Page 168 - The Petroleum System From Source to Trap

160 Downey

Where these seal maps are combined with data on the tive data are valuable and, where properly used
presence (or absence) of hydrocarbon shows, they (Schowalter, 1979), can provide an important starting
become a powerful tool in the search for new accumu­ point for assessment of seal capacity. Unfortunately, such
lations. "microdata" taken from a rock sample may have limited
use when attempts are made to extrapolate rock sample
data to the entire "macro" sealing surface that roofs or
MICRO PROPERTIES bounds an accumulation (Downey, 1984).
Rocks that are seals have pore throats that are too
small and poorly connected to allow passage of MICROPROPERTIES TO
adjoining hydrocarbons. A typical rock-sealing surface MACROSURFACES
does not behave like an impermeable plastic sheet but
more like a "fine mesh" screen. A rock-sealing surface The difficulty of extrapolating precise measurements
holds back hydrocarbons only until the hydrocarbons made on, for example, a piece of core 4 in. (10 em) in
exert sufficient buoyancy pressure to pass through the diameter to the entire entrapping surface can be under­
water-wet rock pores, or membrane seal (Watts, 1987). stood by a simple example. Assuming a domal closure
Laboratory experiments and theory allow an exact area of 6400 ac (2590 ha.), a core sample of the top seal
description of the capacity of a rock to impede the flow would provide a ratio of areas of about 1 to 3.5 billion.
of hydrocarbons. Fundamentally, the quality of a rock What is the probability that the "microproperties" char­
seal at any given time is determined by the minimum acterizing the core are invariant when extrapolated over
pressure required to displace connate water from pores the entire domal sealing surface? Large extrapolations of
or fractures in the seal, thereby allowing leakage. This data are commonly necessary in geologic work, but it is
minimum entry pressure (capillary entry pressure) thus important in assessing seal properties to remember that
describes the buoyancy pressure of the hydrocarbon average values are nearly meaningless in determining
phase that must be attained to allow hydrocarbons to the probability of a seal for a hydrocarbon accumulation.
penetrate through an adjacent surface. If the sealing surface is a homogeneous, very fine grained
Capillary entry pressure (P d) of a water-filled rock is a claystone or evaporite, the sealing capacity of a casually
function of the hydrocarbon-water interfacial tension (y), encountered rock sample is likely to be extremely high.
wettability (8), and radius of largest pore throats (R), Caution is in order, however, since a single flaw or
according to the following relationship (Purcell, 1949): fracture in dense rock can render the apparent seal
totally ineffective. In looking at sealing surfaces, we are
basically concerned with the properties of the "weakest"
point of the sealing surface. The measured values from a
This equation states that capillary entry pressure random core sample, unfortunately, have little relevance
(sealing capacity) of the seal rock increases as (1) the to the problem of determing the most likely leak point
throat radius of the largest connected pores decreases, (2) of the seal.
the wettability decreases, and (3) the hydrocarbon-water Where the sealing surface is a homogeneous, laterally
interfacial tension increases. continuous, fine pore throated lithology, laboratory
Capillary forces of a seal act to confine hydrocarbons measurements of the capillary entry pressure of the seal
within an accumulation. The buoyancy forces of the can provide useful data. Such data are useful in assessing
hydrocarbon column of a static accumulation are given the maximum hydrocarbon buoyancy column that the
by the product of the hydrocarbon column height and seal can resist. If the capillary entry pressure of a random
the difference in density between the hydrocarbon and point on the sealing surface is measured and found to be
the reservoir pore water. These hydrocarbon buoyancy balanced, for example, by the pressure equivalent of a 50-
forces must be matched or exceeded by the resistance of ft oil column, then a maximum of 50 ft of oil should be
the capillary entry pressure that characterizes the pore expected in the trap, no matter how excellent the sealing
structure of the seal. When the buoyancy pressure of an surface is, on average.
underlying hydrocarbon column exceeds the hydro­
carbon water displacement pressure of the seal, the
hydrocarbons pass through. Of course, downward­ MACROCHARACTERISTICS
directed hydrodynamic flow increases the entry pressure Lithology
of seals, while upward-directed hydrodynamic flow
decreases the effective entry pressure of the seal. Any lithology can serve as a seal for a hydrocarbon
One can measure in a laboratory the displacement accumulation. The only requirement is that the
pressure necessary to force a given hydrocarbon through minimum displacement pressure of the lithologic unit
a given rock under specified conditions of temperature comprising the sealing surface be greater than the
and pressure. Such measurements provide quantitative buoyancy pressure of the hydrocarbon column in the
data about the capacity of that seal to entrap those accumulation. In practice, however, the overwhelming
hydrocarbons. Indeed, the displacement pressure of majority of effective seals are evaporites, fine-grained
sandstone seals can be estimated from grain size and clastics, and organic-rich rocks. These lithologies are
sorting data, using the method of Berg (1981). Quantita- commonly found as seals because they typically have