Trinity Energy Group: A Short Introduction to Petroleum Geology

Exploring for oil and gas takes a lot of money and know how to pull off as it requires first a comprehensive understanding of the fundamentals of petroleum Geology. Oil and gas are, after all, essential petroleum resources that are found deep within the earth’s crust, although there are cases where they may be found on the surface as well, but not in abundant commercial quantities.

Liquid oil and liquid natural gas underneath are found in porous and permeable rocks (also called reservoirs) that have collected these precious materials for thousands of years. There are four essential types of geologic features that contain oil and gas deposits, namely:

* Oil and gas source rocks

* Reservoir rocks

* Seals

* Traps  

The figure below shows how the four features can contain oil and gas in a configuration that allows explorers to tap each part separately.

Figure 1. Arrangement of oil and gas source rocks, a reservoir, a seal, and a trap in a way that has allowed the natural accumulation of oil and gas.

Oil and Gas Source Rocks

Oil and gas can be found in sedimentary source-rocks that were deposited in very quiet water, such as stagnant swamps, shallow calm marine bays, or in ancient deep underwater basins. Source rocks are made up of extremely minute mineral components. Within the spaces between these mineral fragments are contained the remains of organic substances, such as wood bits, algae, or pieces of soft plant materials. Once these tiny sediments are gradually overlain through continuous sedimentation, heat and pressure increase, turning these soft organic sediments into solid rock strata. With further accumulation of sediments and subsequent increase of temperatures above 120^o C (250^o F), the organic deposits start to be "cooked", producing oil and natural gas which are then removed from the source-rock strata.

It takes thousands of years for this process to take place before commercial volumes of what is called thermogenic (that is, produced by heat) oil and gas can accumulate. Organic materials mostly made up of wood fragments in source rocks will produce natural gas upon maturation while algae or the soft parts of plants on land will produce both oil and natural gas.

At temperatures above 150^o C (300^o F), organic remains would have generated most of the oil they can produce. The remaining oil in the source rock or any oil that has been trapped in adjacent reservoirs will be converted into natural gas.

Natural gas can also be produced in certain organic-rich sedimentary rocks through bacterial processes in shallow burial depth prior to thermal maturation temperatures are attained. This process called biogenic-gas (that is, produced by organisms) generation occurs at depths of less than 2,000 ft and produces less amounts of gas compared to thermogenic gas.

Oil and Gas Reservoir Rocks

Oil and gas reservoir rocks have high porosity and are also highly permeable, thus, allowing oil and gas that have been expelled from source rocks to enter or seep into adjacent reservoir rocks. Sandstones, limestones and dolomites comprise most oil and gas reservoir rocks.

Seals

Oil and gas that have occupied the air spaces within reservoir rocks freely move about to seek other spaces or less dense environments. Since most reservoir rocks are originally saturated with saline groundwater and since saline ground water is denser than oil and gas, the latter rises upward through the water-saturated pore spaces until they meet a barrier of impermeable rock or what is called a seal. Seals are usually very fine-grained rocks, such as shale, with no pore spaces or are impermeable to fluids.

Oil and Gas Traps

As oil and gas migrate up and within reservoir rocks, they eventually encounter barriers that block their way according to the configuration of the reservoir rock and the presence of one or more seals. This arrangement produces a trap. A trap can be either structural or stratigraphic. Structural traps are created when the reservoir rock and the seal caver have been deformed by folding or faulting of rock layers through geologic eras (Figure 2).

Figure 2. Folded strata that form a structural trap.

Stratigraphic traps are created when the reservoir rock is deposited as a discontinuous layer. Seals are deposited beside and on top of the reservoir. A typical case of this form of trap is a coastal barrier island. Impermeable shale seals are deposited both landward and seaward of the barrier island, which forms an elongate lens of sandstone. What forms is a porous sandstone reservoir within shale seals which may in turn be source rocks.

Figure 3. A discontinuous layer of sandstone that forms a stratigraphic trap.

The primary question now is where such oil and gas deposits can be found exactly in any given region and how to extract them at a commercially viable cost. This is the job for a company such as Trinity Energy Group. Trinity Energy Group has many years of experience in oil and gas exploration, drilling, production and management of operations in North America, Central America and all over the world. It can provide you with the next big step toward achieving your own investment goals.