Prediction of the hydrocarbon system in exhumed basins, and application to the NW European margin

Abstract

Uplift, erosion and removal of overburden have profound effects on sedimentary basins and the hydrocarbon systems they contain. These effects are predictable from theory and from observation of explored exhumed basins. Exhumed basins are frequently evaluated in the same way as ‘normal’ subsiding basins, leading to errors and unrealistic expectations. In this paper we discuss the consequences of exhumation in terms of prospect risk analysis, resource estimation, and overall basin characteristics.Exhumation should be taken into account when assigning risk factors used to estimate the probability of discovery for a prospect. In general, exhumation reduces the probability of trapping or sealing hydrocarbons, except where highly ductile seals such as evaporites are present. Exhumation modifies the probability of reservoir in extreme cases; for example, where a unit may have been buried so deeply before uplift that it is no longer an effective reservoir, or where fracturing on uplift may have created an entirely new reservoir. The probability of sourcing or charging is affected by multiple factors, but primarily by the magnitude of the post-exhumation hydrocarbon budget and the efficiency of remigration. Generally gas will predominate as a result of methane liberation from oil, formation water and coal, and because of expansion of gas trapped before uplift. These factors in combination tend to result in gas flushing of exhumed hydrocarbon basins.Compared with a similar prospect in a non-exhumed basin, resource levels of a prospect in an exhumed basin are generally lower. Higher levels of reservoir diagenesis influence the standard parameters used to calculate prospect resources. Porosity, water saturation and net-to-gross ratio are adversely affected, and (as a consequence of all three) lower recovery factors are likely. Hydrostatic or near-hydrostatic fluid pressure gradients (as observed in exhumed NE Atlantic margin basins) will also reduce the recovery factor and, in the case of gas, will adversely affect the formation volume factor.Hydrocarbon systems in exhumed settings show a common set of characteristics. They can include: (1) large, basin-centred gas fields; (2) smaller, peripheral, remigrated oil accumulations; (3) two-phase accumulations; (4) residual oil columns; (5) biodegraded oils; (6) underfilled traps. Many basins on the NE Atlantic seaboard underwent kilometre-scale uplift during Cenozoic time and contain hydrocarbon systems showing the effects of exhumation. This knowledge can constrain risk and resource expectation in further evaluation of these basins, and in unexplored exhumed basins.

Role of Faults in Hydrocarbon Leakage in the Hammerfest Basin, SW Barents Sea: Insights from Seismic Data and Numerical Modelling

“…Additionally, gas cap expansion due to overburden removal by erosion would contribute to elevated pressures [11,75], enhancing the likelihood of fault slip and increase in permeability.…”

“…Following severe uplift of the Hammerfest Basin, gas expansion and flushing of oil towards the flanks of the basin comprise an additional key mechanism in driving the hydrocarbon re-migration and leakage from structures [11,19,75]. This can explain the oil accumulation in Well 7120/2-1 (Figure 1), Johan Castberg (formerly Skrugard), the Wisting and Goliat oil fields [6], which are located updip from the main kitchen areas [29,34].…”

Synchronous exhumation events around the Arctic including examples from Barents Sea and Alaska North Slope

“…Examples include Barents Sea (Cavanagh et al 2006), Svalbard (Blythe & Kleinspehn 1998), North Slope Alaska (O'Sullivan et al 1993;O'Sullivan 1996O'Sullivan , 1999, Sverdrup Basin (Arne et al 2002), West Greenland (Japsen et al 2005) and East Greenland (Thomson et al 1999;Hansen et al 2001). Implications for exploration include enhanced maturity levels (compared with assessments that underestimate former burial depths), possible loss of reservoired hydrocarbons as a result of seal breach and spillage, and phase changes due to pressure reduction (see Doré et al 2002, for an extensive review of the influence of exhumation on hydrocarbon systems). In addition, the timing of generation becomes a key issue in such areas, particularly for plays involving structures formed in the initial stages of exhumation, as these structures were not available for charging during the main phase of generation, which occurred during burial prior to the onset of exhumation.…”

Episodic uplift and exhumation along North Atlantic passive margins: implications for hydrocarbon prospectivity

“…In terms of hydrocarbon systems, failure to account for the greater depths of burial prior to exhumation can lead to serious underestimation of maturity levels, while the effects of exhumation on the timing of hydrocarbon generation, changes in migration routes and on any reservoired hydrocarbons also require assessment (e.g. Doré et al 2002). In terms of tectonics, the results presented here show that not only southern Norway but also major portions of the offshore shelf underwent exhumation in the early Neogene, and any attempt to understand the underlying processes must take into account the linkage 11.…”

“…The interpretation of what happened during the formation of a hiatus can thus have a profound influence on our understanding of the hydrocarbon system in an area (cf. Doré et al 2002): when did maximum burial occur? How much section has been removed?…”