation to recover the hydrocarbons economically.

Migration of Petroleum A trap consists of an impermeable layer of rock above a porous, permeable layer containing the hydrocarbons. Geologists group traps into two types: structural and stratigraphic. Structural traps occur when the reservoir formation deforms in some way. Stratigraphic traps are those in which porosity or permeability has changed within a formation or where a nonporous layer seals off the top of a reservoir. It is common for reservoirs to be trapped by a combination of folding, faulting, permeability changes, and other conditions.

Reservoir Fluids Reservoir rock usually contains three fluids: oil, gas, and saltwater. When layered, the lightest, gas, is on top; the oil is in the middle; and the heaviest, water, is on the bottom. The pressure of gas and water can help drive oil out of the rock to the surface, making pumping unnecessary.

Reservoir Pressure All reservoir fluid are under pressure. The weight of the formations on the top of the reservoir adds to the fluid pressure. An artesian effect occurs when there is a buildup of abnormally high pressure. Petroleum geoscientists job is to find where the oil reservoirs are located, so drillers can tap the resources.

Summary The petroleum geologist is primarily concerned with rocks that contain oil and gas, particularly rocks that contain enough petroleum to be commercially valuable. A petroleum reservoir is a rock formation that holds oil and gas, somewhat like a sponge holds water. Folds are rock strata that have crumpled and buckled into wavelike structures. Folds are the most common structures in mountain chains. When rocks near the surface break, the two halves might move in relation to each other, producing a fracture known as a fault. Faults are important in geology because their movement creates areas that accumulate oil and gas. Petroleum geologists are mainly interested in sedimentary rocks because most oil and gas formations occur within sedimentary formations. Most of the world’s oil lie in sedimentary rock formations from marine sediments deposited along the edge of continents. The most accepted theory is that oil and gas comes predominantly from the remains of microscopic plant and small animal organisms. Rocks have tiny openings or pores that are visible under magnification. Any oil and gas that exists in the rock is found in these pores, somewhat like the way sponge holds water. A rock is permeable when its pores are connected allowing oil, gas, and water to move from one pore to the other. Shale oil and tar sands require extensive technological innovation to recover the hydrocarbons economically. A trap consists of an impermeable layer of rock above a porous, permeable layer containing the hydrocarbons. Geologists group traps into two types: structural and stratigraphic. Reservoir rock usually contains three fluids: oil, gas, and saltwater. When layered, the lightest, gas, is on top; the oil is in the middle; and the heaviest, water, is on the bottom.

Home Work 1. What is a petroleum reservoir? 2. Why are faults important in geology? 3. What is the most accepted theory as to where oil and gas comes from? 4. What three fluids are contained in reservoir rock? What is on top, middle, and on the bottom?