A Short Introduction to Petroleum Geology by Trinity Energy Group
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
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 120o C (250o 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 150o C (300o 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.