On Friday 16th May, 2003, there gathered at the Bankes Armes Hotel, in Corfe Castle, a select band of WEGA members. They were awaiting the arrival of Geoff Swann, the Department of Trade and Industry's Chief Geologist for Onshore Oil. We were looking forward to finding out about the oil geology of Dorset and, more generally, about the geology of oil.

 

The Bankes Arms Hotel	Members pass the time of day	The castle from the village	Gathering at Kimmeridge; Geoff Swann in centre

The geology part of the excursion started on the Saturday morning when we foregathered at Kimmeridge Bay. There are many reasons for visiting Kimmeridge Bay. Ours was that here we could see a real live Oil Field.

 

The Kimmeridge Clay in Kimmeridge Bay	The Oilfield is at the top of the cliff	The Kimmeridge Bay Oilfield	The nodding donkey hard at work

But first we need some background. The most up to date source of authoritative information is to be found here. The stratigraphy of Dorset is shown here and here. For the moment we will note that the Kimmeridge Clay is shown as a seal to the Corallian reservoir just below it. A plan of the oil field is shown here, and a cross-section here. The oil was produced from the Lias, migrated upward to the Corallian, which could act as a reservoir because it had the Kimmeridgian, to act as a seal, above it.

This is an example of a Petroleum System.

(There now follows a very brief digression on Petroleum Geology. This makes use of Geoff Swann's excellent handout. The examples are taken from the Dorset area. After the digression we get back to what we did on the excursion.)

 

The Main Elements of a Petroleum System
A petroleum system is a combination of factors that must all be present in order to create and preserve oil and gas fields:-
	A Source Rock that is buried deeply enough, for long enough, to produce oil and/or gas
	A Migration Pathway to allow oil and/or gas to move from the source rock to the trap
	A Geological Structure to trap oil and/or gas
	A Reservoir Structure to store oil and/or gas
	A Caprock to stop oil and/or gas leaking from the trap
	BUT ....... TIMING is everything

From the above it can be seen that a petroleum geologist needs to know everything about the geology of his area.

Source Rock

In the case of the Kimmeridge Bay Oil Field the Source Rock is the Lias and the Caprock is the Kimmeridge Clay. But in the North Sea the Kimmeridge is the pre-eminent source rock. How does a source rock produce oil? In most circumstances organic material falling to the sea bed will rot or be eaten or otherwise oxidised. In order for the organic material to be incorporated in the sediment we need to have a reducing layer of water at, or above, the sea bed. And the sea bed needs to be below the depth to which light can penetrate. A diagram of Kimmeridge Clay stratigraphy and possible scenarios for the Kimmeridgian sea bed are given here.

The source rock needs to be buried to a sufficient depth so that it can "cook" and so produce hydrocarbon. A diagram illustrating hydrocarbon generation is given here.

Migration Pathway

Once oil (or other hydrocarbon) has been generated in its source rock, it has to percolate out and get to some place where we can get it. Source rocks are, almost by definition, non-porous and non-permeable. Given sufficient time, and propitious circumstances, the hydrocarbon will, nevertheless, move out. This diagram, of the Wytch Farm Oilfield illustrates the devious ways of such migration pathways. The oil has moved vertically along faults and horizontally along sandstones. It was trapped in the sandstones but later movements have allowed some of the sandstones to be emptied. It is one of the arts of the Petroleum Geologist to predict how hydrocarbon will move and thus know where to drill. It is obvious that one needs an intimate knowledge of the sub-surface geology

Geological Structure

Once hydrocarbon starts moving it will continue to do so until it gets to the surface, where it will seep away, or it runs into a trap. A trap is a geological structure which, once the oil is in it, will not let it out again. The parameters of a trap are shown here. This is an anticlinal trap but there are many other sorts - against faults, abutting lithological changes, salt domes etc.

