Introduction
Oil moved sharply at the start of this week, and many UK investors will have felt it without ever deciding to own a barrel of crude. If you hold a FTSE 100 tracker inside a Stocks and Shares ISA or a general investment account, your FTSE 100 oil exposure is already one of the largest single sector positions you own.
This article covers three things: what happened in the Gulf, how oil reaches a portfolio that was never pointed at oil, and how much of it a typical UK investor is carrying right now. The figures come from the index funds themselves and from official energy data.
This article is for information and education only. It is not financial advice or a recommendation to buy or sell any investment. The value of investments can fall as well as rise and you may get back less than you invested. Past performance is not a guide to future performance.
What Actually Happened In The Strait Of Hormuz
Saudi Arabia shut its East to West crude pipeline on Friday 11 September 2026 as a precaution, after a drone attack launched from Iraq. The line carries crude from eastern oilfields to the Red Sea coast, and its purpose is to reach market without passing through the Strait of Hormuz. Capacity is roughly 7 million barrels a day. No restart date has been given.
Prices responded at once. Brent crude moved towards $108 a barrel and West Texas Intermediate traded near $103, after Brent rallied close to 9 percent over the preceding week. A regional meeting between Iran and Gulf states, due to be held in Salalah in Oman, was postponed, and the United Kingdom Maritime Trade Operations centre reported a tanker attack inside the Strait on the Sunday.
20.9 million barrels a day
The US Energy Information Administration puts average oil flows through the Strait of Hormuz at 20.9 million barrels a day in the first half of 2025, about 20 percent of global petroleum liquids consumption and a quarter of all seaborne oil trade.
That is why a pipeline closure in Saudi Arabia registers on a screen in London. Hormuz is the largest chokepoint in the oil trade, and the East to West line is one of the few routes that can bypass it. Losing the bypass while the Strait itself is disrupted removes an alternative rather than barrels, but markets price the loss of an alternative all the same.
How Oil Reaches A Portfolio You Never Pointed At Oil
Most UK investors do not buy oil. They buy an index fund, and the index decides what they own. A tracker holds companies in proportion to their market value, so the shape of the index becomes the shape of the portfolio. If this is new to you, our explainer on what ETFs are covers the mechanics in plain terms.
The FTSE 100 is an unusual index by global standards. It lists the largest companies traded in London, and the London market leans towards banks, miners, tobacco, pharmaceuticals and oil. Those were the industries that happened to list there. A UK index fund therefore carries a very different set of sector bets from a global one, and FTSE 100 oil exposure is the clearest example of a bet the owner rarely chose deliberately.
Your FTSE 100 Oil Exposure, Measured
The cleanest way to measure FTSE 100 oil exposure is to read it straight off a large tracker rather than estimate it. The iShares Core FTSE 100 UCITS ETF held 11.75 percent of the fund in energy as at 11 September 2026, making energy the fourth largest sector behind financials, industrials and consumer staples.
That weight is concentrated in two names. The factsheet for the same fund, dated 31 August 2026, showed Shell at 7.21 percent and BP at 3.09 percent, a combined 10.30 percent of the whole portfolio held in two oil majors.
Source: iShares Core FTSE 100 UCITS ETF fund page, sector breakdown as at 11 September 2026. Verified 14 September 2026.
Put another way, close to one pound in every nine in a FTSE 100 tracker sits in energy, and most of that sits in two companies. This is not a criticism of the index. It is simply what the index is.
FTSE 100 Oil Exposure Versus A Global Tracker
The contrast with a global fund is the part that surprises people. A developed world tracker spreads across roughly two dozen countries, and energy is a far smaller slice of that opportunity set. The table below sets FTSE 100 oil exposure against the same measure for a global fund, with the ongoing charge alongside it. If you want to see what each platform adds on top of that charge, our UK broker fees calculator works it through.
| Fund | Energy weight | Ongoing charge |
|---|---|---|
| iShares Core FTSE 100 UCITS ETF | 11.75% | 0.07% |
| iShares Core MSCI World UCITS ETF | 4.19% | 0.20% |
Source: iShares Core FTSE 100 UCITS ETF and iShares Core MSCI World UCITS ETF fund pages. Sector weights as at 11 September 2026, ongoing charges verified 14 September 2026.
That is roughly 2.8 times as much energy in the UK fund as in the global one. An investor holding both ends up somewhere in between, weighted by how much sits in each. It is worth knowing where your own portfolio falls in that range before the next headline arrives.
Compare what UK platforms charge to hold a tracker
Capital at risk.
Why Concentration Cuts Both Ways
A heavy energy weight is not automatically a problem, and it is not automatically a benefit either. When crude rises, an index carrying 11.75 percent energy tends to hold up better than one carrying 4.19 percent. When crude falls, the same weight works in reverse. FTSE 100 oil exposure has helped the index in some years and hurt it in others, and neither outcome was predictable in advance.
The risk worth naming is not that oil is volatile. It is that a portfolio can be concentrated in ways its owner has never checked. We made the same point about technology weights during the global chip selloff, and our piece on record refining margins covers how oil profits actually reach shareholders.
The check takes five minutes
Open the factsheet for each fund you hold, find the sector breakdown, and note the energy weight alongside the date it was measured. That single figure tells you how much of this story is yours.
Diversified does not always mean spread out
A fund holding 100 companies can still place one pound in nine in a single sector. Holding many shares and holding many risks are two different things.
Questions UK Investors Are Asking
Key Takeaways
Saudi Arabia halted a 7 million barrel a day pipeline
The East to West line, which bypasses the Strait of Hormuz, was shut on 11 September 2026 after a drone attack, and Brent moved towards $108 a barrel.
A FTSE 100 tracker held 11.75 percent in energy
Measured on the iShares Core FTSE 100 UCITS ETF as at 11 September 2026, energy was the fourth largest sector in the fund.
Two companies carry most of that weight
Shell at 7.21 percent and BP at 3.09 percent made up 10.30 percent of the same fund on its 31 August 2026 factsheet.
A global tracker held 4.19 percent
FTSE 100 oil exposure was roughly 2.8 times the energy weight of the iShares Core MSCI World UCITS ETF on the same date.
Sources: US Energy Information Administration, World Oil Transit Chokepoints; iShares Core FTSE 100 UCITS ETF; iShares Core MSCI World UCITS ETF; UPI; Bloomberg. Fund and price data verified 14 September 2026.
This article is for information and education only. It is not financial advice or a recommendation to buy or sell any investment. The value of investments can fall as well as rise and you may get back less than you invested. Past performance is not a guide to future performance.
