Lesson 7: Source Rocks for Hydrocarbon Generation

In this chapter, we discuss the key elements required for a successful hydrocarbon field. These include source rocks, hydrocarbon migration, reservoir rock with trapping geometry, and an overlying sealing lithology.

A petroleum system encompasses the origin, accumulation, and migration of petroleum. It involves source rocks that generate oil and gas, as well as traps where they accumulate. An example is the North Viking Robin offshore Norway.

- The essential elements for hydrocarbon formation are source rock, reservoir rock, sealing rocks, and overburden. - Hydrocarbons are generated in the source rock and migrate into the trap. - In conventional plays, hydrocarbons need to be generated, migrated, and accumulated within the reservoir rock.

Oil and gas are sourced from organic material, such as plants or animals. This material undergoes a process called hydrocarbon generation under specific conditions like geologic time, depth of burial, and temperature. When the organic matter is produced in plants or animals, it starts to generate molecules of oil and gas.

To determine the presence of organic matter, two factors are important: concentration and source rock. The concentration should be at least 1% by weight in a rock sample to have a significant amount of total organic carbon (TOC). It is preferable to find areas with low input from river systems or other sediments. Additionally, preservation of organic matter requires an environment with low oxygen content.

In anoxic environments, the oxygen available to decompose organic material is reduced. This occurs in stagnant waters or areas with low oxygen levels. In water columns, such as in oceans or lakes, the photic zone produces a lot of organic material that sinks and can reach a minimum level of oxygen close to zero. Organic material buried in these conditions has a higher chance of preservation.

Organic matter can be classified into three types. Type 1 is algal remains, commonly found in deep lakes under anoxic conditions, which generate waxy crude oil. Type 2 consists of planktonic and bacterial remains from marine critters, preserved in low-oxygen environments to produce both oil and gas. Type 3 refers to terrestrial plant matter decomposed by bacteria and fungi, with better preservation chances under sub-oxic conditions.

The properties associated with a source rock are the total organic carbon content (TOC) and the hydrogen index. A rock with over 1% TOC is considered a potential source rock, while those with over 15% TOC are world-class sources. The hydrogen index is important for neutron porosity logs.

To produce hydrocarbon molecules, it is not enough to have high total organic carbon (TOC). We also need the raw material (plant and animal remains) to be buried and undergo a chemical reaction. The temperature plays a crucial role in this process as it controls the reaction. Geochemists use equations like Aranea quai shion's to determine how much organic material can be converted into oil and gas during burial.

Formation of Organic Material The chart on the right shows the process of organic material formation, from its living conditions to being buried deeply. The deposition of organic material occurs along with sediment grains. Through diagenesis, rocks are compacted at mild temperatures and pressures, transforming the organic material into karagin, a waxy substance.

"Catagen ASIS" - Thermal Degradation "Catagen ASIS" refers to the thermal degradation of karagin under increased burial depth and temperature. This results in hydrocarbon chains forming oil molecules and gas molecules. As temperature and depth increase further, longer hydrocarbon chains crack into shorter ones until only gas remains.

Metamorphism - Release of Hydrocarbons During metamorphism or meta genesis phase,the remaining hydrocarbons are driven off,releasing residual carbon.Therefore,different zones can be observed including diagenetic zone

The Van Krevelen diagram is a chart that plots the hydrogen to carbon ratio on the y-axis and the oxygen to carbon ratio on the x-axis. It helps us identify different types of organic matter, such as type 1, type 2, and type 3. By analyzing where a sample falls on this diagram, we can determine its organic material type and its stage in oil generation.

Computers can model how basins have subsided and the occurrence of properties like temperature and hydrocarbon generation over geological time. This can be done in one dimension using well locations or pseudo well locations based on seismic interpretation. Two-dimensional modeling involves cross-sections intersecting multiple wells, while three-dimensional modeling uses grids. The starting point is the present-day stratigraphy, which is back stripped into past times through gia history analysis. Models are calibrated using present-day temperature data and indicators of hydrocarbon generation states.

To generate a burial history, we use existing stratigraphy and paleo data. The software allows us to progressively remove layers and date them based on sea level changes. We can also model how the basin has subsided and filled over time.

Total subsidence in a basin is influenced by two main factors: the tectonic thermal effect and the weight of sediments on top of the basement. By understanding these components, we can gain insights into how the basin formed and filled.

In extensional pull apart Atlantic type margins, we analyze the sediment effect and subtract it out. This analysis is done for margins where the continental crust thickness decreases from 35 to 40 kilometers to oceanic crust. The plot on the bottom shows the amount of crustal extension.

This chapter discusses the theoretical subsidence in extensional pull-apart basins. The authors present a study on the subsidence of mid-ocean ridges using three different models, with a focus on the Dyke ejection model. They analyze geological time and calculate the extent of crust thinning, allowing for estimation of heat flow associated with rifting and sagging phases.

In this chapter, we explore how temperatures have changed over geological time at a specific location. We use a model to simulate these temperature variations and calibrate it with measured values from the well. The black circles represent the actual measurements, while the dashed magenta line shows our model's predictions based on GIA history backstripping and heat flow.

In addition to Vitrinite Reflectance analysis, there is another paleo thermometer called bit reflectance. It measures the heat experienced by rocks over time using a parameter known as veteran height reflectance (R Sub Zero). This allows us to calibrate our temperature model not only with present-day temperatures but also with veterinary Flecktones.

By analyzing subsidence and heat flow history, we can determine the oil and gas windows. The oil window ranges from an R0 of 0.55 to a reflectance of 0.95, while the gas window extends from a reflectance of 0.95 to 2.2.

In source and generation analysis, we use available data to identify potential source rocks. We collect data on total organic carbon, hydrogen index temperature, and bitumen reflectance in the area of interest. Using this data, we model subsidence fill and temperature histories using actual well locations or seismic derived stratigraphy. By comparing our models with existing temperature and reflectance data, we can predict where the source enters the oil window and transitions to the gas window.

The video discusses the transition from the gas window to the over mature window. Diagrams and references on various slides are available for further exploration.

The study focuses on the Norwegian up half of the North Sea, specifically the area around Stavanger and Bergen. The researchers analyzed temperature data, bitter night reflection data, geological tops, and ages to model two main source rocks' entry into oil and gas generation windows. The objective was to analyze prospects for offshore lease development.

The temperature data and bit reflectance data are shown on the left and right respectively. The colored symbols represent the actual temperature and light reflectance data, while the blue curve represents the model temperature for that location. We have a good match in our region of interest.

Covered Units The first seven units of the syllabus have been covered in this session. Reservoir rocks will be discussed next Tuesday, and structure and traps will be covered a week from today.

Role of Extinctions in Organic Matter Production "If there is a mass extinction, it leads to the creation of a lot of organic matter. Limited oxygen in the water column results in widespread anoxic conditions for several million years."

Open-Source Software for Basin Analysis "I am not familiar with any open-source software available for basin analysis. There are vendors who sell systems that do this type of analysis."

Arrhenius Equation "The Arrhenius equation is used by geochemists to predict when oil or gas generation occurs based on different types (type 1, type 2, or type 3)of organic matter."