Stratigraphic Correlation

Stratigraphic correlation is the process of comparing geologic formations. In the oil exploration industry, stratigraphers compare the geology of a known area with unknown formations in nearby locations in order to try to predict where new reservoirs are. They do this by looking at information collected in drillers' logs, sample logs, and electrical logs in order to compare fossils,the composition of formations, and electrical data from one well or area with the same information from another well or area. Rock tex­ture and fossil characteristics in forma-tions, particularly those under the oceans, normally change very gradually. Sudden changes in the sequence of rock types are geologic indicators, and often geologists use particular fossils as markers when they are trying to recognize the continuation of a formation in a new location. If the kind of rock is the same and the fossil markers are the same, the formation is probably a continuation of the one in the previous location.

The sequence, or order, of formations can also be used as a method of correlation. In sequence stratigraphy, geologists take the information from seismic surveys and analyze it to deduce the environment that existed when a rock layer first formed. For example, geologists can distinguish between sandstone laid down in a river from sandstone that formed from a beach. This method depends in part, as do the others, on drilling closely spaced wells. Stratigraphers can match the formations accurately when they corre­late the data derived from several wells.

Maps

Explorationists use various kinds of maps throughout the exploration pro­cess. Geologists use base maps that show existing wells, property lines, roads, buildings, and other manmade surface features to recommend sites for geo­physical studies, exploratory drilling, and reservoir development. Topographic maps show surface features such as mountains and valleys. Maps can also show information about underground formations gathered from explora­tion surveys. Gravity survey results are displayed on a Bouguer gravity map.

Contour Maps

Topographic, gravity, and other surveys can be depicted in the form of contour maps —those most commonly used by petroleum geologists. Contour maps show a series of lines drawn at regular intervals, often connected to enclose an area. The points on each line represent equal values, such as depth or thickness.

To understand a topographic contour map, imagine looking at a mountain from above it. If you could mark a dot every place on the mountain that is 1,000 feet high and then connect the dots, you would have a curve going around the mountain. Do the same for 2,000 feet, 3,000 feet, and so on, and you have a series of concentric curves. Draw these curves on a piece of flat paper, and you have a contour map.

Contour maps for exploration may depict geologic structure and thick­ness of formations. They can also show the angle of a fault and where it intersects with formations and other faults, as well as where formations taper off or stop abruptly.

One type of contour map is the structural map, which depicts the depth of a specific formation from the surface. The principle is the same as that used in a topographic map, showing instead the highs and lows of the surface of the buried layer.

EXPLANATION

Дата добавления: 2015-10-31