The document summarizes the geology of the Lambeth Group in the London Basin based on borehole data. The environment of deposition was a barrier island lagoon or bar-built estuaries based on evidence of burrows, shells, and plant roots indicating alternating marine and terrestrial conditions. The rock types change from east to west, with the westernmost borehole containing structureless sandstone and mudstone deposited by a river, becoming more marine and structured to the east where shells and burrows indicate a lagoon/estuary environment, and the easternmost boreholes containing coastal beach deposits. Potential reservoir sandstones exist but are not sufficiently thick, with the thickest being only 7m

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RESERVOIR GEOLOGY ASSESSED PRACTICAL
1. Background Information:
Two categories of rocks, clastic and carbonate.
Clastic Rocks – rocks made up of rock fragments and minerals derived from older deposits
Conglomerates – made up of pebbles >2mm, these pebbles are held in a Matrix of finer grained particles which
may be mud or sand. Commonly seen as river deposits or as offshore gravel bars in marine environments
Sandstones - composed of 1/16mm – 2mm grains of sand. Common minerals in sandstones include Quartz (hard,
white/translucent) and Feldspar (white-pink, opaque and soft). Small rock fragments may also occur. It is useful
to refer to the maturity of the sandstone which is a reflection of how much clay is in the composition.
Immature sandstones have a significant clay content, mature sandstones have little
Mudrocks – less than 1/16mm in size, grains of quartz and fine mud particles. Because of the platy nature of muds,
these rocks often split into thin leaves. Highly organic rich shales are black and have shiny appearance caused
by the high volume of organic matter present. Marine black shales are often good hydrocarbon source rocks
These rocks can be cemented together – this is usually a mineral formed after deposition.
Fossils & Structures
Shelly fossils – indicate estuarine to marine conditions, some species are characteristic of particular
environments
Trace fossils
Burrows are formed by marine-intertidal soft bodied organisms burrowing in the sands or mud.
Rootlets are formed by plants
Iron concretions
Common in shallow marine sandstones which have been oxidised at the surface for some time.
Similar to this is colour-mottling in sandstones and claystone. This is often red-green and associated with the
formation of soils in mudbanks in estuaries and rivers.
Glauconite
A green mineral formed in shallow marine environments
2. Assessment
Background:
You should attempt this assessment individually
You have been provided an A3 correlation panel and an accompanying key, these are available on the website.
This incorporates the lithological logs from boreholes in the Lambeth Group of the London Basin. These rocks
are about 58 million years old. The boreholes are arranged from west to east in geographical order.
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Q1- Identify the environment of deposition of the Lambeth Group.
Q2 - Give an explanation of the change in rock types, both in time (i.e. vertically) and geographically (i.e. from
east to west). Ensure that you cite your evidence for the explanations given.
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RESERVOIR GEOLOGY ASSESSED PRACTICAL
Q3 - These rocks are analogous to some current hydrocarbon reservoirs. On the cross section, identify
potential reservoir rocks
3. Findings
Q1: The environment of deposition of our correlation panel is probably a barrier island lagoon or
bar-built estuaries (an estuary with a sandbar separating the shallow water from the ocean). Burrows
& shells, validate the theory of a marine environment, and plant roots & iron concretions show that
this marine environment was exposed at the surface sometimes. A lagoon system includes both
these characteristics. It is a shallow water environment that often evolves into a swamp depending
on the amount of sediments filling it and the rise and fall of ocean level. (Hamblin & Christiansen,
2001)
Q2: Newbury borehole shows from bottom to top, a succession of medium-fine sandstone beds,
quite thick layers of mudstone, and then again medium-fine sandstone. The main characteristic of
this borehole however is the absence of structure in the sedimentary layers. Probably this depositary
environment is the mouth of a large river coming from mainland west. Indeed it does not have
enough strength anymore to carry gravels but it also does seem to have any traces of marine life and
the layers series of sediments are not as chaotic as further east. Staines borehole emphasises our
theory, compare to Newbury, it contains plant roots & soil structure and even larger layers of
mudstone but still no shells of burrows. The current of this large river is extremely weak sometimes 2
the level of the river is so thin that some layers of sediments are exposed to the surface and get
oxidised (iron concretions). 404T, A4A, 410, and A6 contains shells & burrows and the layers are
much more chaotic. This would be the lagoon/estuary. The difference type of sedimentary layers,
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3. Oct 2009
RESERVOIR GEOLOGY ASSESSED PRACTICAL
mudstone, sandstone, and the defined bedded structure could be explained by the endless battle
between sea water and river water depending on the rise and fall of ocean level and the amount of
sedimentation filling the lagoon/estuary. A few interesting things to notice are: all these boreholes
contain rocks that have been oxidised (the swamp period), 404T contains quite a large layer of coarse
sandstone which probably comes from a strong channel (Braided system), and A6 has a layer of
pebbles which could be explained by wave and storm action on the sandbar further east (Charlton Pit
& Upnor). Finally Charlton Pit & Upnor has the sedimentary deposition characteristic of coastal
beach (well- sorted, medium grained sandstone perhaps with ripples marks (Upnor) and was
probably a sandbar or shoal that protected the shallow marine environment of the lagoon/estuary
from the ocean. (Marshak, 2005)
Q3: Reservoirs rocks are usually sandstone or carbonate. Shells seem to be mixed up, on the
correlation panel, with mud and don’t seem to have form any limestone or dolomite (potential
carbonate reservoirs). On the other hand we are in presence of sandstone. However a potential
reservoir needs to be permeable, porous, and thick. The better sorted are the grains, the more
porous the reservoir will be. Only medium fine sand seems to fulfil this criterion. However none of
the successive layers of mid-fine sandstone seems to be thick enough to be a potential reservoir. A4A
seems to have the thickest layers but it is no more than 7m thick. (Stoneley, 1995)
Hamblin, W. K., & Christiansen, E. H. (2001). Earth's Dynamic Sytem 9th Edition. New Jersey: Prentice
Hall.
Marshak, S. (2005). Earth Portrait of a Planet. New York: W.W. Norton & Company.
Stoneley, R. (1995). Introduction to Petroleum Exploration for Non- Geologists. Oxford: Oxford
University Press.
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