Reservoir Geology Assessed Practical


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Lithological logs analysis

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Reservoir Geology Assessed Practical

  1. 1. Oct 2009 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. 1 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. Lionel Wandfluh © |
  2. 2. Oct 2009 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, Lionel Wandfluh © |
  3. 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. 3 Lionel Wandfluh © |