b'Seismic window Seismic windowMichael Micenko Associate Editor for Petroleum Figure 1.Seismic amplitude map extracted along an unconformity surface. Stars are hydrocarbon mick@freogeos.com.au discoveries, circle are dry holes and ? is an undrilled amplitude.The subtle trapa case studyFor a seismic interpreter unconformities are hard, but they are also useful in setting up subtle and not so subtle hydrocarbon traps The following example from a well-explored area shows an undrilled amplitude anomaly in an area where several amplitude related discoveries have already been made (Figure 1) Three wells drilled where there were no amplitude anomalies were dry holes The amplitude supported prospect looks attractive, but is undrilled because it is complicated, and complicated means riskyFigure 2.Strike line along a fault terrace. A is the top seal shale, B is the sub-cropping reservoir, C is an Figure 2 is a strike line that runs SW-NEinterpreted shale, D Is a low amplitude sand and E is a thick regional shale.along the prospect area It shows an unconformity that truncates a series of dipping reflectors, one of which has significantly higher amplitude than the others It has been interpreted as a hydrocarbon filled reservoirThere are several elements to the trap and they all have to be working for the prospect to work It is obvious that the overlying shale (A) can provide the top seal in a simple structural trap, but in this case a thin transgressive sand could create leak points Fortunately, the transgressive sand is not present or very thin in surrounding wellsThe reservoir section (B) immediately overlies a bland, low amplitude section (C) interpreted to be a shaley siltstone seal rock In a truncation trap such as this, a base seal is required to prevent hydrocarbons leaking up-dip from the trap, although in the two sections shownFigure 3.A NW-SE dip line across the amplitude anomaly. The reservoir section B sub-crops the here (Figures 2 and 3) this base seal isunconformity which is overlain by the top seal (A). The shale section C provides a base seal. Cross fault seal down dip of the sub-cropping edge Butto the east is provided by the deeper shales in the foot wall and seal to the west is provided by down thrown that may not be the case everywhere younger shales. Structural dip to the west and north provide closure.OCTOBER 2022 PREVIEW 34'