Interpreting hydrocarbon shows

Exploring for Oil and Gas Traps
Series	Treatise in Petroleum Geology
Part	Predicting the occurrence of oil and gas traps
Chapter	Predicting reservoir system quality and performance
Author	Dan J. Hartmann, Edward A. Beaumont
Link	Web page
Store	AAPG Store

One powerful use of petrophysical analysis is in interpreting shows. An understanding of the interrelationship of water saturation, relative permeability, pore throat-size distribution, and height above the free water level lets us interpret the significance of a hydrocarbon show.

Types of shows

Direct indications of hydrocarbons—seen in drilling fluids, cuttings, cores, or formation tests—may be of several types:

Show type	Significance	Origin
Continuous phase	Trapped oil or gas	A slug or filament of oil or gas with a continuous connection through the pore network of a rock
Residual	Migrated hydrocarbons	A discontinuous phase of oil or gas which formed as a result of a breached or leaky trap
Dissolved gas	Gas present in the petroleum system	Gas exsolving from formation water from pressure release
In kerogen	Oil or gas present in source rock	Oil liberated from kerogen in the oil generation phase

Oil show type manifestations

The table below lists similarities and differences between different oil show types.

Show type	Core or cuttings	Mud log	Sw%	Log analysis	Dst	Repeat formation tester (RFT)
Continuous-phase oil	Yes	Yes	<65 for Ss, <45 for Ls or dolomite	Moveable oil	Free oil or oil-cut fluids	Hydrocarbon gradient
Residual oil	Yes	Yes	>65 for Ss, >45 for Ls or dolomite	No moveable oil	No recovery	Hydrostatic gradient
In-kerogen oil	Yes	No	100 or less	No moveable oil	No recovery	Hydrostatic gradient

Gas show type manifestations

Gas show types are similar to oil show types but need to be considered separately. Use the table below to help with gas show interpretation.

Show type	Core or cuttings	Mud log	Sw%	Log analysis	Dst	Rft
Continuous-phase gas	Yes	Yes	<65 for Ss, <45 for Ls or dolomite	Moveable gas	Gas to surface at measurable sustained rate	Hydrocarbon gradient
Residual gas	Yes	Yes	>65 for Ss, >45 for Ls or dolomite	No moveable gas	Short flow period caused by pressure drop near wellbore	Hydrostatic gradient
Dissolved gas	Yes	Yes	100	No moveable gas	Gas-cut water or mud	Hydrostatic gradient

Shows from transition and waste zones

Shows from waste and transition zones are continuous-phase shows. Waste zones occur at the top of traps; transition zones occur at the bottom. The table below lists ways to distinguish transition zone shows from waste zone shows (after Schowalter and Hess).

Characteristic	Transition	Waste
Fluid production	Oil or gas with water, or water only	Oil or gas with water, or water only
Flow rate	High in high-permeability rocks	Low
Water saturation	High, decreases upward	Low, increases upward
Calculated hydrocarbon column	Small	Large

Residual shows

Residual hydrocarbons occur as dead oil or as water displacement residual hydrocarbons. Dead oil forms as a result of water washing, thermal cracking, or biodegradation. Water displacement residual hydrocarbons form in reservoirs as a result of a leaky or breached trap or as a result of production.

Dissolved-gas shows

Dissolved-gas shows are reported as mud-log shows, trip gas, gas-cut fluids, and gas bubbles in samples. Dissolved-gas shows occur when gas is liberated from water as pressure drops in rock cuttings pores as they rise to the surface during drilling. The amount of gas contained in formation water depends on the salinity. Fresher formation water absorbs more gas. Quantities range up to 14 scf/bbl of water. Dissolved gas has no capacity to flow into a wellbore, and dissolved shows are significant only in that hydrocarbons are present.

In-kerogen shows

In-kerogen shows occur when solvents are used on cuttings containing kerogen. The solvents liberate some oil from the kerogen, and this oil can be mistaken for evidence of free oil. Kerogen is the precursor to oil or gas and, when heated, generates oil and/or gas. Inkerogen shows indicate the presence of source rocks that have generated oil or gas.

Residual and continuous-phase shows

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  Figure 1 Drainage and fill-up relative permeability curves for an oil reservoir.

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  Figure 2 Schematic cross section of a stratigraphic trap with transition, reservoir, and waste zones and their corresponding show characteristics.

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  Figure 3 Schematic cross section of a stratigraphic trap with residual hydrocarbon saturation, reservoir, and waste zones and their corresponding show characteristics.

During reservoir water drainage, hydrocarbons enter a trap. The trap starts at 0% hydrocarbon saturation (S_o), or 100% S_w, and ends up with a higher S_o. Water refills the reservoir as hydrocarbons exit a trap (fill-up). During fill-up some hydrocarbons are permanently left behind as a residual accumulation. There is no relative permeability to hydrocarbons in rocks containing residual hydrocarbons. At a point during fill-up, hydrocarbons no longer drain because they are no longer connected in a column but are isolated in pores. A DST of a zone with residual hydrocarbons produces no oil or gas, and an RFT would not show a hydrocarbon gradient.

Figure 1 shows drainage and fill-up relative permeability curves for an oil reservoir. In this example, the reservoir ends up with approximately 60% S_w, or 40% residual hydrocarbon saturation.

Trap with transition zone

Figure 2 is a schematic cross section of a stratigraphic trap with transition, reservoir, and waste zones and their corresponding show characteristics.

Trap with residual hydrocarbon zone

Figure 3 is a schematic cross section of a stratigraphic trap with residual hydrocarbon saturation, reservoir, and waste zones and their corresponding show characteristics.

See also

• Interpreting water saturation
• Interpreting Sw distribution in a reservoir
• Predicting hydrocarbon recovery

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