Sealing mechanisms at pore scale, and consequences for hydrocarbon exploration

Abstract

This dissertation was motivated by the realization that four concepts, which are too simplistic, are widely assumed in the petroleum industry: First, mechanical compaction and associated disequilibrium compaction are frequently assumed to be the main mechanism for overpressure formation, although data are rarely given to support this assumption. This thesis concludes that neither the North Sea nor the Haltenbanken shales compacted mechanically at moderate to deep burial. Therefore, high overpressures in these rocks were not caused by disequilibrium compaction, but more likely by diagenetic processes that were largely unaffected by fluid pressures. Traditional seismic and log-based pore pressure detection methods in these areas should be expected to result in under-prediction of fluid overpressures because the porosities are not higher in overpressured shales than in normally pressured shales. Second, observations of zods (zones of deteriorated seismic signals; at times termed gas chimneys) are often interpreted as evidence of hydrocarbon leakage. This thesis concludes that the occurrence of zods may identify hydrocarbon leakages and where pressure compartments leak. However, prior to interpreting these zones as hydrocarbon leakage, the interpreter must be aware of the various geological processes and non-geological origins that could cause such velocity variations: (a) hydrocarbon leakage, (b) leakage of water with dissolved gas (that could create an inhomogeneous gas saturation), (c) fault or fracture zones themselves, (d) fluid leakage above fault(s) or fault junction, or (e) data quality issues. As a result, applications of zods in hydrocarbon prospect evaluation should be performed more carefully than what is often seen in the industry today. Third, the consequences of high overpressures are often assumed to be hydrocarbon leakage through the caprock – either because of hydro-fracturing or because high water pressure force oil or gas through membrane seals. This thesis concludes that high overpressures are compatible with hydrocarbon preservation. Vertical water leakage from the apex of a trap may take place while oil and gas are retained by capillary forces within the structure. This result is consistent with the fact that several of the largest oil fields on the Norwegian Continental Shelf (Statfjord, Gullfaks, Snorre, Visund, and Kvitebjørn) are highly overpressured and leaky, and yet contain vast amounts of oil and gas. Finally, vertical leakage is often assumed to occur as separate phase oil flow through water-wet caprock shales, and membrane seals will, according to the definition of capillary entry pressure, preserve oil indefinitely as long as the critical pore throats are sufficiently small. This thesis concludes that residual water, which flows through reservoirs and caprocks, carries polar compounds that locally change the caprock wettability, thus resulting in the formation of oil-wet flow paths. This suggestion explains how pore-scale migration can take place through caprock shales without resulting in extensive oil saturation in these shales. The suggestion further implies that membrane seals deteriorate with time, thus promoting the transient nature of oil pools that are observed, but that are not compatible with endless membrane sealing time.