Page 161 - Well Logging and Formation Evaluation

Production Geology Issues 151

Processes that may increase the porosity are:

• Leaching: This can be fabric related or nonfabric related. Fabric-related
leaching is selective due to mineralogical differences in the sediment
and results in mouldic porosity. The nonfabric-related type tends to lead
to large voids or karsts.
2
• Dolomitization: This is the replacement of CaCO 3 by CaMg(CO 3) .
Where this creates porosity, it will generally be vuggy or intercrystalline
in nature.

10.2.3 Faulting and Deformation

In any field, the forces acting on the sediments will be both gravita-
tional and either extensional or compressional in any horizontal direction.
Extensional features may result from (a) lack of lateral support nearby
sediments, (b) movement of basement rocks, or (c) instabilities in the
overburden arising from differential compaction or salt diapirs. Com-
pressional features may result from (a) gravity-sliding of rocks over
inclined basement surfaces, or (b) movement of basement blocks.
The main features observed in extensional tectonics are as follows:

• Growth faults. These are faults in which the thickness of rocks in the
downthrown block are greater than those (in the same time units) in the
upthrown block. The downthrown block has subsided quicker than
the upthrown block and acquired more sediment. The fault plane is
usually curved upward. This is illustrated in Figure 10.2.1.
• Rollover anticlines. These are growth anticlines in which rock units
thicken from the crest toward the flanks. The flanks subsided faster than
the crest and accumulated a greater thickness of sediment.
• Normal faults. Movement in the basement either during or after depo-
sition may cause faulting. In normal faulting the sediments on the upper
side of the fault plane move vertically downward relative to those on
the lower side of the fault plane. This is illustrated in Figure 10.2.1.
• Salt domes. During Permian and Triassic times, large volumes of sea-
water became isolated, and subsequent evaporation created thick salt
deposits. Since the salt is essentially ductile where it is confined, there
is a tendency for it to bulge at the weakest point through the overlying
rock, forming a diapir. This diapir will cause radial fractures in the
overlying rock around it. The diapir may itself form part of the seal,
allowing hydrocarbons in sands to be trapped.