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
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.