CHAPTER 1 BASIC PETROLEUM GEOLOGY

• Jurisdiction: United States

Basic Oil and Gas Technology for Lawyers and Landmen
(Apr 1979)
CHAPTER 1
BASIC PETROLEUM GEOLOGY
Daniel N. Miller, Jr.
State Geologist of Wyoming
Laramie, Wyoming

Introduction to Sedimentary Geology

The concepts of sedimentary geology most often applied to exploration and development of petroleum resources are based on rather fundamental principles and physical laws, extensive data, and more than seventy years of practical application. They evolved over the years as geologists in industry collected and exchanged information among themselves and with geologists in universities and state and federal agencies. Basically, the concepts deal with the types of changes that have modified the earth's surface throughout geologic time.

To a non-geologist the concepts may at first seem vague and highly interpretive, but for the most part such an impression is both temporary and superficial. The following material is arranged to provide non-geologists with a better understanding of the principles that are applied in geological reasoning in an effort to explain definitions and illustrations most commonly used by the petroleum industry, with emphasis on technology and the adequacy and validity of data.

North America — a hypothetical model of sedimentary processes

Assume for the sake of brevity, that sometime long ago in the geologic past a broad bulge developed in the earth's crust, and smaller wrinkles occurred in the bulge as the result of the crust's mobility under the influence of the earth's internal heat and pressure. As the wrinkles developed, great masses of rocky material emerged as islands above sea level where they became exposed to the atmosphere and subject to weathering and erosion. The loosened particles of rock, now under the influence of gravity, wind and water, washed back down the slopes. Some of the sediment was deposited at lower levels on the islands and along the shoreline; but with time, and the addition of more and more material, periferal blankets of sand, silt, and clay, were formed that extended well out to sea.

Assume further that the crustal movements within the earth changed from time to time, and that the size of the bulge and the shape and orientation of the wrinkles also changed. Old wrinkles submerged and became basins of deposition and new wrinkles formed that created new islands, and consequently new patterns of sediment distribution. Again and again the bulging and wrinkling processes continued throughout millions of years. Great land masses emerged above sea level as other areas subsided and filled with layers of sediment.

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Accumulations of mud and silt from nearby islands began to coalesce and over-lap each other forming distinctive bedded layers of strata, and finally consolidated into a broad mass of land more recognizable today because of the position of sea level, as the continent of North America (Fig. 1).

HYPOTHETICAL MODEL OF SEDIMENTARY GEOLOGY

Figure 1 — Assume that the North American continent was once just a bulge in the earth's crust. Local wrinkles created islands that eroded to produce sediment that was later deposited at lower elevations. Assume further that this process was repeated many times and in different parts of the continent during the geologic past, and that it is still going on today.

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In the western interior of the United States there are many deep structural basins where ten to twenty thousand feet of sedimentary rock are still preserved, and it is in these areas that the strata still contain vast accumulations of oil and gas as shown in Figure 1A.

Figure 1A — Illustrates the outlines of the major structural basins in the Rocky Mountain states that still contain thick stratigraphic accumulations of sedimentary rock.

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During the hundreds of millions of years while this activity was going on, the sediments were affected by periodic changes in climate and the local chemistry of their immediate environment. Primative forms of life evolved; first in the sea and later in brackish water swamps and fresh water rivers, and finally on land. Great forests and deserts were formed, and later destroyed. Large rivers contributed great volumes of sediment to the shorelines where huge deltas formed only to be destroyed completely, or in part, by subsequent uplift and erosion. The remains of some of these deposits, containing fossils of early forms of life, are still readily recognizable. Through extensive comparison of fossils and the distribution and type of rock in which they occur, geologists are able to piece together evidence of what the earth's surface must have been during the changing episodes of those primeaval times. In fact, they have gone much farther than that and have subdivided the hundreds of millions of years of geologic time into smaller more usable increments called geologic Eras and Periods, and Epochs, based on some of the major events in earth history.

Many thousands of geologists working in this country and abroad have contributed to our knowledge of uplifted areas, old erosional surfaces, and the relationships of the blankets of sedimentary rocks that surround them. Through their efforts a sub-science called "Stratigraphy" has evolved that permits geologists to interpret with some confidence at least the more important chapters of earth history. Stratigraphy, or stratigraphic geology, is the study of the character and distribution of the layers of sedimentary rock with respect to well-defined periods of geologic time.

In the broadest sense, and for the practical purpose of discussion here, strata are subdivided into three general categories:-those that were deposited in the sea (marine strata), marginal-marine strata that were formed in conjunction with shorelines, and those that were deposited on the land or in lakes and swamps that are called "non-marine strata".

"Marine strata" — are typically siltstone or shale or limestone having broad areal distribution over hundreds or thousands of square miles with relatively uniform composition and continuity.

"Marginal-marine strata" — are typically conglomerates and sandstones distributed in well defined sinuous patterns that in essence represent ancient shorelines.

"Non-marine strata" — are typically combinations of clayey sandstone and siltstone having more heterogeneous composition and more localized and erratic distribution.

Sediments that were deposited in conjunction with beaches and shorelines where wave agitation was greatest are normally the cleanest and have the most

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intergranular (pore) space. At the time of deposition the space between the grains was filled with water, air bubbles and bits of organic debris. Farther offshore the sediments were more poorly sorted and contained greater amounts of organic mud. As the organic matter in the sediments decomposed to form gases such as carbon dioxide, sulfer dioxide and methane, tiny bubbles percolated through the intergranular pore space wherever they could on their way to the surface, displacing the water.

During burial, as new sediments accumulated over the top, the deposits readjusted to the weight and the underlying sediments became more compact. Water and the more volatile gases shifted about in the pore space between the grains until the pressures, acting in all directions, reached an equilibrium. As pressure and temperature conditions changed in the sediment, the pore space, and the distribution of fluids all changed correspondingly. Eventually the physical structure of the sediments reached a more solidified state, normally referred to as rock, due in part to compaction and in part to mineral precipitation. From that point on the beds of rock material responded in a more rigid manner to physical and chemical changes by fracturing, jointing, folding, or even faulting.

Stratigraphic concepts — interpretation of layered rock sequences

Stratigraphy, as a sub-science of geology, is the interpretation of layered, sedimentary, rock sequences in relation to a specific period of geologic time. In other words, a stratigrapher is a geologist who studies the lateral distribution and character of strata. The ultimate goal of a stratigrapher is to be able to correlate individual rock units (strata) with contemporaneous geologic events in other nearby areas. For purposes of this discussion, simply accept the fact that geologists have defined and established nomenclature for the thicker and significant strata and for the intervals of time they represent. For convenience and simplicity in visualization the rock and time units are most often illustrated as a (vertical) geologic column (Fig. 2).

In every case there are two basic assumptions that are fundamental to all stratigraphic interpretation (Fig. 3):

"Law of original horizontality" — It is assumed that most sedimentation takes place, i.e. bedded deposits are formed, on a nearly horizontal surface; in other words the original beds of sediment were deposited as flat-lying beds...