Environmental Engineering Reference
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(a)
(b)
Figure 9.2.4. Layers of clastic sediments
(a) Sandstone. Figure from American Association of Petroleum Geologists.
(b) Highly porous and permeable sandstone reservoir rock of the Farewell Formation,
Whanganui Inlet, Golden Bay, New Zealand. Photo by Lesli Wood, University of
Texas at Austin.
respectively). The dip angle (angle between the horizontal and the slope
of the strata) or a qualitative description thereof (e.g., steeply dipping,
gently dipping) is used to describe the degree to which the sub-horizontal
sediments have been tilted downward. The term closure refers to the
extent of the lowermost fl exure of the caprock, an indicator of the closed
region within which a buoyant fl uid such as oil, natural gas, or CO 2 will be
trapped. A structure with closure all around is referred to as a dome. If you
picture an overturned saucer catching an updraft of smoke, smoke will
spill out from under the saucer at the lowest point, called the spill point, if
the smoke volume exceeds the closure volume of the saucer.
The commonly observed alternating sequence of reservoir rock (high
porosity) and caprock (low porosity) in folded and faulted structures is
well-exemplifi ed by the thick sedimentary rocks of the Great Central
Valley of California. Shown in Figure 9.2.5 is a geological cross-section
of a part of the Southern San Joaquin Valley, California [9.2]. Note that
although the vertical scale of the cross-section is exaggerated, the verti-
cal thickness of the sediments is still considerable (over 3,000 m). The
alternating sandstone and shale formations are the result of continuous
down-warping of the basin and infi lling by sediments eroded, fi rst from
the North American continent and later from the rapidly rising Sierra
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