Life in saline systems

Even though evaporites constitute less than 2% of the world’s sedimentary rocks, one-half of the world’s largest oilfields are sealed by evaporites, and the other half is sealed by shale (Warren, 2016). The exemplary ability of evaporites to act as seals holding back large hydrocarbon columns is seen in the Middle East, where Ghawar, the world’s largest oil field, is sealed by bedded evaporites of the Arab Formation and the overlying Hith Anhydrite seal. This seal holds back an estimated remaining reserve of more than 100-200 billion barrels. Evaporites also seal Safaniya the world’s largest offshore field, also in Saudi Arabia, with estimated reserves of more than 25-30 billion barrels of oil and 5 billion cubic feet of natural gas. Likewise, evaporites are the ultimate seal to North Field in offshore Qatar an Iran, the world’s largest single gas field (non-associated gas) with more than 500 tcf of reserves.

The importance of the evaporite-hydrocarbon association is seen in a compilation of giant oil and gas discoveries across the period 2000-2012 (Bai and Yu, 2014). Of the 120 giant oil and gas fields discovered in that period, some 54.6 % were hosted in marine carbonates and 12% in lacustrine carbonates, meaning less than a third of new giant discoveries were in siliciclastic reservoirs. Some 56% of these oil and gas giants had an evaporite seal, with 82% of the marine carbonates having an evaporite seal and 91% of the lacustrine carbonates having an evaporite seal. Clearly, carbonate reservoirs with evaporite seals constitute the majority of the giant oil and gas discoveries in the period 2000-2012, and the proportions of this association are likely to increase in conventional discoveries across the next decades.

All of the giant gas fields in thrust belts have an evaporite seal (Downey, 1984) reflecting salt’s ability to maintain seal integrity as it deforms, while underlying sediments fracture into salt-sealed reservoirs. Chengzao et al. (2008) conclude that high-quality regional salt cap rocks are essential for the formation of the super-rich, abnormally pressured, giant gas fields, such as Kela-2 in the Tarim basin, while buried anticlines under the overthrust décollement zone are favourable gas habitats. Deforming shallow salt allochthons in the Gulf of Mexico and elsewhere, not only control the seafloor topography focusing deposition of nearby sand and carbonate buildups but later can flow over these deposits to create the sea. Halokinesis creates both suprasalt and subsalt traps. The presence of an impervious salt bed or allochthon also helps maintain higher than expected porosities and pressures in underlying reservoirs.

In the gently-folded belt in front of the main Zagros deformed zone in Iran and Iraq an intimate association exists between sediment deposition, salt flow and the creation of salt-sealed reservoirs. This foreland basin is an area undergoing compression and syndepositional folding, where crests are still growing at the surface (growth folds) and so controlling the distribution and structuration of both reservoir and seal. The same collision system also created the restriction that allowed the Fars-Gasharan seal to precipitate. It is an area with a great deal of potential for future evaporite-associated discoveries, as evidenced by the 1999 discovery of the Azadegan oil field, near Ahwaz in the Zagros Foldbelt of southern Iran. Iranian authorities claim that the Azadegan field has oil-in-place reserves of about 33.2 billion barrels (5.28 billion m3) and recoverable resources estimated at about 5.2 billion barrels (830×million m3). It is the biggest oil field found in Iran in the last 30 years. To put this level of reserves into some perspective, 26 billion barrels of oil in place is more than the total known reserves in China.

Evaporite seals to giant fields discovered in the period 2000-2012. A) Dominant reservoirs for oil and hydrocarbon type. B) seal mineralogy. C) Relations of reservoir to seal mineralogy (after Bai and Yu, 2014)

Over the years, I have come to regard conventional pigeonholes for salt-associated traps (such as depositional, diagenetic, structural) as next to useless in predicting likely positions of hydrocarbons in evaporitic provinces. Instead, as a salt unit is typically the most apparent feature on a seismic section, especially in carbonate terrains, I use a much simpler classification of; “Where’s the salt?” At their simplest, evaporite-associated reservoirs all occur adjacent to either bedded or halokinetic salt, or their solution residues (Table).

With bedded or halokinetic salt or solution breccias, there are three possible positions of a potential reservoir, namely; subsalt, intrasalt and suprasalt. Within a single hydrocarbon fairway, there may be reservoirs sitting in more than one position. Where I can, I further classify each reservoir in terms of the relative roles of deposition, diagenesis and structure as controls on the location of the targeted hydrocarbon accumulations. Understanding and predicting lateral continuity of both bedded and halokinetic salt seals is a critical exploration and development tool.

Role of evaporites in creating hydrocarbon reservoirs and trap

Think about this, when a seal bed with a vertical permeability of 10-8 darcies is a mile long on each side and is cut by a single fracture 6 µm wide, then the drainage through that single fracture is equivalent to the total fluid flux through the square mile of the seal. Downey (1984) calculated that a single fracture 25 µm wide atop a 150 m oil column would leak oil at a rate of more than 150 million barrels per million years. Halite is such an excellent seal as it tends to flow rather than fracture. When it does fracture in the subsurface, the crack quickly reanneals and sutures via pressure-solution recrystallisation.

Continuity of ancient saltern and mudflat seals and their ability to resist fracturing via reannealing means salt beds can gather and constrain subsalt hydrocarbons over large drainage areas. In suprasalt traps, the role of salt in creating reservoirs and traps ranges from stratigraphic seal, through diagenetic trap, to a creator of structural traps (anticlinal [halokinetic], fault and drape traps). A critical factor in assessing the capacity of any seal, including salt, is the timing of any microfracturing or fracturing relative to hydrocarbon emplacement in an underlying or adjacent potential reservoir. In other words, does any loss of seal capacity precede or postdate possible storage of hydrocarbons in the reservoir?

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