Wessex Basin Excursion Report

SECTION 1 - INTRODUCTION

The Wessex Basin excursion examined the exposed stratigraphic sequences displayed in the outcrops in the South Dorset coastal area. The Wessex Basin is part of a larger sedimentary system which stretches through most of southern England, across the English Channel to parts of Northern France. The sedimentation central to the formation of the petroleum system began in the Devonian and ended in the Tertiary. The Wessex Basin petroleum system is host to the largest onshore oilfield in Western Europe, Wytch Farm; however, prospectivity in the area is limited. This paper shall outline the techtono-stratigraphic sequences and the evolution of the petroleum system outlining the factors which have limited the potential for hydrocarbon accumulations in this basin.

SECTION 2 - PRIOR TO PERMIAN

The basement of the Wessex basin is the Devonian-Carboniferous Hercynian basement of Dartmoor Granite. Outcrops of this can be observed in Dartmoor beyond western extent of this report. The basement is important to this system as it has gentle dips in which the primary sedimentation occurred. These dips formed from the compression of the area during a period of orogeny during the late Palaeozoic, known as the Variscan Oregeny, when the continental shelves which now form Europe and Africa collided to form the supercontinent Pangaea. There are steep faults resulting from this compression, which run North-South (Stonely, 1992), these faults play an crucial role during the extensions of the Permo-Triassic and Jurassic.

SECTION 3 - PERMIAN

The Permian sub-era of the Palaeozoic occurred between about 300 and 250Ma. During this time the British Isles were part of a super continent called Pangaea. A period of orogeny caused by the Variscan continental collision ended in the Carboniferous, the mountains which resulted to the south-east of Wessex basin prevented precipitation in the area. The area became an arid, desert climate which stretched across the continent covering all of northern Europe including the Wessex Basin until the Lower Jurassic.

3.1 Dawlish Sandstone

An outcrop of Dawlish Sandstone was observed at Bishop's Court Quarry. It was deposited 260 million years ago, it is a typical continental redbed.

3.1.1 Lithology

The outcrop displays large scale cross-bedding and is homogeneous over the visible area, the cross-bedding is presented in Figure 1. This Sandstone is red and bedded., it is weakly cemented and breaks up easily. The texture is loose and grainy; the grains are coarse to medium in diameter (<1 mm), well rounded and appear spherical under a lens. The grains are uniform in size and are well sorted so no small particles fill the gaps between grains. Lack of cementation result in this sandstone, and the sorted nature mean the porosity (25 - 30%) and permeability (±1000 mD) are high.

3.1.2 Origin

This sandstone formed from fragments of other, pre-existing, rocks which were eroded and deposited by aeolian processes, this is indicated by the highly sorted nature of the sandstone and size of sand grains. The accumulation took place in sand dunes analogous with those seen today in the Sahara Desert. The red colour of the sandstone is evidence that the area was extremely arid, the colour is a result of haematite coating which builds up in a desert environment. Figure 2 represents how the cross-bedding formed, with sand accumulating on the leeward slope of the sand dune.

Digenesis followed sedimentation the pressure of overlying sediments which compacted the sand into sandstone. The sandstone is not cemented which is evidence of the arid nature of the formation environment.

3.1.3 Petroleum System Setting

This sandstone would make an excellent reservoir rock, the high porosity and permeability are two features which qualify a reservoir unit. The lack of cementation means that the pore space remains free to host hydrocarbons, however in the Wessex basin this is not the main reservoir, it is stratigrapicaly below the source rocks due its age and despite the later inversion, migration routes are difficult to conceive.

3.2 Aylesbeare Mudstone Group

The Aylesbeare Mudstone Group was not observed on the field trip; nevertheless this unit has the potential to play an important role in the petroleum system. This group formed in the Late Permian and is the upper seal to the Dawlish Sandstone, it is of sufficient thickness and impermiability to seal off hydrocarbon migration.

SECTION 4 - TRIASSIC

The Triassic is a sub-era of the Mesozoic and occurred between about 250 and 200Ma. The environment in Northern Europe changed significantly in the Early Triassic. The area changed intermittently from a desert to one dominated by rivers and ended the sub-era as a shallow marine environment, this had a significant impact on the sedimentary sequences observed in the Wessex Basin.

4.1 Budleigh Salterton Pebble Beds (BSPB)

The pebble beds are an eastward dipping conglomerate and can be observed in the exposed cliffs at Budleigh Salterton beach dipping 10o towards the east. The BSPB make up the lower part of the Sherwood Sandstone Group and can be observed in both thick sheets and narrow lenses (<3m thick) throughout the cliffs at Budleigh Salterton.

4.1.1 Lithology

The constituents of the BSPB are large quartzite cobbles and pebbles (<150 mm) in a sandstone matrix. They are a horizontally bedded conglomerate. Figure 3 shows in detail the lithology of the BSPB and the proceeding bed, a Limonite Mudstone.

Figure 4 is a field sketch of the banding observed in the lower portion of Budleigh Salterton cliffs, demonstrating the complex environmental cycles in the Sherwood Sandstone Group.

4.1.2 Origin

The origin BSPB is fluvial, the shape and size of the pebbles suggests that the pebbly sands were deposited by a large, fast flowing river system which flowed from the Variscan Mountains across southern Britain in a northerly direction. These rivers were monsoonal, which is the explanation for the layered sequences observed in the conglomerate. The larger banding mudstones occurred in a marine transgression, the limonite layer was produced by meteoric water ingress which reduced the oxidation state of the haematite. The fossils in BSPB group are brachiopods and worm burrows this indicates that the sandstones were deposited in shallow seas and transported to the Wessex basin by a large river which flowed through the desert.