Reservoir Structure

It is all very well having your hydrocarbon trapped but you have to be able to get the stuff out once you have found it! What is needed is a reservoir rock with sufficient porosity and permeability. The porosity gives a measure of how much hydrocarbon the trap will hold - it is the amount of space available. The permeability is a measure of how hydrocarbon can flow through the rock into the borehole. Assuming you have a reservoir of sufficient porosity and permeability, a knowledge of its shape, size and variations across it will be required to exploit it efficiently.

Caprock

The caprock is what stops the hydrocarbon escaping from the trap.

Timing

You can have all of the above but if the timing is wrong you will not get an oilfield. This, perhaps, explains the strangeness of the Dorset Oil Province. There is one large field - Wytch Farm - and a few other tiny fields, but nothing in between. See here. In the case of Wytch Farm one can see that late stage folding or tilting emptied some of the reservoirs near to the main field. See here. It is things like this which can change a decent oil field to a might have been. But it is beyond this writers competence to take this further.

From Kimmeridge Bay and the consideration of source rocks, we moved on to Lulworth Cove which is a great place for appreciating some of the more spectacular aspects of the structure of the Dorset coast. And the building of petroleum traps.

The structural geology of the Wessex Basin is shown here. A glance at the geology of Lulworth Cove (map here, and section here) shows that the structure is east - west folding and faulting which has taken place at two separate times. The earlier is in the Lower Cretaceous, just before the Gault and Greensand; the later is after the Upper Chalk. An east - west cross-section showing the Lower Cretaceous unconformity is shown here.

Most of our time was spent at Stair Hole looking at the Lulworth Crumple and speculating how the two episodes could build and destroy petroleum traps. Then the heavens opened and we had to retreat to the Visitor Centre and learn all about the mysteries of seismic exploration.

 

Stair Hole - The Lulworth Crumple - the Middle and Upper Purbeck Beds	Stair Hole - the sea breaks through the Portland Limestone	Stair Hole - the western end	The western side of Stair Hole

Having seen source rocks and trap formation, heard about exploration, it was time to look at reservoirs. So off to Osmington Mills to look at the Corallian. This is the reservoir rock for the Kimmeridge Bay oilfield. At Osmington Mills there are vestiges of oil. Some horizons look, and smell, oily. Apparently it smells oilier if the weather is warm and the oil is less viscous. Also at one point along the cliffs there is an oil seep into the sea. We were taken to the very spot but it was not flowing for us. It was these indications that started the serious search for oil along the Dorset coast.

How long will the barrier last?	It does sometimes rain on our excursions	The Corallian east of Osmington Mills with oily sands	Doggers in the Corallian

On the Sunday morning we drove to West Bay, near Bridport, to look at the Bridport Sands. These are the reservoir rocks for the Wytch Farm oilfield, and here we were told all about the difficulties of predicting the amount of oil in any given oil field. What is the volume of the reservoir rock? How much of it has oil in it? What is the pore volume? How much will the oil expand when brought to the surface? What is the water saturation?

There are many unknowns and many ways of getting the numbers wrong. And for gas there is an equally complex sum to be done. Assuming one gets all the numbers, a number indicating the amount of oil is obtained.

But the amount of oil recovered will be a different, and smaller, number because of technical factors such as permeability, reservoir pressure, viscosity and - most importantly - economics.

 

The Smugglers Inn at Osmington Mills	The Bridport Sands	Geoff Swann explains it all - near West Sands Bay	Trough in the Bridport Sands

This excursion was a delight. We had seen the rocks before but not through the eyes of a petroleum geologist. What is important for one branch of the profession is not the same for another. Even the least considered aspect for one geologist can be the deciding factor for another.

And Geoff Swann was able to explain it all and make us think, for a little while that we understood the Petroleum Geologist's job.

The Dorset GA have produced a very useful guide book to the geology of Dorset which has been invaluable in writing this article. You can get it here.   Similarly the GA has its official guide book written by the late Professor House. You can get it here, and help WEGA financially.