Dr. Marwa Yousef (Ain Shams University) 10/9/2025 1 For Third Level Students Oil and Gas Chemistry Program Dr. Marwa Yousef Ain Shams University Reservoir Lecture 3 1 2 Dr. Marwa Yousef (Ain Shams University) 10/9/2025 2 For a rock to serve as a petroleum reservoir, it must be porous, capable of storing petroleum. However, for a rock to become an "economically viable petroleum reservoir," additional factors, such as permeability (the ability to flow petroleum), reservoir volume, and compartmentalization, must be considered. These elements help estimate oil or gas in place, and ultimately, how much can be recovered. Factors like formation volume and recovery factor play crucial roles in determining reserves. Oil recovery rates rarely exceed 50%, while gas recovery rates are often higher than 80%. Comparison of Permeable and Impermeable Sandstone. (a) A scanning electron microscope image of permeable sandstone (b) A thin-section photomicrograph of a porous Impermeable Sandstone. 3 4 Dr. Marwa Yousef (Ain Shams University) 10/9/2025 3 Formation Volume Factor (Bo) for Oil and Gas Systems. The formation volume factor (Bo) quantifies the volumetric change between oil and gas under reservoir conditions. Bo is governed by the Bubble Point Pressure (Pb), the pressure at which gas first begins to separate from the oil. The initial formation volume factor (B₀) reflects the oil's compressibility, assuming all gas remains in solution at pressures above the bubble point (Pb). Below the bubble point, the curve demonstrates liquid shrinkage caused by gas exsolution. The intrinsic properties of a potential reservoir rock, critical for estimating petroleum volumes, include: Net to Gross Porosity Permeability Water, Oil, and Gas Saturation 5 6 Dr. Marwa Yousef (Ain Shams University) 10/9/2025 4 Net to Gross It is the ratio of productive reservoir rock to total reservoir rock thickness. It's a key factor for calculating the total volume of oil and gas in a reservoir. Determined by analyzing well logs and core samples to identify which rock layers meet minimum standards for porosity and permeability. The Fractured Reservoir Problem, fractures act as fluid highways that cut through both good and bad rock layers, making it difficult to define what "Net" pay is. In these cases, the classic NTG concept becomes less useful and requires more complex models. Net-to-gross (N/G) is a measure of the proportion of a reservoir interval that can produce hydrocarbons. It is typically determined using a permeability cutoff. 7 8 Dr. Marwa Yousef (Ain Shams University) 10/9/2025 5 Porosity Porosity refers to the void spaces in a rock that may hold petroleum, water, or gases. It typically ranges from 20–30% in many reservoirs but can reach up to 70% in certain limestone reservoirs. High porosity means more space to hold fluids. Porosity types include intergranular, intragranular, and microporosity, each affecting production potential differently. Types of Porosity in Sedimentary Rocks 9 10 Dr. Marwa Yousef (Ain Shams University) 10/9/2025 6 Permeability Permeability defines how easily fluids can pass through a rock. It is measured in darcies (D) or millidarcies (mD). Permeability values vary from 0.1 mD to over 10 D, and corrections must be applied when measuring at surface conditions to reflect reservoir conditions. Water, Oil, and Gas Saturation Reservoirs typically contain a mix of oil, gas, and water, with saturations varying based on reservoir conditions. These percentages affect the recoverable petroleum volume and change over time as production progresses. 11 12 Dr. Marwa Yousef (Ain Shams University) 10/9/2025 7 The most common reservoir rocks are sandstone and limestone (including dolomite). Sandstone reservoirs are prominent in the USA, South America, and North Africa. Limestone reservoirs dominate the Middle East. Most reservoirs are found in younger Cenozoic and Mesozoic rocks. Older rocks are often too deformed by tectonics or clogged by cementation, which destroys the reservoir quality. Fractured reservoirs are crucial. Fractures can either: Be the entire reservoir system themselves, or Act as drains, enabling production from rocks that would otherwise be too tight to flow. Oil fields: This figure illustrates the distribution of ages for the world's largest oil fields. Gas fields: This figure shows the distribution of ages for the world's largest gas fields. 13 14 Dr. Marwa Yousef (Ain Shams University) 10/9/2025 8 Alluvial Fans Alluvial Fans are wedge-shaped deposits of sediment formed where fast-moving streams spill out of mountainous areas and slow down, dropping their load. The sediment is typically poorly sorted (a mix of large boulders, gravel, sand, and mud), resulting in low porosity and permeability. These deposits are relatively small leading to low reservoir quality. Due to their discontinuous nature and rapid changes in sediment type, finding productive zones is challenging. An example is the Triassic Roer Valley Graben in the Netherlands. Alluvial Fans 15 16 Dr. Marwa Yousef (Ain Shams University) 10/9/2025 9 Aeolian Dunes Preserved sand dunes formed by wind in desert environments. The wind action creates well-sorted, rounded sand grains, resulting in high porosity and permeability. This makes them ideal for storing and flowing hydrocarbons, especially gas. While they form vast "sand seas," their preservation in the geologic record is rare because they are easily eroded. When found, they are highly prized. Famous aeolian sandstone reservoirs include the Permian Rotliegend in Europe and the Jurassic Norphlet in the Gulf of Mexico. Aeolian Dunes 17 18 Dr. Marwa Yousef (Ain Shams University) 10/9/2025 10 Lakes Lacustrine systems are generally uncommon as reservoirs because lake sediments have a low chance of being preserved over geologic time. This environment is particularly important in parts of Southeast Asia. Reservoirs are typically small, with reserves usually in the tens of millions of barrels range. Fluvial systems River systems are ancient river deposits that can form extensive sandstone reservoirs, especially in subsiding basins. They are primarily categorized into two types, which have different reservoir characteristics: Braided Rivers: Create sand-rich, high-quality reservoirs with high Net-to-Gross and thick, interconnected sand bodies. A prime example is the Prudhoe Bay Field in Alaska. Meandering Rivers: Form more complex reservoirs with lower Net-to-Gross, as the sands are mixed with mudstones. like the Ness sandstones in the North Sea's Brent Province. 