Page 163 - Well Logging and Formation Evaluation

Production Geology Issues 153

Sand lens
Unconformity trap

Grading
Pinch-out
Fault trap trap
Anticline Salt dome
traps

Buried reef
Figure 10.2.2 Types of Trap

provide a sealing mechanism. Terminology useful to folding is as
follows:
Anticline: convex up
Syncline: concave up
Hinge line: the line of maxiumum curvature
Limb: area of least curvature or flank
Crest: topographically highest point
Trough: topographically lowest point
Axial plane: the plane defined by hinge lines of all horizons
Crestal plane: the plane defined by crests on all horizons
The basic types of hydrocarbon trap are illustrated in Figure 10.2.2.

10.2.4 Abnormal Pressures

Normal pressures are defined by taking a hydrostatic gradient (typically
0.45psi/ft or 1.04g/cc) from surface to a certain depth. Significant devi-
ation from the pressure so determined would be termed abnormal. The
principal mechanisms giving rise to abnormal pressures are:
• Low water table or high elevation. In mountainous areas, the water table
may lie some depth below surface; and in the upper part, any hole
drilled will be through dry rock.
• Hydrocarbons. The presence of a hydrocarbon column above an aquifer
will cause the pressure at a depth in the hydrocarbon column to be