Original gas oil contact

Another type of contour map is isopach map. It shows the variations in thickness of a formations.Geologists use isopach maps to calculate how much petroleum remains in a formation and to plan ways to recover it, as well as an aid in exploration work. Similar to the isopach is the isohore map, which shows the tickness of a layer from top to bottom along a vertical line.

A lithofacies map shows the character of the rock itself and how it varies horizontally within the formation. This type of map has contours representing the variations in the proportion of sand-stone, shale, and other kinds of rocks in the formation. Another type of facies map, the biofacies map, shows variations in the occurrence of fossil types.

Vertical Cross Sections

A vertical cross section is a type of map that represents a portion of the crust as though it were a slice of cake (fig. 2.20). It shows structures and fault patterns. Most cross sections show both structural and stratigraphic features (see chap­ter 1) together. They may show possible anticlinal and fault traps, or they may show only horizontal variations in type of rock or thickness. The fact that cross sections can show gravity anomalies in a contour form has been instrumental in spreading the use of gravity surveys in the exploration industry.

Computer Qraphics and Models

Computer technology has become indispensable in exploration. The huge quantities of data that a computer can store and manipulate, the ease of updating data, and new software for modeling and analyzing data provide explorationists with more information, faster than ever before.

Graphics

The results of some types of surveys, such as seismic surveys, may be analyzed by so-called supercomputers, computers that are so fast they can handle thousands of times the information a PC can work with.

Many graphics that draftsmen laboriously created in the past are now handled at PC workstations running sophisticated software. Skilled com­puter operators can generate maps that integrate all sorts of different data. For example, they can lay a cross section of a well's path over a seismic section showing the geologic structures or display lease bound­aries on top of a porosity map. Computer operators can en­hance Landsat, seismic, and other graphics to create three-dimensional images and add false color to highlight certain features with a keystroke.

Computers can also generate a block diagram, which is a perspective draw­ing of a section of the earth's crust as it would appear if cut out in a block (fig. 2.23). A block diagram shows two vertical cross sections whose faces are at right angles to each other, and the top of the block is either a subsur­face view or the surface topography. Operators frequently use computers to analyze gravity and magnetic data and combine the results with seismic data to produce a complex three-dimensional image of the geological fea­tures of the area of interest.

Models

Reservoir modeling is another technique made practical by the computer. Information is manipulated using a model —a computer program contain­ing a set of parameters, such as the number of wells in a section, the pro­duction rates of the wells, and so on. The operator knows the values of the parameters for the present and for the production history of the reservoir. To predict future production, the operator enters new parameters into the program in the form of mathematical "what if" questions: for example, "What if we add three wells in this area and increase the production rate by 20 barrels per day?" The computer plugs the new information into the model, performs all the calculations, and comes up with pro-duction pre­dictions.This type of analysis is an invaluable aid to the decision maker.

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