19 20 Dr. Marwa Yousef (Ain Shams University) 10/9/2025 11 Deltas Delta systems, where rivers meet the sea, can form excellent petroleum reservoirs. However, they often have mixed lithologies that create barriers to fluid flow. Deltas are shaped by river, wave, and tidal forces, affecting the geometry of the reservoir sand bodies. Notable deltaic reservoirs include the US Gulf Coast, the Niger Delta, and the Mahakam Delta in Indonesia. These systems are often associated with growth faulting, creating ideal trap structures for petroleum. Delta Shallow-Marine Systems Shallow-marine sandstones are often ideal for petroleum reservoirs due to their potential association with both source rocks and seals. These sandstones tend to be simple, homogeneous, and highly connected between wells unless disrupted by faulting. Their thickness can vary significantly based on sediment supply, subsidence, and sea-level changes. Tidal and wave energy also influence their development. Notable reservoirs include the Cretaceous-age sandstones in the Midwest USA and Western Canada Basin, as well as the Late Jurassic sandstones in the North Sea. Another important example is the Jurassic/Cretaceous Toro Sandstone in Papua New Guinea. 21 22 Dr. Marwa Yousef (Ain Shams University) 10/9/2025 12 Shallow-Submarine Fans Submarine Fans are large, underwater deposits formed by sediment gravity flows in deep-water basins. They are a major focus of global deep-water oil exploration, especially in the Gulf of Mexico, West Africa, and Brazil. They vary widely. Large fans may have low overall sand content, while smaller, sand-rich fans can host massive oil pools, like the Forties Fan in the North Sea. Recent discoveries off Angola have found giant oilfields in sand-filled canyons of the ancient Congo Delta, highlighting their current importance. Shallow-Submarine Fans 23 24 Dr. Marwa Yousef (Ain Shams University) 10/9/2025 13 Shelfal and Ramp Carbonates Shelfal and Ramp Carbonates are shallow marine limestone deposits that form the world's largest oilfields, such as Ghawar in Saudi Arabia. These systems cover vast areas, allowing for the development of exceptionally large reservoirs. They are dominant in the Arabian Gulf (Cretaceous age) and the Permian Basin of West Texas. While the initial rock may be good, the best reservoir quality often comes from later processes like dolomitization, leaching (creating pores), and fracturing, which dramatically improve porosity and permeability. Shelfal and Ramp Carbonates 25 26 Dr. Marwa Yousef (Ain Shams University) 10/9/2025 14 Reefs Reefs are biologically-built limestone structures that form major petroleum reservoirs. They are historically important in world-class plays They range from small pinnacle reefs to massive fringing reef complexes that span many kilometers. Even smaller pinnacle reefs can hold substantial oil reserves if they have good porosity and are enclosed by sealing rocks Reefs 27 28 Dr. Marwa Yousef (Ain Shams University) 10/9/2025 15 Deep-water Carbonates Though less common, deep-water carbonate reservoirs exist. Carbonate turbidites have developed from material shed from adjacent platforms. The Upper Cretaceous pelagic Chalks in northern Europe also act as reservoirs for significant oil and gas, notably in the Ekofisk complex. Dolomite Dolomite is a carbonate rock that forms a significant proportion of the world's petroleum reservoirs, especially in North America. In the U.S., about 80% of carbonate-hosted oil is found in dolomite, not limestone. Its reservoir quality is not primary but is created by the process of dolomitization (which can enhance porosity) and karstification (dissolution that creates caves and pores). It frequently forms in environments associated with evaporitic rocks (like salt and gypsum), as seen in the giant Upper Jurassic reservoirs of the Arabian platform. 29 30 Dr. Marwa Yousef (Ain Shams University) 10/9/2025 16 Dolomite Karst Karst as a Reservoir, Karst features (like caves and fractures) can create excellent oil and gas reservoirs by significantly increasing rock porosity and storage space. For a karst feature to become a viable reservoir, two conditions are crucial: The karst features must be preserved and not destroyed by later geological processes. They must not become filled with non-porous material like mud. 31 32 Dr. Marwa Yousef (Ain Shams University) 10/9/2025 17 Karst Fractures can dramatically control the success of hydrocarbon reservoirs, but their effect is dual in nature: Open fractures can create the main pathways for fluid flow (permeability) and sometimes even the primary storage space (porosity), especially in otherwise low-permeability, "tight" rocks. They are crucial for making challenging reservoirs viable. Conversely, closed fractures and faults can act as barriers, leading to reservoir compartmentalization. This divides the reservoir into isolated sections, complicating extraction. 33 34 Dr. Marwa Yousef (Ain Shams University) 10/9/2025 18 Fractured reservoir 35 36