Petroleum Geology - Integrated Geochemical Interpretation Limited IGI Ltd

Petroleum Geology & Geochemistry Field Courses in Dorset

The Chapman’s Pool section lies to the east of Kimmeridge Bay and on the eastern flank of the ‘Kimmeridge Anticline’.The total Kimmeridge Clay in the east Dorset area is some 508m thick (Cox and Gallois, 1981), and the Chapman’s Pool section allows inspection of the Upper Kimmeridge Clay and its boundary with the Purbeckian.The east side of Chapman’s Pool baya slumped and vegetated Kimmeridge Clay outcrop capped by vertical cliffs of the Portland limestones.

Kimmeridge Bay: The Upper Jurassic Kimmeridge Clay is seen in an anticline with minor faults.The cliff outcrop displays laminated organic-rich and fossiliferous mudstone, and occasional limestones but with surprising sedimentological and geochemical heterogeneity for what is typically cited as a homogenous oil source rock. During the trip, we discuss bulk, molecular and isotopic characteristics as basis for oil-source rock correlations.We visit the ‘nodding donkey’ of the Kimmeridge oil field (Liassic sourced), and historic aspects of the local oil-shale industry (18th-19th Centuries).

Lulworth Cove is a geo-tourism site – very busy in the summer season and surrounded by a military training area that is often closed in the winter.It lies on the major Purbeck-Isle of Wight line of tectonic inversion. This brings a wide variety of rock to the surface - from the Upper Jurassic Portland and Purbeck carbonate beds via the Lower Cretaceous Wealden sands to the Upper Cretaceous Chalk.It also features structuring – the famous ‘Lulworth Crumple’.

At Mupe Bay we see the Lower Cretaceous in "Wealden" facies - sands and claystones with minor coals and wood. The fluvial Wealden sands are oil stained, this indicating a migration pathway in contrast to the oil trap at Osmington.  A further complexity is that differential oil staining is seen in channel sands and the associated lag deposits of bank collapse clasts.  This has been interpreted as a palaeo-oil seep, indicating very early migration within the Basin.
The outcrop lies within a military training area, and permission is required to visit.  Allow 45 minutes to walk to Mupe Bay from the car park at Lulworth Cove.  Access is via steep steps.  

Osmington Mills: Here we see the breached oil reservoir in Upper Jurassic (Oxfordian, Corallian) fluvial & shallow marine (oil-stained) sands and muds with impressive septarian nodules and a variety of intensively bioturbated surfaces.  Intra-reservoir segregation of asphaltene-rich bands can be seen within a fresh oil column. Oil analyses will be discussed in the context of source rock correlation, and the maturity of the oil biomarkers in the context of the burial history of the source rock kitchen in the basin centre (Isle of Wight).  

Portland Bill: The fossiliferous Portland Stone has been quarried since Roman times and is found in some of the most prestigious of the nation’s buildings (e.g. the rebuilt St Paul’s Cathedral in the 1680s).The stone was normally quarried, but, at the turn of the Century (2000) demand was such that a completely new mine was being trialled (Albion Stone’s Portland Stone Mine).The accessible quarries on the western side of the tip of Portland show the most prized stone to be a shallow marine oolitic micrite. Its fine grain and absence of lamination makes it a ‘freestone’ capable of being worked (carved) in all planes. The shallow water & lagoonal Purbeck beds above and the deeper water Cherty Series and Portland Sand below the freestone contain abundant fossils, ranging from turritellid gastropods, via brachipods and the areas signature giant ammonites, to dinosaur footprints and fossilized tree stumps.

Travel to West Bay or Burton Bradstock to see the Bridport Sands, a reservoir of clean shallow marine sand with carbonate-cemented 'doggers' providing horizontal barriers and small faults providing potential lateral compartmentalisation.  This unit provides one of the two main reservoirs in the Wytch Farm oilfield to the east. The shallow water fine sands show abundant bioturbation and large scale dune bedding at the base with an increasing number of storm-related shelly carbonate bands towards the trap.  The sands are underlain by clays and capped by the condensed Junction Bed of highly fossiliferous ferruginous oolites.

Further west (and lower in the stratigraphic column) at Lyme Regis, the Liassic source rock of the Wessex Basin petroleum system is well exposed in cliffs in Chippel Bay (Monmouth Beach).  Comparisons can be made with the Kimmeridge Clay Formation at Kimmeridge Bay, and distinct differences are observed.  Cyclic interbedding of black shales, claystones and limestones characterize the Blue Lias, whilst the overlying "Shales with Beef" and Black Ven Marls are more homogeneous and still contain very good source potential.

Budleigh Salterton has a good cliff exposure of the Middle Triassic Sherwood Sandstone and the underlying Pebble Beds.  The Sherwood Sandstone is the main reservoir of the Wytch Farm oilfield much further east in the basin, where the facies may show some interesting differences.  This locality also offers a useful comparison with the Bridport Sands exposed at West Bay.