Geology Reference
In-Depth Information
1.7.2
Wells
Wells provide subsurface information on the location of formation boundaries and
the attitude of planes. Measurements of this information are made by a variety of tech-
niques and recorded on well logs. Sample logs are made from cores or cuttings taken
from the well as it is drilled. In wells drilled with a cable tool, cuttings are collected
from the bottom of the hole every 5 or 10 feet and provide a sample of the rock pen-
etrated in that depth interval. In wells that are rotary drilled, drilling fluid is circulated
down the well and back to remove the cuttings from the bottom of the hole. The drill-
ing fluid is sampled at intervals as it reaches the surface to determine the rock type
and fossil content of the cuttings. Depths are calculated from the time required for the
fluid to traverse the length of the hole and are not necessarily precise.
A wire-line log is a continuous record of the geophysical properties of the rock and its
contained fluids that is generated by instruments lowered down a well. Lithologic units
and their contained fluids are defined by their log responses (Asquith and Krygowski
2004; Jorden and Campbell 1986). Two logs widely used to identify different units are the
spontaneous potential (SP) and resistivity logs (Fig. 1.44). In general, more permeable
units show a larger negative SP value. The resistivity value depends on presence of a pore
fluid and its salinity. Rocks with no porosity or porous rocks filled with oil generally have
high resistivities and porous rocks containing saltwater have low resistivities. A variety of
other log types is also valuable for lithologic interpretation, including gamma-ray, neu-
tron density, sonic and nuclear magnetic resonance logs. The gamma-ray log responds to
the natural radioactivity in the rock. Very radioactive (hot) black shales are often wide-
spread and make good markers for correlations between wells. A caliper log measures the
hole diameter in two perpendicular directions. Weak lithologies like coal or fractured
rock can be recognized on a caliper log by intervals of hole enlargement. In wells drilled
with mud, fluid loss into very porous lithologies or open fractures may cause mud cakes
that will be recognized on a caliper log by a reduced well-bore size.
Logs from different wells are correlated to establish the positions of equivalent units
(Levorsen 1967; Tearpock and Bischke 2003). Geologic contacts may be correlated from
well to well to within about 30 ft in a lithologically heterogeneous sequence or to within
inches or less on high-resolution logs in laterally homogeneous lithologies. The cable
that lowers the logging tool into the well stretches significantly in deep wells. The
recorded depth is corrected for the stretch, but the correction may not be exact. Dif-
ferent log runs, or a log and a core, may differ in depth to the same horizon by 20 ft at
10 000 ft. Normally, different log runs will duplicate one of the logs, for example the SP,
so that the runs can be accurately correlated with each other.
The orientation of the well bore is measured by a directional survey. Some wells,
especially older ones, may be unintentionally deviated from the vertical and lack a
directional survey, resulting in spatial mislocation of the boundaries recorded by the
well logs (if interpreted as being from a vertical well) which will lead to errors in dip
and thickness determinations. The most common effect is for a well to wander down
dip with increasing depth.
A dipmeter log is a microresistivity log that simultaneously measures the electrical
responses of units along three or more tracks down a well (Schlumberger 1986). The
