Limitations

Weathering and transportation is followed by the sedimentation of material. The depositional environment can be defined as an area with a typical set of physical, chemical and biological processes which result in a specific type of rock. The characteristics of the resulting sediment package are dependent on the intensity and duration of these processes. The physical, chemical, biological and geomorphic variables 79 show considerable differences between and within particular environments. As a result, we have to expect very different behaviour of such reservoirs during hydrocarbon production. Depositional processes control porosity, permeability, net to gross ratio, extent and lateral variability of reservoir properties. Hence the production profile and ultimate recovery of individual wells and accumulations are heavily influenced by the environment of deposition. For example, the many deepwater fields located in the Gulf of Mexico are of Tertiary age and are comprised of complex sand bodies which were deposited in a deepwater turbidite sequence. The BP Prudhoe Bay sandstone reservoir in Alaska is of Triassic/Cretaceous age and was deposited by a large shallow water fluvial-alluvial fan delta system. The Saudi Arabian Ghawar limestone reservoir is of Jurassic age and was deposited in a warm, shallow marine sea. Although these reservoirs were deposited in very different

depositional environments they all contain producible accumulations of hydrocarbons, though the fraction of recoverable oil varies. In fact, Prudhoe Bay and Ghawar are amongst the largest in the world, each containing over 20 billion barrels of oil. There exists an important relationship between the depositional environment, reservoir distribution and the production characteristics of a field (Figure 5.3). Depositional Reservoir Production Environment Distribution Characteristic Deltaic (distributary channel) Isolated or stacked channels usually with fine grained sands. May or may not be in communication Good producers; permeabilities of 500-5000mD. Insufficient communication between channels may require infill wells in late stage of development Shallow marine/ coastal (clastic) Sand bars, tidal channels. Generally coarsening upwards. High subsidence rate results in 'stacked' reservoirs. Reservoir distribution dependent on wave and tide action Prolific producers as a result of 'clean' and continuous sand bodies. Shale layers may cause vertical barriers

to fluid flow Shallow water carbonate (reefs & carbonate muds) Shelf (clastics) Reservoir quality governed by diagenetic processes and structural history (fracturing) Sheet-like sandbodies resulting from storms or transgression. Usually thin but very continuous sands, well sorted and coarse between marine clays Prolific production from karstified carbonates. High and early water production possible. 'Dual porosity' systems in fractured carbonates. Dolomites may produce H2S Very high productivity but high quality sands may act as 'thief zones' during water or gas injection. Action of sediment burrowing organisms may impact on reservoir quality Figure 5.3 Characteristics of selected environments 80 It is important to realise that knowledge of depositional processes and features in a given reservoir will be vital for the correct siting of the optimum number of appraisal and development wells, the sizing of facilities and the definition of a reservoir management

policy. To derive a reservoir geological model, various methods and techniques are employed; mainly the analysis of core material, wireline logs, high resolution seismic and outcrop studies. These data gathering techniques are further discussed in Sections 5.3 and 2.2. The most valuable tools for a detailed environmental analysis are cores and wireline logs. In particular the gamma ray (GR) response is useful since it captures the changes in energy during deposition. Figure 5.4 links depositional environments to GR response. The GR response measures the level of natural gamma ray activity in the rock formation. Shales have a high GR response, while sands have low responses. PLAN VIEW deltaic & shallow marine ~ , A' A .~ ~-.~..:-.~ =B== ======================= _ channel SECTION GR Log ~• •,,:,,,:1, :,1, ,:.,.:,, .: ,l i:i~i , • ~ ~ . :. :.. :. : . : ~ ~ ~ ~ : ' , ' , ' ' , ' ' - - - - ~ 2 . . . . . . . . . . . . . . 2222 A I A' 0 150 "Bell" shape ":."':-"'.'" "- ~S 0eta Funne sha0e B B' Figure 5.4 Depositional Environments, sand distribution and GR log response

81 A funnel shaped GR log is often indicative of a deltaic environment whereby clastic, increasingly coarse sedimentation follows deposition of marine clays. Bell shaped GR logs often represent a channel environment where a fining upwards sequence reflects decreasing energy across the vertical channel profile. A modern technique for sedimentological studies is the use of formation imaging tools which provide a very high quality picture of the formations forming the borehole wall. 5.1.2 Reservoir Structures As discussed in Section 2.0 (Exploration), the earth's crust is part of a dynamic system and movements within the crust are accommodated partly by rock deformation. Like any other material, rocks may react to stress with an elastic, ductile or brittle response, as described in the stress-strain diagram in Figure 5.5. 03 yie_ld_ p~,o~~ ductile e l a s t i c - ~ ,,mit / \ Strain Figure 5.5 The stress - strain diagram for a reservoir rock It is rare to be able to observe elastic deformations (which occur for instance during earthquakes) since by definition an elastic deformation does not leave any record. However, many subsurface or surface features are related to the other two modes of deformation. The composition of the material, confining pressure, rate of deformation and temperature determine which type of deformation will be initiated.

If a rock is sufficiently stressed, the yield point will eventually be reached. If a brittle failure is initiated a plane of failure will develop which we describe as a fault. Figure 5.6 shows the terminology used to describe normal, reverse and wrench faults. Since faults are zones of inherent weakness they may be reactivated over geologic time. Usually, faulting occurs well after the sediments have been deposited. An exception to this is a growth fault (also termed a syn-sedimentary fault), shown in Figure 5.7. They are extensional structures and can frequently be observed on seismic sections through deltaic sequences. The fault plane is curved and in a three dimensional view has the shape of a spoon. This type of plane is called listric. Growth faults can be visualised as submarine landslides caused by rapid deposition of large quantities of water-saturated 82 sediments and subsequent slope failure. The process is continuous and concurrent with sediment supply, hence the sediment thickness on the downthrown (continuously downward moving) block is expanded compared to the upthrown block. '~ al Fault ~ _ ~ "Thrust Fault" if displaced over "~. long distance (km range) Wrench Fault Figure 5.6 Types of faulting A secondary feature is the development of rollover anticlines which form as a result of

the downward movement close to the fault plane which decreases with increasing distance from the plane. Rollover anticlines may trap considerable amounts of hydrocarbons. Growth faulted deltaic areas are highly prospective since they comprise of thick sections of good quality reservoir sands. Deltas usually overlay organic rich marine clays which can source the structures on maturation. Examples are the Niger, Baram or Mississippi Deltas. Clays, deposited within deltaic sequences may restrict the water expulsion during the rapid sedimentation / compaction. This can lead to the generation of overpressures. Fault plane Axis of rollover anticline t.:.:.:.:.:.:.:.:.:.:.:. ~