Origin and Accumulation of Oil and Gas

Introduction

Progress in solving the secrets of the origin and accumulation of petroleum took a giant step forward in 1859 with the drilling of the first oil well. This initial well was drilled to a depth of 69 feet. Oil and gas deposits had been encountered at various locations since ancient times, but these instances were relatively rare in 1859. Today we have widely accepted geologic theories along with good supporting evidence that helps to explain how oil and gas were formed.

Once formed in the sedimentary source beds, the oil and gas then migrated to other sedimentary rocks where we find them today. This two-step sequence is the starting place for this introduction. Life on earth possibly began hundreds of millions of years ago in vast seas and inland lakes. This is one of the initial concepts in developing the current geologic organic theory of petroleum. These marine areas are thought of as being reasonably shallow.

The hydrogen and carbon material that makes up the composition of petroleum is presumed to have come from the decomposed plants and animals that were living on land and in the sea. It is probable that the greatest contribution of organic material was deposited in a marine environment rather than a continental environment. Also, it is believed that the small plant and animal forms were of more importance than the larger form as a petroleum source.

Organic Theory of Origin

The figure below illustrates the vast seas that at several times in the geologic past covered large portions of the present continents and near offshore areas supported abundant populations of marine plant and animal life. As these organisms died, their remains were buried and preserved in the sedimentary record. Some evidence of ancient seas is found in the rocks on, and underlying much of, the present land area.

The Mid-Continent United States, for example, is part of one of these old seas. Throughout millions of years, rivers flowed down to these seas and carried with them great volumes of mud and sand to be spread out by currents and tides over the sea bottoms near the constantly changing shorelines. During these times, plant and animal life flourished.

The ocean floors slowly sank under the increasing weight of the accumulating sediments, so that thick sequences of mud, sand, and carbonates were formed and preserved. these sequences were squeezed by the weight of thousands of feet of overlying organic and inorganic material and eventually became what are called sedimentary rocks-the sediments that nature has turned into rocks. These sedimentary rocks include the dark marine shales and marine limestones that scientists think are the source beds of petroleum. Also grouped in this series of marine sedimentary rock are the sandstones, limestones and

dolomites are the reservoir rocks in which we sometimes find oil and gas.

A large amount of very small plant and animal remains came into the shallow seas with river silts and muds. This material joined a much greater volume of similar tiny remains of marine life already settled to the sea bottom. These small organisms, dying and settling to the bottom of the sea, were repeatedly buried by mud and sealed from the air. They were further protected from ordinary decay by the salty sea

water.

Through geologic time, as more and more layers of organic material, sand, silt, clay, and lime accumulated, the deeper sediments were compressed and eventually hardened into rock. As time passed, the weight of the overlying sediments caused tremendous pressure to be exerted on the 'deeper sedimentary layers. Then this pressure, along with high temperature, bacterial action, and chemical reactions, produced the changes that caused the formation of oil and natural gas. The continued squeezing of these source rocks resulted in pressures and temperatures sufficient to cause primary oil and gas migration out of the source rocks into adjoining porous and permeable rocks. One common form of permeable rock in which oil and gas are found is sandstone.

It is made up of sand grains usually mixed with particles of other material. Porous limestones and dolomites are other types of sedimentary rocks in which petroleum occurs. After this primary migration, secondary migration occurred wherein the oil and gas migrated from the tiny spaces or pores between the particles in the sediments to the reservoir where it accumulated. This accumulation occurred as the underground rock masses were folded in certain forms and shapes that halted the oil movement and caused the petroleum to be trapped and gathered in large quantities.

The movement of petroleum from the place of its origin to the traps where the accumulations are now found was both vertical and lateral. This movement took place as the result of the tendency for oil and gas to rise through the ancient seawater with which the pore spaces of the sedimentary formations were filled when originally laid down.

An underground porous formation or series of rocks, which occur in some shape favorable to the trapping of oil and gas, must also be covered or adjoined by a layer of rock that provides a covering or seal for the trap. Such a seal, in the form of a layered, dense, non-permeable rock, halts the further upward movement of petroleum through the pore spaces.

Reference :

Introduction to Oil and Gas Production, Exploration and Production Department BOOK ONE OF THE

VOCATIONAL TRAINING SERIES FIFTH EDITION, JUNE 1996