Pressure and Fluid Contact Evolution of “Basin Centered Gas” Accumulations
Keith W. Shanley and Robert M. Cluff
The Discovery Group Inc., 1560 Broadway, Ste 1470, Denver, CO 80202
Abnormal pressure and the absence of obvious fluid contacts are two of the defining attributes of “basin centered gas” regimes. We propose that both characteristics are the result of the burial history, gas charging and subsequent exhumation of a tightly sealed sedimentary section.
Published burial history models for Rocky Mountain basins predict early hydrocarbon generation and charging associated reservoir rocks while the basins were still subsiding and receiving additional sediment load. Models for the northern Green River basin, as one example, show Cretaceous source rock intervals entering the gas window in the earliest Tertiary and continuing to generate gas until maximum burial depths were attained pre-Miocene. Coupling burial history and compaction models to a sandstone porosity prediction suggests the reservoir rocks had much higher porosity at the time of peak generation and migration than they have today - perhaps 1.5 to 2.0 times their present day porosity. This implies corresponding reservoir permeabilities were in the tens of millidarcies at the time of trap filling. Reservoir rocks with high permeability typically have thin capillary transition zones and sharp fluid contacts.
Subsequent additional burial and diagenesis results in additional chemical compaction and loss of porosity and permeability, until at maximum burial the rocks attain a state similar to their present day conditions. Reservoirs continue to receive additional gas charge as generation continues in deeper source beds and as adjacent source rocks enter the gas window. This continuing gas charge coupled with the relentless decrease in reservoir quality results in a net downward movement of fluid contacts and significant broadening of transition zones as burial proceeds.
Precise timing of exhumation of the Rocky Mountain basins are poorly constrained but likely began in the late Miocene or early Pliocene. Upon initiation of relative uplift, hydrocarbon generation at all levels drops to insignificant rates. This is because generation is a kinetic process dependent on temperature, and as source rocks cool with uplift most of the C-C bonds capable of being broken at the now reduced temperatures were split during deeper burial. Therefore no additional hydrocarbons are introduced into the reservoir system once uplift commences. Nonetheless fluid contacts continue to move down and smear out as gas expands with uplift and tries to escape from the reservoir- in most cases the only way out is down. Furthermore, reservoir pressure changes with temperature, confining pressure, and degree of sealing of each individual reservoir compartment.
The final product of the process of burial, early charging, continued reservoir degradation with additional burial, and finally uplift of relatively closed system will be what were originally totally conventional accumulations, with sharp fluid contacts and normal fluid gradients, have had their fluid contacts smeared out as gas expanded downward into progressively poorer quality rock. Tightly sealed intervals and compartments will also show abnormal fluid pressures as the trapped gas expands and attempts to escape. The end result we have come to call “basin centered gas”.