Stratigraphic and Structural Controls on Overpressure at Jonah Field

DuBois, Dean P.
EnCana Oil & Gas (USA) Inc, Denver, CO

Onset of sustained high gas background and flare near top of Upper Cretaceous Lance Formation during early drilling at Jonah Field led operators to consider this to be the top of overpressure. A relatively low net sandstone-bearing interval (often named Lance Shale) in the lower Unnamed Tertiary that overlies the Lance was considered the top-seal for the gas accumulation at Jonah. Limited pore pressure data acquired during the early years of development did not dispel nor support this interpretation. Additional pore pressure data acquired since year 2000 has shown that overpressure conditions extend up into the Unnamed Tertiary. Pore pressure equals hydrostatic (pressure-depth ratio (PDR)=0.433 psi/ft) at top of Unnamed Tertiary (base of Tertiary Fort Union Formation) while all pressures below exceed PDR=0.433 psi/ft. 

The base of Fort Union (aka “Bois Marker”) is a key stratigraphic horizon because a dramatic increase in depositional energy occurred at basal Fort Union time. The Unnamed Tertiary and Lance section below is dominated by mudstone (about 65% of gross thickness) while the Fort Union above is dominated by coarse clastics and contains virtually no mudstone. 

Pore pressure data plotted against depth below base of Fort Union shows that on average pressure increases by about 1.15 psi/ft. Data from wells located immediately east of faults that bound sub-compartments within Jonah exhibit the highest rates of change in pressure with change in depth. Elevated rates of hydrocarbon migration into that sub-compartment may be occurring adjacent to the eastern side of faults that bound sub-compartments at Jonah Field.