The research related to the origin, occurrence, movement, accumulation, and exploration of hydrocarbon fuels is known as Petroleum Geology.
There are many ways the field of Geology contributes to the Petroleum industry. It explores a higher prospect of the source of oil and where it might be found. The basics are Structural Geology which is the study of rock bodies, their composition, tectonic history and the events that might have changed their features or might do so in the future.
When the nature of these rocks is determined, petroleum geologists can discover whether there is any presence of petroleum, natural gas, or other natural resources trapped within the rock.
Searching for oil in this vast earth and that too thousands of feet below the ground is impossible to find without the study of the rocks and the structural geology of the place.
- 1 Types Of Rock to study under Petroleum Geology
- 2 Common Rocks Containing Oil And Gas:
- 3 Analysis Of Sedimentary Basin
- 4 Exploration Techniques
Types Of Rock to study under Petroleum Geology
The study of rock is the primary step and very important before determining the particular geology of a specific area to find petroleum. As we know, the rocks are classified into three types based on their structural features and chemical composition. They are:
- Igneous rocks crystallize from molten rock or magma. They are of two types based on their depth of the formation. Plutonic igneous rocks crystallize slowly below the surface of the earth and have large crystals. Granite, Gabbro, and Diorite are typical examples. Volcanic igneous rocks crystallize quickly at the earth’s surface and have small crystals. Basalt, Rhyolite, and Andesite are typical examples.
- Sedimentary rocks are the rocks which contain the hydrocarbons such as oil and natural gas. As the name suggests, the sedimentary rocks are formed by the consolidation of the sediments. Sandstone and Limestone are the standard examples.
- Metamorphic rocks form from the alteration of the pre-existing rocks by changes in changes of pressure and temperature. Gneiss, Schist, Slate are typical examples.
Common Rocks Containing Oil And Gas:
The types of rocks that contain oil and natural gas are all sedimentary rocks, and because these rocks are cemented together, they are porous, full of spaces in which energy-rich carbon compounds can settle, later to be liberated in the form of oil or gas.
Shale is a sedimentary rock seen as a natural fuel source, mainly because of its abundance. 42% of sedimentary rocks are shale. It is created when carbon-rich mud layers are compressed into a hardened rock. One type of shale is known as “oil-shale” as it contains a large amount of kerogen that gets processed into oil and gas. ( kerogen is an organic matter capable of producing petroleum.)
Porous rocks that trap the low-density carbon compounds lying above shale beds are known as sandstones. They formed from the grains of minerals like quartz bound by other compounds such as silica. Generally, carbon compounds exist in sandstone beds in liquid form as crude oil.
Carbonates are formed from the remains of marine life, mainly shells, and bones, combined with other minerals. The spaces between their fused fragments are where oil and gas may be found.
Analysis Of Sedimentary Basin
Petroleum geology is principally concerned with the evaluation of the following seven critical structures in a sedimentary basin:
- Source: Source rock refers to the rock which is capable of generating hydrocarbons or the rocks from which hydrocarbons have been derived.
- Reservoir: The porous and permeable rock which have hydrocarbon reserves inside its pores is known as a reservoir rock. Sandstone and limestone are the most common type of reservoir rocks.
- Cap: Cap rocks play an important role in acting as the geological feature which restricts the further migration of the hydrocarbons from the reserves.
- Trap: It is a structural feature that ensures the entrapment of oil into the subsurface so that it cannot escape and hence is lost.
- Maturation: The maturity of a source rock reflects the ambient pressure and temperature as well as the duration of the conditions favorable for hydrocarbon generation. Understanding maturation is significant in shale reservoirs as shales play the dual role of both a reservoir and a source rock.
- Migration: The movement of hydrocarbons from their source to the reservoir rock is known as migration in petroleum geology. It typically occurs from a structurally low area to a higher area because of the relative buoyancy of the hydrocarbon in comparison to the surrounding rock.
Hydrocarbon exploration is the search by petroleum geophysicists and geologists for hydrocarbon deposits beneath the earth’s surface. Petroleum exploration is grouped under the science of petroleum geology.
The exploration techniques for petroleum can be separated into two ways: surface geology and sub-surface geology.
Surface Geology: 1st Exploration Technique in Petroleum Geology
- Direct indications: Seepage of oil and gas through any fault line, the porous rock may indicate the presence of hydrocarbons.
- Surface data: They may include the study of the rock on the surface. Lithology, stratigraphy, porosity, and thickness of significant formations help in explorations.
- Field geology: It is a traditional method that involves mapping the surface geology and predicting the types of rocks in the sub-surface.
- Gravimetric method: This method has the usage of gravimeters. Force of gravity at any point on the earth’s surface influences the magnitude and direction of the rocks of different densities in the area. Older rocks tend to have a stronger gravitational force than sediments of newer basins.
- Magnetic method: A sensitive instrument known as magnetometer is used to measures the variations in the intensity of the earth’s magnetic field which are originated from the varying magnetic properties of the underlying rocks.
- Seismic method: The seismic method of exploration is used to search the availability of petroleum using seismic waves. artificial shock waves are generated on land and the reflected waves are recorded and are analyzed for the indication of an economically valuable source of energy like crude oil.
- Electrical method: Depends on the differences in the resistance to electric currents by rocks of various types.
Sub-Surface Geology: 2nd Exploration Technique in Petroleum Geology
In this method, the drilling is done first. After or while drilling, a sample rock is obtained from beneath which is known as core, and the process is known as coring. A core sample is a cylinder section of the sample rock that is collected for investigating the rock properties such as its porosity, permeability, etc.
It is analyzed by different techniques and equipment depending on the type of data required. Primary information is obtained from the rock sample, and further measurements such as radioactive, acoustic are made on the properties of the rock.
All of these measurements opens a channel to a lot of information such as density, radioactivity, porosity, permeability, resistivity, depth, thickness, fluid type, etc. Analysis of such rock samples yields primary data to the evaluation of the productive potential of a hydrocarbon reservoir. It provides valuable information about the makeup of the rock being drilled.