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Nov 10, 2020
UNIVERSITV OF DELAWARE
DELAWARE GEOLOGICAL SURVEY
REPORT OF INVESTIGATIONS No" 33
EXPLORINGJ DRILLING~ AND PRODUCING PETROLEUM OFFSHORE
STATE OF DELAWARE
Public Access Copy DO NOT REMOVE from room 208.
EXPLORING, DRILLING, AND PRODUCING
, Nenad Spoljaric
Delaware Geological Survey
ABSTRACT. • • •
. . . . . . . . . . . . . . . . . . 1 2
THE NATURE AND ORIGIN OF PETROLEUM. · . 2 MIGRATION AND ACCUMULATION OF PETROLEUM • 3
SEARCH FOR PETROLEUM. • • • •
Leasing of Offshore Areas.
. . . . . . .
Geological and Geophysical Investigations ••
Drilling Equipment and Drilling.
Testing for Petroleum.
Obtaining Rock Samples •
· . . . . . . . . • 20 • 23
. . .
PRODUCTION OF PETROLEUM • • •
Getting Petroleum Onshore ••
Probability of Oil Spills.
PROBABILITY OF FINDING PETROLEUM. •
• • 24
. . . • . 28
SELECTED LIST OF REFERENCES • • • •
· . . . . . . . . • 34
Figure 1. Types of traps where petroleum can accumulate in large quantities • 5
2. Continental shelf areas of the U. S. considered for exploration or already being explored for petroleum • • • • • 7
EXPLORING, DRILLING, AND PRODUCING
A Non~Technical Review
This report was prepared to provide a concise des- cription of offshore operations related to exploration for petroleum (oil and natural gas} from the initial geologic and geophysical investigations to production.
Petroleum deposits differ in their physical and chemical properties and are associated in the rocks with saline water. The origin of petroleum and its migration through rocks are not well understood. Commercial accumulations are found in certain suitable rocks or geologic structures - strati- graphic and structural traps, respectively.
Prospective areas offshore are leased to exploration companies by the federal government. Exploration begins with geological and geophysical investigations that lead to the selection of smaller, promising areas. Detailed studies and drilling are then carried out and, if petroleum is found, various tests are performed to determine the volume of oil or gas or both. If the quantities are large, production facilities are designed and located on the site. The petroleum produced is transported to refining facilities or gas companies onshore by pipelines or tankers.
Experience has shown that large, damaging oil spills are very rare. The most common cause of spills is marine transportation.
To find new, large petroleum accumulations exploration will have to be expanded into deeper waters and into less hospitable regions.
Exploration offshore for petroleum (oil and natural gas) began in the United States about 30 years ago. Since that time more than 23,000 offshore wells have been drilled and about 8.5 billion barrels of oil and more than 50 trillion cubic feet of natural gas have been produced.
The need for resources and decline in production from existing oil and gas fields, both onshore and offshore, have made it necessary to intensify the search for new signifi- cant petroleum accumulations throughout the world. In view of the fact that offshore exploration on the Atlantic Shelf off the United States' East Coast has begun, the Delaware Geological Survey has prepared this brief, generalized report. Its purpose is to provide laymen with a summary of procedures involved in offshore oil and gas exploration and development.
THE NATURE AND ORIGIN OF PETROLEUM
Crude oils (unrefined or "natural") vary greatly in their physical and chemical properties. They are composed mainly of carbon and hydrogen, hence the term "hydrocarbon." Other elements, such as sulphur, nitrogen, oxygen, and heavy metals (iron, calcium, magnesium, nickel, copper, titanium, and others) are present in varying amounts ranging from a fraction of one percent to a few percent.
Commercial quantities of crude oil are associated in the subsurface with natural gas. Natural gas is a mixture of gases that can contain both hydrocarbons and non-hydro- carbons. The principal component of this mixture is methane (generally more than 90 percent) with small amounts of ethane, propane, and butane.
Water is also found associated with oil and gas. It is called "interstitial water" and is saline; the total amount of solids in solution usually exceeds that in sea water.
The origin of petroleum is still a subject of contro- versy. Although a great majority of petroleum geologists
believe that oil and gas formed from organic matter (decay- ing plant and animal remains) by decomposition and trans- formation, there are some geologists who subscribe to the hypothesis of inorganic origin of petroleum. This contro- versy in itself suggests that the process by which the ori- ginal material, whether it be organic or inorganic, changes into oil and gas is not well known.
Nearly all petroleum is found in sedimentary rocks ranging in age from Cambrian (550 million years ago) to Quaternary (one million years ago). But there are some important oil fields found in crystalline rocks. Whether or not oil migrated into such rocks from sedimentary rocks in the vicinity is unclear.
The fact that the origin of petroleum is still not well understood is one of the difficulties encountered in the search for new, large, and economic petroleum deposits. Exploration is further complicated by problems involving the location of deposits hidden deep underground. Statis- tical data available for the period between 1949 and 1968 show that it took to drill more than 1,000 new field wild- cat (exploratory) wells to discover one large oil field of 50 million or more barrels of oil or the equivalent in natural gas.
MIGRATION AND ACCUMULATION OF PETROLEUM
The generation of petroleum is thought to have occurred in source rocks. The concentration of petroleum in the source rocks is generally relatively small, and such rocks are normally very fine-grained sediments, for example shales, with low porosity (small amount of open spaces between individual sediment grains). Petroleum must migrate from source rocks into reservoir rocks that are porous and permeable and allow accumulation and extraction of petroleum if sufficiently large quantities are present.
Migration is believed to take place in two phases. First, petroleum moves from the source rocks into the .reservoir rocks (primary migration), and second, it accumulates in the reservoir rocks (secondary migration). Tertiary migration refers to any subsequent migration of petroleum that may be brought about by specific geologic processes.
How migration is accomplished is not well understood. It is believed that water, moderately high temperature,
pressure of the overlying sediments, and buoyancy play important roles in the movement of petroleum through the rocks.
In some instances petroleum migrates to the land surface or to the bottom of the ocean and can be observed in the form of natural oil and gas seeps. A study done by the National Academy of Sciences (1975) has shown that these natural seeps of petroleum in the world oceans amount to about 0.6 million metric tons, or about 4 million barrels, per year. Small seeps of natural gas can be observed in many places in Delaware marshes; this gas is mainly methane and the marsh sediments in which it develops can be considered source rocks.
How do oil and gas fields develop? The consensus is that they form by primary and secondary migration. The necessary secondary migration is possible only if the reser- voir rocks are "capped" with impervious layers to prevent seepage or flushing of petroleum. Such capped reservoir rocks are appropriately called traps. There are basically two different types of traps: stratigraphic and structural (Figure 1).
Stratigraphic traps develop when the migrating petroleum encounters very fine sediments that prevent further migration. Such traps are common and can be sometimes recognized in the subsurface by the utilization of various geological and geophysical techniques.
Structural traps are in fact structural obstructions, such as faults, anticlines, and domes (Fig. 11 that prevent further migration of petroleum. In case of the faults it is important, for example, that the fault plane itself is plugged with impermeable material and that the rocks on the opposite sides of the fault are also impervious so that further migration or seepage of petroleum can not occur.
There are many specific types of both stratigraphic and structural traps, but their discussic,n here is not essential to this report. Interested readers may refer to Selected List of References for additional information on this subject.
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