Abstract: Pressure Trends in Lower Viosca Knoll and Mississippi Canyon, Gulf of Mexico Deep Water: Implications for Seals, Column Heights and Hydrocarbon Migration
WAGNER, BRUCE E., Amoco Production Company
Analysis of pressure data from forty-one deepwater wells in the northern Gulf of Mexico (GOM) reveals consistent patterns in the rates of increase of both pore pressure and fracture gradient with increasing depth. Several conclusions may be drawn from these patterns, including:
Pore pressure (PP) and fracture pressure (FP) trends are not parallel with increasing depth. PP and FP converge at the mudline. The small differences between pore pressure and fracture pressure in the shallow section of a well directly influence the openhole drilling distance allowable between casing sets. In addition, these narrow tolerances can exacerbate problems in controlling shallow water or gas flows that may be encountered.
Pore pressure is elevated above a “normal” 8.5 ppg hydrostatic trend at shallow sediment burial depths. Indications are that top seals form with as little as 1500'-2000' of sediment burial. Such early seal formation is favorable to the formation of stratigraphic traps. It also sets a maximum depth for conventional riserless drilling.
Pore pressure and fracture pressure trends diverge, DPP < DFP, with increasing depth in the mildly overpressured section. This section has high sedimentation rates which may be a factor. This separation directly influences column height. As the difference between pore pressure and fracture pressure increases, the maxinnun column height also increases.
Conversely, pore pressure and fracture pressure converge, DPP > DFP, in deeper sections associated with lower sedimentation rates and unconformities. As a result, maximum possible column heights decrease in these intervals and may actually preclude sealing any significant volumes of hydrocarbons.
The trends of increasing/decreasing sealing capacity have application to models of generation, expulsion, primary and secondary migration, and accumulation of hydrocarbons in the GOM deepwater. Areas of reduced sealing capacity in the deeper sedimentary section will “frac”, allowing vertical migration of fluids to zones with higher sealing capacities.
AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah