Seal Characteristics: Processes Controlling Sealing Capacity
Comparison of hydrocarbon column heights (HCHs) calculated from seals recovered in conventional cores with HCHs calculated by using cuttings from the same interval indicates that mercury/air capillary pressure measurements of cuttings can be extremely useful to estimate seal capacity. An empirical adjustment factor (EAF), expressed in psi, needs to be added to the capillary pressure value determined on cuttings to approximate that measured with mercury/air capillary pressure of conventional cores.
For top and lateral seals that are the result of lithologic changes (as opposed to fault seals), good to excellent agreement is found between the hydrocarbons actually trapped in fields and the HCH calculated from mercury/air capillary pressure curves of vertical plugs cut perpendicular to the sealing surface. The plugs are sealed with epoxy so that mercury can enter only from the top and base of the plug. The mercury/air capillary pressure curves are generated using a system that can inject mercury at pressures up to 60,000 psi [8703 kPa] (equivalent to a hydrocarbon column of >10,000 ft for 35° API gravity oil and normal saline water).
Depending upon seal type, high-pressure mercury/air injection curves (HPMIC) of cuttings can be used to approximate those of samples from conventional cores. Injection pressures for cuttings are usually lower than those from equivalent cores for a particular percent pore volume occupied by mercury. Empirical adjustment factors (EAFs), expressed in psi, for different seal types are derived from comparisons of HPMIC on epoxy-sealed vertical conventional core plugs with cuttings or “simulated cuttings” of the same seal interval. The EAF values are added to the capillary pressure measurement of cuttings to obtain the approximate value of mercury/air capillary pressure of a vertical plug. The EAF vary from ~1900 psi (mercury/air) for type “A” seals to ~25 psi for type “D” seals, using 7.5% mercury pore volume saturation as the reference saturation.
Careful sample preparation and accurate closure corrections are critical to obtaining accurate HPMIC measurements and corresponding EAF values for HCH calculations.
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Seals, Traps, and the Petroleum System
This memoir provides the information to help explorationists greatly increase their understanding of seals and traps and thereby markedly improve their ability to forecast hydrocarbon occurrences.