Page 38 - Basic Well Log Analysis for Geologist

BASIC RELATIONSHIPS OF WELL LOG INTERPRETATION

Figure 7B. Example of Dual Laterolog* - Microspherically Focused Log (MSFL)* curves through a hydrocarbon-bearing zone.
Given: the drilling mud is saltwater-based (Ry = R,,).
We've seen that where saltwater drilling muds invade a hydrocarbon-bearing zone (S, << 60%), there is low resistivity in
the flushed zone (R,,.), an intermediate resistivity in the invaded zone (R;), and high resistivity in the uninvaded zone (R,).
The reason for the increase in resistivities deeper into the formation is because of the increasing hydrocarbon saturation.
See Figure 5 for review.
Ignore the left side of the log on the opposite page, and compare the three curves on the right side of the log (tracks #2 and
#5). Resistivity values arc higher as distance increases from the left side of the log.
Log Curve LLD—Decp Laterolog* resistivity curves measure true resistivity (R,), or the resistivity of the formation deep
beyond the outer boundary of the invaded zone. In hydrocarbon-bearing zones (in this case from 9,306 to 9,409
ft), the curve will read high resistivity because of high hydrocarbon saturation in the uninvaded zone (Rj).
Log Curve LLS—Shallow Laterolog* resistivity curves measure the resistivity in the invaded zone (Rj). Ina
hydrocarbon-bearing zone. the shallow Laterolog* (LLS) will record a lower resistivity than the deep Laterolog*
(LLD) because the invaded zone (R,) has a lower hydrocarbon saturation than the uninvaded zone (R,).

Loz Curve SFL—Microspherically Focused Log* resistivity curves measure the resistivity of the flushed zone (R,,). fn
hydrocarbon-bearing zones, the curve will record low resistivity because saltwater mud filtrate has low resistivity
and the residual hydrocarbon (RH) saturation in the flushed zone (R,,) is low. Therefore, ina
hydrocarbon-bearing zone with saltwater-based drilling mud. the uninvaded zone (R,) has high resistivity, the
invaded zone (R;) has a lower resitivity, and the flushed zone (R,,) has the lowest resistivity.