By Stephen A. Sonnenberg, Larry Meckel
The driving forces for conventional accumulations (structural or stratigraphic traps) are Forces of Buoyancy which are due to
differences in
densities of hydrocarbons and water. In contrast, the driving forces for unconventional tight accumulations are Forces of Expulsion which are
produced by high pressures. That is an enormous difference and creates unconventional petroleum systems that are characterized by very
different and distinctive characteristics. The Force of Expulsion pressures are created by the
significant increase in volume when any of the
three main kerogen types are converted to hydrocarbons. At those conversion times in the burial history, the rocks are already sufficiently tight
so the large volumes of generated hydrocarbons cannot efficiently escape through the existing tight pore system,
thus creating a permeability
bottleneck that produces an overpressured compartment over a large area corresponding to the proper thermal oil and gas maturities for that
basin. The forces initially created
in these source rocks can only go limited distances into adjacent tight reservoirs (clastics or carbonates)
above or below the source. The exact distance will vary depending on the pressure increase, matrix permeability, and fractures of that specific
tight reservoir system. In general, the distances are small, in the orders of 10s to 100s of feet for oil and larger for more mobile gas systems.
Those exact distance numbers are subject to ongoing investigations.
A plot of the pressure data versus elevation
for a given formation is critical in determining whether an accumulation is conventional or
unconventional. Conventional accumulations will have hydrocarbon columns of 10s to 100s of feet with the pressure in the hydrocarbons and
that in the water equal at the bottom of the accumulation (at the HC-water contact). In contrast, the unconventional accumulations will show
HC column heights of 1000s of feet with the pressure in the hydrocarbon phase and the water phase being the same at the top of the
accumulation (at the updip transition zone). Those significant differences are critical for understanding and differentiating these two play types.
Because the system is a pore throat bottleneck with very little or minimum lateral migration, the type of hydrocarbon
s are closely tied to the
thermal maturity required to generate those hydrocarbons. Thus the play concept begins with two important geochemical considerations: (1)
where are the source rocks and what are the kerogen types and organic richness (TOC), and (2
) where are they mature in the basin for oil,
condensate, and gas in the basin. These parameters will very quickly define the fairway for the play. Then one has to add the
critical
information on the reservoirs themselves: composition (brittleness), thickness, and reservoir quality (matrix porosity and permeability). In
summary, these tight unconventional petroleum systems (1) are dynamic
,
and (2) create a regionally inverted petroleum system with water over
oil over condensate over gas for source rocks wit
h Type I or II kerogen types.
Structural Traps,
Stratigraphic Traps,
Tight Gas Sands,
Sedimentology and Stratigraphy,
Carbonates,
Clastics,
Geochemistry and Basin Modeling,
Kerogen Typing,
Maturation,
Migration,
Petroleum Systems,
Thermal History
American Association of Petroleum Geologists (AAPG)
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Our Current Working Model for Unconventional Tight Petroleum Systems: Oil and Gas